90 Plants and their Ways in South Africa 



ately this element is generally lacking in South African soils and 

 must be supplied by artificial fertilizers. 



Some salts, as calcic nitrate, magnesium sulphate, and 

 ammonium sulphate are readily soluble in water and so should 

 be applied at the time of seeding, while calcic phosphate should 

 be applied some time before in order that the acids in the soil 

 may set the phosphorus free and render it in a soluble form 

 for the plants. 



Oxygen, as food material, is probably obtained from salts 

 and from water rather than from the free atmospheric oxygen 

 which is the source of that used in respiration. 



Water not only serves to dissolve salts for absorption by 

 the plant but it supplies hydrogen, which is in all plant food, 

 and also serves to keep the cells turgid. 



The service performed by some of the minerals found in the 

 ash of plants is not clearly understood. Substances which 

 alone in the soil are poisonous to plants, have their poisonous 

 effects neutralized when balanced by other substances. It has 

 been found that the poisonous effects of a sodium salt disappear 

 when calcic chloride is added. Similarly magnesium and po- 

 tassium salts which are poisonous when found separately lose 

 their poisonous effects when their action is combined.^ 



A great drawback to South African farmers is the large 

 areas of brak or alkaline soil due to sodium or magnesium 

 salts. In times of drought an abundance of white crystals ap- 

 pears by evaporation on the surface. Rain dissolves these salts 

 which are readily re-absorbed by the soil. Much of the under- 

 ground water brought to the surface by driven wells contains 

 so much of these salts that the soil becomes sterile. Progres- 

 sive farmers are finding means to remedy this condition. Under- 

 draining or in the case of clay or iron hard pan, dynamiting the 

 soil is effective in carrying away the accumulation of salts. 

 Another method consists in treating the soil with gypsum 

 (calcic sulphate) where sodium carbonate produces the black 

 brak. The calcium of the gypsum replaces sodium and forms 

 calcium carbonate, the sodium combines with the sulphur to 

 form sodium sulphate. The calcium compound thus formed 



* Osterhout, " Bot. Gazette," August, igoS, August, igog. 



