NUTRIENT MANAGEMENT PLAN FIELD MONITORING 1. Bradford D. Brown ABSTRACT

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1 NUTRIENT MANAGEMENT PLAN FIELD MONITORING 1 Bradford D. Brown ABSTRACT Nutrient Management Plan Field Monitoring enables producers to evaluate the effectiveness of their Nutrient Management Plan implementation using periodic sampling and analysis of soil and plant tissues. Effective tests for monitoring include soil test P, N, K, salts, and sodicity. Useful plant and forage tests include forage protein, K, and nitrate. Keywords: Nutrient management, nutrient monitoring INTRODUCTION The purpose of the NMP is to help manage nutrients such that returns from crop and livestock production are maximized while avoiding degradation of soil and water resources. As with any plan, it is appropriate to periodically evaluate its effectiveness in meeting the goals. Specific goals may include increasing, lowering, or maintaining nutrient concentrations in particular fields. Nutrient Management Plan Field Monitoring (NMPFM) enables the producer to evaluate the plan's implementation in meeting the goals related to soil and plant nutrient concentrations in specific fields. Nutrient Management Plan Field Monitoring involves the periodic measurement of soil or plant nutrient concentrations. A variety of soil and plant tests are useful for evaluating the plan's effectiveness. Monitoring can be accomplished with soil and/or plant samples normally collected in producing and marketing crops or growing feed for livestock. Those may be sufficient for monitoring purposes. The combined use of several soil and plant analyses may be the most accurate measure of NMP effectiveness. Most soil and plant tests were developed for ensuring the adequacy of nutrients for crop production, but they are increasingly used as indexes of the risk for nutrient transport to surface or ground waters, or soil degradation. Whereas they have less frequently been calibrated for use as environmental indicators of excessive nutrient enrichment, they can be used in a relative sense to indicate the trend in nutrient accumulation. Normally nutrient concentrations in entire fields are monitored. However, soils in fields can differ appreciably in soil type and may occur in large enough areas to be treated and monitored as individual management units (MU). Some MU will accommodate higher or lower nutrient applications than others. Modern GPS positioning and mapping facilitates the 1 B. Brown, Extension Soil and Crop Management Specialist, Univ. of Idaho Parma R & E Center, U of I Lane, Parma, ID Phone ( x216); (bradb@uidaho.edu). Published In: Proceedings Idaho Alfalfa and Forage Conference 7-8 February 2005, Twin Falls, ID, University of Idaho Cooperative Extension. 16

2 division of fields into MU. For additional information on MU consider the publication PNW 527, "Monitoring Soil Nutrients Using a Management Unit Approach". The ability to monitor the effectiveness of a NMP depends on the accuracy of monitoring soil, or plant and feed tissue, which in turn depends on collecting representative samples. For all materials, a number of subsamples (25-30) from within the field or MU should be collected, added together to make a composite sample, mixed well, and a sample used from this composite to submit for analysis. SOIL TESTING Crop land is generally the final destination of waste from livestock operations. Soil testing is the primary tool for directly monitoring changes in field nutrient concentrations. Some nutrient concentrations change more rapidly than others and are more sensitive to nutrient applications or cropping practices. For example, soil tests for NO 3 -N change more rapidly from season to season than other nutrients and are more easily influenced by applied N. Soil test P varies less from year to year. Soil test NO3-N consequently may need to be monitored more frequently than soil test P. A variety of soil tests are available for monitoring purposes. Many of these described here are particularly relevant to monitoring NMP for livestock enterprises. Guidelines for nutrient applications to crops can be found in UI Fertilizer Guides for a variety of crops. Phosphorus (P) -- The most critical soil test for monitoring the NMP for livestock enterprises is available soil test P as it largely determines the options available to the producer for manure/compost applications. Available P in soils is measured using different extractants depending on soil acidity. Most southern Idaho soils are neutral to calcareous and the available P is determined with a 0.5M NaHCO 3 extraction (Olsen P). The test was developed for indicating fields with inadequate available P for maximum crop production. Olsen P has more recently been used to indicate excessive available P that poses a risk for degradation of surface