REPORT OF THE CHEMIST. 165 



gneisses, limestones, and dolomites, which contain it without exception ; 

 volcanic soils i)0ssess it in large quantities, whilst alluvial soils and 

 those lands that are periodically swept by floods, are much poorer. 

 Soils containing less than .05 per cent, of it will be sterile and unfer- 

 tile, as a general rule, unless accomiiauied by a large amount of lime. 



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

 ble form arising from the disintegration of feldspathic and other rocks. 

 In the majority of cases the natural supi)ly of the soil is suflBcient to fur- 

 nish to the plants the potash of which they are in need ; a soil contain- 

 ing .125 per cent, should furnish potash enough for a century, without 

 its being necessary to add to the manures used on such soils any salt of 

 potash. Besides this available potash the soil often contains very con- 

 siderable quantities of this element which the acids do not attack and 

 which form the reserve for the future supply of the plants. 



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 gen- 

 erally 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 .8 to .5 per cent. ; lighter loams 

 from .45 to .30 per cent. ; sandy loams below .3 per cent., and sandy 

 soils of great depth may contain less than .1 per cent, consistently with 

 fertility, depending on the amounts of lime and phosphoric acid with 

 which it is associated. A high percentage of potash in a soil seems ca- 

 pable of making up for a low percentage of lime, and, conversely, a soil 

 very rich in lime and phosphoric acid may be very fertile notwithstand- 

 ing a low percentage of potash. The average annual consumption of 

 potash for raising crops is 45 pounds per acre, or about .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 it exceeds .10 per cent, in quantity. 



Magnesia is found in all fertile soils, in diff"erent 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 lime in 

 soil improvement. 



Sulphuric acid and chlorine occui' very sparingly in most soils. From 

 .02 to .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 of phosphoric acid. 

 Nitrogen and 7iitrates.—T\ie 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 rapidly as crops 

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

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

 portance to agriculturists. This is due to a minute hactarium present 

 in all soils, whereby the humus and ammonia are oxidized and the nitro- 

 gen converted into nitric acid. This process does not take place unless 

 the soil is moist and has free access of air, and some base, generally 

 lime, is present with which the nitric acid can combine. Nitrification 

 is thus most active in summer and ceases apparently in winter. 



Messrs. Lawes and Gilbert have for some years past been devoting 

 their attention to the sources of the nitrogen of crops, and in the pages 

 of the Journal of the Koyal Agricultural Society and of the Journal of 

 the Chemical Society will be found their reports in foil. 



