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Some Factors Influencing the Utilization of Phosphorus from Crop Residues¹

ABSTRACT

A large proportion of phosphorus of crop residues was found to be readily available to succeeding crops. Some of the factors that influence the rate of release of phosporus from plant residues added to soils for use by succeeding plants were studied. They were: (a) maturity of the crop residue, (b) phosphorus content of the residue, (c) rate of application of the residues, (d) time of contact of the crop residue with the soil, (e) kind of crop residue, and (f) plant part involved. The study was conducted under greenhouse conditions in large glazed pots that held 1 kg. or more of soil.

The influence of the above factors on the utilization of phosphorus by several test crops is less pronounced in soils well supplied with available phosphorus than in soils low in available phosphorus. The rate of availability of the phosphorus of the crop residues to the test crops, ryegrass and tomato, was inversely related to the stage of growth of the residues. For example, green manures cut prior to blooming supplied plants with about as much phosphorus as did liquid H₃PO₄ fertilizer. The phosphorus of straw, on the other hand, was absorbed by plants to a lesser extent than that of the liquid H₃PO₄. The phosphorus of residues high in total phosphorus was more readily available to plants than was that of residues low in phosphorus. The threshold phosphorus content of the crop residue above which the phosphorus of the residue was not wholly immobilized by microorganisms for use in decomposition was found to be about 0.2%. Phosphorus from root residues was less readily available than that from top residues only if the residues came from fully mature plants.

¹ Journal paper No. 369 of the University of Arizona Agr. Exp. Sta., Tucson, Ariz., presented before Div. III, Soil Science Society of America, Davis, Calif., Aug. 16, 1955.

² This investigation was supported in part by the U. S. Atomic Energy Commission, Contract No. AT(11-1)-103. Associate Professor and former graduate students in Agricultural Chemistry and Soils.

Received for publication July 29, 1955.