Date of Award:

1948

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Plants, Soils, and Climate

Advisor/Chair:

D. W. Thorne

Abstract

One of the principal fertility problems of calcareous soils is the lack of available phosphates. The factors which control this availability are not completely understood. One hindrance to the solution of the problems involved results from confusion of the concepts of solubility and availability. Availability is the net effect of the chemical state of a plant nutrient element and the ability of the plant to utilize the forms of the element present under the existing chemical and physical environmental conditions. In this paper, the term solubility will be used to designate the chemical solubility of an element in water or in specified extracting reagents. The term physiological availability will be used to designate the ability of the plant to assimilate the soluble forms of the element, and the term net availability will be used to designate the net effect of chemical solubility and physiological availability. These limited definitions seem justified in this study because the sodium and calcium systems investigated are alkaline and plants growing in these systems would probably be largely limited to soluble phosphates. Solubility is known to be one of the main factors which control availability of phosphates, but there is now conclusive evidence as to the importance of physiological availability.

Some investigators (26,13)1 have produced evidence which they claim supports the hypothesis that the H2PO4- ion is the only phosphate ion absorbed by plants. Since the relative concentration of this ion decreases as the pH increases above neutrality, they then conclude that physiological availability is largely a function of pH. Other investigators (41,7) have produced evidence which suggests that solubility is probably the only factor which limits the availability of phosphates in alkaline and calcareous soils. The purpose of this study is to add evidence which will help clarify the relative importance of solubility and physiological availability of phosphates in sodium and calcium soils.

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