and shallow ground waters. The Olsen P test is an index of P availability and does not quantify the absolute amount of the P shortage or excess. The Olsen P test concentration required for maximum production ranges from 8 to 25 ppm depending on the crop and the lime content of the soil. Research has shown that soluble P in runoff water is directly related to the Olsen P test value. The 40 ppm value is used as a threshold in the NRCS Idaho 590 standard. Above 40 ppm, P additions from all sources (manure or fertilizer) are limited to the amount of P removed with the cropping system. Olsen P has been measured for some manured fields in excess of 100 ppm. Clearly, appreciably more P has been applied from manure to these fields than was removed with annual cropping. Soils above the 40 ppm P threshold will drop only a few ppm each year with normal cropping, assuming no additional P is applied. It would take several years with normal cropping to reduce available P to levels that would limit crop growth. With annual P additions that match annual crop P removal, Olsen P values may not change much. Annual testing is useful for indicating whether the value is increasing or decreasing, and the rate of 17

3 change with implementation of the NMP. Olsen P does change slightly depending on the time of sampling. Typically, spring collected soil samples are slightly higher than those sampled in the fall. Therefore, samples should be collected consistently at the same time of year for comparison to previous sampling for effective monitoring. Soil samples for monitoring P should be collected from 0-12" and 12-24" depths. The first foot of soil is the first to be enriched with P applications. Thus, Olsen P in the first foot is the most sensitive indication of P accumulation in the system. Available P is less subject to movement in water percolating through soil than soluble N. But P can move to depths below 12", especially with preferential flow; flow through soil cracks, earthworm casts, or decayed old root channels when the soil is saturated with moisture. Olsen P in the second foot is used as an index of risk for P enrichment of shallow water tables or springs that can contribute to the enrichment of surface waters. Olsen P will increase from P additions to soil regardless of P source. Manure, compost, food by-product, and fertilizer P additions all influence the Olsen P value, although organic P sources may be less immediately available and cause STP to change more slowly than inorganic fertilizer P. Other processes may also influence STP. Regardless of whether a field is cropped, or P applied, available P typically declines slowly as the P is incorporated into soil microbial tissue or P reacts with Ca, Fe, Mn, or Al to precipitate as sparingly soluble P. This slow process of soil test P decline represents a slow conversion of readily available P to more stable less available forms. It occurs simultaneously with available soil P depletion due to crop removal. It is difficult to quantify which mechanism, crop removal or P immobilization, is the most responsible for soil P decline. The soil test P value is the net effect of P additions that increase available P and the various ways (removal, immobilization, and precipitation) that available P is reduced in soil. When Olsen P exceeds the 40 ppm threshold, the current Idaho 590 standard allows P to be applied at the beginning of the rotation at rates that match the estimated P removal during an entire rotation sequence. This will result in higher soil test P initially and declining soil test P with each crop harvest. Monitoring could involve comparisons between crops in the rotation (annual sampling) but monitoring should be done at least before or after the same crop in the rotation. Nitrogen (N) Nitrate and Ammonium - Soil test N is the measure of readily available N in the form of nitrate (NO 3 -N) and ammonium (NH 4 -N) nitrogen. Nitrate and ammonium N are easily extracted from soil in the laboratory and measured. Plants take up both of these forms of N from soil and they represent the most immediately available N to plants. These forms are routinely measured in commercial agriculture for estimating the residual N remaining from previous cropping and to guide N applications for crops. All soil testing labs offer these tests. In most well aerated soils, NO 3 -N is the predominant form of N available. High nitrates after a crop is removed generally suggests that the N provided for the previous crop was more than 18

