402 THE HOME, FARM AND BUSINESS CYCLOPAEDIA. 



under consideration, and held safely for the use of plants. Thus avail- 

 able plant food accumulates in a fallow field or permanent pasture, and 

 the ground becomes richer by rest. 



ACTIVE AND DORMANT CONSTITUENTS OF SOILS. The preceding section 

 will have shown that in every soil the material from which the tissues of 

 plants may eventually be elaborated, exists in two physical conditions. 

 First, there is the mass of earthy matter which has already been seen to 

 consist of sand, clay, lime, vegetable matter, and stones ; with the excep- 

 tion of lime none of these substances are taken up by the roots of plants. 

 But associated with them in much smaller quantities are a number of 

 substances which constitute the active ingredients of a soil. Of these last 

 a small portion is already soluble and available, and a larger quantity is 

 insoluble, and therefore for the present unavailable. 



The mass of soil may be regarded as the hunting-ground in which the 

 roots of plants ramify and search for food ; also as a suitable material for 

 preserving proper conditions relating to moisture and to temperature. It 

 is indispensable to the well-being of the plant, but at the same time it is 

 not the general mass of the soil which feeds it. The entire mass may be 

 thus divided : First, the part comprising potential, or possible (future), 

 plant food as yet insoluble in water and in acids. The second, the por- 

 tion soluble in water, or dilute acid, representing the active or available 

 plant food of the soil. 



What then is the nature of these changes which are for ever taking k 

 place in a soil ? 



By the removal of crops the available plant food becomes exhausted, 

 and the field ceases to be productive. But after a few years of rest fer- 

 tility is found to be restored. During the interval of rest the soil has 

 been exposed to changes of temperature, to the action of air and moisture, 

 and it may be to vegetative force in the action of roots of plants which 

 have taken possession of the surface. The consequence is, that under the 

 combined influences of these forces, fragments of quartz, felspar, apatite, 

 phosphorite, gypsum, and other mineral substances, become " weathered." 

 They part with small quantities of their substance, which pass over into, 

 the soluble and available condition, and thus a store of plant food once 

 more accumulates. 



This mineral matter rendered soluble will not be suffered to wash 

 through the staple of the soil, but will be only carried a very short dis- 

 tance before it is seized upon and appropriated by that singular force of 

 surface-attraction already noticed ; and hence we see a satisfactory rea- 



