- Tell me about the presidedress soil nitrate test (PSNT)
- What have we done at Rutgers?
The goal of the soil management plan for a sustainable systems should be to “feed the soil.” Healthy soils produce healthy crops.
What is soil texture?
The relative amounts of sand, silt and clay. Sand particles are the largest, clay particles are the smallest, and silt particles fall in between the two.
There are 12 textural classes. The Rutgers Soil Testing Laboratory classifies soils into 4 broad classes: loamy sand, sandy loam, loam, silt loam. However, they also offer an individual special test for soil texture that provides the percentages of sand, silt, and clay. With these percentages, you can determine the soil texture using the NRCS soil texture calculator.
Why is soil texture important?
Texture influences water-holding capacity, internal drainage, ease of compaction, aeration, and cation exchange capacity or nutrient holding capacity. For instance, water drains quickly through sandy soils and slowly through clayey soils.
Can I change the soil texture?
Soil amendments can be used to improve or enhance soil texture, but cannot be used practically to change texture significantly.
What is soil pH?
Soil pH is a measure of the concentration of active, not total, acidity.
Most grains, vegetables, trees, and shrubs produce optimally on soils with pH 6.2-6.5 (slightly acidic). Potatoes produce optimally on soils with pH 4.5-5.5, and alfalfa and cabbage produce optimally on soils with pH 6.5-6.7.
Can I change the soil pH?
Yes. To decrease the soil pH (increase acidity), use acidifying organic materials, such as sphagnum peat moss, or elemental sulfur-containing amendments. To increase pH (lower acidity), use limestone.
How can I determine the liming requirements for my fields?
There are two methods by which to determine a soil’s liming requirements:
- Determine soil texture and pH; use tables appropriate for the crop to be grown
- Find the soil buffering capacity; use the buffering capacity along with the initial soil pH and the target pH to calculate lime requirements. Typically, soils with low buffering capacity (e.g. sandy soils, soils with low organic matter) need less amendment to change the pH. Soils with high buffering capacity (e.g. clay loam soils, soils with high organic matter) will need greater amounts of amendment to change the pH. The Rutgers Soil Testing Laboratory offers a special individual test for soil pH and lime requirement.
See: Liming New Jersey Soils for Fruit Crops, Liming New Jersey Soils for Field and Forage Crops, Liming New Jersey Soils for Vegetable Crops, and Soil pH and Lime Requirements for Home Grounds Plantings (PDF).
Can I use calcium silicate in place of calcium carbonate limestone?
Yes. Calcium silicate may even production benefits beyond correcting soil pH. Research done by Rutgers soil fertility specialist Joseph Heckman demonstrated suppression of powdery mildew in pumpkin and wheat grown on silicon-amended soils. Organic growers should use wollastonite, the naturally-occurring form of calcium silicate. More information on using calcium silicate as a liming alternative can be found in Silicon and Soil Fertility (PDF), Volume 20 of the Soil Profile by Joseph Heckman.
What is organic matter?
Organic matter includes plant and animal residues at various stages of decomposition, as well as soil microbes both alive and dead.
What are the benefits of organic matter?
Organic matter provides energy for biological and chemical processes in the soil and enhances soil biological activity. We can get a good idea of the levels of biological activity in the soil using the Solvita soil respiration test, now offered at Rutgers Soil Testing laboratory. Organic matter provides a source of nitrogen, improves soil tilth and structure, increases porosity and infiltration, reduces soil crusting and erosion, increases water-holding capacity, and improves nutrient holding and release (cation exchange capacity).
How can I increase soil organic matter?
To increase organic matter, plant cover crops, practice minimum or no tillage, and consider rotational grazing. Annual ryegrass, winter rye, sweet clover, or sorghum-sudangrass hybrids are good cover crop choices for building soil organic matter. (See: Economics and Soil Improving Methods Using Cover Crops (PDF) and Cover Crops and Green Manure Crops: Benefits, Selection, and Use)
Can I have too much organic matter?
Yes! For general agronomic and horticultural use, 10% organic matter is suggested as a maximum. Soils with too much organic matter may exhibit poor drainage, increasing the potential for root disease. Conversely, under hot, dry weather conditions, organic matter will repel water.
Know the symptoms of nutrient deficiencies (PDF).
If you suspect a nutrient deficiency, get the soil tested. Collect three different samples: 1) from an area where the crop is growing; 2) from an area where symptoms are most severe; 3) from an area were symptoms are just starting to appear.
