24 



Potash. All soils suitable for cultivation contain potash in an avail- 

 able form, arising from the disintegration of feldspathic and other rocks. 

 In the majority of cases the natural supply of the soil is sufficient 

 to furnish to the plants the potash of which they are in need; a soil 

 containing 0.125 per cent, should furnish potash enough for a century, 

 without it being necessary to add to the manures used on such soils any 

 salts of potash. Besides this available potash the soil often contains 

 very considerable quantities of this element which the acids do not at- 

 tack, and which form the reserve for the future supply of the plants. 

 (Peligot.) 



The quantity of potash varies in the different soils from the merest 

 traces up to 1 and 2 per cent. Sandy and peaty soils and marls are 

 generally deficient in this alkali, whilst soils rich in alumina are, with 

 some exceptions, also rich in potash. It exists in the soil in combination 

 with silica, forming a silicate, which is somewhat soluble in water. 

 Heavy clay soils and clayey loams vary from 0.8 to 0.5 per cent; lighter 

 loams from 0.45 to 0.30 per cent.; sandy loams below 0.3 per cent.; and 

 sandy soils of great depth may contain less than 0.1 per cent, consist- 

 ently with fertility, depending on the amounts of lirne and phosphoric 

 acid with which it is associated. (Hilgard.) A high percentage of pot- 

 ash in a soil seems capable of making up for a low percentage of lime, 

 and conversely, a soil very rich in lime and phosphoric acid may be 

 very fertile notwithstanding a low percentage of potash. The average 

 annual consumption of potash for raising crops is 45 pounds per acre, 

 or about 0.002 per cent. 



Soda. This is a less important constituent in soil than potash, and 

 unless near the sea-coast, is present in even smaller quantities. Under 

 the form of common salt, however, its presence is a cause of sterility in 

 the soil when this exceeds 0.10 per cent, in quantity. 



Magnesia is found in all fertile soils, in different proportions, often 

 amounting to a mere trace. In the majority of cases the percentage of 

 magnesia is greater than that of the lime, but it does not seem capable 

 of performing to any appreciable extent the general function of lirne in 

 soil improvement. 



Sulphuric acid and chlorine occur very sparingly in most soils. From 

 0.02 to 0.04 per cent, of the former seems to be adequate to most soils. 



There does not exist any affinity between the quantities of lime and 

 magnesia contained in soils and those of potash and phosphoric acid. 



Nitrogen and nitrates. The natural sources of nitrogen in crops are 

 the nitrates and ammonia salts, which are seldom present in large quan- 

 tities, and should be used on or generated in the soil as rapidty as crops 

 require them. The process of nitrification, whereby inert or unasai Bail- 

 able nitrogen becomes converted into nitric acid, is thus of great im- 

 portance to agriculturists. This is due to a minute Bacterium, present 

 in all soils, whereby the humus and ammonia are oxidized and their 

 nitrogen converted into nitric acid. This process does not take place 



