FERTILIZERS, OTHER THAN NITROGENOUS 225 



shows that the nitrogen of both barnyard manure and of green manure 

 crops plowed under in April or May becomes available only gradually for 

 plant growth during the latter half of the growing season.-"^ 



Phosphorus. — Though experiments have shown little or no direct 

 benefit to deciduous fruits from the application of phosphatic fertilizers 

 these are often useful in stimulating the growth of intercultures or in 

 promoting desirable changes and reactions in the soil. 



The leading phosphatic fertilizers available for use in the orchard are 

 rock phosphate or "floats," acid phosphate or superphosphate and ground 

 bone. The phosphorus in raw rock phosphate or "floats" and in ground 

 bone is held in the form of tri-calcium phosphate, which is very nearly 

 insoluble in water or in the soil solution and hence becomes available for 

 plant growth very slowly as it is acted upon gradually by various soil 

 agencies. The phosphorus of acid phosphate or superphosphate is held 

 as mono-calcium phosphate, which is soluble and is the form in which 

 plants are supposed to absorb most of their phosphorus. When added 

 to the soil it unites with more calcium to form di-calcium or "reverted" 

 phosphate which is intermediate in solubility between the mono- and 

 tri-calcium compounds. Gradually this di-calcium phosphate unites 

 with more calcium to form tri-calcium phosphate and it finally exists in 

 the soil in the same form as in raw rock phosphate. For this reason 

 "floats" or raw rock phosphate might be inferred to have equal value with 

 the acid phosphate as a fertihzer. This is not the case, however, since 

 the acid-treated material, being readily soluble, goes down into the soil 

 and becomes fairly evenly distributed throughout the area reached by 

 the roots. Furthermore, the plants are able to obtain considerable quan- 

 tities before it becomes "reverted" or certainly before it is changed to 

 the very nearly insoluble tri-calcium form. Mention may be made again 

 of the possibihty that some of the benefit from acid phosphate is due to the 

 sulphur that it carries as well as to the phosphorus. Unlike nitrogen, 

 phosphorus is not lost from the soil in large quantities through leaching. 

 The reasons for this have been brought out in the preceding discussion. 

 Some indication of the phosphorus fixing power of soil is afforded by an 

 experiment with a light sandy loam in Florida m which it was found that 

 at the end of four years only 0.05 per cent, of the amount applied in 

 fertilizers had been lost through the drainage water, i"^" 



Potassium. — Though there are a number of different forms in which 

 potassium may be applied, the two most common are the muriate and the 

 sulphate. Where these two forms of potash have been used side by side 

 in the fruit plantation the sulphate has usually, though not always, given 

 more striking results. The suggestion may be repeated that when there 

 is an apparent need of potash fertihzers, as indicated by a material 

 response from the use of the sulphate, the possible need of sulphur be 

 thoroughly investigated. In marked distinction to the case afforded by 