4 that required for the crop's maximum production. It is a fairly sensitive indicator of the N management provided for the previous crop. While it may suggest excessive N was available for the previous crop, it may also indicate that the combined N and water management were such as to avoid leaching of the nitrate. Nitrate-N is soluble and moves with percolating moisture. Excessive watering results in leaching and loss of NO 3 -N from the root system. Nitrates should be measured to the effective rooting depth in order to monitor the movement of NO 3 -N. Therefore, NO 3 -N measurements enable growers to monitor not only the effectiveness of their N management but also the effectiveness of their water management. Appreciable NO 3 -N after crop removal at lower depths indicates that irrigation scheduling and possibly the timing and amount of N applied may need adjustment to maintain N in the root system and avoid leaching of NO 3 -N to shallow aquifers. Soil NO 3 -N is measured generally prior to planting to determine whether pre-plant N should be applied for crops that can effectively utilize pre-plant applications. Since most crops are spring planted, most soil NO 3 -N is measured in the spring. Late spring soil tests are also effective in indicating the N available from pre-plant applied N and spring N mineralization. Post-plant soil NO 3 -N measured in late spring in furrow irrigated fields is more problematic in that available N is re-distributed in the bed with each wetting. Post-harvest soil NO 3 -N measurement indicates the residual N from the previous crop. Postharvest NO 3 -N measurement avoids the influence of continued fall N mineralization that increases extractable NO 3 -N. Publications are available to help interpret post-harvest nitrates such as OSU EM 8832-EPNW, "Post-harvest soil nitrate testing for manured cropping systems in western Oregon and Washington". Many soil samples are collected in the fall prior to fall bedding for potatoes, onions, sugarbeets, beans and other crops. Nitrates are frequently determined from these samples but the fall NO 3 -N measurement is less useful for predicting N requirements for spring planted crops than are spring pre-plant samples. The late fall samples do provide a measure of NO 3 - N available for leaching during winter or with early season irrigation. Mineralizable N - Whereas nitrate and ammonium are readily available to plants, mineralizable N is the N released over time from organic matter decomposition. Soils can provide appreciable mineralizable N to plants, particularly where manures, composts or effluents are routinely applied to soils. Knowing the mineralizable N in soils can be helpful in determining appropriate rates of fertilizer or organic N sources to apply for crop N needs. Unfortunately, measuring mineralizable N in laboratories is not as straightforward as measuring nitrate and ammonium. Whereas several lab tests have been proposed, few are routinely used by commercial soil testing labs. The most frequently used index of N mineralization is soil organic matter (SOM). However, SOM provides a crude and imperfect index of N mineralization because the N readily mineralized (the active organic N fraction) is a small and inconsistent fraction of the total N bound in SOM. In addition, many factors affect the rate of mineralization including moisture, temperature, and previous management to name a few. 19

5 A small number of labs provide an index of mineralizable N based on the extractable N before and after a short-term anaerobic incubation of the sample in the lab. The test has been calibrated for specific areas and cropping systems but has not been calibrated across widely different production systems in the PNW. Nevertheless, the incubation N value can be useful in monitoring the mineralizable N in different fields or in the same field over time. Salinity and Sodicity - Excessive salts can reduce the productivity of soils. Salts include sulfate, nitrate, chloride, and bicarbonate anions associated with potassium, calcium, magnesium, sodium and ammonium cations. Manures and composts, fertilizers, all contain salts to some degree. Irrigation waters are also important sources of salt. Some animal rations include salts for balancing nutrients or buffering ph in the rumen. Monitoring salts in soils can help avoid the accumulation of excessive salts that reduce productivity. Salinity is measured generally by saturating soils and measuring the electrical conductivity of the saturated paste extract. The greater the conductivity, the greater the salt content. Most all soil laboratories offer the electrical conductivity test for salts. Salts should be measured periodically in the first foot. Sodium affected (sodic) soils are unproductive soils characterized by high concentrations of sodium relative to calcium and magnesium and have high ph (>8.5). Sodium bicarbonate is a frequently used buffering agent in feed rations. Besides ph, another measure of the accumulation of sodium is exchangeable sodium percentage (ESP). Both ph and ESP tests are performed in most soil test labs. Potassium (K) - Potassium concentrations in soil are a concern because