114 ELEMENTARY AGRICULTURAL CHEMISTRY 



contains about 50 per cent. K2SO4, 34 per cent. MgS04, and 

 11*5 per cent, water, corresponding to about 27 per cent, of 

 potash, K2O. 



Potash is most needed on light sandy or calcareous soils, 

 particularly for potatoes, grasses, clovers, peas, and beans. 

 Potash manures are best applied in the autumn or winter, and 

 little loss need be feared from drainage. 



(D) Miscellaneous Manures. — Common salt is generally 

 regarded as possessing no true manurial value, but as being 

 of service because of its action upon the potash, lime, and 

 magnesia compounds in the soil. It has a good effect on 

 mangolds and cabbages. It increases the solubility of phos- 

 phates and silicates. 



Gypsum^ CaS04.2H20, is found to improve clover apd 

 turnips. It perhaps acts as a source of sulphur, but probably 

 its good effects are indirect, partly by liberating potash from 

 insoluble silicates and partly by aiding nitrification. 



Lime, CaO, slaked lime, CaH^Og, and chalk, marl, or limestone, 

 CaCOg, are very often useful as manures. They all contain 

 small quantities of phosphates, and thus provide plant food, but 

 their most important effect is that of acting as basic material, 

 and thus (1) neutralising the vegetable acids produced by the 

 decay of organic matter and (2) promoting nitrification. 

 Caustic lime — i.e., either quicklime, CaO, or slaked lime, CaH^Oj 

 — always has a more decided effect than chalk or limestone, 

 .although within a very short time of its application to the soil 

 it is converted into the same compound, CaCOg. This is due 

 to the solubility of calcium hydrate, leading to its more uniform 

 distribution throughout the soil than is possible with even the 

 finest ground chalk or limestone. 



Another valuable action of lime is its flocculating effect upon 

 clay. It must be remembered that the too frequent applica- 

 tion of lime to a soil may exhaust the nitrogenous matter 

 contained in the humus. Lime from magnesian limestone, 

 which contains magnesia, MgO, is not so good for agricultural 



