238 



Transformation of Mineral Substances 



of the soluble phosphate that they require for their eell synthesis. 

 This is similar to the needs of the microorganisms for nitrogen, 

 although the amounts of phosphorus required for this purpose are 

 less. 



2.4 



E 120 - 



E 

 = 3.4 2 



4.4 



5.4 







15 



240 



200 



E 

 160 1" 



- 120 .£ 



80 



40 



5 10 



Incubation period, days 



Fig. 95. Course of sulfur o.xidation and transformation of insoluble phosphate 

 into soluble forms l>y Tliiolxicilhis tliiooxiclans in lifjnid media (from Waksman 



and Joffe ) . 



The estimation of available phosphate in the soil, or the potential 

 response of a crop to a dressing of phosphate, is complicated by sev- 

 eral factors, notably the root system of the plant, the depth of pene- 

 tration of the root system, aud rainfall. The favorable effect of addi- 

 tion of available phosphate to the soil will be an increase in the root 

 system, which leads to a greater uptake of the soil phosphate. The 

 ability of the roots of plants to absorb the less easily available phos- 

 phates depends on the phosphate nutrition of the plant and its root 

 systeui. Some plants excrete organic acids from their roots. All 



