CROPPING SYSTEM 123 



2. Exhaustion of some necessary element, usually lime, 

 nitrogen, phosphorus, or potassium. 



RESTORING PRODUCTION 



91. When low production is due to the exhaustion of 

 organic matter, then any cheap system of restoring that 

 matter, as plowing under a green manure crop, will usually 

 restore production in a measure. One effect of this de- 

 caying organic matter is the reaction on the minerals of the 

 soil, thus increasing solubility, and restoring the physical 

 condition. The -physical effect is to make the soil more 

 loamy in character by increasing granulation of clay, on 

 the one hand, and on the other hand, in the case of sandy 

 soils, binding the particles together. In this case no new 

 supply of plant food is added to the soil, as the organic 

 matter is grown on the land and only adds to the soil the 

 carbon compounds taken from the air. Adding organic 

 matter from an outside source, in addition to the above, 

 also adds its own supply of elements. 



When a certain element has been exhausted from the 

 soil, that element may be added. 



Nitrogen may be added in three ways : (1) by growing 

 leguminous crops ; (2) by adding organic matter from an 

 outside source ; (3) by adding nitrogen salts. 



Phosphorus, potassium, and lime can be restored in two 

 ways : (1) by adding organic matter from an outside 

 source ; (2) by adding salts of phosphorus, potassium, or 

 lime. 



Aside from a proper system of drainage where needed, 

 the whole problem of devising a cropping system, includ- 

 ing the application of fertilizers for maintenance or in- 

 crease of production, is involved in the above statements. 



