106 



Pi^RT I. 



SOILS. 



The fectility of a soil is dependent upon its chemical composition and its 

 mechanical texture. Soils to be fertile must contain the elements of plant food in 

 such forms that thej can be readily used for the nutrition of vegetation. At the 

 same time its condition must not be too loose, else a firm hold will not be afforded 

 the roots of plants, and there will be too much drainage and evaporation, nor must 

 it be too heavy and plastic, for then air and water could not freely permeate it nor 

 the roots extend themselves beyond a very limited area. Generally speaking, light, 

 loose soils are not as rich in plant food as those in which clay predominates ; yet, on 

 account of their excellent condition of tilth, they often yield in favourable seasons 

 heavier crops than the latter. Stiff, heavy clays, though rich in inorganic plant food 

 (potash and phosphoric acid) are often poor in nitrogen, while their condition is 

 such as to prevent thorough aeration and the penetration of the roots. It is these 

 soils especially that are benefited by drainage. By a system of drainage the water 

 which saturates the surface soil is carried off, air allowed to permeate, the whole 

 rendered more friable and easily worked, and much plant food is converted into 

 assimilable forms. 



Where sand largely preponderates the soil is not retentive of moisture and 

 fertilizing material, especially if the subsoil be light, and though easily worked is not 

 so desirable in dry seasons as a heavier soil. 



A proper proportion of sand and clay, therefore, for many reasons, makes the 

 best soil. 



With the clay and sand, varying amounts of peaty matter or humus (derived 

 from the decomposition of vegetable matter), and of calcareous matter (principally 

 carbonate of lime) are usually associated, and a right proportion of the two latter 

 exerts a beneficial influence upon the tilth of a soil. From the presence of these 

 predominating materials soils are known respectively as clay, sandy, peaty and 

 calcareous, according as one or the other is in excess. 



By the slow decomposition of the clay and the peaty and calcareous matter, 

 plant nutrients are liberated in a soluble form, and therefore the function of these 

 soil fundamentals is not only mechanical but chemical. 



The constituents of soils may be divided into two classes : inorganic and 

 organic. 



In the first of these is the material formed by the disintegration of the i"Ocks at 

 the earth's surface by atmospheric agencies. This mineral matter consists pi-incipally 

 of lime, magnesia, oxide of iron, alumina, potash and soda, combined with silica, 

 phosphoric, sulphuric and carbonic acids. 



The actual and relative amounts of these constituents in soils vary according to 

 the nature and composition of the rocks from which they are derived. 



The organic portion of a soil consists largely of semi-decomposed vegetable 

 matter (roots, underground stems, leaves &c.), otherwise known as humus, peaty 

 matter, &c. The elements which enter into its composition are carbon, hydrogen, 

 oxj'gen and nitrogen, but it is the latter only that has an agricultural value. 



INORGANIC CONSTITUENTS. 



The most important inorganic constituents of a soil are potash and phosphoric 

 acid. These together with nitrogen, are known as the essential elements of plant 

 food. 



