USE OF RADIOACTIVE PHOSPHORUS IN A STUDY OF THE AVAILABILITY OF PHOSPHORUS TO GRAPE VINES UNDER FIELD CONDITIONS

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Abstract
Phosphoric acid tagged with radioactive P was applied by 2 methods (to the surface and at a depth) to a loam soil with a high phosphate-fixing power, and which supported grape vines low in soluble P. The results showed that small amounts of phosphate were taken up and translocated within 40 hrs. after the phosphoric acid application. At the 11-and 43-day periods, vines of the deep placement showed P increases that were approximately twice those of the surface application. Calculations based on the radioactive measurements indicated that < 1 % of the P added to the soil was contained in the aerial part of the vines. Radioactive analysis of the soils showed that the phosphoric acid had penetrated 11 inches in 43 days while the water had penetrated at least 20 inches. Of the P added to the soil surface, 99% and 86% was in the first 6 inches of the profile after 11 and 43 days, respectively. Radioactive measurements of plant samples from vines only 3 to 4 feet from the point of fertilization indicated there were no active roots at even this short distance.

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