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MISCELLANEOUS PUBLICATION NO. 1065, U.S. DEPARTMENT OF AGRICULTURE 



Phosphorus 



Research on the field application of phosphatic 

 fertilizers has been underway in the United States 

 for over 80 years. A number of the State agricul- 

 tural experiment stations — Illinois, Pennsylvania, 

 Ohio, and New Jersey, for example — initiated this 

 early research. As time went on, there also de- 

 veloped an extensive program of laboratory studies 

 towards understanding the chemical behavior of 

 phosphate in soils. 



Research showed that some phosphorus is pres- 

 ent as a natural component of soil-forming min- 

 erals in all soils. The amount in the 7-inch plow 

 layer ranges from less than 100 to as much as 

 4,000 pounds per acre (of soil to the plowed depth) . 

 The average content runs about 1,000 pounds per 

 acre or about 0.05 percent of the 2 million pounds 

 of soil in an acre to plow depth. To illustrate, the 

 Clyde, Brookston, and Claremont soils of Indiana 

 and Ohio averaged 2,540 pounds per acre, whereas 

 the Norfolk and Tifton soils of the Atlantic 

 Coastal Plain averaged only 340 pounds per acre. 



Much of the mineral phosphorus content in soils 

 is not only insoluble in water, but unavailable to 

 plants. The part of the phosphorus that is not ac- 

 tually present as crystals of insoluble phosphates 

 or occluded within grains of other minerals is ad- 

 sorbed very tenaciously on the surface of the soil 

 colloids. Phosphate derived from water-soluble 

 fertilizers is rapidly converted to insoluble forms. 



Much research has been done on thousands of 

 soils as to the soil chemical factors that control 

 phosphorus availability. In acid soils, freshly ap- 

 plied fertilizer phosphate is rapidly converted to 

 various forms of aluminum phosphates, some of 

 which are crystalline. With the passage of time 

 these compounds disappear, as the phosphate is 

 converted to forms of iron phosphate by reaction 

 with the iron oxides that are an almost universal 

 component of the soil. In virgin (undisturbed) 

 soils most of the mineral phosphate is present as 

 poorly defined but highly insoluble iron phos- 

 phates. 



In neutral and alkaline soils the principal forms 

 present are basic calcium phosphates. At pH values 

 above 6 these are very stable and highly insoluble. 

 In addition to the mineral forms, significant quan- 

 tities of phosphorus can be present in organic 

 forms, in amounts ranging from 5 to 75 percent of 

 the total depending upon the character of the soil. 



These are also very stable compounds ; the availa- 

 bility of this kind of phosphorus depends mainly 

 on its rate of conversion to mineral forms. 



A marked step forward was made possible in 

 studying the phosphate chemistry of soils in the 

 middle twenties by discovery of the ceruleo- 

 molybdate reaction in which traces of phosphate 

 reacting with molybdenum in the presence of a tin 

 catalyst produce a bright blue color that is propor- 

 tional to the concentration of phosphate. This 

 colorimetric technique made possible assay of phos- 

 phorus in soil solutions that contained only 0.1 

 p.p.m. 



A tremendous step forward in research and un- 

 derstanding of the behavior of phosphate in soil 

 solutions was made in the midf orties when a radio- 

 active isotope of phosphorus, P 32 , became availa- 

 ble from atomic research. Use of this isotope ena- 

 bled the researcher to distinguish between the 

 phosphorus that was inherent in the soil and that 

 which was applied as a fertilizer. Phosphate ions 

 that entered into crop plants, into drainage waters, 

 or became residual in the soil itself could be traced. 

 The great mass of chemical evidence that has been 

 attained from research on soil phosphorus shows 

 very conclusively that phosphate applied to soils 

 does not move downward into the ground water 

 or drainage water except in very minute amounts. 

 Studies of soils in lysimeters have shown this very 

 clearly. For example, in lysimeter studies at the 

 Illinois and Wisconsin agricultural experiment 

 stations (68, 107) phosphate content of the per- 

 colate was not even measured because there was 

 such a minute trace. 



Cooperative research by the U.S. Department of 

 Agriculture and the South Carolina Agricultural 

 Experiment Station (9) involving lysimetric 

 studies of a Lakeland sand is of special interest. 

 If phosphorus would percolate downward through 

 the soil and into the effluent from any soil, it would 

 do so in Lakeland sand. Phosphate was added to 

 the soil in the lysimeters at rates varying from 

 zero to 288 pounds of P 2 5 per acre. A number of 

 different crops were grown on the soils so treated. 

 The amount of phosphate that accumulated in the 

 leachate was measured over a 5-year period follow- 

 ing fertilizer application. In the lysimeter that re- 

 ceived no phosphate fertilizer, the total loss of 

 P 2 5 over the 5-year period was 1.48 pounds per 

 acre. The four lysimeters that had received 288 



