Section 1: Principles of nutrient management and fertiliser use

Transcription

1 Section 1: Principles of nutrient management and fertiliser use The need for fertilisers 3 Understanding soil physical properties 3 Soil acidity and liming 5 Nitrogen for field crops 8 Phosphorus and potassium 17 Magnesium 22 Sulphur 22 Sodium 24 Trace elements 24 Fertiliser types and quality 25 Fertiliser application 27 Protection of the environment 27 1

2 THE NEED FOR FERTILISERS All plants require adequate amounts of water, light, carbon dioxide and nutrients in order to allow them to grow to their maximum potential. A shortage or excess of one or more of these raw materials, or the presence of disease or any other restriction to growth, can cause serious reductions in crop growth, yield and the quality of the crop produce. Many crops show large and very profitable responses to the correct use of lime and fertiliser in terms of both the yield and quality of the crop produce. Nitrogen, phosphate, potash, sulphur and magnesium are the most commonly used fertiliser nutrients but lime is also widely needed to correct soil acidity. The essence of good nutrient management and fertiliser use is to ensure that the necessary quantities of the essential crop nutrients are available when required for uptake by the crop, and that losses of nutrients to the environment are minimised. The application of a nutrient as fertiliser is normally justified where the supply of the nutrient from all other sources is expected to be insufficient to meet the crop requirement. Where the crop requirement will be met from nutrients already in the soil, or supplied from other sources such as organic manures, then there is no need to apply fertiliser. Any application of fertiliser which exceeds the crop nutrient requirement or which is not properly applied, will waste money and, for nitrogen and phosphorus, will increase the risk of pollution. There is a wide range of essential plant nutrients. Major nutrients such as nitrogen, phosphorus, potassium magnesium, sulphur and calcium are required in relatively large quantities, whilst trace elements such as manganese, copper and boron are required in very small quantities. It is important to avoid either a deficiency or an excess of each nutrient. Although a deficiency of any one major nutrient or trace element can severely restrict crop growth, a nutrient excess can also result in crop growth problems (e.g. lodging of cereals, disease, poor crop quality) and may, for nitrogen and phosphorus, increase the risk of environmental pollution. Maintaining an appropriate balance between nutrients is also important. Nutrients can interact with each other so that an excessively high level of one nutrient may reduce the availability of another nutrient for crop uptake. UNDERSTANDING SOIL PHYSICAL PROPERTIES A good knowledge of the soil type in each field is essential for making accurate decisions on lime and fertiliser use. It will not be possible to use this book effectively unless the physical properties of the topsoil and subsoil are known (see Appendix 1 for a description of soil types used in this book). Soil properties often change between the cultivated topsoil layer and the subsoil. Unless good soils information is already available, investigations by augering or digging inspection holes will be needed. The most important soil physical properties are: Soil texture The proportion of sand, silt and clay in each soil layer. Texture influences: the likelihood of drought effects the likely natural fertility (soil nutrient status) the risk of leaching of soluble nutrients the potential of the soil to be acid options for soil management systems soil structural stability, drainage and aeration the likelihood of trace element deficiencies 3

3 Soil organic matter Organic matter influences: the amount of nitrogen released (mineralised) for crop uptake the likelihood of trace element deficiencies the need for lime soil structural stability drainage and aeration Potential rooting depth Many crops can root to 1 metre deep or more. In some soils, rooting is restricted by hard rock or compact layers. Rooting depth influences: the volume of soil exploited by the crop for water and nutrients the likelihood of drought effects the risk of leaching of soluble nutrients Soil stone content A high content of hard, impermeable stones in the soil may cause: a more rapid leaching of soluble nutrients a higher risk of drought Soil parent material The underlying geological material from which the soil has been formed. Parent material can influence: the natural soil ph level the efficiency with which crops use fertiliser nitrogen the quantity of potassium reserves in the soil Investigating Soil Physical Properties Once investigated, known and understood, inherent soil physical properties such as texture, potential rooting depth and soil organic matter will not substantially change for many decades. If unfamiliar with the soil types on a farm, it is worthwhile spending time investigating the nature and patterns of the soils that need to be managed and fertilised. Existing information is available from soil surveys carried out within the last 50 years. Representative and detailed surveys have been carried out on most landscape types in England and Wales. Published reports of these surveys give maps and information on a wide range of soil properties (available from the Soil Survey and Land Resource Centre at Cranfield University). The Soil Code (MAFF PB0617) contains general guidance on practices which will maintain and increase the ability of the soil to support plant growth. In contrast, soil chemical properties (ph and nutrient levels) can change more quickly and should be monitored by laboratory analysis on a routine basis. Soil structural condition can also be quickly changed depending on management practices. Unless different parts of the field are going to be managed in different ways, the predominant soil type in each field needs to be identified when making lime and fertiliser decisions. Where there is a large variation in soil type within one field, it may be necessary to make different decisions for each soil type. The correct assessment of soil texture is important. This can be done in the laboratory but, with adequate training and practice, hand texturing of soil in the field is usually sufficiently accurate. The method for hand texturing is given in Appendix 1. A texture triangular diagram, defining the particle size distribution for each named texture class, is given in Appendix D of Controlling Soil Erosion (MAFF PB4093). An estimate of organic matter content can be based on the feel and colour of the soil, though laboratory analysis will give a more accurate measure. 4

