Understanding Soil Nitrogen Behavior:

Soil testing is used effectively to determine the availability of phosphorus (P) and potassium (K) in agricultural soils and fertilizer recommendations for these nutrients. Penn State Coop Extension PDF (1)   Unfortunately, due to the complex behavior of nitrogen (N) in the soil, matching supply of N from soil with plant demand for this nutrient is one of the nutrient management challenges of crop production. Rutgers Coop Extension PDF (2)

To understand when to perform nitrate soil testing, one must understand soil N behavior. N behavior in soil is very complex. Over 90% of the N found in the surface layer of most soils occurs in organic forms, found in soil organic matter (SOM), with most of the remainder being present as NH4. The roles of SOM are divided into three broad categories: biological, physical, and chemical. CSIRO Land & Water 2004 (3)    The soil biological activities help drive the increase of SOM. Although SOM represents a small percentage (<6% of soil weight), it is a major contributor to soil productivity, controlling over 90% of the soil functions. Nichols, K. USDA-ARS 2004 (4)

The soil N cycle is driven by SOM, which contains approximately 50% carbon (C) and 5% N. Only 1 to 5% of the N in SOM is in a labile form, readily available soil C. This labile form represents microbes or soil properties that are readily used by microorganisms. Jackson, S. UC Davis 2010 (5)  Younger organic material, from recently deposited roots and residue, dead organisms, or waste products, is the most biologically “active” fraction of the SOM, meaning that it serves as a food source for the living soil biological community. The younger fraction is the “labile” SOM fraction, indicating that it is decomposed more rapidly than the passive/stable fraction. UMN Extension 2015 (6)  Most of this labile N exists in the form of proteins derived from the decomposition of dead plant and animal tissue. When an organism dies, protein is hydrolyzed by protease enzymes excreted by soil bacteria to produce polypeptides and amino acids. International Journal of Science and Technology doc 2012 (7)  Amino acids are small molecular weight compounds (monomers) that are recycled and reused by a huge range of microbial populations. Predation of microbes by the soil fauna also releases labile N. Jackson, S. UC Davis 2010 (5) 

Regardless of the form in which N is present in the soil, originally or subsequently added, soil N undergoes many changes that determine whether it becomes available primarily as nitrate-N or is lost to the environment. Most of these changes are carried out by microorganisms in the soil that are very sensitive to rainfall, temperature, soil drainage and SOM levels. Because of this sensitivity to environmental factors, changes in N availability and losses are generally highest during the spring and fall of the year (rainy seasons) when the crop is not actively taking up N, resulting in greater potential for leaching and denitrification. Penn State Coop Extension PDF (1)   

When to Perform Soil Nitrate-N Testing:

In view of the above, the key question is - what contributes to the effective and efficient transfer of soil N to plants, with little potential for N loss? The answer is in the timing - when active microbial populations are rapidly mineralizing N from labile SOM, and at the same time, plant roots are taking up NH4 and NO3 at rapid rates. Jackson, S. UC Davis 2010 (5) 

Striking a balance between having too little or too much N available from soil is important to yield, crop quality, farm profitability, and the environment. N availability from soil can change rapidly, and soil testing for nitrate should be viewed as a snapshot in time of the soil’s current N status, which is reflective of the ability of the soil to supply available N to the crop. Crop demand for N uptake also changes during the course of the growing season. Thus, time of soil sampling is critical for the proper use and interpretation of soil nitrate test data. Therefore, soil nitrate testing should be guided by both an understanding of the dynamic behavior of nitrate in soil and crop demand for nitrate uptake. Rutgers Coop Extension PDF (2)

Soil nitrate testing is most suitable for use with annual crops which accumulate N rapidly within a single growing season. Typical patterns of biomass and N accumulation for annual crops are shown below (Fig. 1). (2) These patterns are similar, suggesting that the accumulation of biomass and N are closely linked. Thus, the rate of plant growth roughly approximates the rate of plant N accumulation or plant demand for N. Rutgers Coop Extension PDF (2)   

In annual crops, N accumulation more closely follows the pattern of dry matter accumulation during vegetative growth than during reproductive growth and maturation. This is because in the maturing crop, N uptake slows because much of the N already in the plant is remobilized from vegetative tissues to reproductive growth.  The important point is that the pattern of N uptake by an annual crop is approximated by its pattern of growth, and that this pattern suggests the appropriate times for soil nitrate testing and N fertilization as seen in Table 1 below: Rutgers Coop Extension PDF (2)     

Types of Nitrate Testing:

There are two types of nitrate tests used for improving the efficiency of N fertilizer applications. The Preplant Profile Nitrate Test (PPNT) predicts site-specific N fertilizer needs, measuring residue nitrate-N in the crop root zone. The Presidedress Soil Nitrate Test (PSNT) allows adjustments to N recommendations based on the soil’s nitrate content.  This test measures the amount of N released from previous legume crops, manure application, and soil organic matter including N carried over from the previous growing season. Cornell University 2007 (8)

