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This article in SSSAJ

  1. Vol. 21 No. 1, p. 25-28
    Received: Dec 15, 1955
    Accepted: June 14, 1956

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Salt Concentration a Factor in the Availability of Phosphorus from Rock Phosphate as Revealed by the Growth and Composition of Alfalfa1

  1. D. O. Howe and
  2. E. R. Graham2



A study of the availability of phosphorus held in rock phosphate in relation to the soluble salt content of sandclay cultures was made. An effort was made to relate the availability of the phosphorus to the relative bonding energies of Putnam clay for calcium, magnesium, potassium, and hydrogen. Alfalfa was grown as the test crop. The less than 0.2 µ Putnam clay was brought to desired cation saturation by making the system unsaturated by treating the Na suspended clay with hydrogen-Amberlite (IR-120), electrodialyzing to reconcentrate, and then adding carbonates or hydroxides of Ca, Mg, and K. Trace elements and nitrogen as ammonium nitrate were added in small amounts. Phosphorus was added as either rock phosphate or phosphoric acid. Calcium, magnesium, potassium, and sodium salts were added. Calcium and magnesium salts were added in series at 0.010, 0.020, 0.041, 0.081, and 0.163 moles per liter and potassium and sodium salts at 0.020, 0.041, 0.081, and 0.163 moles per liter of soil water.

The results showed all salts influenced the availability of phosphorus held in rock phosphate. However, calcium salts reduced phosphorus uptake with small reduction in plant yields. Variable effects on plant yields and phosphorus uptake resulted when magnesium salts were used. Potassium salts of different anions produced different degrees of response, with the first increments of the carbonate and sulfate giving a positive yield response while the chloride depressed growth and phosphorus uptake. Sodium sulfate gave a reduction in crop yields and phosphorus uptake indicating an influence of excessive sodium.

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