SOIL EXHAUSTION AND ROTATION OP CROPS. ]g^ 



appreciable amount. You do not commonly find this substance in 

 the water of wells or springs, except in the minutest quantities. 

 It is very rare to detect it in waters, except those which have 

 passed through a very heavily manured soil, or unless it is other- 

 wise especially abundant. Potash, for another example, rarely 

 wastes from the soil, unless it is from light, coarse, sandy land, 

 having but little fine material in its tilth. 



If the substances which feed the crop, one or all, have become 

 reduced in quantity or are not in proper condition as to solubility, 

 we may remedy the exhaustion either by applying the materials in 

 the form of some fertilizer which contains them, or we may omit 

 that, and rely upou those processes by which the original rocks of 

 the earth's surface have been converted into nutritive soil ; the 

 processes by which those substances, once totally unavailable for 

 crops, have been made available. We can wait the operation of 

 the natural agencies which are involved in what we call " weather- 

 ing;" the action of water, and of the carbonic acid and oxygen 

 in the air. When we leave land in fallow — a thing which is prac- 

 ticed much less now than formerly — these processes go on in the 

 soil, and prepare a quantity of plant-food for the crop of another 

 year. This "weathering" process is in constant progress and is 

 of great importance in supplying the materials which our crops de- 

 mand. If that process should be suspended, farming would 

 become a very difficult- business. That certain fields will produce 

 crops of the same kind for years and years without any fertilizing 

 addition whatever, is due to the fact, that as fast as the crop re- 

 quires and removes the materials given in our table they are sup- 

 plied by the soil itself; they exist in the soil, were originally 

 stored up there, and they are made soluble day by day, as the 

 crop may need. The rate at which this weathering process goes 

 on determines, other things being equal, the natural yield in a 

 given case. By active tillage, throwing up the soil, so that it is 

 exposed more fully to the arr, and by drainage, if this be neces- 

 sary to ensure access of the atmosphere, this process can be 

 hastened. Most saline fertilizers, such as common salt, nitrate of 

 soda, superphosphate of lime, and plaster of Paris, also act in a 

 similar way to dissolve the elements of the soil, and thus prepare 

 them for the crop ; so that, although these fertilizers may in some 

 cases do nothing towards feeding the crop directly, they help to 

 feed it, by this indirect action in dissolving and bringing into an 



