Search

Summary The humus form is recognized in a soil profile as the group of A horizons in which organic matter is concentrated, and it is thus the upper part only of the genetic humus type, the true measure of which is the complete organic profile extending into B and C horizons. Grassland humus forms are subdivided on a basis of field morphology into two groups: mull, in which the organic residues are largely incorporated as an intimate clay-humus mixture, and mor, in which organic residues in varying stages of decomposition overlie the mineral soil with a sharp boundary but may show weak mechanical admixture of mineral grains. A mor-like mull intergrade is recognized where the mull is very high in intimately incorporated organic matter. According to variation in horizon thickness, structure, and consistence, sixteen humus forms are recognized: strongly granular, weakly granular, massive or blocky, fine, matted, laminated, and lenticular mulls, strongly granular, weakly granular, and laminated mor-like mulls, and granular, massive, matted, laminated, banded, and mullized mors. Where mineral grains are very abundant in mor a sandy phase is recognized. Each layer or horizon of a humus form may have one or more distinct micro-structures or fabrics. Twelve fabrics have so far been recognized under grassland: weak mull humus, strong mull humus, mull-like moder, mull-like rendzina moder, leached mull-like rendzina moder, silica moder, swollen moder, rendzina moder, leached rendzina moder, raw soil humus, sclerotial humus, and raw humus.

Grazing land is here taken to include all areas where grazing by vertebrates is important. This means that all vegetation types dominated by herbaceous plants have to be included. These range from tundra and temperate grassland to savanna and desert.

The savanna lands have been seen by some writers as an undeveloped area of great potential. Mean annual rainfall mostly exceeds 500 mm and reaches 1,300 mm. in places. Temperatures are high throughout the year, allowing the growth of a wide range of crops The vegetation is less dense than the rain forest and can be cleared more economically; savannas typically occur on level terrain, and so the use of machinery is possible. Much of the savanna is covered by grass, which in places gives high yields of dry matter, which would seem to augur well for livestock production. Not surprisingly, some have seen the savannas playing a role in the twentieth century comparable with that of the temperate grasslands in the nineteenth; it was with this hope that the British and French Governments launched a number of development schemes in the African savannas in the 1940s and 1950s. Many were failures, and few have been commercially successful, so that the optimism of that period is now much qualified. Whilst food production can — and indeed must — be increased, the savannas are unlikely to be radically transformed in the near future.