518 SOILS: PBOPJERTIES AND MANAGEMENT 



of this substance : (1) kind of plant grown; (2) degree of 

 basicity of soil; (3) fermentation of organic matter; 

 (4) character of the accompanying salts. 



437. Ejffect of plants on the availability of tricalcium 

 phosphate. — It is to be expected that the various kinds 

 of plants should not all exert an equal influence on the 

 availability of the phosphorus of tricalcium phosphate. 

 Prianischnikov ^ found that lupines, mustard, peas, 

 buckwheat, and vetch responded to fertilization with raw 

 rock phosphate in the order named, while the cereals did 

 not respond at all. He did not include maize in his 

 experiments, but that crop is said to respond well to difB- 

 cultly soluble phosphates. It is generally considered 

 that those plants which have a long growing season are 

 better able to utilize tricalcium phosphate than are more 

 rapidly growing plants. An explanation for the ability 

 of some plants to utilize the phosphorus of difficultly 

 soluble phosphates more successfully than do other plants 

 has been sought in the rate of excretion of carbon dioxide 

 by plant roots. It has already been stated (par. 324) that 

 Stoklasa and Ernst found that the capacity of a plant to 

 absorb phosphorus from difflcultly soluble phosphates is 

 proportional to the rate at which carbon dioxide is given 

 off by the roots, but that the experiments of Kossowitch 

 and BarakofI failed to confirm these^ results. This ques- 

 tion is bound up with the larger one involving the solvent 

 action of plant roots, regarding which little is now known. 



438. Effect of basicity on the availability of tricalcium 

 phosphate, — It is recognized that raw rock phosphate is 

 more available to the same plant in some soils than in 

 others, and a number of persons have stated, as the result 



1 Prianischnikov, D. Berieht uber Verschiedene Versuehe 

 mit Rohphosphaten unter Reduction. Moscow. 1910. 



