Advice Sheet 6: Understanding Soil Nitrogen Why is nitrogen the first nutrient we think about? Nitrogen is critical for plant growth. It is used in the formation of amino acids, which are the essential building blocks of proteins. It is important in the formation of chlorophyll, which the plant uses when converting carbon dioxide (CO 2 ) and sunlight into energy. Nitrogen (as N 2 gas) is the most abundant element in the atmosphere, but for the majority of plants, this rich pool of nitrogen is not available to them because the energy required to breakdown the gas is too high, as a consequence nitrogen is the most common, and most significant deficiency in plants. The plant takes up the majority of nitrogen in mineral forms, such as NH + 4 ammonium ions or NO - 3 nitrate ions. This is the Soil Mineral Nitrogen (SMN). In a natural ecosystem these ions are supplied to the plant by microbial mineralisation of unavailable organic nitrogen in organic matter. This is a slow process that does not necessarily release nitrogen when the plant needs it. Thus most agricultural soils contain too little naturally occurring plant available Nitrogen to meet the needs of a crop throughout the growing season. Although it should be noted that the supply of Nitrogen via mineralization can be significant in organic/peaty soils or where organic manures have been applied for many years. Consequently, supplementary Nitrogen applications have to be made either in the form of organic manures or inorganic fertiliser products. The correct amount of Nitrogen needs to be applied at the correct time of year. This is an essential feature of good crop management. What are the symptoms of nitrogen deficiency? Nitrogen deficiency symptoms are some of the most obvious. Stunted, yellow growth is typical, where cell production and extension are limited and chlorophyll production low. The addition of nitrogen to a plant typically has a large effect on crop vigour, yield and quality. As cell production and growth increases more protein is formed and the plant will turn a much darker green as the chlorophyll content is raised. The farm nutrient management programme should aim to apply Nitrogen at a rate which will increase yields up to the on farm economic optimum. Application of Nitrogen at this rate will result in large increases in yield but please note this will not be the maximum yield possible. Maximum yields are achieved when the nitrogen is applied at a greater rate than the on farm economic optimum but this increase will be worth less than the cost of the extra Nitrogen and thus is never a target if farm profits are to be maximised. It maybe prudent to note that excessive application of Nitrogen above the maximum yield can infact reduce yield by aggravating problems such as lodging of cereals, foliar diseases and poor silage fermentation. Excess applications thus have a financial cost but there are also environmental costs. When too much Nitrogen is applied a larger proportion is unused by the crops and thus there is a larger surplus of residual Nitrogen, usually as Nitrate in the soil after harvest. This Nitrate is at risk of loss in ways that can cause environmental problems such as leaching to ground or surface water & denitrification to Nitrous oxide, a greenhouse gas).
So how do we determine the Crop Nitrogen Requirement (CNR)? The key to applying the correct amount of Nitrogen is by determining the Soil Nitrogen Supply (SNS) index. The SNS is the amount of Nitrogen (kgn/ha) in the soil (apart from that applied for the crop in manufactured fertiliser and manures) that is available for uptake by the crop throughout its entire life, taking into account N losses. SNS = SMN + estimate of N already in the crop + estimate of mineralisable soil N Where: Soil Mineral Nitrogen (kgn/ha) is the Nitrate-N plus Ammonium-N content of the soil within the potential rooting depth of the crop. Nitrogen already in the crop (kgn/ha) is the total content of Nitrogen in the crop when the soil is sampled for SMN. Mineralisable Soil Nitrogen (kgn/ha) is the estimated amount of Nitrogen which becomes available for crop uptake from mineralization of soil organic matter and crop debris during the growing season after sampling for SMN. The SNS index can be determined by using specific field information (Field Assessment Method or FAM) without sampling and analysis, or by using the results of soil sampling and analysis for SMN and an assessment of any Nitrogen already taken up by the crop (Measurement Method). The reader can then refer to the N- recommendations in the RB209 which are based on seven SNS indices and each index is related to a quantity of SNS in kgn/ha. In turn because of the amount of Nitrate leached is heavily dependant on the soil type and the amount of water draining through the soil (the