Crops and crop rotations require different levels of nitrogen (N), phosphorus (P) and potassium (K). The mobility of N makes it difficult to buildup in the soil. Requirements of N minus the N-supplying power of the soil will indicate the N rate necessary to produce the yield goal for a selected crop. N requirements are adjusted on the basis of yield goal and the assumed plant population required to achieve a given yield level.

P and K can be built up to sufficient levels so as not to be limiting in general crop production. The desired soil P test level for row crops and small grains is about 45 lbs/a whereas most hay and pasture grasses need only 40 lbs/a. Depending upon soil type and crop grown, the desired soil K test level for most fields ranges from 160 to 220 lbs/a. Above these optimum levels, the potential for response to additional P and K is low. Typically, soil fertility recommendations are based on an 8 year buildup because of economics and crop response. Fields that test only slightly lower than the desired levels may be built up in a period of 4 years.

Fields with the above soil test levels can produce optimum crop yields (from a soil fertility point of view) for at least one year without adding more fertilizer. However, a maintenance amount of fertilizer is recommended each year to replace the nutrients removed by the crop yields. Some of the fertilizer removal values, identified by the University of Missouri in Columbia, are listed below for various crops:

Crop	N (lbs)	P2O5 (lbs)	K2O (lbs)
Corn (bu/a)	0.9	0.45	0.30
Corn silage (ton/a)	9.0	3.60	9.00
Soybeans (bu/a)	0.0	0.84	1.44
Grain sorghum (lbs/a)	0.014	0.0093	0.006
Wheat (bu/a)	1.26	0.60	0.30
Alfalfa hay (ton/a)	0.0	10.00	45.00

Fertilizer maintenance requirements are determined by the product of yield goal and nutrient removal. For example, a 100 bu corn yield will remove the equivalent of 45 lbs/a of P2O5.

Due to its high mobility, N can be lost from the soil by runoff, leaching, and denitrification. All of these methods are favored by high rainfall conditions. Anhydrous ammonia applications are less susceptible to these losses than are other N sources. Nitrates (NO3) continue to be detected with increasing frequency in private and public water supplies in both surface and ground water. High levels of NO3 in shallow wells in northeast Missouri have often been linked to septic system problems. Regardless, producers need to be careful when applying N before rainy weather especially on poorly drained soils.

Soon after P and K are applied to the soil, they are bound or tied up on the surface of soil particles. At this point, they are essentially immobile and move downward in the soil very gradually. These nutrients are usually not a problem with the environment, except in areas of severe erosion. When soil particles wash off a field, they carry fertilizer with them. Soil conservation practices (e.g. vegetated waterways, strip cropping, etc.) slow runoff, trap sediment, and therefore reduce fertilizer loss.

In the past few years, the loss of phosphorus in agriculture runoff has been a primary concern in the Midwest. Excessive amounts of available phosphorus in surface water can lead to various problems such as eutrophication. Since most farmers can not afford to apply excessive amounts of P to pollute the environment, the culprit is often believed to be the production of phosphorus in manure from confined animal operations. Because of the unfavorable N:P ratio of most manures, field applications can build soil P levels beyond the optimum range for most crops. Besides possibly causing some environmental problems, extremely high P levels can create an imbalance of micronutrients. In addition, producers need to be especially cautious when applying manures on frozen soils.

Unfortunately, it is really difficult to track down non-point sources of fertilizer in our drinking water. There has been much speculation and blame from both urban and agriculture groups, but to date, there is no conclusive evidence from either side of the fence. For more information about building your soil fertility bank or fertilizer losses, call me at (573) 769-2177.