WHAT CAUSES CHANGES IN SOIL TEST PHOSPHORUS  OVER TIME    (EVEN WHEN RECOMMENDED FERTILIZER WAS APPLIED)?  To answer this question one must understand soil contains many forms of phosphorus (P), that vary from readily available to very unavailable. As plants use the available forms, forms that are only slightly unavailable gradually become available.     When conventional dry or liquid fertilizer P is added to the soil, the P is 100 % available. However, it immediately begins reacting with the soil to become less and less available. These reactions are slow, and enough roots come in contact with fertilizer that the crop gets enough fertilizer P to eliminate the deficiency that would otherwise occur that year. This relationship between fertilizer or available soil P and slowly available soil P can be illustrated by considering the two forms of P as being contained in reservoirs connected by a tube that lets the P slowly flow from one to the other .      Soil that has a soil test P of 45 would require about 50 lb. P2O5 per acre. The fertilizer P would fill the small reservoir and provide enough available P to meet crop needs for the entire growing season (b). However, only about 10 to 15 % of the 50 lb. of P2O5 applied as fertilizer would be used by the plant. The unused fertilizer would slowly react with the soil, becoming less available as it moves from the small reservoir on the left to the big reservoir on the right.     The soil test provides an index of slowly available soil P. For fine textured soils (loam to clay loam) it will take about 20 lb. P2O5 per acre to raise the soil test value one unit, that is from 45 to 46. What this means in relation to the above diagram, is that if we fertilize established alfalfa with 50 lb. P2O5 per acre each year to make sure P is adequate, about 40 lb. P2O5 per acre will be added to what is already in the soil (large reservoir on the right). If the crop did not remove any soil P from the reservoir on the right then we would expect our soil test to increase two units each year.     Good quality alfalfa removes about 14 lb. of P2O5/ton. So a field that produces five ton per acre per year could remove 70 lb. of P2O5 per acre. If, as in the above example, 50 lb. of fertilizer P were added each year, this would about equal the amount used by the crop and the soil test should stay about the same.     The end result is that the soil test may remain nearly the same as we are growing alfalfa. If production is high and the soil test was initially high, then the soil test may decrease over time because less fertilizer P is added to the soil than is removed by the crop each year. If the soil test was initially low, so the fertilizer recommendation is 100 lb. P2O5 or more, then we may find the soil test increase over time.     The reason there is still some uncertainty in all of this is because soils react differently. The size of the reservoir on the right is of different sizes for different soils. It is very large for fine textured soils, for example, and small for sandy soils. Extremely high and low soil pH also contributes to a larger size reservoir. Soil testing on a regular basis is the best way to evaluate how the soil is behaving to intensive cropping and fertilizing.    Additional Information     Alfalfa Stand Establishment. OSU Extension Facts F-2089

Gordon V. Johnson, Nutrient Management Specialist Plant and Soil Sciences  Oklahoma State University

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