286 CITRUS FRUITS AND THEIR CULTURE. 



(a). Humus is added and the water holding capac- 

 ity of the soil is increased. 



(b) . The soil is opened up, hard layers are prevented 

 from forming and clay soils from becoming compact. 



(c). Moisture is removed from the soil during the 

 rainy season in Florida and the Islands. 



(d). Prevents surface washing in rainy season. 

 2. Affecting the fertility of the soil. 



(a). Prevents leaching of nitrates during the period 

 of excessive rainfall. 



(b). Promotes nitrification. 



(c). Adds plant food (leguminous crops). 



(d). Breaks up the plant food in the soil and ren- 

 ders it available. 



Cover crops are usually divided into two classes: 

 nitrogen collectors and nitrogen consumers. 



To the first group belong the leguminous plants, such 

 as clover, cowpeas, beans, peas, beggarweed, velvet beans 

 vetch, lupines, alfalfa and many others. 



Buckwheat, rye, oats, grasses of different kinds, rape, 

 and, in short, all cover crop plants not belonging to the 

 Family Leguminosae are placed in the second group. 



The groups are based upon the fact that upon the 

 roots of leguminous plants, nodules of various shapes and 

 sizes are found. Two kinds of these are shown in figures 

 67 and 69. There are usually various different shapes for 

 each species of plants, each having its own form, and they 

 vary considerably in size. They are formed by the action 

 of bacteria which find entrance to the roots through the 

 root-hairs. These nodules are distinct in form, and must 

 not be confounded with the irregularly shaped swellings 

 caused by the action of the root-knot, producing nematode 