4 Potential rooting depth is best assessed in early summer or when roots are fully grown. Digging inspection holes is the only effective method of assessment. Roots of most arable crops will explore soil to at least 1 metre depth though crops such as potatoes and some vegetables are more shallow rooted. SOIL ACIDITY AND LIMING (See section 6 for additional soil ph and liming recommendations for Fruit, Vines and Hops, and section 7 for recommendations for Grassland) Soil ph is a measure of acidity or alkalinity. It ranges from about ph 4 (very acid), when most crops will fail, to about ph 8 for soils which are naturally rich in lime or have been over-limed. Soil ph influences the availability of most plant nutrients which can be reduced where the ph is either too high or too low. Although natural processes (e.g. rainfall, leaching and crop growth) and some farming practices (e.g. high nitrogen rates) tend to make soils gradually more acid, some topsoils are naturally rich in lime (calcareous) and will never need liming. Correct liming practice is important in order to maintain optimum ph levels in the topsoil in all parts of the field. Insufficient use of lime on acid soils can cause large yield losses, but excessive use is wasteful and can aggravate trace element deficiencies. Optimum soil ph levels vary depending on soil type and cropping as shown below. Optimum soil ph a Mineral soils Peaty soils Continuous arable cropping Grass with occasional barley crop Grass with occasional wheat or oat crop Continuous grass or grass/clover swards 6.5 b a) The optimum ph is based on soil that has been correctly sampled (see Appendix 3) and where ph has been measured following drying and grinding in a 1 soil : 2.5 water suspension shaken for 15 minutes as described in The Analysis of Agricultural Materials (MAFF RB427). In some soil samples, fragments of free lime can give a misleadingly high ph when analysed following grinding in the laboratory. b) In arable rotations containing acid sensitive crops such as sugar beet, maintaining a ph of between 6.5 and 7.0 may be justified. 5

5 Lime Recommendations The amount of lime needed will depend on the current soil ph, soil texture, soil organic matter and the optimum ph needed. Clay and organic soils need more lime than sandy soils to raise the soil ph by one point. A normal recommendation should give the tonnes/ha of ground limestone or ground chalk needed for a 20 cm depth of soil (cultivated land) or a 15 cm depth of soil (grassland). The table below gives the recommended amounts of lime (t/ha of ground limestone or chalk, NV 50-55) needed to raise the soil ph of different soil types to achieve the optimum ph level. Soil ph Sands and loamy sands Sandy loams and silt loams Clay loams and clays Organic Soils Peaty soils Arable Grass Arable Grass Arable Grass Arable Grass Arable Grass t/ha In cultivated systems where soil is acid below 20 cm, a proportionately greater quantity of lime should be applied. Where 10 t/ha or more is required, half should be deeply cultivated into the soil and ploughed down with the remainder applied to the surface and worked in. No more than 7.5 t/ha per year should be applied to established grassland or other situations where soil cultivation is not carried out. Liming Materials The effectiveness of a liming material is dependent on its neutralising value (NV), its fineness of grinding and the hardness of the parent rock. The NV is the relative effectiveness of a liming material compared to pure calcium oxide (CaO). Lime recommendations are usually given in terms of ground limestone or ground chalk (NV 50-55), but other forms of lime can be used provided the application rate is adjusted to take account of differences in NV of the materials. The Fertilisers Regulations (as amended) give details of the meaning and required declarations of different named liming products. In addition, materials such as sugar beet factory lime and lime treated sewage cake have a useful liming content. Appendix 7 gives typical NVs of some common liming materials. The booklet Agricultural Lime the Natural Solution (Agricultural Lime Association) gives more information on liming materials. Where alternative materials are available but at different cost, they can be compared by calculating the cost per unit of NV and assessing any differences in fineness. Some liming materials contain other useful nutrients which should be taken into account when selecting the most suitable source of lime. For example, magnesian limestone contains large amounts of magnesium and is effective for correcting soil magnesium deficiencies. However, many years of using magnesian limestone can result in excessively high soil Mg Indices which may induce potassium deficiency in crops. 6

6 Example Ground limestone has an NV of 50 and costs 12/t delivered and spread. An alternative liming material (A) has an NV of 30 and costs 10/t delivered and spread. Ground limestone costs (12 x 100) / 50 = 24 pence per unit of NV. Liming material A costs (10 x 100) / 30 = 33 pence per unit of NV. Provided the two materials have the same physical characteristics, the ground limestone is the most cost-effective liming material. Lime Application Lime should be applied well before sowing or planting since it can take some months to have its full effect on soil ph. Crops vary in their sensitivity to soil ph which can influence the way that lime is applied within a rotation. For instance: potatoes can tolerate acid soils. Since lime can increase levels of potato scab, avoid applying lime immediately before potatoes are grown sugar beet and barley are sensitive to soil acidity. The lime requirement should be applied well before these crops are sown clover is more sensitive to soil acidity than pure grass swards It is unwise to grow a crop which is sensitive to acidity immediately after liming a very acid soil. Where a rapid effect is required, use of a fast acting liming material may be worth considering. 7

7 NITROGEN (N) FOR FIELD CROPS (Section 7 contains additional information for grassland) Basis of the Recommendations Nitrogen usually has a larger effect on crop growth, yield and crop quality than any other nutrient. The chart below shows a typical nitrogen response curve. It is clear that using nitrogen gives a large increase in yield but that using too much nitrogen can reduce yield. Additionally, using too much nitrogen will be financially wasteful and can aggravate problems such as lodging of cereals, foliar diseases and poor silage fermentation. Excessive use of nitrogen will also increase the risk of causing nitrate pollution of water. A Typical Nitrogen Response Curve and Corresponding Nitrate Leaching Losses C D Crop Yield B The economic optimum nitrogen rate (190kg/ha N) Nitrate leached A Nitrogen (kg/ha N) Without nitrogen (A) yield is very low. At low nitrogen rates (B), there is a large and very profitable yield response to increasing rates of nitrogen. At the economic optimum nitrogen rate (C), the cost of any more nitrogen would be greater than the value of the extra crop yield produced. This is the rate of nitrogen that will give maximum profit, supplied as fertiliser and/or from organic manures. At nitrogen rates up to and including the economic optimum rate, there is a roughly constant amount of residual soil nitrogen that may be lost by leaching after harvest. Nitrogen rates above the economic optimum rate (D) are not justified and can result in losses of yield and quality. There may also be an increased need for agro-chemicals such as fungicides and growth regulators. Use of nitrogen above the economic optimum rate will result in lower profits. At nitrogen rates above the economic optimum, there will be a large surplus of residual soil nitrogen after harvest that is likely to cause substantially more nitrate leaching and pollution of water. 8

