80 MANUAL FOR SUGAR GROWERS. 



soil as well as a gain of vegetable matter or humus ; 

 whereas green dressings of other classes of plants 

 merely restore the nitrogen taken from the soil. 



It is found that some of the plant-food constitu- 

 ents of the soil are soluble in water, as for instance, 

 nitrates and chlorides, and therefore these substances 

 are found in the drainage-water ; on the other hand, 

 some of the constituents are insoluble, as potash, 

 phosphates, and ammonia ; these things are seldom 

 found in drainage-water (see page 21). Pure sand 

 possesses little or no retaining power for the various 

 food constituents ; and thus, if soluble phosphates, 

 or potash salts, or ammoniacal chemical manures are 

 mixed with sand, it is found by experiment that they 

 are readily washed out by water. If, however, these 

 substances be mixed ^ith a good soil containing 

 clay and humus, it mil be found that they are not 

 washed or removed by water. Potash, ammonia, and 

 phosphoric acid are retained by soil by virtue of 

 chemical action ; precisely how this takes place is 

 not clear, but the hydrated oxides of iron and 

 alumina, the hydrated fc^-ms of silica and humus, 

 doubtless play an important ]3art in the process. 

 (See Warington, " Chemistry of the Farm," p. 28.) 

 The retention of the phosphoric acid is largely due 

 to the oxides of iron and alumina and to lime ; the 

 potash is probably chiefly retained by humus and 

 the hydrated silicates of alumina ; the ammonia is 

 chiefly retained b}^ humus. In the case of potash 

 and ammonia salts, it appears necessary that they 

 should present themselves in the form of carbonates ; 

 if the sulphates, etc., of the substances be used, 



