SOILS AND MANURES 549 



302. Available Plant-food. — Some of the phosphoric acid, CHAP, 

 potash, etc., present in the soil is in a state of chemical com- VIIL 

 bination in which the plant cannot make use of it for food, 



until a certain amount of "weathering" and decay has taken 

 place through the action of moisture and air. Estimations of 

 the total quantities of such salts present in the soil are there- 

 fore of comparatively little value, alone, as indications of its 

 actual fertility or of its manurial requirements, without a know- 

 ledge of the amounts available as plant food at any one time. 



303. Recuperative Power of Soils. — -Soils are possessed of 

 great recuperative power, and if the conditions are favourable 

 the renewal of the available plant-food may take place with 

 considerable rapidity. Under the action of moisture and air a 

 process of "weathering" is constantly taking place, and the 

 salts are thereby rendered soluble. This is one reason why 

 summer fallowing often proves so beneficial. Even if the 

 available salts had been quite exhausted before fallowing was 

 resorted to, the soil would not long remain unproductive ; the 

 available plant-food would soon be restored by the action of 

 moisture and air on the mineral matter. 



304. Character of South African Soils. — As a general rule 

 South African soils are not rich in the total amount of salts 

 required by plants, as compared with those of many parts of 

 the world. Yet the peculiar fact remains, as pointed out by 

 Ingle (1), that luxuriant crops are yielded by soils which, on 

 analysis, appear to be extremely deficient in plant-food. This is 

 partly explained by the favouring influences of abundant sun- 

 shine and high temperature. In tropical and sub-tropical 

 countries the processes of soil renewal appear to go on more 

 rapidly than in other climates. Soils poor in available salts 

 may, under these conditions, give the plants actually greater 

 nutriment than soils containing a considerably larger percent- 

 age of salts under conditions less favourable {Ingle, 1). 



305. Soils Suitable for Maize-growing.— -Maize as a sur- 

 face-rooting plant is quickly affected by change of climate 

 {Burtt-Davy, 16). It is also sensitive to variations of soil — 

 perhaps more so than other cereal crops. To succeed well 

 it requires a good deal of moisture, but standing water or 

 water-logged soils are injurious ; to secure the best results the 

 soil should be moist, but well drained. It should also be of 



