274 PLANT BIOCHEMISTRY 



For a long period ol time gypsum has been employed for this purpose. 

 The reaction results in the production of calcium carbonate and am- 

 monium sulfate. 



CaS04 + (NH4)2C03 T^ CaCOs + (NH4)2S04 



Superphosphate, which contains both gypsum and monocalcium I 

 phosphate, functions better than either of these compounds alone, the 

 reaction taking place according to the equation: 



2CaS04 + CaH4(P04)2 + 2(NH4)2C03 -> 



Ca3(P04)2 + 2(NH4)2S04 + 2H2O + 2CO2 



Tricalcium phosphate, unlike calcium carbonate, does not react with 

 (NH4)2S04, and the above reaction is not reversible. Thus the addi- 

 tion of superphosphate accomplishes two results: it serves as a preserva- 

 tive for ammonia, and it supplies phosphorus to create a better-bal- 

 anced fertilizer. 



Many materials having bactericidal properties have been employed 

 as preservatives. These include formalin, chloropicrin, and the sulfates 

 of iron, zinc, and copper. Strong acids, such as sulfuric, phosphoric, 

 and hydrochloric, have been used to a certain extent. The latter com- 

 pounds prevent the loss of ammonia through the formation of am- 

 moniimi salts and change the pH to a value which inhibits fermenta- 

 tion. Whether the retention of ammonia compensates for the inhibi- 

 tion of bacterial life is open to question. 



Effects of manure on soil. The beneficial effect of an application 

 of manure as reflected by plant response may be very great. This effect 

 may be apparent for only 1 year when manure is applied to gravelly or 

 sandy soils, but when it is applied to heavier soils profitable crop 

 increases may be noted from 2 to 4 years later. At the Rothamsted 

 Station the effects of eight yearly applications of 14 tons each were ap- 

 parent 40 years after the last treatment. 



CHEMICAL EFFECTS. Additions of manure to a soil increase the quan- 

 tity of mineral nutrients available for plant growth. The increase of 

 available nutrients results in part from the liberation of plant nu- 

 trients carried by the manure and in part from materials made avail- 

 able from insoluble soil minerals. This is true especially for calcium, 

 magnesium, potassium, manganese, and to a certain extent for other 

 elements. The exchange capacity of manured soils is considerably in- 

 creased over that of non-manured soils, reflecting the effect of added 

 humus. 



PHYSICAL EFFECTS. When manure undergoes a rapid decomposition 

 in heavy soils, it improves the structure, allowing better aeration. The 



