PRESIDENTIAL ADDRESS SECTION C. 57 



there is notliiug- to retain it ; almost the whole supply rushes 

 off to the river; little or none of it reaches the subsoil, and the 

 death of deep-rooted plants results; the small supplies wbicli 

 reached the surface soil soon evaporate, and so the surface- 

 rooting* plants die, and the result is that a change of vegetation 

 from mesophytic to xerophytic is inevitable, usually taking' the 

 form of a change from red grass (Anthistirio) or blue grass to 

 wire grass (Atistida). 



Repeated burning — whether annual, biennial or at longer 

 intervals — only accentuates the evil, and in the absence of 

 humus protection the half-dry crowns and growth-buds of the 

 better types are scorched or burned, while the xerophytic types 

 adapted to such treatment manage to survive and so become 

 the dominant vegetation. 



Summer burning is even more regularly destructive, for 

 though each species has its season of soft growth, more species 

 are liable to damage during summer than during winter, hence 

 " summer burns " often leave their mark for many years. 



In what is really good grass-veld tliere is always the 

 tendencj^ to recover some time if the opportunity is given, 

 i.e., the rainfall and general conditions favour the better kinds 

 rather than the xerophytes, but in too many cases the farmer, 

 finding his veld mostly composed of wire grass (Arisfida), 

 which is only edible during the first feM' weeks of its growth, 

 resorts to fire again on purpose to clear oft' the indigestible 

 old growth and allow stock to feed for a fortnight in spring 

 on the more tender young leaves. Thus it happens that sooner 

 or later the wire grass gives place to bare patches, or to 

 patches of summer annuals or to xerophytes of more pronounced 

 type, the run-off of rainfall is aggraA^ated, erosion begins and 

 the locality eventually becomes devoid, not only of useful 

 vegetation, but also of soil. It is eroded to the rock, and the 

 surface is only covered by whatever disintegrates from the 

 rock below — even that is often waslied away in slabs or stones 

 as these become free, instead of being reduced to soil or clay, 

 as happens when the disintegration takes place under dense 

 vegetation. 



In a recent pamphlet, " Soil Erosion and Conservation," 

 I have dealt with over-stocking, A-eld tramping, water con- 

 centration and bad farming — all leading to donga formation, 

 erosion, desiccation and desolation in tlie same way as grass 

 burning, and often acting in concert with that practice. 



Thus while under a constantly unl)urned and undamaged 

 vegetation-blanket humus accumulates and the soil deepens 

 and becomes more and more fit to retain moisture and to 

 maintain the higher standard of vegetation known as plant- 

 succession up to a higher climax tvpe, the reverse is the case 

 where the vegetation is burned off. or tramped off, or over- 

 grazed. What vegetation remains graduallv dies off', 

 xerophytes hold the bare surface until erosion displaces both 

 the plants and the soil, and then when the rock is readied even 

 that breaks up by insolation and radiation, and wlien a flood 

 does happen, these stones, by friction, aid the torrents in 

 causing further erosion. These two courses — the one upward 



