Date of Award:


Document Type:


Degree Name:

Master of Science (MS)


Plants, Soils, and Climate

Department name when degree awarded

Soil Chemistry


R. L. Smith


Soil-phosphorus relations have attracted the interest of many investigators since Liebig introduced his famous theory about the importance of the mineral matters to the plant in 1840. It was soon realized that phosphorus nutrition was a problem not easily solved for two reasons. 1. The added phosphorus fertilizers, soon after soil application are converted by some reactions in the soil to complex compounds far less soluble, consequently less available to the plant. Conclusions about this process led to controversial debates until it was discovered that a general statement covering all soils was impossible since the reactions involved in each case are different. 2. There was confusion caused by the use of two terms coined to express the plant's need for phosphorus. The first term, soluble phosphorus, was based on the assumption that nutrient absorption is a simple diffusion of ions from the soil solution into the roots while the second term, available phosphorus, was based on the observation that the plant absorbed more than that which could be estimated as soluble phosphorus from some insoluble sources. A general definition was given to the term available phosphorus as "that part of soil phosphorus which may be absorbed by ordinary crop plant in the production of plant substance." Later some restrictions were applied to regard the physical conditions of both soils and plants.