146 ELEMENTS OF AGRICULTURE 



who expect to cure sick soils know the cause of the 

 trouble before applying remedies. 



There are many ways in which a soil may become 

 poor, but, as already pointed out, the loss of moisture 

 and organic matter is the chief cause of poor soils. 

 Therefore, in improving a poor soil, the first point to 

 be inquired into should be its supply of water and 

 organic matter. A soil well supplied with organic 

 matter is, if well cultivated, usually well supplied with 

 water and plant food; so it is safe to begin the 

 improvement of worn soils by looking to the supply of 

 organic matter. Increase the soil's supply of organic 

 matter, and you increase its supply of moisture and 

 plant food. The usual method of increasing the soil's 

 store of organic matter is to grow some crop and plow 

 it under while it is growing. Leguminous crops, such 

 as clover and beans, are the best for the purpose, as 

 they gather from the air stores of nitrogen which, 

 when they decay, are added to the store in the soil. 

 The plowing under of growing crops is called green 

 manuring, and is much practiced on worn soils. The 

 green crops, decaying in the soil, form valuable com- 

 pounds of nitrogen and humus. The humus increases 

 the soil's power to hold water, and combining with 

 the insoluble phosphates and potash compounds in the 

 soil forms available plant foods. As the organic matter 

 of the green manure decays slowly, and the humus 

 which is formed acts slowly on the insoluble com- 

 pounds, the good effects of one application of green 

 manure may last throughout several years. For green 

 manuring there is no better crop than the old-field pea, 



