304 NATURE AND PROPERTIES OF SOILS 



Before the question of possible soil exhaustion can be dis- 

 cussed adequately, the losses of nutrients in the drainage 

 water must be considered as another source of loss in addition 

 to the cropping influences already noticed. 



161. Qualitative composition of drainage water. — In 

 theory, at least, the qualitative composition of drainage water 

 should be the same as that of the soil solution ; that is, in it 

 should be found all of the common bases and acid radicals. 

 Actually, however, due to the absorptive power of the soil, 

 certain constituents appear in very slight amounts. Phos- 

 phorus, for example, often occurs in drainage only in traces, 

 as do the nitrites, ammonia, and carbonates. The principal 

 bases lost by leaching are calcium, magnesium, potassium, 

 and sodium. The important acid radicals of drainage water 

 are the nitrates, chlorides, sulfates, and bicarbonates. 



As might be expected, the constituents appearing in drain- 

 age are extremely variable not only when different soils are 

 compared but also within the same soil at different periods. 

 Phosphorus may be leached from some soils in measureable 

 quantities, while from others the amount may be negligible. 

 Nitrate nitrogen is usually an important constituent in all 

 drainage water during the summer, especially that from a bare 

 soil. In the winter and early spring nitrates decrease in 

 amount. The method of soil treatment as to cultivation, ma- 

 nuring, liming, or fertilizing may also markedly influence the 

 qualitative composition of the water draining from field soil. 



162. Quantitative composition of drainage water. — While 

 but little reliable data regarding the composition and 

 especially the concentration of the soil solution are available 

 at the present time, much exact information has been obtained 

 regarding drainage water. The concentration of drainage 

 water is much lower than that of the soil solution and much 

 less variable. The total concentration seems to be governed 

 more by the amount of water leaching through than by any 

 other factor. Other seasonal conditions of course come into 



