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The Phosphorus Requirements of Root Crops

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Title:The Phosphorus Requirements of Root Crops
Authors:Zaag, Peter Vander
Date Issued:1979
Abstract:The objectives of this study were: to determine the external and internal P requirements of five root crops (cassava, potatoes, sweet potatoes, taro and yams); to test a methodology for transferring soil management information from one site to another and to determine the phosphate fertilizer requirements of soils which have been used or have potential for growing root crops.
In order to fulfill these objectives, P experiments were identified in 10 countries. Soil samples and the field experimental data were secured. This was supplemented with experiments in Hawaii for each of the five root crops. All analytical work on the soils was done in Hawaii. P sorption curves were developed from which it was possible to predict the initial level of P established in solution and to convert each P fertilizer treatment to P in solution. The procedure used to determine P sorption curves consists of equilibrating 3 g samples of soil for 6 days at 25®C in 30 ml of 0.01 M CaCl2 containing graded amounts of Ca(H2P0^)2- The samples were shaken for 30 minutes twice daily. Crop yields were plotted as a function of the P concentration in solution. From these curves the external P requirements were determined.
Yield responses for the five root crops varied considerably. A composite yield response curve for seven experiments revealed that cassava has an external P requirement of 0.005 ppm— the lowest of the five root crops studied. This low P requirement was attributable not only to a very extensive root system but also to a mycorrhizal association which allow for a greatly increased surface area through which diffusion of P into the roots could take place. Three experiments did not conform to the above results. In two of these experiments, foliar analysis indicated that P nutrition was inadequate at soil concentrations which would have been adequate elsewhere. These data suggest some factor, perhaps related to inefficient mycorrhiza, was responsible for these anomalous results.
Yam results were obtained from five locations. Only in Hawaii was a response to P evident and that for high yield potential varieties. The external P requirements were from 0.01 to 0.02 ppm. The roots were highly mycorrhizal suggesting that the mycorrhiza were responsible for the efficiency of yams in utilizing P at relatively low solution concentrations.
Based on results from three locations, sweet potatoes responded gradually to increasing levels of solution P. Seventy-five to 80 percent of maximum yield was obtained at 0.003 ppm P while 95 percent of maximum yield was not obtained until a P concentration of 0.1 ppm in solution was reached.
Results for three taro varieties were obtained in Hawaii. The first increments of P produced a pronounced response. The indicated external P requirement was 0.02 ppm. The taro root system was extensive but few roots developed mycorrhizal associations.
Based on results from five experiments, potatoes have a high (0.2 ppm) external P requirement. The lowest P level employed (0.003 ppm) was associated with 42 percent of maximum yield.
Based on the results of this study, it appears that P sorption curves can be used as a basis for transferring information about P fertilizer requirements from one geographic location to another irrespective of differences in sorption capacity. A survey was made of the P status of more than 300 soils from 20 countries. The various root crops seem to lie within fertility boundaries. Careful consideration should be given to the P fertilizer requirements for prospective soil-crop combinations prior to bringing new root crops into areas where they have not been grown. Phosphate sorption curves are useful tools for making predictions about phosphate fertilizer requirements in areas where detailed experimentation has not been done.
Appears in Collections: Ph.D. - Agronomy and Soil Science

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