I goo] Shutt — Soils. 



39 



The carbonic acid g-as of the air, though present only to the 

 extent of 4 parts in 10,000, furnishes all the carbon required for the 

 organic compounds of plants, of which starch, sugar and albumen 

 are the chief. These compounds constitute fully 80 per cent, of 

 their water-free substance, and are those which give to all vege- 

 table tissues their chief value as food for man and beast. Some 

 idea of the extent to which plants appropriate their nourishment 

 from this source may be gained from the statement that an acre of 

 wheat, by virtue of the green colouring matter of its foliage in the 

 presence of the sunlight, will remove during its season of growth 

 nearly one ton of carbon, or as much as would be contained in a 

 column of air over that area three miles in height. Though this 

 is a very large amount, the practical agriculturist needs not to 

 concern himself with this class of food; for nature always furnishes 

 an abundant, a practically unlimited, supply. 



Water is mvariably found in all the tissues of plants, from 75 

 to 95 per cent., as in green stem and foliage, to 8 or 10 per cent, 

 as in the seed. From one point of view, water is to be regarded 

 as the most important of all forms of plant food, since without it 

 all other nourishment is unavailable. Though not of the soil 

 looked at geologically, it is only water present in the soil which is 

 of use to plants. Their whole supply is drawn by the rootlets 

 from this source. Apart from irrigation, we can only indirectly 

 control this supply. Indirect methods for the conservation of soil 

 moisture, chief of which are under-drainage and surface cultiva- 

 tion (which by the formation of a dry earth mulch arrests or 

 checks surface evaporation), are now considered matters of the 

 greatest importance and worthy of equal consideration with 

 problems for supplying plant food. For indeed water not only 

 forms a large proportion of all plant tissues, but it is the vehicle 

 whereby all soil food is appropriated and assimilated. That 

 nourishment which plants take from the air is certainly in the form 

 of a gas, but that which they absorb from the soil must be in the 

 form of a dilute solution. Solids, as such, cannot be utilized ; 

 they must first be dissolved. If they cannot be attacked either by 

 the soil water or the slightly acid fluid that exudes from the root- 

 lets, then no matter how rich such solid materials may be in food 

 constituents, they are of no value to crops. The knowledge ofthi§ 



