MAINTAINING FERTILITY OF LAND 185 



(4) The nitrogen and lime of the soil may be carried 

 away in drainage water. 



(5) The constant cropping may exhaust the available 

 supply of some plant-food. Each crop removes a certain 

 amount of nitrogen, phosphoric acid, potash, and lime. 

 In time this may limit the available supply. Usually it 

 is not a shortage of the absolute amount of such food in 

 the soil, but a shortage of that which the plant can secure 

 in soluble form. This may be remedied by drainage, 

 tillage, additions of humus, lime, fertilizer, and manure. 



(6) The exhaustion of the organic matter is the most 

 frequent cause for decrease in crop yields. This affects 

 crops in many ways. It may result in an unfavorable 

 physical condition of the soil that will limit the crop 

 when there is no shortage of food. The soil may " bake " 

 or it may lose its water-holding power. Since the organic 

 matter furnishes the nitrogen by its decomposition and 

 encourages the fixation of free nitrogen, its exhaustion 

 will be accompanied by a shortage of nitrogen. Or 

 becauso of the lack of organic matter, the mineral ele- 

 ments may not be rapidly enough dissolved, although 

 present in abundance. In such a case, the addition of 

 phosphoric acid or potash might increase the crop, but 

 it would usually be wiser to supply organic matter so as 

 to render available the food that is already in the 

 soil. 



Many soils are losing their fertility in all of the ways 

 mentioned above. 



(7) In arid regions, the accumulation of alkali is one of 

 the most frequent sources of decreased production. Too 

 heavy applications of water make the problem worse. 

 This may be remedied by tile drainage. 



