The Problem of Impoverish p:d Lands. 105 



Fortunately, tliis great store of plant-food is largely' locked up, 

 else it would have leached from the soil or have been used up long 

 ago. By careful husbandry, a little of it is made usable year by 

 year ; and the better the management of the land the more of this 

 food is available to the plant. When the farmer has done his best to 

 get out of the land all that it will give him, then he may add 

 fertilizers for bigger results. 



Plant-food is available when it is in such condition that the plant 

 can use it. It must be both soluble and in such chemical form that 

 the plant likes it. Plant-food which is not soluble in rain water, 

 may still be soluble in soil water (which contains acids derived from 

 the humus) ; and the acid excretions from the roots may render it 

 soluble. But solubility is not necessarily availability, for, as we have 

 said, the materials must be in such combination that the plant will 

 take them. Thus, nitrate of soda (Na IsO-i) is available because it 

 is both soluble and in the form in which the plant wants it. But 

 nitrite of soda (N-a, NO2) is not available although it is soluble, — the 

 plant does not like nitrites. 



11. Nitrogen must probably be in the form of nitrates before it 

 can be used by inost plants. — Nitrogen is abundant. It is approxi- 

 mately four-fifths of the atmosphere, and it is an important content 

 of every plant and animal. Yet, it is the element which is most 

 difficult to secure and to keep, and the most expensive to buy. This 

 is because the greater part of it is not in a form to be available, and 

 because, when it is available, it tends to leach from the soil. It is 

 available when it is in the form of a nitrate — one part of nitrogen, 

 three parts of oxygen, united with one part of some other element 

 (Na NO3, nitrate of soda ; K NO3, nitrate of potash or saltpetre ; 

 H NO3, nitric acid, etc.). The process of changing nitrogen into 

 nitrates is called nitrification. This process is the work of germs or 

 microbes in the soil ; and these germs work most efficiently when the 

 soil is not water-logged, and when it is well tilled. The farmer 

 should make his available nitrogen supply as he goes along ; and he 

 makes it with tile drains, plows, harrows and cultivators. 



But there are some plants which have the power of using the 

 nitrogen which is in the air in the soil. These are leguminous 

 plants,— clovers, peas, beans, vetch, alfalfa. If, therefore, the 