8 All recommendations given in this book are for the economic optimum rate of nitrogen. It requires substantial changes in the value of the crop produce or the cost of fertiliser to alter the recommendations. Where appropriate, different recommendations are given so that the crop quality specifications of different markets can be targeted. Nitrogen Supply and Loss Processes It is convenient to think about nitrogen in terms of Nitrogen Supply and Nitrogen Losses. Nitrogen may be supplied from the soil, atmosphere and organic manures as well as from fertiliser. However, nitrogen may also be lost by leaching, ammonia volatilisation and denitrification. Nitrogen Supply Soil Mineral Nitrogen (SMN). Nitrate-N (NO 3 -N) and ammonium-n (NH 4 -N). Depending on the recent history of cropping, organic manure and nitrogen fertiliser use, soils can contain small or large quantities of these forms of nitrogen which are potentially available for crop uptake. Nitrogen mineralised from organic matter. Mineralisation results in the conversion of organic nitrogen to soil mineral nitrogen. This source of nitrogen can be large: on organic and peaty soils where organic manures have been used for many years where nitrogen-rich, leafy crop debris has been left after harvest (e.g. sugar beet tops) Nitrogen from the atmosphere. Small amounts of nitrogen are deposited in rainfall and directly from the atmosphere. Leguminous crops can fix atmospheric nitrogen into a form that can be used directly by crops. Peas, beans and clover are examples of leguminous crops. Nitrogen from organic manures. Most organic manures contain large quantities of nitrogen and other nutrients. Some of this nitrogen is equivalent to fertiliser nitrogen; the remainder of the nitrogen becomes available more slowly (see section 2). Fertiliser nitrogen. Fertiliser nitrogen is used to make up any shortfall in the crop s requirement for nitrogen. Nitrogen Losses Leaching. Nitrate-N is soluble. Once the soil is fully wetted, nitrate may leach into field drains or sub-surface aquifers as drainage water moves through the soil. Leaching is more rapid on light sand soils compared to deep clay or silt soils which are less free draining and therefore more retentive of nitrogen. The amount of winter rainfall has an important influence on the proportion of soil nitrate-n leached. Although ammonium-n is strongly fixed to clay particles and as such is resistant to leaching, under normal conditions ammonium-n in the soil is rapidly nitrified to nitrate-n. In practice, sources of nitrogen as ammonium- N will have a similar risk of leaching as sources containing nitrate-n. Denitrification. In anaerobic soils (poorly aerated soils), nitrate-n can be denitrified and lost to the atmosphere as nitrous oxide and nitrogen gases. Denitrification is most significant in wet and warm soils where there has been a recent nitrogen application. Ammonia volatilisation. Nitrogen may be lost to the atmosphere as ammonia gas. Significant losses commonly occur from livestock housing, livestock grazing and where organic manures are applied to fields. There can also be losses when urea is used as a straight nitrogen fertiliser. All supplies and losses of nitrogen, and the efficiency of fertiliser nitrogen recovery by the crop must be considered when calculating how much fertiliser nitrogen to use. 9

9 Factors Influencing Nitrogen use Decisions The nitrogen requirement of a crop is the amount of nitrogen that is economically justified under given economic conditions (the economic optimum ). This will depend on: 1 The crop requirement for nitrogen. 2 The amount of nitrogen that the soil can supply for crop uptake. 3 Fertiliser costs and the likely value of the crop produce. The crop nitrogen requirement can be considered as the maximum amount of nitrogen contained in the crop when the economic rate of nitrogen fertiliser has been applied. However, when assessing the nitrogen recommendation, the following other factors are also important. Crop rooting depth and the efficiency of nitrogen use by the crop. Market requirements for crop quality. The prevailing economic conditions. Assessing the Soil Nitrogen Supply (SNS) For purposes of using this book, the Soil Nitrogen Supply is defined as follows: The Soil Nitrogen Supply (SNS) is the amount of nitrogen (kg/ha N) in the soil that becomes available for uptake by the crop from establishment to the end of the growing season, taking account of nitrogen losses. Research has shown that there is a close relationship between the measured amount of SNS within the crop rooting depth and the amount of nitrogen taken up by the crop in the absence of applied fertiliser. This relationship confirms the importance of knowing the SNS. The SNS is different to, but includes Soil Mineral Nitrogen (SMN). The calculation of SNS must include three separate components of nitrogen supply as follows. Soil Nitrogen Supply (SNS) = SMN + estimate of total crop nitrogen + estimate of mineralisable nitrogen where: Soil Mineral Nitrogen (as kg/ha N) is the nitrate-n plus ammonium-n content of the soil within the potential rooting depth of the crop, allowing for nitrogen losses. Total crop nitrogen (as kg/ha N) is the total content of nitrogen in the crop when sampling for SMN is carried out. Mineralisable nitrogen (as kg/ha N) is the estimated amount of nitrogen which becomes available for crop uptake from mineralisation of soil organic matter and crop debris during the growing season. The SNS depends on a range of factors which will commonly vary from field to field and from season to season. It is therefore important to assess the SNS for each field and in each year. The key factors influencing SNS are: the nitrogen residues left in the soil from fertilisers and organic manures used for the previous crop the use of organic manures for the last crop and in previous seasons the soil type and soil organic matter content losses of nitrogen by leaching and other processes. The amount of winter rainfall is important nitrogen made available for crop uptake from mineralisation of soil organic matter and crop debris during the growing season 10

