Agriculture and Soils of the Cape Province. 4ii5 



It is evident that the importance of water to plant life is diflficult 

 to over-estimate, especially in this country of ours where the rainfall 

 is both uncertain and light. 



Food Supply. — If crop yields are consistently poor the farmer 

 more often than not ascribes the bad yields to a deficient food supply 

 in the soil. Sometimes he is right, but just as often wrong, for 

 food supply is only one of several factors whose presence is necessary 

 to plant growth. Before attributing bad yields to a lack of food, 

 one should always make certain that none of the other factors is 

 wanting. 



Doubtless there are many instances on record in which soils have 

 been deficient in plant food right from the start, also in which the 

 supply of food has seriously diminished owing to long continued 

 cultivation. Many also are the instances in which the yields of 

 crops have been considerably increased by the addition to the soil 

 of one or more plant foods. 



It will have been gathered that there is more than one kind of 

 plant food; as a matter of fact there are at least a dozen kinds. Of 

 the foodstuffs required by plants one, i.e. carbondioxide, is obtained 

 from the atmosphere. The plant takes this gas in through microscopical 

 openings on its leaves, from which one may reasonably conclude that 

 good leaf development is essential to good yields. The rest of the 

 food substances are taken from the soil through the roots. There 

 are, however, no openings on roots comparable to those on leaves 

 through wdiich the plant takes its food from the soil ; but the food 

 enters the plant by its roots. These latter are constructed of material 

 which allows water to pass through it, carrying with it substances 

 (including food substances) which it has dissolved from the soil. No 

 matter how finely substances are divided .they cannot pass through 

 the roots of a plant unless they are soluble in water and are actually 

 dissolved in it. 



Water is therefore rightly regarded as the vehicle Oy which food 

 is transported from the soil to the plant. It therefore follows that 

 if the water factor is deficient, so must the food factor be in effect; 

 also that no benefit can be expected from the artificial increase of 

 the food supply (manuring) until the defect in the water factor has 

 been remedied. 



It should, however, be stated here that, given two soils identical 

 in every respect, except that the one contains plenty of all kinds of 

 available plant food, the other being somewhat deficient in that 

 respect, the former will produce crops at a less water cost than the 

 latter. This means that up to a point an abundant food supply 

 economizes the water supply. 



It was stated higher up that no plant food substance can enter 

 the roots of a plant unless it be first dissolved in the water of the soil. 

 Now, only a very small fraction of the plant food substances in the 

 soil are soluble in water; that is, almost the whole quantity ie 

 insoluble and therefore unavailable for immediate use by the plant. 

 Consequent on these facts, two groups of food substances are 

 recognized in the soil, i.e. — 



(a) Dormant or unavailable food substances. 



(b) Water-soluble or available food substances. 



