THE AVAILABILITY OF MINERAL PLANT FOOD. 



A MODIFICATION OF THE PRESENT HYPOTHESIS. 



By NORMAN M. COMBER. 



{Department of Agriculture, The Urdversity, Leeds.) 



The Present Hypothesis. 



Scientific soil researches have pussed through one fairly well defined 

 phase. The important and pioneer work of empirical field trial has 

 established the main physical and chemical requirements of crops and 

 laid down the routine method of examining soil amendments. 



The researches of recent years have inaugurated a second phase : an 

 enquiry into the constitution of the soil and into the mechanism of the 

 growth of plants in soil. In connection with this second phase of soil 

 investigations much progress has been made on the botanical side in 

 developing knowledge of the mechanism of processes which go on inside 

 the plant, and in the last decade views of the constitution of the soil have 

 changed fundamentally. But the relation of the plant to the soil has 

 received very scanty consideration in modern literature. Views on that 

 subject are very much as they were after Daubeny's publication in 1845. 

 Outstanding work has been done by Dyer^, by Cameron and Whitney-, 

 and by HalP and his collaborators. There have been some changes of 

 interpretation and some differences of opinion, but the fundamental 

 assumption that the soil solution is the nutrient medium of the plant, 

 that the state of solution is necessary to availability has remained almost 

 unquestioned. 



The basis of practically all teaching on the chemistry of soil fertility 

 and on the value of fertiUzers, is the behef that nutrient substances pass 

 into solution in the soil water externally to the plant and subsequently 

 diffuse into the root hairs. Much well-known discussion has arisen about 

 the solvents concerned, particularly about the possible excretion of 

 organic acids by the plant. 



' Tran.-i. Chem. Sue. 189-t, 65. 



= U.S. Dept. Aijrk. Bureau vj Soils Bull 1903. No. 22. 



3 Phil. Trans. 1913. 