excess winter rainfall) separate SNS index tables are given for different rainfall situations, when using the FAM. Refer to RB209. The measurement method is preferable where the rooting depth may contain large or uncertain amounts of plant available Nitrogen or in those fields where organic manures have been used regularly in recent years, or following on from a vegetable crop that has left N-rich residues. However SMN measurements are the most useful on silt and clay soils in low rainfall areas. They are less worthwhile on light and shallow soils and should not be used on peat soils. Even if the Field Assessment Method is employed, measuring the SMN on a few fields can provide a useful check on how the SNS on a farm compares to that estimated by the FAM. Why is Nitrogen not included in the standard soil analysis? In order to measure the amount of soil mineral nitrogen it is essential to perform the analysis on a fresh sample that is kept in cool conditions. If the sample is allowed to warm, microbial activity will be encouraged, mineralising more of the unavailable organic nitrogen pool to ammonium and nitrate giving an over inflated result. This means that a soil must be kept cool, either by refrigeration or the use of insulated packaging and cool packs. Due to the nature of this packaging, the urgency of the analysis and the relatively complex methodologies used, soil mineral nitrogen is not a routine analysis. There are also other factors to consider, soil mineral nitrogen content is temperature dependent (as it is microbially controlled) therefore values change throughout the year
and it is therefore important to sample as near to application as possible. The optimum sampling period is typically in early spring when the crop is established and not in early autumn when the majority of samples are taken for standard soil analysis (ph, P, K, Mg). It is possible to have ph, P, K and Mg analysed on the spring sample for soil mineral nitrogen but typically these elements are applied when the crop is sown/planted. Please do not confuse the analysis of soil mineral nitrogen with total nitrogen. This can be conducted on standard soil samples, but is of little use when considering fertiliser application rates. This is because the amount of soil mineral nitrogen is a small fraction (a few percent) of the total amount of nitrogen, which includes relatively large amounts of unavailable organic forms of nitrogen. Soil Sampling for Soil Mineral Nitrogen When to sample In most situations, sampling in spring (February) before Nitrogen fertiliser is applied, is preferable especially in high rainfall areas or on shallow or light sand soils, because account is taken of overwinter leaching. On soils less prone to leaching, sampling earlier in winter is possible. Avoid sampling within 2-3 months after application of Nitrogen fertilisers or organic manures. Depth of sampling Each position should have samples at three depths: 0-30cm, 30-60cm and 60-90cm. Sampling to the full 90cm is recommended when sampling from January onwards. Sampling to 60cm is satisfactory when sampling in the autumn. If sampling to 60cm and deep rooted crops are to be grown, an estimate of the mineral nitrogen in the 60-90cm layer must be made. In the absence of any better estimate, uniform distribution of nitrogen in the profile should be assumed. For shallow rooted crops (e.g some vegetables), sampling to 30cm or to rooting depth is all that is needed. Whatever the case, to identify the correct SNS index an estimate of mineral nitrogen to 90cm will need to be made. Method of Sampling Samples must be taken to be representative of the area sampled. Do not sample from unrepresentative areas, such as ex-manure heaps or headlands. If differences in soil type or past management are large within a field, then these areas should be sampled separately. A minimum of 15-20 soil cores should be taken per field (based on a 10ha field) and samples from each depth bulked to form a representative sample. If the bulked sample is too big, sub-sample by taking many small representative portions: do not mix excessively. Avoid cross contamination of samples from different depths. Samples can also be mixed from the whole 90cm into one sample but when samples are mixed it is important to state the deepest depth extracted. It is this depth that is used to calculate the available Nitrogen. Prepare! Freeze ice packs the night before Prepare! Label bags before going into field Place the cores in the sample bag provided and squeeze the sample into a ball to expel excess air estimating stone content if possible Put the samples in an insulated box with frozen ice packs and dispatch to laboratory via courier (if this is not possible practically then samples must be refridgerated but not frozen until ready for collection).
