36 FIRST PRINCIPLES OF AGRICULTURE. 



substances which form the soil are of such a character 

 as to permit of the change of but a small proportion of 

 its dormant into active constituents, while, in the other, 

 a large proportion of the constituents are annually ren- 

 dered available. 



Soils overlying limestone and granite, and those formed 

 by the gradual accumulation of vegetable matter, as in 

 the prairies of the Western States, possess a high natu- 

 ral strength. They contain large quantities of the dor- 

 mant constituents, which are of such a character as to 

 be readily changed into activity under ordinary condi- 

 tions of season and farm practice, and large crops are 

 possible each year for a long period. 



Sandy soils, perhaps, are the best examples of soils of 

 low natural strength ; in these the purely mechanical con- 

 stituents are in great excess, no considerable quantity of 

 dormant or reserve substances exist, and the constituents 

 made active are only sufficient for minimum annual yields. 



Texture of Soils. Another point to be taken into 

 consideration, when studying the composition of a soil, 

 is the power it possesses of absorbing and retaining such 

 constituents as may become active. This characteristic 

 of a soil is termed its " texture ; " and it has not only a 

 bearing upon the adaptability of the soil to the growth 

 of plants, but also exercises a decided influence upon 

 such growth. 



Soil so open in texture as to freely admit the circula- 

 tion of water is more liable to be depleted in its active 

 constituents than a soil which is close in texture, and 

 retains for a considerable time the water which falls 

 upon it. 



