CHAPTER XIII. 



LOSSES AND GAINS BY THE SOIL. 



Under natural conditions, a large portion of the material taken 

 from the soil by plants returns to it again. The plant dies and 

 decays. Droppings from animals which have eaten plants are 

 distributed on the soil. Nevertheless there is some loss of 

 material due to leaching. We have seen that in the weathering 

 of rocks into soils, large percentages of material are removed, but 

 this process has taken long periods of time. Soils which are 

 highly weathered contain much less plant food than those less 

 weathered ; this shows that losses by percolation occur under 

 natural conditions. Under cultivation, there may be much greater 

 losses, due to the smaller amount of vegetation on the soil at cer- 

 tain seasons, and to the removal of the crops. 



Gain by Rainfall. In Chapter III we saw that the rain dis- 

 solves and brings down small amounts of ammonia, nitrates, dust, 

 and other substances. The quantity of the most important con- 

 stituent, nitrogen, brought down by the rain, has been shown to 

 average about 8.0 pounds per acre. (Chapter III.) 



Loss in Percolation. Of the water which falls on the soil, a 

 portion runs off, a portion evaporates or is transpired by plants, 

 and a portion penetrates through the soil, and either reappears in 

 springs, drains, wells, or seepage water, or else sinks deep into 

 the earth. This is the percolating water. The water which per- 

 colates dissolves some of the material that comes in contact with 

 it, thereby causing a loss of material. The water contains silica, 

 organic matter, potash, soda, lime, and magnesia, in the form of 

 carbonates, sulphates, phosphates, chlorides, and nitrates. The 

 more important of these constituents, from an agricultural point 

 of view, are the potash, phosphoric acid, nitrogen, and lime. 



The amount of loss by percolation depends upon a number of 

 factors : 



(i) The Quantity of Percolating Water. This depends upon 

 the amount of rainfall, whether the land is bare or covered with 

 vegetation, the character of the soil, etc. Unless the rainfall is 



