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The PSNT is a soil test for nitrate-nitrogen (NO3-N) developed for use at the 4 to 6 leaf stage of corn to help in making more accurate N fertilizer recommendations at sidedressing time. The test, designed by Dr. Fred Magdoff at the University of Vermont, was initially developed to help in estimating the amount of N available where manure or other organic wastes have been applied, or corn is grown in rotation with a forage legume. The PSNT was initially developed to identify fields that would not be expected to respond to additional N. The test uses the NO3-N content of the top foot of soil as an estimate of the amount of N available to the crop. Research in a number of states has confirmed that this test can be useful in managing N on corn. Work in Massachusetts, California and Florida has also shown the PSNT to be useful for some vegetable crops. By far, the major use of PSNT is in the production of field corn, which is the focus of this paper.
As the PSNT has been adopted by agriculture, there has been a great demand to make calibrated N recommendations based on the test results. In other words, agronomists have been asked to make N recommendations that are adjusted for, or inversely proportional to the amount of residual NO3-N found in the soil. Given that the original purpose of the PSNT was simply to identify soils that were non-responsive to N, we may be asking a lot of this test by using it to accurately calibrate soil N status levels or N recommendations. However, some progress in this area has occurred. There is agreement on the general range of soil NO3-N that is considered critical for adequate corn growth without additional N applications. This range is from 21 to 30 ppm NO3-N in the top 12 inches of the soil when the corn is in the 4 to 6 leaf stage. Many Universities consider the critical NO3-N value to be 25 ppm.
There is less agreement on how much supplemental N to recommend when the PSNT result is between zero and the critical level. In other words, how do we calibrate N recommendations to PSNT results? Most of us understand that agronomists often disagree on fertilizer recommendations, and the PSNT is no exception. There is some research that supports higher N recommendations when the PSNT result is low and lower recommendations when the result is closer to the critical level. However, many factors of weather, soil conditions, corn hybrid, crop management, and others may change crop response to a particular PSNT level. It is not within the scope of this paper to thoroughly discuss all of the factors that can influence the reliability of N recommendations based on PSNT results. However, like all soil testing, the PSNT should be used as a guide, not a guarantee. Neither the PSNT result nor the N recommendation should be over-simplified to a simple recipe. The PSNT is simply one part of the complicated process of crop management.
Another occasional point of confusion occurs when we try to reconcile the ppm of NO3-N with the amount of N required by a corn crop, or the amount of N applied prior to taking the PSNT sample. The PSNT result is an “index” of available N, not a calculation factor. You should not try to perform calculations to reconcile the amount of N applied or crop uptake with PSNT results, because they will not likely “add up”. We must keep in mind that a crop typically utilizes N from below the 12 inch sampling depth; it will receive N from mineralizing soil organic matter; it may lose access to N due to denitrification or leaching later in the season; or other factors may change the N available to the crop, either before or after the PSNT is taken.
It is common to hear people discuss fertilizer recommendations in terms of crop removal, plus or minus soil test buildup as if they were somehow disconnected from each other. In fact, they are two aspects of the same subject. Most of us fall into the habit of thinking that we are fertilizing plants. Except for foliar fertilizer or tree trunk injection, we do not fertilize plants… we fertilize soil. Because of this, soil chemistry will determine how much of the applied nutrients the plants will be able to take up. If a soil is low in phosphorus (P) or potassium (K), it will tie-up or “fix” much of the applied fertilizer P and K (P2O5 and K2O) into forms that are not available to the plants. This nutrient fixation is simply another way of saying that the soil is trying to build itself up in these nutrients… whether that is your intention or not. The soil is a reservoir for the nutrients that have been applied or generated by other means over the years. The nutrients that a plant gets in any one season are likely to be ones that have been in the soil for many years. Therefore you can think of fertilization as putting nutrients into one side of a reservoir while the plants are taking them out of another end. What happens inside of this nutrient reservoir is soil chemistry and microbiology. These processes, along with weather, determine how much access the plants have to the nutrients within the reservoir.
