THE CHEM1STKY OF THE SOIL. 81 



MANURES AND MANURING. 



In order to understand properly the action of the different substances 

 as manures, it is accessary to study in some detail the food requirements 

 of the different crops and the manner in which these are satisfied. 



The Chemical Composition of the Plant. 



This subject will be dealt with as broadly and generally as possible, so 

 aa to avoid technical language, and t<> bring into prominence only those 

 essential points which will enable us to understand the connection between 

 the chemical requirements of the plants and the part played by the soil and 

 by manures for their provision. 



We shall not go far wrong if we assume that all plants are built up of the 

 same elements combined together in different proportions. These elements 

 are carbon, hydrogen, oxygen, nitrogen, sulphur, phosphorus, silicon, chlorine, 

 potash, and about half-a-dozen metals in combination. It is to the relative 

 proportion of these elements and their compounds that the great differences 

 in the chemical composition of the various plants are due. 



Different combinations of carbon, hydrogen, and oxygen give us the large 

 class of bodies known as carbohydrates, including such dissimilar substances 

 as starch, sugar, the woody structure, cellulose, gum, <fcc. Hydrogen and 

 oxygen combined give us water ; nitrogen, united with the above elements 

 and sulphur, forms the nitrogenous constituents of the plant, such as the 

 gluten in wheat. Little is known regarding the manner of formation of 

 these substancas in the living plant. Still less is known of the metals — their 

 disposition in the plant, and the changes they undergo. We are able to 

 detect their presence in the ash, in combination with oxygen, phosphorus, 

 sulphur, and chlorine. 



Not only do the quantities of the different elements vary in different 

 plants, but in the same plant some elements are present in quantities out of 

 all proportion to the others. Over one-half of the bulk of most growing 

 plants consists of water. In watery fruit, such as the melon or cucumber, 

 the percentage of water is as high as 90 to 96. Of the remaining portion of 

 the plant the compounds of the metals rarely exceed 2 per cent., and are 

 more often about 1 per cent. This you may readily prove by burning a 

 weighed quantity of any green crop until a white ash, or nearly so, is left. 

 The ash from every pound of such crop will not weigh more than 100 grains. 



The point of importance for us to keep in mind is that each of these 

 plant-constituents is as necessary for the well-being, even for the existence, 

 cf the plant as any other, whether that constituent be present in large or 

 small quantities. The substances found in the ash, though present in minute 

 proportions, are as indispensable to the well-being of the plant as the elements 

 found in large quantities. Not only that, but if any single one of the ash- 

 constituents is insufficiently supplied — if the potash is deficient or the iron — 

 the plant suffers, although the amount of such element does not in many 

 cases amount to more than a fraction of 1 per cent. 



The volatile portion of the plant — that is, the water and the carbo- 

 hydrates — is for the most part abundantly supplied to it by air and rain 

 (with the exception of nitrogen in some plants) — that is to say, about 98 

 per cent, of the food of the entire plant is derived from air and water. From 

 the soil it extracts the remaining 2 per cent., consisting for the most 

 part of iron, lime, magnesia, potash, and soda, combined with sulphuric, 

 hydrochloric, and phosphoric acids, together with the nitrogen as already 



