SCIENTIFIC AGRICULTURE. 191 



The remains of plants, together with the excrements and car- 

 m of animals, if returned to the soil before decomposition, 

 must contain all the mineral, organic and gaseous elements,, 

 which the plants derived from the soil or the atmosphere. 

 These must pass through the different processes of decompo- 

 sition, before they assume their original gaseous and earthy 

 forms, and become again available for the food of plants. 



The whole science of manuring consists in supplying to the 

 soil, those indispensible elements which have become exhaust- 

 ed. The richest manure may be applied to a failing soil, and 

 if it lacks a particular element which the crops require, and 

 which the soil does not contain, the soil grows barren notwith- 

 standing the manuring. Farm-yard manure, probably contains 

 the greatest number of elements necessary to fertility ; but par- 

 ticular plants require special manures. 



Manures operate beneficially on the soil in several ways. 



1. By serving directly in some instances as the food of plants. 



2. By causing chemical changes in the soil, by which other 

 substances are prepared to be taken up as nutriment by their 

 roots. 3. By neutralizing noxious substances in the soil which 

 prevent the growth of vegetation. The operation of lime on a 

 cold, sour, peat soil, or one which abounds in sulphate of iron, 

 is an example of this principle. 4. Manures change, accord- 

 ing to their bulk and texture, the mechanical properties of 

 soils, 5. They may change more or less, according to their 

 various properties, the physico chemical character of a soil, in 

 relation to light, heat, air and water. Sand, used upon a clay 

 soil, for the purpose of rendering it more loose and friable, 

 would be as properly a manure, as farm yard, or any other 

 variety. Clay used to ameliorate a sandy soil, is also in effect 

 a manure. 



Manures have been classified in various ways, according to 

 their supposed operation and nature. The most simple and 

 convenient division, and one which is usually adopted at pre- 