excessive K in forages can contribute to milk fever in dairy livestock. K is readily taken up by plants in excess of their requirements, accumulating as much as two or three times the K that is required for growth. Manures and composts are important sources of K in soils. Potassium carbonate is a frequently used buffering agent in dairy rations and most of the K in the ration is excreted. Soil K in southern Idaho is generally measured using the same extractant as is used for P. Therefore, monitoring for both P and K can be done using the same soil sample. PLANT (FEED) TESTING Plant testing provides useful corroborative documentation for measuring the adequacy of the NMP for providing adequate nutrients for production as well as documenting the nutrient balance (net difference between nutrient addition and removal) for each field. Forages are routinely tested by many livestock enterprises and their dry matter and nutrient content determined for balancing the ration. If the production and dry matter content of the forage are known then these with the nutrient content can give the nutrient removal from the field. Forage nutrient contents are generally expressed on a dry matter basis. The values are indicated either as percentages or as parts per million (ppm). Ten thousand ppm is equivalent to 1%. To convert the reported ppm values to a percentage divide by 10,000, or simply move the decimal point four points to the left. To convert from the reported percentage value to 20

6 ppm multiply by 10,000, or move the decimal point four places to the right. The terms ppm, µg/g, or mg/kg are equivalent terms. Forage protein - Protein is the most common forage analysis and is used to adjust livestock rations. It can also be used to reflect the N available to the crop as it is directly related to the N content of the forage. Forage protein on a dry matter basis divided by 6.25 gives the actual N content as a percentage. Forage protein decreases with plant development and maturity. Therefore, interpretation of forage protein, particularly for alfalfa, must take into consideration the growth stage of the harvested crop. Alfalfa protein concentrations lower than 12% are likely N deficient and indicate poor or ineffective nodulation. Whole plant corn protein at silage harvest below 6% (0.96%N) may indicate that N was inadequate for production. Conversely, whole plant protein above 12% may indicate excessive available N. Nitrates - Whole plant forage NO 3 -N concentrations > 2000 ppm at harvest can indicate either excessive available N or stress conditions that interfered with the plant's normal metabolism of N taken up by the plant. Drought conditions at harvest in particular can cause high forage NO 3 -N. But excessive available N will exacerbate high forage NO 3 -N. Specific tissues are also collected to indicate available N during the season. Lower corn stalk NO 3 -N below 700 ppm at harvest indicates a shortage of N probably limited production. Lower corn stalk NO 3 -N exceeding 2000 ppm indicates excessive N available to the corn from all N sources, provided the silage corn was not under stress at harvest. Forage P, Ca, K, Mg - Forage P, Ca, K, Mg content is also frequently tested. Sufficiency concentrations of P in whole forage plants range from 0.2 to 0.3 % and for K they range from 1.5 to 2.5 %. However, forage concentrations of these nutrients are seldom used for indicating excess amounts in soil, although they have potential for this purpose, especially with K. Of these nutrients, plant K concentrations best reflect the entire range of available soil K. The uptake of nutrients in excess of the actual needs of the plant is termed luxury consumption and the term is commonly associated more with K than other nutrients. Whole plant K concentrations higher than 4% probably indicate K enrichment of soil. LABORATORY QUALITY Results from test labs can differ appreciably for the same sample as indicated by several blind sample exchanges. Blind samples are those received in labs for which the analytical result was unknown. Lack of uniformity in test results undermines the credibility and utility of testing in general. The North American Proficiency Testing (NAPT) program was designed to help labs identify problems they have with specific tests but is not a lab certification program. It simply prepares and distributes a uniform soil or tissue sample to all participating labs and enables each lab to compare their lab results with the median of the results from all participating labs that run the same tests. A pilot program in the Pacific Northwest is currently underway for extending the NAPT to include double blind samples. The results from this proficiency assessment program (NAPT-PAP) should be available in The outcome of the program will be a list of labs with demonstrated proficiency. 21