46 AGRICULTURE. 



Soils differ as to their composition ; some, such as mucky 

 soils, may contain plenty of nitrogen but not enough phosphate 

 or potash. In this case the use of a phosphate such as ground 

 bone, or of potash such as wood ashes, would change a barren 

 soil into a fertile soil. Such a soil as a light-colored clay may 

 require nitrogen compounds to make it complete. 



Again, a soil may contain plenty of food, but it is locked up, 

 it is unavailable ; that is, it is not soluble or in form ready to 

 be taken up by the plants. If \ve drain and cultivate it so that 

 the air can get in, these will in time be changed into soluble 

 forms. But sometimes we can hurry up or assist in this work, 

 as when we apply land plaster (sulphate of lime) to a soil 

 bearing clover, salt to a root crop or to grain, and quicklime to 

 to a heavy clay or to a fresh mucky soil. The plaster, salt, and 

 lime are not direct foods, but they act upon the constituents of 

 the soil, setting free potash and nitrogen compounds. 



NITRIFICATION. Wheat and other cereals take up their 

 nitrogen from the soil in the form of nitrates. These are 

 sometimes supplied in fertilizers in the form of nitrate of soda. 

 Nitrate of potash, or saltpetre, is now too expensive to be so 

 used. Humus contains nitrogen, and in its decay forms 

 nitrates, especially nitrate of lime. The change from the 

 insoluble forms of nitrogen in humus to the soluble nitrates is 

 brought about by ferments. These are minute forms of plant 

 life too small to be seen by the eye. Yeast that is used in 

 fermenting dough is a ferment somewhat similar. In order to 

 do their work, these tiny nitrate ferments in the soil require 

 warmth, air, and moisture. Humus, of course, must be pre- 

 sent. If the soil is sour, they will not work. Good drainage 

 and tillage, therefore, assist. The fermentation of the manure 

 pile in the barnyard is brought about by ferments. Green- 

 manuring adds material for making nitrates, and barnyard 

 manure adds not only humus but also the ferments. The 

 making of nitrates in the soil is called nitrification. 



