195 



with these bacteria, and these plants are thus easily supplied with 

 an abundance of nitrogen in a form which they can use. When 

 these nitrogen-fixing bacteria are present in a soil on which a le- 

 guminous crop is growing, the bacteria invade the roots of the 

 legume and live there. Their presence is usually made manifest 

 by swellings — the so-called tubercles — on the roots of thrifty plants 

 of clover, alfalfa, beans, peas, and other legumes. Nitrogen from 

 -the soil filters into the roots, where the bacteria appropriate 

 it, manufacture an abundance of nitrates, and give a portion to 

 the plant in exchange for starch. The tissues of leguminous plants 

 become very rich in nitrogenous compounds, and when they 

 decay in the soil they set free large amounts of nitrates for the use 

 of any crop which may be growing at the time. 



The cultivation of leguminous crops is one of the most im- 

 portant and economical means of maintaining a supply of nitro- 

 genous plant food in the soil. Nitrates may, of course, be supplied 

 in commercial fertilizers ; but fertilizers containing nitrogen are 

 very expensive, and it usually pays better to supply nitrogen by 

 growing legumes or by the application of stable manure, which is 

 rich in nitrogen when properly handled. In good farm practice 

 both stable manure and leguminous crops are used as sources of 

 nitrogen. 



SOIL MOISTURE AND HUMUS. 



In order to produce a ton of dry hay on an acre of land it is 

 necessary that the growing grass should pump up from that acre 

 approximately 500 tons of water. In order to supply this enormous 

 quantity of water, the soil must not only be in condition to absorb 

 and hold water well, but it must be porous enough to permit water 

 to flow freely from soil grain to soil grain. The presence of large 

 quantities of deca5''ing organic matter (humus) adds enormously to 

 the water-holding capacity of the soil. One ton of humus will 

 absorb 2 tons of water and give it up readily to growing crops. 

 Not only that, but the shrinkage of the particles of decaying 

 organic matter and the consequent loosening of soil grains keep 

 the soil open and porous. 



Furthermore, humus of good quality is exceedingly rich in 

 both nitrogen and mineral plant food. The maintenance of fertil- 

 ity may almost be said to consist in keeping the soil well supplied 

 with humus. The first step in renovating worn-out soils is to give 

 them an abundant supply of humus of good quality. Perhaps the 

 best source of humus is stable manure containing both the liquid 

 and the solid excrement, especially when the stock are fed with 

 rich nitrogenous foods. Even a poor quality of barn-yard manure, 

 which has had much of the plant food leached out of it, has con- 

 siderable value because of the humus it makes. 



Another cheap and valuable source of humus, but one which 

 must be used understandingly, is crops grown to turn under as 

 manure. The legumes are especially valuable for this purpose 

 because of the nitrogen they contain, but other crops, such as corn 

 sown thickly, may sometimes be made to supply large quantities 



