Fertilizer Evaluation: IV. Use of P32‐Labeled Fertilizers

Abstract Two different measures of the yield of fertilizer phosphorus were defined as follows: ( 1 ) The net increase in yield of phosphorus from fertilization is equal to the yield of phosphorus in the phosphate‐fertilized crop minus the yield of phosphorus in the control. ( 2 ) The yield of true fertilizer phosphorus is the yield of the atoms of phosphorus actually applied in the fertilizer and is equal to the product of ( a ) the yield of phosphorus in the phosphate‐fertilized crop and ( b ) the ratio of the specific activity of phosphorus in the crop to that in the fertilizer. Measurements of these two kinds of yields of fertilizer phosphorus were made on a crop grown in greenhouse cultures on 6 soils with 4 particle sizes of P 32 ‐tagged monobasic sodium phosphate applied at 4 rates. The net increases in yield of phosphorus from fertilization exceeded the yields of true fertilizer phosphorus, and the difference between these two values increased with the quantity of labile phosphorus in the soil. These observations can be explained on the basis of ( a ) enhanced growth of roots and absorption of soil phosphorus in the nonfertilizer‐affected soil in the presence of fertilizer, ( b ) enhanced absorption of soil phosphorus in the fertilizer‐affected soil, or ( c ) a combination of ( a ) and ( b ), but no evidence permitting a choice of hypotheses was obtained.