188 

 PART VII. PHYSICAL COMPOSITION OF SOILS. 



The foregoing pages have been devoted all but entirely to the con- 

 sideration of questions pertaining to the chemical nature of the soil, and 

 more particularly to the proportions of plant food present therein. Im- 

 portant, however, as the chemistry of the soil is, there is often a danger 

 of its looming so large in the field of vision that other factors which go- to 

 make up soil fertility are lost sight of. Chemical analysis alone can 

 never suffice to measure a soil's fertility, and even still less its productive- 

 ness. The fertility of the soil depends upon other inherent properties 

 besides the presence of plant food; for instance, its texture and general 

 physical condition : its productiveness is dependent on variable environ- 

 ments and incidental circumstances, such as rainfall, atmospheric tem- 

 perature, conditions of drainage, and methods of cultivation; in other 

 words, on factors which are altogether extraneous to the soil itself. Hence 

 a soil well supplied with plant food is not always fertile, and a fertile soil 

 is not necessarily productive : the most fertile soil cannot be productive 

 when climatic conditions are unfavourable, and the methods of cultiva- 

 tion adopted unsuitable. These distinctions require to be well kept in 

 "view. Leaving out of sight, however, the wider subject of crop produc- 

 tion, and turning again to the more restricted one of soil fertility, it would 

 be well clearly to understand that even the chemical aspects thereof are 

 closely connected with the soil's mechanical condition. The supply of 

 water which circulates within the soil, and which is there tenaciously re- 

 tained for plant use, is directly dependent upon the state of subdivision of 

 the soil particles, other conditions being equal. The very availability of 

 the plant food constituents, moreover, as there has been occasion pre- 

 viously to- remark, is regulated by the fineness of division in which they 

 exist in the soil. Under these circumstances, as Dr. Wiley observes: * 



"It is not, therefore, a matter of surpiise that the fertility of a soil is found, 

 cccteris paribus, to be commensurate, to a certain limit, with the percentage of fine 

 silt and clay which it contains. It is true that two soils, quite different in fertility, 

 may have approximately the same silt percentages, but in such a case it is demon- 

 strable that even in the poorer soil the measure of fertility is largely the percentage 

 of fine particles in connection with its actual content of plant food. Many soils may 

 have large quantities of plant food, but thse stores, owing to certain physical con- 

 ditions, are not accessible to the rootlets of plants. . . . The full value of silt 

 analysis will only be appreciated when many typical soils, from widely separated 

 areas, are carefully studied in respect of their chemical and physical constitution 

 and the character of the crops which they produce." 



We see then how closely a study of the chemical composition of the 

 soil must be interlocked with that of its mechanical condition. The one 

 is never complete without the other, and it is much to be regretted that 

 the exigencies of circumstances compelled the jettison of a systematic in- 

 vestigation into the mechanical nature of the various agricultural soils of 

 the Colony at the time when the chemical survey was commenced. 



In the mechanical analysis of a soil we aim at determining the relative 

 proportions in which the different sizes of soil particles are present. There 

 is, however, a sad lack of uniformity in the nomenclature by the medium 

 of which these various grades are wont to be designated : it may hence 

 prove profitable to examine into this subject somewhat closely. 



In popular language the generality of soils has for many years been 

 divided into " sandy soils " and " clay soils," as though all soils consisted 

 essentially or solely of either sand or clay. For the purposes of rough- 

 and-ready classification a broad differentiation on these lines may pass, but 



* " Principles and Practice of Agricultural Analysis," "Vol. 1, 2nd ed., 1906. p. 310. 



