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The Influence of Straw and Straw-Fertilizer Composts on the Uptake of Fertilizer Phosphorus by Plants¹

ABSTRACT

The application of solutions of inorganic phosphorus fertilizers to crop residues before the residues are tilled into the soil is now a widespread practice in the Southwest. This practice is centered around the hope that the fertilizer phosphorus will remain at a higher level of availability as a result of this procedure than if it were added directly to the soil. The hypothesis is that phosphorus fixed by microorganisms during the decomposition of the crop residue is only temporarily unavailable to plants. Phosphorus fixed by this method presumably would be made readily available to plants during the latter stages of decomposition of the residues in contrast to the irreversible fixation by the inorganic constituents of the soil.

Results with wheat straw and wheat straw composts containing tagged phosphorus indicate that a relatively greater amount of phosphorus is utilized by plants from compost than from noncompost material. The compost had lost 40 to 50% of its original weight as a result of decomposition before being added to the soil. On the other hand, results with phosphorus-free extracted oat straw showed that the less decomposed material (oat straw plus KH₂PO₄) supplied more P to ryegrass growing on Mohave clay loam than the compost (oat straw composted with KH₂PO₄). However, the extracted oat straw composts did not decompose to an extent to exceed 20% before being added to the soil.

According to the data obtained, it must be concluded that there is no particular advantage of adding liquid phosphoric acid fertilizers directly to straw before incorporating them into calcareous soils over the practice of applying the fertilizer and straw residues independently.

¹ Contribution of the University of Arizona Agr. Exp. Sta. This project was supported in part by the United States Atomic Energy Commission. Presented before Div. III, Soil Science Society of America, Cincinnati, Ohio, Nov. 15, 1956.

² Professor and Head of Department, and former graduate student in Agricultural Chemistry and Soils, respectively.

Received for publication November 23, 1956. Accepted for publication January 30, 1957.