170 FARAI-YARD MANURE. 



conditions of its fertility, that is, after a course of years of 

 cultivation it loses tlie power of again producing this crop, 

 plant, or part of a plant. A thousand grains of corn requke 

 from the soil a thousand times as much phosphoric acid as 

 one grain ; and a thousand straws demand a thousand times 

 as much silicic acid as one straw. When, therefore, the 

 soil is deficient in the thousandth part of phosphoric or 

 silicic acid, the thousandth grain or the thousandth straw 

 will not be formed. If a single stalk of corn is taken 

 away from a field, the consequence is that the field no 

 longer produces one straw in its room. 



Hence it follows that a hectare of ground, containing 

 25,000 kilogrammes of the ash-constituents of wheat, 

 uniformly distributed, and presented to the roots of the 

 plants in a perfectly available condition, can, up to a 

 certain point, continue to give in succession remunerative 

 crops of various cereal plants, without receiving any 

 restoration of the mineral constituents taken away in the 

 corn and straw, provided that the uniform mixture of the 

 soil be maintained by careful ploughing and other suit- 

 able means. The succession of crops is determined by 

 this principle, that the second plant must always take 

 away fi.'om the soil less than the first, or possess a greater 

 number of roots, or generally a larger absorbent root- 

 surface. After the average crop of the first year, the 

 crops would go on yearly diminishing. 



The farmer, to whom uniform average harvests are the 

 exception, and an alternation of good and bad crops 

 dependent upon change of weather is the rule, would 

 hardly notice this constant diminution, even supposing his 

 field to be actually in that favourable chemical and 

 physical condition wliich would enable him to cultivate 

 wheat, rye, and oats for seventy years in succession, 

 without restoring any of the mineral constituents removed 



