1 0.2 



contain much clay all the more retentive of moisture, and of the nutritious 

 substances therein, when sands, although abounding in mineral plant food, 

 may be unfitted for cultivation simply through the lack of soil moisture. 



Moreover, the finest particles of the soil are those which contain the 

 largest amount of the elements of plant food in an available form, so that 

 in every respect a soil containing a large proportion of silt and clay is there- 

 by the better fitted for plant sustenance, and, altogether apart from any 

 question of direct chemical analysis, the mechanical analysis of a soil will 

 frequently bo a good index to its probable fertility. Above and beyond 

 all this it is to be remembered that a fine-grained soil not only presents its 

 plant food in a form better fitted for the plant to assimilate, but it gen- 

 erally has more of it to present. 



By " clay," it need hardly be said, is meant not only the term as used 

 in the strictly chemical sense, namely, silicate of alumina, but, in a wider 

 signification, it denotes that finely divided material which when stirred up 

 with water, does not settle down readily, but remains suspended in the 

 liquid for a long time, a time generally to be measured by days some- 

 times by weeks.* 



Now the effect of rain beating upon the surface of the soil is to stir 

 up the soil particles, and, as the water percolates down through the soil, it 

 constantly carries with it in suspension a quantity of clay, naturally there- 

 fore in a rainy district the sub-soil becomes in time more clayey than the 

 surface soil, and as clays contain more plant food than coarser gained soils, 

 it also follows that the subsoil will, as a rule, be richer than its surface 

 soil. Hence, too, subsoils are more retentive of moisture, and less easily 

 penetrated by the rain -owing to the accumulation of clay. 



The mechanical analyses of soils are thus of far-reaching importance ; 

 not only does such an analysis serve its direct purpose of determining the 

 proportion of particles of various sizes in the soil, but, indirectly, it sheds 

 light upon the soil's power to retain moisture, and also upon its permea- 

 bility to plant roots, and it even aids as stated above in forming a con- 

 jecture as to its chemical potentialities, since the soils that contain most 

 silt and other fine grades of soil particles, are frequently the richest from 

 a chemical point of view. 



Dr. Wiley's words, already quoted, partially answer the question why 

 soils of certain class are required in order to cultivate particular crops, but 

 it does not solve the problem completely. Reference has been made to 

 Snyder's apportionment of differently graded soils to specific crops, but just 

 why such classes of soil should be so specially suited to some crops, and not 

 to others, has not been fully stated. A partial solution may be found in 

 the fact that it is a characteristic feature of each kind of plant to require 

 for its development a certain degree of soil heat, and specific amounts of 

 moisture : both of these are dependent upon the mechanical condition of 

 the soil. Now, according to Snyder, soils of the class that he prescribes as 

 best suited for potatoes are so suited because such soils generally contain 

 from 5 to 12 per cent, of water; similarly, those which he recommends for 

 fruit usually retain from 10 to 18 per cent, of water : those which are 

 recommended for corn should, according to the same authority, contain 

 about 15 per cent, of water; grain soils, he says, ought to hold from 18 to- 

 20 per cent, of moisture. Now all these conditions, naturally, are intended 

 to apply to certain parts of the United States of America. In the Cape 

 Colony we know as yet neither (1) whether soils physically composed as 

 above would retain the proportions of moisture stated, nor (2) whether such 

 proportions would be the optima for the classes of crops mentioned. What 



" As used in a physical sense ' clay ' may be silica, felspar, limestone, mica, kaolin, 

 or any other rock or mineral which has pulverised until the particles are less than -C05. 

 mm. diameter." (Snyder : "The Chemistry of Soils and Fertilisers," 1899, p. 13). 





