WHY DO ALFALFA FIELDS CONTINUALLY TEST DEFICIENT IN PHOSPHORUS?

This question is often asked by alfalfa producers who fertilize their fields according to a soil test each year and do not see any improvement in their soil test P value. The easiest way to answer this question is to consider the soil as a "black box" and that in order for the level in the black box to raise, more has to be put in each year than what is taken out.   

We know that good quality alfalfa removes about 14 lb of P (expressed as P₂O₅)/ton of hay. So, a field that produces 5 ton/acre/year could remove about 70 lb of P₂O₅/acre each year.   

If the soil test P index for this field before it was planted was 30 (OSU soil test), the soil test calibration would identify a deficiency of 80 lb P₂O₅/acre. Applying 80 lb P₂O₅/acre each year would about equal the P removed each year if 5-ton yields were maintained.   

If the soil test is a measure of the level of P that will become available to the crop during the growing season, then we should expect the soil test to remain about the same each year and for it to continue to show a P deficiency and the need to keep applying about 80 lb P₂O₅/acre.   

Although the uptake of P by alfalfa, reactions of fertilizer P with the soil, and the dynamics of soil P are quite complex, the "black box" approach provides a fairly true explanation for what happens to the soil test P value when the deficiency amount identified by the soil test is close to the amount of P removed by the crop.   

We cannot always take this approach, and in fact do not recommend as a "general rule" adding back the amount of P that is removed in harvests each year. In the next issue of the NEWS we will attempt to answer the more difficult question of "why does the soil test continue to decrease over time, even when the recommended amount of P fertilizer is added each year?"