10 Assessing mineral nitrogen residues post harvest but before leaching Residues of mineral nitrogen (nitrate-n and ammonium-n) in the soil following harvest of the previous crop will vary depending on the crop type grown. Residues following cereals are generally lower than those following break crops. On nitrate retentive soils (e.g. deep clay or silty soils), a large proportion of these residues can persist to the spring and will then be available for uptake by the following crop. On nitrate leaky soils (e.g. light sand soils) most of these residues will be leached in an average winter. The management and performance of the previous crop can have a significant effect on the actual level of nitrogen residues. Residues are likely to be lower in a high yielding season or where nitrogen use has been less than normal, but may be higher than average if the crop has performed badly due to problems such as disease or drought. The SNS Index assessments assume that all previous crops have been managed well and that previous nitrogen fertiliser use has been close to the recommended rate, taking account of any use of organic manures. In ley-arable rotations, the nitrogen released from grass leys may persist for up to 3 years following ploughing out. Most useful nitrogen becomes available within the first 2 seasons after ploughing out. Assessing nitrogen leaching losses using excess winter rainfall The amount of nitrogen leached will depend on the amount of nitrate-n in the soil at the start of winter, the soil type and the amount of water draining through the soil (the excess winter rainfall). The excess winter rainfall is the actual rainfall between the time when the soil profile becomes fully wetted in the autumn (field capacity) and the end of drainage in the spring, less evapo-transpiration during this period. The Meteorological Office can provide estimates of excess winter rainfall in different locations and for different soil/cropping situations. Excess winter rainfall (mm) = Rainfall between field capacity and end of drainage evapotranspiration transpiration Light sand soils and some shallow soils can be described as leaky. Nitrate residues following harvest are fully leached in an average winter even where substantial residues are present in the autumn. Previous cropping has little effect on SNS which is nearly always Index 0 or 1 except in low rainfall areas or after dry winters. Deep clay and silt soils can be described as retentive. The leaching process is much slower and more of the autumn nitrogen residues will be available for crop uptake in the following spring. Differences in excess winter rainfall will have a larger effect on SNS in these soils. Low levels of SNS (Index 0 and 1) are rare. Other soil types are intermediate between these two extremes. Because of both regional and seasonal differences, separate SNS Index tables are given for three different rainfall situations (see section 3): 1 Up to 600 mm annual rainfall ( mm excess winter rainfall) mm annual rainfall ( mm excess winter rainfall). 3 Over 700 mm annual rainfall (over 250 mm excess winter rainfall). 11

11 Assessing nitrogen released from mineralisation of organic matter In organic and peaty soils, mineralisation of soil organic matter in late spring and summer results in large quantities of nitrogen becoming available for crop uptake. Since soil temperatures are too low for much mineralisation in late winter/early spring, small amounts of nitrogen may need to be applied in early spring for crops that have a significant nitrogen requirement at this time. Long season crops (e.g. sugar beet) will utilise more mineralised nitrogen than crops which are harvested in mid summer. For example, cereals will make little use of nitrogen mineralised after June. Organic soils contain between 6 and 20% organic matter and are likely to contain a wide range of SNS levels depending on the actual amount of soil organic matter. However, the actual SNS cannot reliably be estimated by knowing the soil organic matter concentration. The recommendations for organic soils are based on soils with a topsoil organic matter content of 15%. Peaty soils contain over 20% organic matter. They are always at SNS Index 5 or 6 irrespective of previous cropping or manuring history, or excess winter rainfall. This is because the large amounts of nitrogen mineralised will usually be much greater than variations in the nitrogen residues due to previous cropping. The amount of nitrogen mineralised from past applications of organic manures (over 1 year old) is difficult to estimate without analysis for SMN. Generally, this nitrogen contribution will be small but will be greater where there has been a history of large regular applications. Nitrogen in crop debris from autumn harvested crops usually mineralises quickly and is subject to over-winter leaching losses in the same way as mineral nitrogen residues. Mineralisation of nitrogen-rich leafy debris will be quicker than from nitrogen-poor woody debris. The organic nitrogen which is not quickly mineralised becomes available over a long time and may contribute little to the following crop. Examples of cropping situations where mineralised nitrogen can make a useful nitrogen contribution are: Following sugar beet on light sand soils, mineralisation of the sugar beet tops provides a useful contribution to the SNS for the next cereal crop. Where a second cauliflower crop is grown in the same season. Large amounts of leafy, nitrogen-rich crop debris will be returned to the soil after harvest of the first crop, and will quickly release mineralised nitrogen. The Soil Nitrogen Supply (SNS) Index System The SNS Index system used in this book has been substantially revised compared to the Nitrogen Index system used in the 6th Edition of RB209. Nitrogen recommendations are based on seven SNS Indices and each Index is defined in terms of a quantity of SNS in kg/ha N. This means that an SNS Index can be determined using field specific information (the Field Assessment method) without sampling and analysis for SMN, or by using the results of sampling and analysis for SMN (the SMN Analysis method). A nitrogen recommendation is obtained by determining the SNS Index of the field using one of these methods, then referring to the appropriate crop table to obtain the nitrogen recommendation for the selected Index level. The SNS Index system is not applicable for established grassland or established fruit crops. Note: The SNS Index system does NOT take account of organic manures applied since the last crop was harvested, or any manures that will be applied during the growing season. The nitrogen contribution from these manure applications must be calculated separately (see section 2). Full details of the SNS Index system and how to use it (with examples) are given in section 3. 12

