CHAPTER XXVIII 



CROP STIMULANTS AND PROTECTIVE AGENTS 



A CLEAR distinction should be made betweeen the use of plant 

 food in systems of permanent agriculture and the use. of crop 

 stimulants or crop " protectors." Unquestionably there are con- 

 ditions under which the use of some particular substance, other 

 than plant food, will produce a sufficient increase in the yield of the 

 crop for which it is applied to more than pay the cost; and, further- 

 more, the use of such material may in some cases be advisable, but 

 it should be used with intelligence and full understanding of its 

 effect. 



Land-plaster. Land-plaster (native calcium sulfate) is a well- 

 known crop stimulant, but it contains neither nitrogen, phos- 

 phorus, potassium, nor lime. Thus it supplies no plant food of 

 value and has no power to correct soil acidity, and its physical 

 effect on the soil is probably injurious rather than beneficial. 

 In fact, it is the common report that the soil tends to become hard 

 and more compact with the long-continued use of land-plaster; 

 but whether this effect is wholly due to the wearing out of the 

 organic matter, or in some part due to the cementing properties of 

 the calcium sulfate, cannot be stated with certainty. When de- 

 hydrated, calcium sulfate becomes plaster of Paris, and it is a 

 constituent of different cementing materials. 



The temporary beneficial effect of land-plaster is probably due 

 to its chemical action in the soil. It may convert more or less of 

 the supposedly difficultly available iron phosphate into the more 

 readily available tricalcium phosphate, as indicated by the follow- 

 ing equation : 



2 FeP0 4 + 3 CaS0 4 = Ca 3 (PO 4 ) 2 + Fe 2 (SO 4 ) 3 . 



Very possibly this or some similar reaction occurs to a limited 

 extent when calcium sulfate is applied to a soil containing iron 



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