Agriculture and Soils of the Cape Province. 481 



The mechanical composition of the soil, taken in conjunction 

 with the climate of a particular locality, enables one to form an 

 estimate of the temperature suitability of that soil for a particular 

 crop. 



Reference to Table 1 hereunder will show that the mechanical 

 analysis classifies the soil particles into groups of various sizes, the 

 coarsest being sand and the finest clay. Now, the coarser the soil, 

 the hotter it becomes during the heat of the day, while the greater 

 the proportion of fine particles (viz., silt and clay) it contains, the 

 cooler it will keep. Similar differences exist with reference to soil 

 temperature changes as winter gives place to spring. The sandier 

 the soil, the sooner it warms up to the general air temperature. Thus, 

 under the temperate climatic conditions of, say, England, sandy 

 soils are often valuable because they are " early," while clay soils 

 are commonly known as cold and " late." 



Speaking rather generally, the temperature properties of clay 

 soils are undesirable in cool climates, but very valuable in hot 

 localities; while the reverse holds with reference to sandy soils. 



One must evidenth' therefore always consider climatic condi- 

 tions and the mechanical composition of the soil in conjunction. To 

 come to actual instances, a sandy soil which is just right for 

 potatoes under Witkop conditions, would be too hot under 

 Burghersdorp conditions ; or a sandy soil situated in the Karroo 

 would give only a poor crop of wheat, whereas the same soil situated 

 in the Witkop district would return quite good yields. We have 

 dealt with this matter somewhat lengthily, because in our experience 

 the farmer often ascribes to a lack of plantfood in the soil what 

 ought really to be ascribed to unsuitable climatic conditions, par- 

 ticularly temperature conditions. 



Air Supply. — It is only below ground that the air (oxygen) 

 supply is likely to be deficient, and then only in the case of soils 

 which contain large percentages of finest particles, i.e. the loams and 

 clays. Even the most compact soil contains a considerable quantity 

 of air, the presence of which is revealed when a lump is placed under 

 water; as the w^ater enters the soil the air is driven out and escapes 

 as bubbles. 



Soil is therefore not to be regarded as a solid mass, but rather 

 as a porous mass whose solid particles are packed more or less closely 

 together. The particles touch one another at one or more points ; 

 between them are spaces which, if the soil is quite dry, are entirely 

 filled with air. If, however, the soil is moist, part of the air-space, 

 i.e. that contiguous to the soil particles, is taken up by water. (A 

 fair picture will be obtained if one imagines the soil particles to be 

 covered with a film of water whose thickness will depend on the 

 amount of moisture in the soil.) 



Since sand particles are relatively large (see sizes in Table I 

 hereundei) the spaces between them are correspondingly big; there 

 is therefore little friction impeding the movement of air in a sandy 

 soil. 



In the case of soils which contain large percentages of finest 

 particles (i.e. silt and clay), the spaces between particles are so small 

 that the movement of air is very much impeded. 



Speaking generally, the movement of air is easy in sandy but 

 difficult in clay soils ; the renewal of the air of sandy soils is there- 

 fore accomplished much more readily than the renewal of the air of 

 clay soils. 



