Phosphorus (P) and Potassium (K) Recommendations
Recommendations are given as pounds of P2O5 and K2O required per acre for each crop. The P and K recommendations are intended to aid in building below-optimum testing soils up into the optimum range. Once an optimum level has been established, the recommendation is to maintain that level by applying P and K to offset the amount that is removed by the harvested crop (Table: "Typical crop nutrient removal for phosphorus and potassium"). Once the soil level is above optimum, no P or K is recommended.
Typical Crop Nutrient Removal for Phosphorus and Potassium
|Crop (units)||Per unit of yield||Typical yield/A||Removal for given yield|
|Corn (bu)||0.4||0.3||150 (bu)||60
|Corn silage (T)¹||5.0||11.0||25 (T)||125||275|
|Grain sorghum (bu)||0.6||0.8||125 (bu)||75||100|
|Forage sorghum (T)¹||3.0||10.0||15 (T)||45||150|
|Alfalfa (T)2,3||15.0||50.0||5 (T)||75||250|
|Red Clover (T)2,3||15.0||40.0||3.5 (T)||55||140|
|Trefoil (T)2,3||15.0||40.0||3.5 (T)||55||140|
|Cool-season grasses (T)2,3||15.0||50.0||4 (T)||60||200|
|Bluegrass (T)2,3||10.0||30.0||2.5 (T)||25||75|
|Wheat/rye (bu)4||1.0||1.8||60 (bu)||60||110|
|Oats (bu)4||0.9||1.5||80 (bu)||70||120|
|Barley (bu)||0.6||1.5||75 (bu)||45||110|
|Soybeans (bu)1||1.0||1.4||50 (bu)||50||70|
|Small grain silage (T)1||7.0||26.0||6 (T)||40||160|
Reference: Table 1.2-8, The Agronomy Guide
Very high soil test levels should be avoided as much as possible. High soil nutrient levels not only represent an economic loss, but they also may indicate potential crop, animal, or environmental problems. Very high P levels in the soil may lead to crop production or feed quality problems and may result in potentially harmful P loss to the environment. Best management practices may be necessary to reduce the potential for environmental problems with P. The PA Phosphorus Index can be used to estimate the risk of loss of excess P to the environment. If fields are very close to water (within 150 feet), have very high soil test levels (greater than 200 ppm P), are in a special protection watershed, or if large amounts of manure P are to be applied, the P Index should be used to estimate the risk of P loss and suggest management changes to protect the environment. (see The Pennsylvania Phosphorus Index for more details).
Very high K levels in the soil can lead to nutrient imbalances in forage crops, which can cause serious health problems in animals. Use caution when grazing forage crops, especially if the soil magnesium is not also in the high range. It may not be possible to correct these soil imbalances in the short term. Feed rations must be balanced accordingly. Very high soil test levels are often a side effect of using manure to supply the N needs of crops. Usually, when manure is applied to meet the N requirements of a crop, excess P and K will be applied. Over time, this can lead to very high P and K levels in the soil. This should be monitored with regular soil testing, and appropriate management action should be taken to limit applications in excess of crop needs, or to minimize the potential for negative crop, animal, or environmental consequences.