76 IRRIGATION PRACTICE 



King found that eleven successive extractions of soil with 

 water removed more than eleven times the quantity of 

 some constituents that was extracted the first time. The 

 continuous solubility of soils is well established, and it has, 

 no doubt, an important bearing on the permanent pro- 

 duction of crops. 



Whenever, therefore, irrigation water is applied to 

 the soil, a part of the soil is dissolved, providing that the 

 substances dissolved by the previous irrigation have been 

 somewhat thoroughly removed by plant roots or by 

 drainage. Naturally, not all soil constituents are extracted 

 at the same rate by successive applications of water. 

 Approximately the same quantity of potash goes into 

 solution from extraction to extraction, while a very large 

 part of the nitrates is extracted during the first applica- 

 tion of water, leaving little for the later ones; unless, 

 indeed, during the interval between irrigations, nitrates 

 have been added or cultural treatments have permitted 

 a very rapid rate of nitrification. 



53. Absorption by soils. The solution of soil con- 

 stituents occurs most readily at the surfaces of the soil 

 grains. The dissolved substances, under the influence 

 of somewhat obscure manifestations of the laws of attrac- 

 tion, are held in high concentration very near the sur- 

 faces, and the outward movement through the water- 

 film of the dissolved materials is very slow. This property 

 of firmly holding certain soluble substances near the sur- 

 faces of the soil particles, known as absorption, is of tremen- 

 dous importance in conserving the fertility of agricultural 

 soils, whether under humid or arid conditions. The first 

 water added to a soil, as has already been explained, is 

 held as thin films around the soil grains. Drainage through 

 the soil occurs only after these films have acquired a cer- 