12 The Field Assessment Method In most situations the SNS Index will be identified by using the Field Assessment method which is based on field specific information for previous cropping, previous fertiliser and manure use, soil type and winter rainfall. There is no requirement for soil sampling or analysis. This method provides a satisfactory assessment of the SNS. However, use of soil sampling and analysis for SMN (the SMN Analysis method) is recommended where high or uncertain amounts of soil nitrogen are expected. The SMN Analysis Method Direct measurement and estimation of the key components of SNS (SMN, total crop nitrogen content and mineralisable nitrogen) will usually result in the most accurate assessment of the amount of soil nitrogen available for the crop, and therefore the most accurate nitrogen fertiliser decision. SMN is the most important component. Soil Nitrogen Supply (SNS) = SMN + estimate of total crop nitrogen content + estimate of mineralisable nitrogen Since soil sampling and analysis for SMN is an expensive procedure, the use of this method should be targeted to fields where soil nitrogen residues are expected to be large and uncertain, and particularly where organic manures have been used in recent years. The SNS will often result from the cumulative effects of farming practice over several years and can be difficult to predict. The SMN can be directly measured by soil sampling and analysis, which should be carried out carefully using the procedure recommended in Appendix 2. Sampling and analysis for SMN is NOT recommended on peaty soils or in the first year after ploughing out grassland. This is because the contribution of mineralisable nitrogen is likely to be a high proportion of the SNS. It is also not recommended for use in established grassland systems. The total crop nitrogen content of cereals and oilseed rape at the time of SMN sampling can be estimated using the scheme given in Appendix 2. There are no schemes available for estimating the crop nitrogen content of other crops. It is much more difficult to obtain a reliable estimate of the nitrogen that will be made available from mineralisation of organic matter. However on soils of low or average organic matter concentration, this will be small and not practically significant. Large quantities of nitrogen can be mineralised on organic and peaty soils, or where large quantities of organic material (crop debris or organic manure) have been applied in recent years. Some typical values of mineralisable nitrogen are given in Appendix 2. Where SMN has been measured to 90 cm depth, or to maximum rooting depth in shallow soils over rock, the results can be used to identify the SNS Index of the soil. It is important to ensure that an estimate of crop nitrogen content and mineralisable nitrogen is added to the SMN result when calculating the SNS. Where shallow rooted crops are to be grown (e.g. some vegetables), soil sampling for SMN may not be carried out to the full 90 cm depth. In these cases, the results of analysis should not be used to determine the SNS Index. There are existing decision support systems that can be used to interpret these results to provide a fertiliser recommendation. 13

13 Nitrogen Use Efficiency by Crops For well-grown crops, the efficiency of uptake of different sources of nitrogen by crops varies. SMN within rooting depth is usually used efficiently, approaching 100% (i.e. for every 100 kg/ha of SMN in the soil, approximately 100 kg/ha is taken up by the crop). This is because it is evenly distributed within the soil and readily accessible for root uptake. Fertiliser nitrogen is used less efficiently than SMN. This is probably because fertiliser nitrogen is less well distributed within the soil. For crops suffering from disease, soil, drought or other growth problems, nitrogen uptake efficiency will be much lower. Research has shown that the efficiency of use of fertiliser nitrogen by winter wheat and winter barley varies depending on the soil type. Light sand soils. 70% efficiency of use (i.e. 70 kg/ha N taken up by the crop for every 100 kg/ha fertiliser nitrogen applied). For many crops on light sand soils, the lower nitrogen recommendation takes account of this factor and the lower yield potential of these soils. Medium, clay, silty, organic and peaty soils. 60% efficiency. Shallow soils over chalk and limestone. 55% efficiency. The winter wheat and winter barley recommendations in this book are adjusted to take account of these differences in nitrogen fertiliser use efficiency. There has been insufficient research to show if these differences also apply to other crops. Timing of Nitrogen Applications Correct timing of nitrogen fertiliser application is important so that crops make best use of the nitrogen applied. As a general principle, nitrogen should be applied at the start of periods of rapid crop growth and nitrogen uptake. The chart below shows the typical pattern of nitrogen uptake by a winter cereal crop. It is easy to see why there is no benefit from applying autumn nitrogen to winter cereal crops. The nitrogen requirement is low during the autumn and winter and the supply from soil reserves is adequate to meet this. However, autumn nitrogen is recommended for some winter oilseed rape crops, reflecting the larger nitrogen requirement of this crop at this stage of the year. 14

14 Nitrogen Uptake by a Winter Cereal Crop in Relation to Available Soil Nitrogen Winter cereal crop nitrogen content Soil mineral nitrogen 200 Nitrogen (kg/ha) A 50 B 0 Nov Dec Jan Feb Mar Apr May Jun July Aug For a winter cereal crop in the autumn/winter (A), there is a low crop nitrogen requirement which can easily be met by soil nitrogen reserves. There is no need for autumn applied fertiliser nitrogen for autumn sown cereals. The main period of nitrogen uptake (B) is March-June. During these growth periods, there is usually insufficient soil nitrogen to support unrestricted growth. Nitrogen fertiliser should be applied at the start and during this growth period. Nitrogen timing can also have a range of other important effects on crop growth and quality. Too much seedbed nitrogen can reduce the establishment of small seeded crops. Early spring nitrogen will increase tillering of cereals. This may be beneficial, but too much nitrogen at this stage can increase lodging. Late applied nitrogen will increase the grain nitrogen/protein concentration of cereals. Recommended timings of nitrogen applications for individual crops are given in the recommendation tables. 15