For more information on nutrient deficiencies, see: Plant Nutrients from Penn State, Guide to Symptoms of Plant Nutrient Deficiencies (PDF) from University of Arizona Cooperative Extension, and Crop Nutrient Deficiencies and Toxicities (PDF) from MU Extension.
How do soil nitrogen levels change over time?
When organic matter decomposes, nitrogen initially accumulates as ammonium. In warm soils with favorable pH, ammonium is rapidly converted to nitrate. Several factors, including soil temperature and residue composition, affect the rate at which nitrogen becomes available to plants. Nitrogen is released more slowly in cold soils and more rapidly in warm soils. The carbon-to-nitrogen ratios of decomposing materials also impact nitrogen availability. The decomposition of “brown” materials (high carbon-to-nitrogen ratio) requires nitrogen and thus temporarily “ties up” available soil nitrogen. Conversely, the decomposition of “green” materials (low carbon-to-nitrogen ratio) quickly releases available nitrogen into the soil.
Concentrations of nitrate in the soil are susceptible to rapid changes. For instance, heavy rains can cause nitrate leaching. In saturated soils, nitrate can be converted to gaseous forms of nitrogen which are released into the atmosphere.
Over longer time periods, manure applications and legume cover cropping will increase soil nitrate levels.
How do soil nitrate levels change over the course of one season?
As soil warms in the spring, nitrate concentration increases. During the course of the season, crops take up nitrogen, and the soil nitrate level declines. Initially, crops take up nitrogen slowly. Then, during the rapid growth phase, plants take up nitrogen more quickly. As crops mature, uptake slows and nitrogen within the plant is redistributed from vegetative to reproductive organs. Nitrates remaining in the soil after harvest are susceptible to leaching over the winter. Late-season plantings and cover crops reduce nitrogen losses.
How do soil properties affect nitrate levels?
Soil texture can impact soil nitrate levels. For instance, sandy soils typically have low concentrations of nitrate.
How do I take a proper soil test sample?
For a basic soil test, take samples 6-8” deep. Take multiple samples from each area and combine them. Take different samples for different crops and management practices.
Always record important information, such as the site from which the sample was taken.
Evaluate results and keep records to chart progress over time.
When should I take soil test samples?
Some tests, such as pH and fertility, can be done nearly any time during the year.
Nitrate-N tests should be done prior to crop planting or just prior to rapid growth. The period of rapid nitrogen uptake is extended in some crops, such as tomato. For these crops, nitrate-N tests can be taken multiple times during the growing season.
How do I interpret the results of my soil test?
SLAN and BCSR are two methods for evaluating soil fertility.
SLAN stands for sufficient level of available nutrients. Based on data from historical field trials, SLAN determines whether soil fertility levels are below optimum, optimum, or above optimum.
BCSR stands for base cation exchange ratio. This method is based on maintaining percent base saturation or ratio between bases and can be useful in soils with low cation exchange capacity to determine if potassium and magnesium competition is occurring.
See: Soil Fertility Test Interpretation – Phosphorus, Potassium, Magnesium, and Calcium.
When do I use PSNT?
PSNT is useful for annual, but not perennial, cropping systems.
The PSNT is generally not useful on sandy soils, which are expected to have low nitrate levels.
The PSNT is useful on manured fields and fields which were planted in legume cover crops to determine if the supply of available nitrogen is adequate. The PSNT is also useful in double-cropping systems to determine if there is sufficient nitrogen for the second crop.
What do the results tell me?
In organic systems, results from the PSNT can be useful for evaluating the current fertility program.
For field corn, sweet corn, and cabbage, 25ppm NO3-N is sufficient for crop growth and no additional nitrogen is needed. For pumpkin, squash, celery, lettuce, and pepper, 25-30ppm NO3-N is sufficient. Subsequent weather events, such as excessive rainfall, can impact soil N levels and, in turn, crop growth.
How do I take a sample for PSNT?
Take 15 to 20 soil cores 0-12” deep from the area to be sampled.
In 2000-2001, we worked with 3 organic farms in the state on a study evaluating the use of soil NO3-N testing as a nitrogen sufficiency test in tomato. Tomato, as an indeterminate crop, exhibits season-long demand for nitrogen, and fields were sampled throughout the growing season.
As expected, nitrate rose as soils warmed, peaking late spring. Levels declined as plants rapidly removed nitrogen from the soil.
This research demonstrated that nitrate testing should be done approximately 2-3 weeks after transplanting, just before the period of rapid nitrogen uptake and rapid growth. 25-30 ppm NO3-N was shown to be optimal for tomato.
Even more valuable information can be found in the Soil and Nutrient Management section of the