lo April. i9I--] Scil Mniifurc and Cropping. 



Secondly: — .4^ to zvaicr necessary for the soil processes ivlierehy food 

 materials arc rendered available for crops. — On the average a soil will 

 contain loo times more plant food than the crop then growing on it re- 

 quires. But the crop cannot use it because the soil constituents must be 

 easily soluble before thev can be taken up or ab.sorbed as plant food. The 

 soil constituents which the crop finds most difficulty in obtaining enough of 

 in soluble forms are nitric and phosphoric acids, and sometimes potash. 



There is extremely little available phosphoric acid and also sometimes 

 little available potash left in a poor soil at harvest. For the next crop, 

 unless manured, the chief source of supply is the amount which can be 

 rendered available in the intenal. A year's fallow gives longer time. But 

 in order that the process whereby the phosphoric acid is made available may 

 go on, the soil must be sufficiently moist. Chemical change scarcely occurs 

 between dry solids. In addition to chemical change, solution of mineral 

 plant foods is to some extent effected by tHe soil bacteria, but these also 

 require moisture. In fallow, it is desirable to conserve moisture apart from 

 the water requirement of the next crop, because the moisture facilitates the 

 solution of mineral plant food during the period of fallow. 



But probably soil moisture is most important from the point of view 

 of nitric acid formation. Deficiency of phosphoric acid can be made goo*! 

 at relatively small cost by a small application of superphosphate and its 

 use is perfectly safe. Nitric acid on the other hand is not only much 

 dearer, but its application artificially as nitrate tends to force on a kind of 

 growth which renders the crop more subject to damage by drought at a 

 later date. The process whereby nitric acid is formed from the insoluble 

 nitrogenous compounds of decaying crops is termed nitrification. 



The change is brought about by several different kinds of bacteria. 

 There are three stages. One kind attacks the nitrogen of the decaying 

 crop residues ; a second carries the work through another stage ; the third 

 yields the finished nitric acid. It is known that sufficient moisture must 

 be present to allow these germs to work, and that they may be killed by 

 se\'ere drought. Exactly how much water they require to do their best 

 work, and at what stage of dryness they cease work altogether is a subject 

 however upon which there appears no reliable information. 



Nitrification is a necessary antecedent to crop-production on any land 

 growing grain or roots, and to provide suitable conditions for it is of 

 primary importance. It is of particular importance on soils continuously 

 under crop without any return of fresh vegetable matter to them, because en 

 such land nitrification becomes each year more difficult. 



Thirdly: — As to water lost inevitably from the soil. — There are three 

 ways in which water is lost from soils : — - 



(i) Surface drainage into ditches and watercourses. 



(2) Percolation into underground springs. 



(3) Evaporation from the surface. 



(i) Surface drainage causes most loss on baked surfaces, lying on the 

 slope, and when the rainfall is concentrated in a short period of time. 

 N[ore particularly in autumn and winter, when the land is hard, is much 

 water lost in this way. Early ploughing after harvest avoids loss, as 

 not only is the surface left rough to impede surface flow, but at the same 

 time the upper portion of the soil it rendered sufficiently open and porous 

 to take in what falls upon it. 



(2) Percolation into underground springs. This loss is unavoidable^ 

 and in many ca.ses it is an advantage preventing the soil becoming water- 

 logged. In some classes of land, particularly clays, when no natural 