15 The Effect of Economic Changes As a general principle, the recommendations are insensitive to changes in the value of the crop produce or the cost of nitrogen fertiliser. It would normally require a large change in the ratio of the crop price and nitrogen fertiliser price to alter the recommendation. The breakeven ratio is the crop yield (kg) needed to pay for 1 kg of nitrogen. Large increases in the breakeven ratio may justify a small reduction in the nitrogen recommendation, and vice versa. Breakeven ratio = Cost of nitrogen (pence/kg) Value of crop produce (pence/kg) Example Ammonium nitrate (34.5% N) costs 100/t or 100 x 100 = 29.0 pence/kg N 34.5 x 10 Wheat is sold for 70/t or 70 x 100 = 7.0 pence/kg 1000 The breakeven ratio is 29.0 = 4.1 kg crop produce/kg N 7.0 If there are large changes to current crop or nitrogen prices, small reductions to the cereal and winter oilseed rape recommendations may be appropriate if the breakeven ratio increases substantially. Specific advice is given in these crop recommendation tables. Applying Nitrogen Fertiliser In most situations, nitrogen applied across the whole of the cropped area is recommended, but for some wide row crops such as vegetables and maize, it may be beneficial to place some nitrogen. During the early stages of growth, roots of wide row crops only exploit a small proportion of the soil, yet young root systems still need to be able to absorb enough nitrogen from the small volume of soil in which they are growing. To meet this requirement, the concentration of nitrogen in this soil must be high. Band placement of nitrogen fertiliser can achieve this without applying nitrogen to soil areas where there are no roots. Nitrogen placement can reduce fertiliser costs and improve crop performance. The risk of nitrate pollution of water will also be reduced. Where applicable, more detailed recommendations are given in the crop recommendation tables. Alternative Approaches to Nitrogen Decisions There is ongoing research into new approaches and techniques for deciding on nitrogen fertiliser use for cereals. The canopy management technique and use of sensors are two examples. At the time of writing, neither of these techniques is yet sufficiently developed to be recommended for general use. In future, one or both of these new approaches may become sufficiently tested and proven to allow them to be adopted with confidence. 16

16 PHOSPHORUS (P) AND POTASSIUM (K) (Section 7 contains additional information for Grassland) Crops need a balanced and adequate supply of other essential nutrients, including phosphorus and potassium. Phosphorus and potassium may be supplied from soil reserves, fertilisers or organic manures. Account should always be taken of the nutrients supplied from any organic manure applications; large savings in fertiliser use are usually possible where organic manures are used. The amount and availability of phosphate, potash and other nutrients will depend upon the amount and type of manure used. Section 2 gives details of how to calculate the quantity of nutrients supplied from different types of organic manures that are equivalent to inorganic fertilisers. Particular care should be taken to avoid the build up of unnecessarily high levels of phosphorus in soil. This is financially wasteful and is likely to increase phosphorus loss from soils, which can cause pollution of surface waters. Phosphorus enrichment is a significant factor contributing to the eutrophication of still or slow moving freshwaters. This can result in algal blooms and can disturb the ecological balance of aquatic life. The Water Code (MAFF PB0587) advises that soil P levels should NOT be raised above those necessary for economic crop production (see page 28 for more details). Planning a Fertiliser Policy for Phosphate and Potash To plan a policy for using phosphate and potash fertilisers, it is necessary to know: 1 The results of soil analysis. 2 The target soil Index for the rotation. 3 The need to build up or run down the soil P or K Index. 4 The responsiveness of the crop grown to fresh fertiliser. 5 The quantity of nutrients removed from the field in crop produce. 6 The quantity of nutrients supplied from organic manure applications (see section 2). 7 The residues of phosphate and potash from previous fertiliser and manure applications. Detailed recommendations for phosphate and potash are given in sections 4 to 7. Guidance on how to use the recommendation tables is given in section 3. The recommendations in this book are given as kg/ha of phosphate (P 2 O 5 ) and potash (K 2 O). Conversion tables (metric-imperial; element-oxide) are given in Appendix 8. Soil Sampling and Analysis The use of phosphate and potash fertilisers should be based on regular soil sampling and laboratory analysis. Under most cropping systems, the soil nutrient status only changes slowly and it is safe to use soil analysis results as a basis for fertiliser recommendations for up to 4 years from the date of sampling. Soil sampling and analysis should therefore be carried out approximately every 4 years. The analysis results will only be meaningful if an adequate and representative soil sample is taken. The recommended procedure for soil sampling is described in Appendix 3. Soil analysis provides a measure of the quantity of nutrient available for uptake by the crop. Results should be given as milligrams of phosphorus and potassium per litre of soil (mg/litre P; mg/litre K) and may also be expressed as an Index. Indices range from 0 (deficient) to 9 (very large). Most field soils have Indices of between 1 and 4. Appendix 4 gives details of the Index scheme classification. 17

17 Internationally, there are different soil analysis methods that may be used. The recommendations given in this book should only be used in conjunction with the results of soil analysis using the following standard laboratory methods which are appropriate for soils in England and Wales. Phosphorus measured in a sodium bicarbonate soil extract at ph 8.5 (Olsen s P) or measured in an anionic resin soil extract (resin P) Potassium and Magnesium measured in an ammonium nitrate soil extract Full details of the analytical methods are given in Specification for Topsoil (British Standard 3882) or The Analysis of Agricultural Materials (MAFF RB427). The resin P analysis method is described by Hislop and Cooke (1968). Potassium-releasing clay soils Depending on the nature of the clay parent material, some heavy clay soils contain large quantities of potassium which gradually becomes available for crop uptake but which is not measured by routine soil analysis methods. Potassium-releasing clay soils can release around 50 kg/ha of potash each year which should be allowed for when calculating potash application rates. Potassium-releasing clays. Chalky boulder clay, Gault clay, Weald clay, Kimmeridge clay, Oxford clay, Lias clay, Oolitic clay Clays which do not release much potassium. Carboniferous clay Local knowledge and past experience of soil analysis results and manuring history is useful when assessing the potassium releasing characteristics of clay soils. Target Soil Indices Fertiliser applications should aim to raise the soil P and K Index to a target level for the rotation and then to maintain this level in the soil. Where the soil Index level is below this level, even high rates of fertiliser application may not achieve the same yield compared to a soil which is at the target Index level. This is particularly likely where soil P is Index 0. Until this phosphate has been thoroughly incorporated into the topsoil, roots are not able to take it up effectively. As potassium is more mobile than phosphorus in the soil, freshly applied potash fertiliser can often make up for a soil K deficiency. However this should not be relied upon. The target soil Indices for soil P and K are given below. Soil P Soil K Arable and forage crops, grassland Vegetables mg/litre (Index 2) mg/litre (Index 3) mg/litre (Index 2 ) mg/litre (Index 2+) 18