The Pre-Plant N Test (PPNT) measures soil nitrate or soil nitrate-plus-ammonium in the soil (typically from 0 to 60 cm) early in the season to guide N fertilizer applications at planting. It is generally recommended for cases with either high residual inorganic N from the previous season, or with organic N inputs such as manure, where it provides some guidance for adjusting early N fertilizer application rates. Its accuracy, however, is deemed limited for determining the economically optimal N rate (EONR) in humid regions where the inorganic N is highly susceptible to losses during the early season. UW Extension 2001 (9)

The Presidedress Soil Nitrate Test (PSNT) is primarily used by growers to determine whether a crop will respond to sidedress N fertilizer. It is used by agronomists to improve N fertilizer recommendations and can provide information about residual and post-harvest soil nitrate. Improved, site-specific N fertilizer recommendations will maximize N fertilizer efficiency while concurrently minimizing groundwater contamination by nitratePenn State Extension 2009 (10)Timely N fertilizer recommendations using in-season tests like the PSNT require that the tests be performed at the recommended critical crop growth stage and the results should be rapidly available, because growers often obtain the recommendations by cell phone while waiting in the field to be fertilized. Penn State Extension 2009 (11)

The PSNT is highly recommended on fields that have received manure, sewage sludge, or other organic residual products or have grown a forage legume (alfalfa or clover) or a legume cover crop. Use of the PSNT is appropriate if the manure or sludge had been applied recently (this year) or if it has been several years since application. The PSNT is not recommended on fields where commercial N fertilizer is, and historically has been, the only source of N for corn production. UMD Extension 2010 PDF (12)     

How PPNT and PSNT Should Be Used:

Both of these tests are recommended for use primarily on fields where there are significant organic N contributions such as a history of manure applications or a use of forage legumes in the crop rotation cycle. These tests have limited value on most fields without organic N contributions since these fields generally have low N levels and thus the standard recommendations are suitable. UMD Extension 2010 PDF (12)     

In order for the PSNT to be a reliable tool for determining corn sidedress N rate, the total N fertilizer application prior to sidedress must not be greater than 50 lb. N/acre. This total max. of 50 lb. N/acre includes pre-plant broadcast and starter band applied N. UMD Extension 2010 PDF (12)     

In corn, a critical level of soil NO3-N measured by the PSNT has been established above which the yield response to sidedress N fertilizer is not expected. If PSNT NO3-N concentrations are greater than or equal to 21 parts per million (ppm or mg N/kg soil) do not apply sidedress N fertilizer. The PSNT critical level of 21 ppm NO3-N is a revised value that is lower than the critical level previously used for the PSNT (which was 25 ppm). This revision is the result of continuing research and PSNT calibration in commercial cornfields. UMD Extension 2010 PDF (12)     

If the PSNT NO3-N concentrations are less than 21 ppm, refer to the existing nutrient management plan. If the measured PSNT NO3-N concentrations are less than 21 ppm and the Nutrient Management Plan recommends sidedress N fertilization, then apply the sidedress N rate recommended in the Nutrient Management Plan. UMD Extension 2010 PDF (12)        

Research conducted on other crops such as sweet corn, celery, lettuce, cabbage, peppers, pumpkin, winter squash, and sugar beets suggests that sufficient soil N is available to grow these crops when the soil nitrate test is between or above 25 to 30 ppm NO3-N. Some research has shown even lower concentrations were considered sufficient, but until additional research has been conducted, 25 to 30 ppm NO3-N should be used as the generally recommended sufficiency level. In vegetable crops that set fruit, such as peppers, pumpkin, and squash, the application of sidedress N when the soil NO3-N concentration is greater than 30 ppm may cause crop yields to decrease. Rutgers Coop Extension PDF (2)

Use of the PSNT to estimate sidedress N fertilizer requirements is more appropriate under certain agronomic or site situations than in others. Factors to consider when identifying testing sites: Rutgers Coop Extension PDF (11), UDEL Extension 2013 doc (13)    

 1. Coarse soils or soils with a sandy texture with low organic matter content often mineralize only small amounts of nitrate, and in humid regions like the Northeast US, nitrate that is mineralized in coarse soils is easily leached.  Soil nitrate concentrations are typically low in these soils.  (13)  There is little useful information to be gained by performing the PSNT on such soils because crop need for N fertilizer is already predictable. It is generally better to focus efforts in nitrate testing on soils that have relatively high organic matter contents. Rutgers Coop Extension PDF (2)  

2. Manure or compost amended soils increase the mineralization potential of soils and make these soils good candidates for the PSNT.  How much the soil mineralization potential has been increased is difficult to predict.  The PSNT reduces the uncertainty. Rutgers Coop Extension PDF (11)UDEL Extension 2013 doc (13) 

