Agriculture and Soils op the Cape Province. 437 



to attacks by disease and insect pests. The nitrogen factor would 

 not appear to be deficient in our average soil ; on the other hand, it 

 is doubtless often excessive. 



Phosphoric Oxide. — Suitable compounds of phosphoric oxide 

 exert a very beneficial effect on root development and on tillering 

 (stooling). Its effect in this respect is particularly marked on soils 

 of poor aeration ; not so much on sandy soils. An abundant supply 

 of phosphatic food shortens the period necessary for the plant to come 

 to maturity, and, when this food factor is deficient in comparison 

 with nitrogen, artificial additions result in a considerable increase in 

 the yields of tubers to top, seed to straw, etc. 



Furtherj an adequate supply of phosphatic food produces crops 

 of much higher feeding value than if the supply is inadequate. 



The average soils of the Union would seem to be more deficient 

 in phosphatic food than in any other. 



Potash. — Potassium is that food element which is responsible for 

 the " filling out " of grain. Other factors being normal, an 

 inadequate supply of potash leads to grain of dull appearance, 

 " thin " and often of low germinating vitality. A plentiful supply 

 of potash tends to keep the plant growing in droughty times, and 

 confers on the plant a greater resistance to certain diseases. 



Potatoes, beet, mangels, and tobacco are among the crops which 

 require more potash than is usually found in average soil. 



Rectifying Deficiencies in Food Supply. — If the food supply is 

 deficient, matters may be improved by adding substances called 

 manures or fertilizers to the soil. 



Manures are substances which add water-soluble food substances 

 to the soil or food substances which, when added to the soil, become 

 readily soluble in the soil water. Superphosphate is an example of 

 a water-soluble phosphatic manure, while basic slag and bone, if 

 ground to a sufficient degree of fineness, are examples of phosphatic 

 manures which are not water-soluble, but which become readily 

 water-soluble when applied to the soil. Nitrate of soda and 

 ammonium sulphate are examples of water-soluble nitrogenous 

 fertilizers, blood and meat meal being insoluble, but becoming readily 

 soluble in the soil. Sulphate of potash is a water-soluble potassic 

 fertilizer, as are also certain plant ashes. These latter are, however, 

 not generally usable owing to the fact that under certain conditions 

 (deficient rainfall among them) their use would introduce a harmful 

 factor, as well as food, into the soil. 



Kraal manure, stable manure, Government guano, and bat guano 

 are examples of fertilizers which supply all three food substances, and 

 all of them contain water-soluble and water-insoluble food substances. 

 Further, the amounts of plant-food substances present in these 

 manures are not well balanced ; phosphatic food substance, in which 

 our soils are most deficient, is also most deficient in these fertilizers. 



Manures are not to be used in any haphazzard fashion ; the 

 correct choice depends not only on soil conditions, but also on the 

 crop it is desired to grow and on climatic conditions. There is no 

 such thing as a "best" fertilizer; what is best in one case may be 

 worst in another. 



The reader having perused the above sketch of factors in plant 

 growth should now be able to appreciate the discussion of the 

 analytical figures. 