18 Phosphate and Potash Application Policies A Nutrient requirement (kg/ha) B C Soil Index level Where the soil Index is below the target level (A), crop responses to phosphate and potash are possible. High rates of application will often be recommended. Since maximum yield may only be obtained from soils maintained at the target Index level, the recommendations allow for a nutrient surplus to achieve a long term build up in the soil Index (see page 21 for more details). Where the soil Index is at the target level (B), the application rate should be sufficient to replace the nutrients removed in crop produce. For responsive crops (e.g. potatoes), a higher rate will be needed to ensure there is no limitation to yield. Where the soil Index is above the target level (C), fertiliser rates may be reduced or omitted for one or several years until the soil Index falls to the target level. This can result in significant financial savings. Recent research has shown that there is an increased risk of phosphorus loss from soils which are at soil P Index 4 or over. To reduce the risk of loss, The Water Code (MAFF PB0587) advises that application of phosphate fertilisers should not exceed the amounts recommended in this book. For fields at soil P Index 3 or above, care should be taken to avoid the total inputs of phosphate from organic manures and fertilisers exceeding the total amount of phosphate removed by crops in the rotation. This will help avoid raising the soil P level above that necessary for economic crop production. Magnesium deficiency can be induced at high soil K Indices or where excessive amounts of potash fertiliser are applied. Light sand soils Because of their very low clay content, some light sand soils have a limited capacity to hold potassium. It is therefore not sensible to try and maintain the target soil K Index level on these soils. For loamy sands it is generally possible to maintain soil K at 150 mg/litre (Index 2-) but for genuine sand textured soils, the realistic upper limit is 100 mg/litre (upper Index 1). Attempting to maintain soil K at any higher levels will result in movement of potassium into the subsoil and some loss by leaching. On genuine sand textured soils therefore, the maintenance (M) recommendation should be applied where soil K is 100mg/litre or higher. 19

19 Responsive Situations At P and K Index 0 and possibly Index 1, extra yield may be obtained from using freshly applied phosphate or potash fertiliser. At these Indices, fertiliser should be applied annually for each crop and at a time when it can be thoroughly incorporated into the topsoil before the crop is established. The nutrients are then easily accessible for uptake by the growing crop. Placement of the fertiliser may be justified. Crops such as potatoes, some field vegetables and forage maize may respond to fresh applications of phosphate even where the soil P Index is at or slightly above the target Index level for the rotation. For these crops, special methods of application such as placement or band spreading may be recommended. Residues from these applications should be allowed for when calculating the fertiliser requirement of unresponsive crops in the rotation. Maintaining the Soil Index level Where the soil Index is at the target level, sufficient fertiliser should be applied to replace the quantity of nutrient removed from the field in harvested crop material. This is called the Maintenance rate of application (shown as an M in the recommendation tables) and can be calculated from knowing the crop type, yield and nutrient content of the crop material removed from the field. Typical values for the content of phosphate and potash in crops are given in Appendix 5. Example A soil is at P Index 2 and K Index 2-. Winter barley is grown which yields 8t/ha. The straw is baled and removed. Because the soil P and K Indices are at the target levels (2 and 2 respectively), the fertiliser application should be calculated so that the nutrients removed in the harvested crop are replaced during the rotation. From Appendix 5, winter barley (straw removed) contains 8.6 kg of phosphate and 11.8 kg of potash per tonne of grain yield. The following application rates should maintain the existing soil Index levels. Phosphate = 8 multiplied by 8.6 = 69 kg/ha phosphate (P 2 O 5 ) Potash = 8 multiplied by 11.8 = 94 kg/ha potash (K 2 O) Because soil Indices only change slowly, the maintenance application may be applied in several equally acceptable ways provided that the total quantity of nutrients removed in crops during the whole rotation is replaced. Where no responsive crops are grown in the rotation and soil P and K are at or above the target Indices, the phosphate and potash requirement may be applied at any time of the year. Except for potash on light sand soils, the whole requirement for up to 3 years may be given in a single application. Where there is a significant risk of surface run-off entering watercourses (e.g. poorly drained clay soils on sloping land) and phosphate is applied every 2 or 3 years, the application should, where practically realistic, be worked into the soil surface before the winter period. This will reduce the risk of phosphorus run-off which can pollute surface waters. On light sand soils (sands and loamy sands), potash must be applied annually. It is best applied in late winter or early spring after ploughing. Care should be taken to avoid large applications of potash in the seedbed which can reduce emergence and root development especially under dry conditions. On some heavy clay soils, recent research has indicated that there may be a very slow decline in the soil P level even where maintenance rates of phosphate are applied. Where this effect is significant, it will be detected by regular soil analysis. 20