3. Application of pre-plant broadcast N is susceptible to leaching in humid regions.  The PSNT can be used to check if there is sufficient nitrate in the soil if heavy rainfall has occurred after application of pre-plant fertilizer. Rutgers Coop Extension PDF (11)UDEL Extension 2013 doc (13) 

4. Soils double cropped with a late summer crop like cabbage after sweet corn will sometimes have sufficient carryover of nitrate in the soil to reduce the amount of N fertilizer needed for the second crop.  The PSNT can estimate the carryover and provide information about the amount of nitrate available from the incorporation of residue from the first crop. Rutgers Coop Extension PDF (11)UDEL Extension 2013 doc (13) 

5. Legume crops and cover crops can supply large amounts of nitrate to soils.  The timing and amount of nitrate available from the decomposition of the legume residues is difficult to predict.  The PSNT can reduce the uncertainty about nitrate availability from legumes. Rutgers Coop Extension PDF (11)UDEL Extension 2013 doc (13) 

6. Organic nutrient management systems rely almost entirely on N mineralization from organic amendments for their N supply.  The PSNT can be used to estimate if sidedress N is needed in these systems or if more or less organic amendments are needed in the future.  The options for sidedress application of N in these systems is limited because there are few materials with a high percentage of soluble N, but some materials are available.  The most important use of the PSNT in organic systems is to ensure sufficient but not excess nitrate availability from organic amendments. Rutgers Coop Extension PDF (11)UDEL Extension 2013 doc (13) 

References:

  1. Fox, R., Roth, G., Piekielek, W., Beegle D. 1999. Presidedress Soil Nitrate Test for Corn. Penn State College of Agricultural Sciences Research and Extension Program
  2. Heckam, J.R. Ph.D., Extension Specialist in Soil Fertility. 2003. Soil Nitrate Testing as a Guide to Nitrogen Management for Vegetable Crops. Rutgers Cooperative Extension, New Jersey Agricultural Experiment Station, Rutgers, The State University of New Jersey
  3. Krull, E., Skjemstad, J., and Baldock, J. 2004. Functions of Soil Organic Matter and the Effect on Soil Properties: A Literature Review. Report for GRDC and CRC for Greenhouse Accounting. CSIRO Land and Water Client Report. Adelaide: CSIRO Land and Water
  4. Nichols, K. Ph.D., Soil Microbiologist, Northern Great Plains Research Laboratory, USDA-ARS. July 27, 2011 Webinar. Role of Soil Biology in Improving Soil Quality.  MS River Basin Healthy Watersheds Initiative, Role of Healthy Soils in Nutrient Management to Improve Water Quality
  5. Jackson, S. Ph.D., eOrganic. March 11, 2010. Soil Microbial Nitrogen Cycling for Organic Farms. Organic Agriculture. University of California Davis
  6. Overstreet, L.F., North Dakota State University, DeJong-Huges J., University of Minnesota. April 20, 2015. The Importance of Soil Organic Matter in Cropping Systems of the Northern Great Plains. Tillage: University of Minnesota Extension
  7. D. Narendra et al., Department of Pharmaceutical Biotechnology, Lydia College of Pharmacy, Ravulapalem, A.P, India. July, 2012. Isolation and Characterization of Protease Producing Bacteria from Soil. International Journal of Science and Technology. Research Article ISSN 2319-2119.
  8. H.M. van Es*, B.D. Kay, J.J. Melkonian, and J.M. Sogbedji. May 7, 2010. Integrating Weather Variability into Nitrogen Recommendations. Managing Crop Nitrogen for Weather Proceedings of the Symposium Nitrogen Management for Maize in Humid Regions: Case for a Dynamic Modeling Approach
  9. Bundy, L.G., S.J. Sturgul, and R.W. Schmidt. 1995. #A3512 - Wisconsin’s Preplant Soil Nitrate Test. UW-Extension, NPM. R-5-95-3M
  10. Hartz, T.K. and S. Breschini. 2000. Determining nitrogen requirements of lettuce through presidedress soil nitrate testing (PSNT). HortScience 35:387-519
  11. Heckman, J.R. 2002. In-season Soil Nitrate Testing as a Guide to Nitrogen Management for Annual Crops. HortTechnology. 12:706-710. Rutgers Cooperative Extension 2009
  12. Environmental Science and Technology, University of Maryland Extension. Revised April 2010. Making Decisions for Nitrogen Fertilization of Corn Using the Pre-Sidedress Soil Nitrate Test (PSNT). Soil Fertility Management, SFM-2
  13. A.L. Shober, K.L. Gartley, B.L. Vasilas, and J.T. Sims. 2013. Nitrogen Management for Corn in Delaware: The Pre-Sidedress Nitrate Test. Department of Plant and Soil Sciences, University of Delaware