44 FIRST PRINCIPLES OF AGRICULTURE. 



Sandy soils are improved when made more compact and 

 tenacious; this may be accomplished by adding clay, or 

 organic matter, or both. Clay soils are improved as they 

 are made more porous and open; this may be accom- 

 plished, in part at least, by the addition of sand. Clay- 

 ing and sanding are expensive processes, and are seldom 

 resorted to in this country except in cranberry culture; 

 though in districts where clay marls are abundant, the 

 same object is accomplished by the application of this 

 material. Marling, however, materially improves their 

 chemical character, because of the mineral constituents, 

 potash, phosphoric acid, and lime, one or all of which 

 may be contained in them. 



Green Manuring. The addition of organic vegetable 

 matter to soils, for the purpose of improving both their 

 physical and chemical character, is readily accomplished 

 by means of green manuring. The term " green manur- 

 ing " is used when the crops themselves are plowed under 

 in their green state. Any plant, of course, may serve for 

 this purpose, though those most commonly used are red 

 and crimson clover, cow pea, rye, and buckwheat. 



Plants most Useful. Of these crops the clovers and 

 peas are more useful than the others. Clovers, peas, 

 beans, lupins, vetches, and a number of others of less 

 importance, belong to a class of plants called "legumes," 

 which have the power of securing nitrogen from the air, 

 and can, therefore, make perfect growth under proper 

 conditions without depending upon soil nitrogen. This 

 function of the legumes has long been known by practi- 

 cal farmers, but the method by which the nitrogen is 

 obtained is a quite recent discovery. 



