Pre-Sidedress Soil Nitrate Testing for Managing Nitrogen
Posted: December 19, 2011
Unfortunately, a reliable test for determining available nitrogen in the soil has not yet been developed. Nitrogen availability is a very complex and varies throughout the year.
Fortunately, nitrogen recommendations have been developed for most vegetable crops based on years of scientific studies. Recommendations can be used to wisely apply nitrogen to optimize yields and minimize the chances of excess nitrogen resulting in environmental problems.
Nitrogen can also be supplied by organic nutrient sources including compost, legume cover crops and manure. Nitrogen availability from these sources is dependent on organic forms of nitrogen being converted into inorganic, plant-available forms of nitrogen. Even after numerous years of scientific research, predicting the amount of available nitrogen from organic nutrient sources is imprecise. This is because the conversion to inorganic forms is dependent on many interacting variables including temperature, moisture levels, type and number of soil microorganisms, particle size of the nutrient sources, etcetera.
Pre-sidedress soil nitrate testing (PSNT) was developed for field corn production, but has been adapted for vegetable production. PSNT results in more efficient nitrogen use and has been found to be particularly useful in locations with residual soil nitrogen such as from the use of compost or cover crops; fields where a portion of the nitrogen recommendation has been broadcast preplant; and where leaching is limited (high tunnels are an example). PSNT currently cost $6 per sample through Penn State’s Agricultural Analytical Services Laboratory (http://www.aasl.psu.edu/PreSideress.HTM).
Dr. Joe Heckman, a Professor at Rutgers, has conducted years of research on using PSNT for vegetable production. He developed the publication Soil Nitrate Testing as a Guide to Nitrogen Management for Vegetable Crops. It can be found at http://njveg.rutgers.edu/assets/pdfs/soil/e285-jhSoil%20Nitrate_Testing_as_Guide_to_Nitrogen_Management_for_Vegetable.pdf.
To use the PSNT, 12 inch deep soil samples from 15 to 20 places in the field are taken when vegetables reach various stages; see the table below (Source: Soil Nitrate Testing as a Guide to Nitrogen Management for Vegetable Crops). Samples are pooled and dried immediately before being sent to the lab for analysis.
Results can be used to determine whether sidedress applications of nitrogen are needed based on the table below (Source: Soil Nitrate Testing as a Guide to Nitrogen Management for Vegetable Crops).