20 Building up or Running down Soil Indices Where the soil P or K Index is below the target level for the rotation and there is a long term requirement for maximum yields, the fertiliser policy should be to raise the Index gradually to the target level. Maximum yields are unlikely at P Index 0 even where high rates of phosphate fertiliser are used. Organic manures are a useful source of phosphate and potash for building up soil Indices. Where the soil Index is well above the target level, then the policy should allow for a gradual decline. At P or K Index 3, the full maintenance dressing may be applied if wished. It can take large surpluses of nutrient to raise the soil analysis level by one Index though the exact amounts are not well defined. To increase soil P by 10 mg/litre, it may need kg/ha of phosphate in addition to crop removal, irrespective of soil type. Much more is needed on clay soils if the initial level is below 10 mg/litre (Index 0). To increase soil K by 50 mg/litre, it may need kg/ha of surplus potash. Where the policy is to run down the soil Index level, similar amounts of phosphate or potash must be removed in crop material before the Index level will change. The speed of building up or running down soil Indices will depend on the amount of surplus nutrients applied each year (for build up) and the nutrient deficit (for run down). A fertiliser policy to build up or run down soil Indices will usually take several years to have its effect. Where the soil Index level is above or below the target Index level, the recommendations in the tables should result in the soil Index increasing or decreasing by one level over a year period for arable crop rotations and grassland, or 5-10 years for vegetable rotations. The adjustment values (see table below) have been calculated using the mid-point values for each Index (as mg/litre) compared to the mid-point value of the target Index. Arable and forage crops, grassland Vegetables Phosphate (P 2 O 5 ) Potash (K 2 O) Phosphate (P 2 O 5 ) Potash (K 2 O) kg/ha Index 0 Index 1 Index 2 Index 3 M+50 M+25 M M-50 M+50 M+25 M (Index 2-) M-25 (Index 2+) M-70 M+150 M+100 M+50 M M+150 M+100 M+50 (Index 2-) M (Index 2+) M-90 In some situations (e.g. phosphate for potatoes), the crop is likely to respond to higher rates of nutrient application than would be recommended using the above adjustments. The recommendations given in the tables in sections 4 7 are the higher of: the rate of nutrient required for maximum crop response the rate of nutrient based on the maintenance amount adjusted for building up or running down the soil Index according to the above table Where there is no plan to raise the soil Index level (e.g. on short term rented land), the above amounts for buildup may be deducted from the recommended rates given in the tables. The application rate will then be the larger of the maintenance (M) rate and the rate needed for full crop response. Where a more rapid increase in the soil Index level is required, higher rates may be applied. More frequent soil analysis may be worthwhile during a buildup policy especially where a rapid change is planned. 21

21 MAGNESIUM (Mg) (Section 7 contains additional information for Grassland) As with phosphorus and potassium, soil analysis gives the quantity of available magnesium in mg/litre of soil, along with an Index (see Appendix 4). Potatoes and sugar beet are susceptible to magnesium deficiency and may show small yield responses to magnesium fertiliser when soil magnesium is deficient. Other arable crops may show deficiency symptoms at soil Mg Index 0 but seldom give a yield response to applications of magnesium. Transient symptoms of magnesium deficiency are commonly caused where crop rooting is restricted due to soil compaction or poor drainage, rather than due to a soil deficiency. Residues from magnesium fertiliser applied to potatoes or sugar beet will satisfy the needs of other crops in the rotation. Where sugar beet or potatoes do not feature in the rotation, magnesium fertiliser is only likely to be justified if the soil is at Index 0. In these situations apply 50 to 100 kg/ha MgO every three or four years. Where soil magnesium reserves are small and liming is necessary, the use of magnesian limestone may be costeffective. An application of 5 t/ha of magnesian limestone will provide at least 750 kg/ha of MgO. However, many years of using magnesian limestone can result in soil Mg Index 5 or over which may induce potassium deficiency. Magnesium recommendations are given as kg/ha of magnesium oxide (MgO) not as Mg. Conversion tables are given in Appendix 8. SULPHUR (S) (Section 7 contains additional information for Grassland) Sulphur is an important plant nutrient and needed by plants in similar quantities to phosphorus. Historically, the crop s requirement for sulphur has been met from atmospheric deposition and fertilisers that have contained sulphur as a by-product. However, sulphur deposition from the atmosphere has fallen rapidly in recent years and deposition in 1999 was about 15% of that in Sulphur deposition will continue to fall in the future. The map shows sulphur deposition in England and Wales in 1996 and is useful for indicating the relative differences between areas. Sulphur recommendations are given as kg/ha of sulphur trioxide (SO 3 ) not as S. Conversion tables are given in Appendix 8. 22

22 Annual Sulphur Deposition Sulphur (S) kg/ha per year Above Below 13 CEH Edinburgh The Occurrence of Sulphur Deficiency There is an increasing risk of sulphur deficiency in England and Wales in a wide range of crops including cereals, oilseed rape, brassicas, peas and grass. Oilseed rape and grass grown for silage are particularly sensitive to deficiency. As atmospheric deposition of sulphur continues to decline, it is likely that the risk of deficiency will affect an increasingly wide range of crops and soil types. A knowledge of soil type and field location is currently the best guide for assessing the risk of deficiency. Sandy, shallow or medium textured soils that are low in organic matter content are most susceptible to sulphur deficiency. Sulphate-S (the form of sulphur taken up by crops) is soluble and is easily leached from these soils. Deep silty or clay soils, or fields that have received regular manure applications, are less likely to show deficiency. Organic soils and fields close to industrial centres emitting sulphur are not likely to show deficiency. Diagnostic Methods Sulphur deficiency causes paling of young leaves and crop stunting that can easily be confused with nitrogen deficiency. In oilseed rape, middle and upper leaves can show interveinal yellowing, and flower petals are a pale yellow colour. Leaf analysis is a useful guide to diagnosing deficiency in cereals, oilseed rape and grass but interpretative criteria have not yet been established for other crops. Although analysis results may be available too late to correct deficiency in the current crop, they can be useful for decisions on sulphur use for future crops. The procedures for plant sampling and interpretation of analysis results are given with each crop recommendation table. Soil analysis to 90 cm depth can help identify severely deficient soils but in other situations is not as reliable a guide as leaf analysis. 23