THE SOIL. 35 



it seldom contains more than a trace of ammonia, potash, 

 or phosphoric acid. Because nitrates are found in 

 relatively large quantities in bare fallow, and because they 

 are not fixed in the soil, the heavy watering of such land 

 results in a loss of valuable plant food. The compounds 

 in the soil causing fixation vary ; phosphoric acid is 

 chiefly fixed by forming insoluble phosphates of lime 

 and iron; potash, ammonia and lime are fixed chiefly by 

 interaction with certain chemical compounds called zeolites 

 which are hydrated double silicates of alumina and some 

 other metal. Thus a potassium salt (soluble) with hydrated 

 silicate of soda and alumina (insoluble zeolite) gives a 

 sodium salt (soluble) with hydrated silicate of potash and 

 alumina (insoluble zeolite). When the base of a salt is fixed 

 in a zeolite, the base set free from the zeolite passes into 

 the drainage water combined with the acid of the decom- 

 posed salt. It is chiefly because potash is fixed and soda 

 is not that the sea water contains so much more of the 

 latter salt. In the presence of a large amount of salt 

 however, the soda may replace some of the potash in the 

 zeolite, and hence saltness of the soil results in the gradual 

 exhaustion of the fixed plant foods of the soil. Humus 

 in the soil has a certain power of fixing ammonia, lime, 

 etc., probably through the formation of insoluble double 

 humates. 



The power possessed by soil to fix plant foods chiefly 

 depends therefore on the presence of zeolites, humus, lime, 

 and iron. In loams and clay soils, these substances occur in 

 much greater quantity than in sandy soil, and accordingly 

 the latter soils are much less active in fixing plant foods. 