Prepare! Arrange all courier bookings in GOOD TIME and always only for pick up Monday- Thursday and incorporate a minimum 4 hour window for collection Ensure paperwork is submitted with the samples so we know who the samples belong to and what analysis is required This protocol must be adhered too. Unless samples are representative and are kept cool, misleading calculations of Nitrogen application rates could be made, resulting in severe economic losses. Soil Mineral Nitrogen Results Once the samples arrive at the laboratory they will be analysed for Nitrate-N and Ammonium-N using Potassium Chloride as the extracting solution. The results will be reported within 2 days as Ammonium-N and Nitrate-N in mg/kg and if the depth is given this will be converted to kgn/ha of Mineral-N in each soil layer. If an estimated stone content was submitted with the sample this will be incorporated into this final calculation. Estimation of nitrogen that will be released from Mineralisation of Organic Matter and the Crop Nitrogen content Nitrogen mineralised from soil organic matter and crop debris after sampling is a potentially important source of Nitrogen for crop uptake, although in mineral soils of low to average organic matter content, the amount of mineralisable Nitrogen or Additionally Available Nitrogen (AAN) will be small and has historically been ignored. Adjustments may be needed where the soil organic matter is above average or there is a history of multiple manure applications. Currently NRM offers a biological method which gives a good indication of the soils mineralisable potential. This involves a short-term incubation of the soil under anaerobic conditions. Biological activity during incubation ensures development and maintenance to anaerobic conditions, eliminating possible nitrification-denitrification reactions at the soil-water interface that would lead to low results. The sample is incubated at 40 o C for 7 days, and the amount of ammonium nitrogen formed is determined in the same way as in the analysis above. The amount of ammonium nitrogen in the soil before incubation and the mineralisable nitrogen is calculated from the difference in the results of this analysis. Because this mineralisable nitrogen is measured in the laboratory (incubated at 40C), this value will be an over-estimation of what will be available under field conditions. So to arrive at the mineralisable potential in the field the values are adjusted utilizing a factor. This value is often reported as mineralisable potential. By considering this source of nitrogen too, targeting the amount of fertiliser to be applied can be improved.
Mineralisable Nitrogen (kgn/ha) Mineralisable potential <31 10 31-80 20 80.1-110 30 110.1-150 40 150.1-180 50 180.1-220 60 220.1-310 70 310.1-500 85 >500 90 However if an actual analysis is not undertaken, as a crude guide around 10kg/ha more SNS maybe expected for each 1% increase in soil organic matter above 4% in England an Wales or above 10% in Scotland and Northern Ireland. Where soil organic matter is less than this, mineralization has generally been ignored, but recent reseach indicates that an estimate of around 20kg/ha may be appropriate, at least after cold winters. This mineralisable nitrogen becomes available gradually through the year. In contrast, the nitrogen in crop debris is released rapidly following incorporation into the soil. To estimate the final component of the SNS equation ; the Crop Nitrogen content, we refer the reader to RB209. Retrieval of this estimate will then allow for the calculation of the SNS. Utilisation of the SNS index system can then be employed to obtain the nitrogen recommendation and the amount of Nitrogen fertiliser finally applied will be as close to the on farm economic optimum as possible. Adopting changes to N use Large SMN measurements can overestimate SNS and small SMN measurements can underestimate SNS. If SNS estimates indicates that large increases or decreases in N fertiliser use are required caution must be applied and confirmatory evidence should be sought. Crops should be monitored closely through spring for signs
of N deficiency or excess and the planned N strategy should be adjusted if necessary. It may be prudent to introduce the changes in N use to be introduced gradually over a few seasons. More information on soil nitrogen For more information on nitrogen in soil see the new 8th edition of RB209, it contains in depth information on the topics covered in this technical bulletin and the HGCA Topic Sheet 115 Estimating Soil Nitrogen Supply. References Defra Fertiliser Manual 8 th edition (RB209) ISBN 978 0 11 243286 9