Agriculture and Soils of the Cape Province. 433 



for what crops a particular soil is suited ; crops wliicli require good 

 aeration cannot be grown on soils whose particles consist mainly of 

 clay and silt. 



Wate7' Supply. — Water is to plant growth what a mainspring is 

 to a watch ; it is that which sets the processes of growth in motion 

 and keeps them going. All crops need laige quantities of water, 

 some more than others, and they obtain all of it from the soil through 

 their roots. Provided the amount of water in the soil is not so great 

 as to interfere with the air supply, the crop yield, other things being 

 equal, will be proportional to the water supply. 



Not only must a certain quantity of water be available to the 

 crop; it must be there when the crop needs it. Spells of drought are 

 always liable to reduce crop yields, especially at certain critical 

 periods of growth, as at the flowering stage. 



A farmer can, therefore, only depend on the rainfall to supply 

 the moisture factor when rains are sufficiently abundant and 

 frequent. Given one and the same rainfall, however, soils of different 

 types absorb and retain it to very different degrees. 



■ Even violent rains are absorbed completely by sandy soils, but 

 th^re would be a very considerable run-off in the case of the finest 

 types of soil, because water can only penetrate a soil as quickly as 

 air escapes from it. 



On the other hand, sandy soils have very little power of retain- 

 ing rainfall,, so that much of which falls escapes as drainage. Clay 

 soil, however, has very great power to retain water, and is therefore 

 in this respect a more desirable type of soil than a sand. Since 

 water sinks into a clay soil much slower than into a sand, the latter 

 type of soil is ready for ploughing after rain much sooner than the 

 clay, and because the latter type of soil remains wet at the surface 

 longer there is a greater loss of moisture from it through evaporation. 



Coarse sandy soils have very little power to lift water, so that 

 although there may be ground water at no great depth, the sand 

 above it may be quite dry. Clays, on the other hand, have very great 

 water lifting power, but the water ascends very slowly ; so slowly in 

 the case of heavy clays that crops growing on them may suffer 

 severely in times of drought, although there is plenty of moisture 

 in the lower levels of the soil. 



It is evident that both sands and clays have good as well as bad 

 points. Neither type is a good one for farming on rainfall. The 

 ideal soil for that purpose is one in which water moves readily up, 

 down, and sideways, and one which, at the same time, is able to 

 absorb and retain a fair amount of the rain that falls upon it. Since 

 it is the finest particles which ar^ responsible chiefly for the ability 

 of the soil to retain moisture, and since these same particles are 

 responsible for its water lifting power, for the slowness of movement 

 of water in it, and for its slow absorbing properties, the ideal soil 

 will be one which contains enough fine particles to make it retentive, 

 yet not enough seriously to impede the movements of water in it. 

 Such soils are the loams, particularly the fine sandy loams, a type 

 of soil which contains from 10 to 15 per cent, of clay. 



The mechanical analysis of the soil, by indicating the propor- 

 tions of fine to coarse particles, therefore assists one in forming an 

 opinion as to the water properties of it. 



It will be noted that " humus " figures in the mechanical 

 analysis. Humus is even more retentive of moisture than clay, and 



