THE PEINCIPLES OP MANUBING 183 



soluble are less liable to loss by drainage on stiff land 

 than on open sandy soil. 



The chemical composition of soils is sometimes abnor- 

 mal. There may be a deficiency or there may be an 

 excess of some particular ingredient, or the soil may be 

 more or less deficient in all the essential elements of 

 plant food. Clay soils derived from granite are usually 

 well supplied with potash. Sandy soils and humous 

 soils are very often deficient in that ingredient. The 

 proportion of nitrogen in humous soils is generally 

 large compared with that of the other ingredients ; owing 

 to the rapid oxidation and free drainage, sandy soils are 

 often conspicuously deficient in nitrogen. The growth 

 of leguminous crops tends to enrich the soil in nitrogen 

 and to exhaust the potash and phosphates. Other crops, 

 especially roots and potatoes, tend to exhaust the potash 

 and phosphates without increasing the nitrogen. The 

 manures applied to previous crops generally prevent 

 exhaustion and leave a certain residue for those which 

 follow. 



It is obviously superfluous to add those ingredients 

 which are already present in excess, and it is essential 

 to add those of which there is a deficiency. This has 

 been formulated in a statement sometimes called the law 

 of maxima and minima. It asserts that the crop is 

 governed by the constituent which is present in minimum 

 quantity. According to this view, each kind of crop 

 requires a certain minimum quantity of each of the 

 essential constituents, and a deficiency of one is not in 

 any way compensated by excess of any or all of the 

 others. For example, assuming that a crop requires, say, 

 50 Ibs. of nitrogen, 30 Ibs. of potash, and 20 Ibs. of 

 phosphoric acid, a full crop could only be obtained if 

 the soil were able to furnish these quantities ; if it 

 could only provide half the quantity of one of them 



