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A Fixation Method for Determining the Phosphorus and Potassium Requirements of Soils
Author(s) -
Purvis E. R.,
Blume J. M.
Publication year - 1941
Publication title -
soil science society of america journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.836
H-Index - 168
eISSN - 1435-0661
pISSN - 0361-5995
DOI - 10.2136/sssaj1941.036159950005000c0034x
Subject(s) - soil water , citation , potassium , phosphorus , fixation (population genetics) , session (web analytics) , library science , computer science , information retrieval , environmental science , chemistry , world wide web , soil science , biochemistry , organic chemistry , gene
I T has been known for well over a century that soils will absorb phosphate and potassium ions from .water solution. That the capacity of soils for absorbing these ions is influenced by various chemical and physical properties is also common knowledge. Since the absorptive power of a soil markedly influences the amount, and kind, of ions which can remain in free water solution in the soil, it would seem that a measure of the capacity of soils to absorb these ions would offer an excellent index to soil fertility. Emerson (2) made use of this theory in the development of a potash test in which he assumes that iodine is liberated when the soil absorbs potassium from a solution of potassium iodide. Thornton has also devised an absorption method* for determining both phosphate and potash requirements of soils. Field and laboratory tests conducted at this Station have shown that the phosphorus extracted by a weak sodium acetate solution of pH 5.00 may, or may not, indicate the phosphate requirement of a soil, depending upon the degree of saturation existing in the phosphate fixing mechanisms of the soil. Justification for a phosphate requirement test based on absorption lies in the assumption that the higher the fixing power of a soil for phosphate, the more phosphate must be applied to maintain an optimum concentration of this ion in the soil solution. Since fertile soils contain phosphorus in the soil solution, as has been shown by Pierre and Parker (4), and others, it would seem safe to assume that a soil, which will remove practically all of the phosphorus from a standard phosphate solution is deficient in this element. The same reasoning may be applied to justify an absorption method for determining the potash requirement of soils since it is still believed by many that most of the nutrient supply required by a plant must be in the water soluble state before utilized. However, unlike phosphate, which becomes rather permanently fixed in chemical combination when absorbed by the soil, most of the absorbed potash enters • into the loosely bound exchangeable state where it has been generally thought to remain in equilibrium with the soluble potash of the soil solution. As the soluble potash of the soil solution is reduced by leaching, or crop removal, the exchangeable potash moves back into solution to reestablish this equilibrium. Recent work by Jenny (3) has shown that this is true only to a certain extent in that the percentage potash saturation of the exchange complex and not the total amount present, governs the amount of potash which may be obtained from the exchangeable fraction by the plant. Albrecht ( i) has shown the same to be true with regard to calcium. This being the case, it must follow that the amount of potassium absorbed from a potash solution of given strength would be determined by the exchange capacity of a soil, as well as the amount of exchangeable potash in the soil. Since extraction tests determine only the amount of exchangeable potash present, an absorption test would seem preferable.