330 FIELD CROPS 



grain smut? and rusts, fruit rots, and other fungous diseases. 

 In the case of the nitrifying bacteria, however, the relation 

 toward the host plant is a helpful rather than a harmful one. 

 If a healthy clover or pea or bean plant is dug up very care- 

 fully and the earth washed away from the roots, many little 

 knots or bunches will be found on them. These knots, or 

 tubercles, which vary greatly in shape and size according to 

 the plant on which they grow, are filled with thousands of 

 bacteria, too small to be seen without a very powerful micro- 

 scope."^ These bacteria take the nitrogen from the air and 

 change it into a form which can be used by the plants. 

 Nitrogen is the most expensive fertilizer to purchase; and, 

 as the legumes add it to the soil and at the same time yield 

 a valuable crop of hay or seed, we can see their importance. 



440. Conditions Necessary for Nitrifying Bacteria. Air 

 is one of the essentials for the growth of nitrifying bacteria. 

 Unless the soil is in good condition to admit a plentiful supply 

 of air, these bacteria are unable to do their work. Tillage 

 is beneficial to them, as it stirs the soil, loosens it, and admits 

 air. Drainage is also helpful, for a soil which is full of water 

 cannot admit the necessary air. Few leguminous plants 

 grow well on low, wet, sour land, though alsike clover thrives 

 in such situations. Sour, or acid, soils are not suitable for 

 the growth of these bacteria. This condition can be remedied 

 by the addition of lime. The acidity of the soil can easily 

 be tested in a general way by applying a little of the moist 

 soil to litmus paper. If the soil turns blue litmus paper red, 

 it is acid and needs lime. The application of half a ton or 

 a ton of lime to the acre, or double that quantity of ground 

 limestone, will generally correct this acidity. 



441. Inoculation. Though the forms of nitrifying bac- 

 teria on the roots of our various legumes are very similar, 

 they usually can not be transferred from one kind of plant 



lA number of illustrations of typical forms of tubercles on leguminous and 

 other plants will be found in the Yearbook of the Department of Agriculture for 

 1910, pp. 213.218. 



