SOIL MANAGEMENT. 45 



usually indicate a sufficiency of available food. A deficiency in 

 respect of any one plant food implies an insufficient produc- 

 tion of available food from tbe soil's reserve supply of that 

 particular constituent. This will generally correspond with 

 an insufficient reserve supply, but not necessarily so. 



The generation of readily available plant food in soil is 

 influenced largely by the mechanical condition, particularly in 

 regard to aeration. Another important consideration is the 

 presence of actively decaying organic matter, as the 

 by-products of this decay process aid in rendering plant food 

 constituents soluble. Virgin soils generally contain an 

 accumulated supply of readily available plant food, and are, 

 therefore, usually highly productive when first brought under 

 cultivation. Continuous cropping without the addition of fer- 

 tilisers or manures, however, rapidly exhausts the available 

 food supply and leads to diminished yields, the condition being 

 aggravated by the depletion of humus and the consequent 

 deterioration in mechanical condition. 



Plant Food Deficiencies. — In connection with the rela- 

 tive supplies of the various plant foods, it is well to bear in 

 mind the " law of the minimum " as applied to soils. This 

 may be stated as follows : — " Any essential plant food that 

 exists in the soil in relatively small amount as compared with 

 the other important constituents naturally becomes the con- 

 trolling food factor in crop development. Any reduction or 

 increase in the supply of this constituent will cause a reduc- 

 tion or increase in the crop yield " ; or, more briefly : " The 

 crop yield is determined by the quantity of that plant food 

 which is most deficient in the soil." Rational fertihser prac- 

 tice is based upon the application of this law, and obviously 

 implies the recognition of the specific plant food deficiencies of 

 the soil. 



The manurial requirements of soils may be determined 

 either by chemical analysis or from the results of accurately 

 controlled field experiments. 



(a) Soil Analysis. — By chemical analysis we obtain an 

 invoice of the soil's total reserve supplies of the various essen- 

 tial plant foods. A marked deficiency in respect of any one 

 constituent generally indicates that addition of this constituent 

 is necessary. Availability is, however, not always propor- 

 tioned to the total supplies, and deductions from chemical 

 analyses may be somewhat at variance with field observations. 

 Analytical methods have also been devised for the determina- 

 tion of readily available plant food in soils at the time of 



