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Topics : Soil Fertility for Forage Crops : Maintenance


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Maintaining Phosphorus & Potassium Levels

Phosphorus and potassium management for forages should be based on a regular soil testing program. The goal is to maintain the soil test levels in the optimum to high range. Soil test recommendations are designed to achieve this goal by estimating rates of crop removal from the field and recommending an equivalent amount of nutrients be applied. Because crop removal is determined from the expected yield, it is critical that good estimates of the expected yield be included when a soil sample is submitted. Too high or too low an estimate of expected yield can result in large errors in fertilizer recommendations. Most forage crops remove between 15 and 20 pounds of phosphate per ton of hay equivalent and between 45 and 60 pounds of potash per ton of hay equivalent (Table 4).

If the soil test does not give a recommendation for phosphorus and/or potassium, this means that there is enough of these nutrients in the soil to provide the needs of the crop and still maintain the soil test in the optimum range or higher. For example, consider a field with a soil test level of 500 pounds of K2O per acre and an expected crop yield of 5 tons of alfalfa hay per acre. This crop will remove approximately 250 pounds of K2O per acre. When the crop has removed this amount of potash, this would leave 250 pounds of K2O per acre in the soil. This is above the optimum soil test level of 230 pounds K2O per acre in a typical Pennsylvania soil . Thus, the potash recommendation on this field would be zero. The details for these calculations are given in ST-4 "Interpreting Soil Tests for Agronomic Crops," which is sent out with each soil test run by the Penn State Soil Testing Laboratory.

The timing of phosphorus and potassium applications for forages depend on the situation. When the soil test levels are in the optimum range and the recommendations are low, the timing of fertilizer application is not critical. It can be applied after one of the cuttings or in the fall. There will be no advantage to splitting the fertilizer application in this situation. These low recommendations on an optimum testing soil are only to replace what the crop will remove so that the test level is still in the optimum range going into the following season. When high rates of fertilizer are recommended, there may be an advantage to splitting the application, some after first cutting and the balance in the fall. Many plants will take up potassium whether they need it or not. This is called luxury consumption. If all of the fertilizer is applied at one time, the next cutting may take up more than it needs and leave the crop deficient later on. Splitting the application improves the efficiency of potassium use because there will be less luxury consumption. Also, if the soil test levels are low enough to result in a large recommendation, particularly for potassium, applying some of the fertilizer in the fall before the plants are dormant may improve winter survival.

Triple superphosphate (0-46-0), diammonium phosphate (18-46-0), monoammonium phosphate (11-55- 0), and ammonium polyphosphate (10-34-0) are the more common fertilizer sources of phosphorus. Triple superphosphate is the best source for use on legumes because it does not contain nitrogen. A fertilizer based on one of the ammonium phosphates is best on grasses. All of these materials contain readily available phosphorus.

The most common source of potassium fertilizer is muriate of potash or potassium chloride (0-0-60). Muriate of potash is a readily available source of potassium. It does have a relatively high salt index which can, at very high rates, cause some salt injury to the crop. This is another reason for splitting high rates of potassium fertilizer into several applications.

Manure is an excellent source of phosphorus and potassium (Table 1). The phosphorus and potassium in manure can be considered equivalent to commercial phosphorus and potassium for building soil fertility. However, as was discussed before, the potential problems must be considered in applying manure directly to legumes. The best time to apply manure phosphorus and potassium to a legume is when the field is in corn. Remember that when manure is applied to corn to meet the nitrogen needs of the corn, excess phosphorus and potassium will be applied. This excess phosphorus and potassium will accumulate in the soil and can be used by the forage crops later in the crop rotation (see Figure 2).