If food production is to keep up with population growth, then sensible land management is essen­tial.

The basic resource on which food is produced is soil; the maintenance of the nutrient level of soil and the control of erosion are therefore objectives of the utmost importance.

Stream-flow erosion in semi-arid areas is responsible for the formation of dustbowls and badlands strewn with gullies.

Much less obvious is sheet erosion, in which a broad sheet of surface run-off washes soil particles downhill. On ploughed land, such erosion may be imperceptible at first, and it often passes unnoticed until crop yields start to decline because of the reduced level of nutrients in the soil.

Losses in production can be considerable, as shown by a 16-year study in Tennessee, where maize yields were found to be 32 per cent lower on eroded than on un-eroded land.

Contour plowing and terracing

Terraces VietnamOne requirement of land management is that the land should be used only for purposes for which it is suitable.

Because soil loss is greatest on sloping land, the natural vegetation is best left undis­turbed on particularly steep slopes.

On gentler slopes, contour plowing can reduce erosion by half, because the furrows retain rainwater and so reduce the velocity of the run-off. Contour plowing becomes even more effective if there is strip cropping along the contours, because the plant roots also check the downhill flow of water.

In parts of Asia and South America, the popu­lation density is so great that steep slopes have to be cultivated as well as flat areas, in which case the slopes are terraced.

Terracing is expensive, but by breaking a slope into a series of level steps, the run-off never attains much erosive power. Bench-type terraces are bordered by rock walls or by earth banks covered with vegetation. On gentler slopes, low mounds of earth are thrown up along the contours to form terraces.

Soil protection

Fertilizers and crop rotation are essential parts of any conservationist land management pro­gram.

When fallow, the soil should be pro­tected by grass crops such as clover. Clover is especially useful when it is eventually plowed back into the ground because it also helps to fer­tilize soils that are deficient in nitrogen.

In fields of maize, tobacco, cotton and other crops which are grown in rows, the intervening strips should be covered either by the planting of some other crop or - a costlier method - by covering the soil with a protective mulch, which decomposes to become humus. Humus not only enriches the soil, but also increases its water-holding capacity.

Oil-rich Middle Eastern governments halted the advance of desert sands in the early 1970's by financing the spraying of petroleum on the dunes. The oil dries to a gray crust which stabilizes the loose sand and retains moisture beneath the surface. In such improved conditions, grasses and such trees as acacias, eucalyptus and pines, whose roots bind the soil particles, can take root. The plants return humus to the soil which helps to retain more moisture. In this way, once desolate regions were reclaimed.

Consequences of mismanagement

Any regional land management program should be appropriate to the natural factors that determine the local environment.

Erosion has often been the result of the introduction of inap­propriate farming methods. For example, the enormous damage done to the soils in much of eastern North America came about because the pioneering farmers from Western Europe had brought with them agricultural techniques that proved unsuitable in North American conditions.

In the nineteenth century, European colonizers introduced into tropical regions plantation agri­culture and intensive ranching, which were forms of land management that often proved more damaging to the soil than the traditional shifting agriculture.

In the 1970's, a commercial adapta­tion of shifting agriculture was successfully applied in Zaire. It involved clearing strips of land in the forest, practicing crop rotation until the soil fertility started to decline and then reforesting the land and moving on to new strips.

Often, tradi­tional farming methods have to be stopped, such as livestock grazing in semi-arid regions. In this case government aid is needed to find other employment for the herdsmen.

In the USA, the Appalachian Mountains were largely deforested in the nineteenth and early twentieth centuries. The effects of the consequent soil erosion in the upper courses of the Tennessee River included rural poverty.

In addition, the Ten­nessee River carried massive quantities of silt into the Mississippi River. The bed of the Mississippi silted up and the water level rose so that disas­trous floods were common on the plain.

The es­tablishment of the Tennessee Valley Authority (TVA) in 1933 provided for the control of the Tennessee River and its tributaries through a se­ries of large dams. The TVA also launched a reforestation and soil conservation program, including the instruction of farmers.

Similarly, the problems of the overgrazed, drought-stricken Sahel zone in sub-Saharan Africa apply to several countries. In these cases land management pro­grams are most effective when there is multi­national co-operation.

All such programs should take account of ecological factors. For example, the creation of some large reservoirs in tropical Africa, which provide irrigation water and energy to generate hydro-electricity, has in the past led to epidemics of bilharzia.

Another project that has had some dire conse­quences was the building of Egypt's Aswan High Dam. It stopped flooding in the Nile valley, but is also deprived the valley of fertile silt, which now accumulates on the floor of Lake Nasser. In addi­tion, before the dam was built, the supply of silt reaching the Nile delta roughly balanced the ero­sive power of the sea. But by the early 1980s, the rate of erosion at Rosetta headland (Masabb Rashid) near Alexandria was 23m a year and salt water was advancing inland.

So, when you consider the bigger picture, the science of Land Management is enormously complex, and even now in the 21st century we are a long way from fully appreciating its intricacies.