Application of collected municipal leaves to agricultural land improves soil quality and provides a solution to a disposal problem. Farmers are permitted (New Jersey Register, NJAC 7:26, 1.12. Nov. 7, 1988) to apply up to a 6-inch layer of leaves annually. Application at this rate, which is equivalent to approximately 800 cubic yards/acre or 20 tons/acre of dry matter. This will increase soil organic matter content, and improve soil tilth and water holding capacity.
A chemical analysis of 100 municipal leaf samples collected from across New Jersey shows that leaves are a valuable source of all crop nutrients (Table 1). Although nutrient concentration values vary considerably, the application of 20 ton/acre of leaves would add on average 400 pounds of nitrogen, 40 pounds of phosphorus, and 152 pounds of potassium. Assuming values of $.30/pound N, $.23/pound P, and $.18/pound K, the nutrients from this example are worth $156.56.
Application of leaves at 20 ton/acre would also add on average 656 pounds of calcium, 96 pounds of magnesium, 44 pounds of sulfur, 1.5 pounds of boron, 58 pounds of iron, 22 pounds of manganese, 50 pounds of chloride, 4 pounds of sodium, 0.3 pounds of copper, and 3 pounds of zinc. The actual amounts of nutrients applied can vary considerably as shown by the concentration ranges in Table 1.
Although leaves add agronomically significant amounts of nutrients, only a portion of the nutrients are available immediately after application for use by the crop. The increase in the soils total nutrient content will, however, contribute to the long term fertility of the soil as the nutrients are released over time.
Much of the nutrients in leaves are part of the organic structure of the plant tissue and require microbial decomposition to release them. The carbon-nitrogen ratio of an organic material undergoing decomposition is an important indicator factor in the rate of release of its nitrogen in available form. The average carbon nitrogen ratio of leaf waste is 50 and it ranges from 27 to 72. For comparison, the carbon-nitrogen ratio of compost is generally about 25.
The abundant carbon (carbohydrates which provide energy) content of leaves leads to extensive development of fungi and bacteria in the soil which uses up the supply of available nitrogen for the production of microbial cell tissue. As decay proceeds, the carbon-nitrogen ratio decreases and some of the nitrogen becomes available to plants. Because of the high carbon content of raw leaves relative to their nitrogen content, there will likely be very little of the organic nitrogen in leaves available to crops for a period of time after application. Observations of crops (including legumes) planted on soil to which leaves have been applied indicate that plants suffer from a temporary N deficiency unless additional N fertilizer is added.
Crops grown on soils the year after leaf application likely will need additional N fertilizer. Legume crops, such as soybean, may benefit from 20 to 30 pounds of starter N banded beside the row at planting. This will supply a readily available N source to be used by the legume until it forms nodules to supply N by fixation. An additional 50 to 100 lbs of N fertilizer is recommended for corn grown the first year after leaf application. The additional fertilizer that is required increases the cost of crop production on the soil the first year after an application of leaves.
The amounts of P, K, and other nutrients present in leaves are not easily translated into nutrient credits that may be used to reduce fertilizer application. These nutrients are relatively stable in soil and can be monitored simply through soil testing. As soil fertility levels increase as a result of leaf applications, take credit for these nutrients by fertilizing accordingly.
Of the three major nutrients, potassium is the most easily released from leaves and is the most readily available to crops in the first year after leaf waste application. A minimum nutrient credit of 50 lbs K2O per acre may be used for 20 tons of leaves.
Application of collected municipal leaves to soil should not significantly change its agricultural limestone requirement. Three years of municipal leaf application caused no decrease in the soil pH compared to unamended soils.
|Table 1 Nutrient concentrations in municipal leaves (dry weight basis).|
|Phosphorus (P2O5)||0.02 (0.05)||0.29 (0.66)||0.1 (0.23)||2.0 (4.6)|
|Potassium (K2O)||0.09 (0.11)||0.88 (1.06)||0.38 (0.46)||7.6 (9.1)|
|Nutrient||Parts per million||Lb/ton|