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tion has spread, and applied to soils to which they were unsuited. Even in North America, where our stock seems to have taken root naturally, our agricultural methods seem to have been applied too precipitately, with the result that 40 per cent of the whole United States has lost from to of its top soil, and only of the soil of the country is immune." When applied to the hot parts of the world, our methods are even more disastrous. The case of Africa, culminating in that of South Africa, is said to present the most serious problems of soil erosion in the world.

Let us consider the section X-Y across Africa, in association with the Vegetation map. We see how winter rainfall, in both hemispheres, gives the characteristic Mediterranean vegetation in the extreme north and the extreme south of the continent; and how, as rain decreases equatorwards from these extremes, it gives rise successively to thorn-bush, semi-desert and desert. Continuing towards the equator, the rain now begins to increase, "following the sun", i.e. causing summer maxima, slight and single near the Tropics of Cancer and Capricorn, but increasing to heavy double maxima at the equinoxes over the equator itself. The vegetation follows in the wake of the rain, and expands from semi-desert through thorn-bush to grassland (savanna), then park-land, to true dense equatorial forest.

What soils correspond to these variations of rainfall and vegetation? The Mediterranean has its own red soils which grade into the brown-yellow and grey-forest soils showing varying degrees of eluviation according to the rainfall they receive. Equator-wards from these eluviation decreases, the zones of down-washed lime and other material approach and finally reach the surface in the desert, where a loose structureless soil is at the mercy of every wind that blows! (Hygroscopic salts at the surface have a certain restraining influence on desert soils, but when they are removed by irrigation, they join the mass of alkali a little below, and the upper soil may break down to a structureless dust in dry weather, or an uncultivable syrup in wet weather'. This is the "black alkali" so dreaded by the irrigation farmer.)

Continuing towards the equator, the increasing rainfall is seen to induce ever more luxuriant vegetation through tall grasslands with Chernozem soils, to parkland as trees increase until the true equatorial forest is reached. Here, to the intensely eluviating effect of torrential rain must be added the speeding up of chemical reactions due to high temperatures. (Van 't Hoff's law states that the speed of a chemical reaction is doubled for every 10° C. (=18° F.) rise of temperature. Lehenbauer's work on the growth of maize seedlings shows that, between certain minimal and optimal temperatures for the various plant functions, this law of temperature still obtains.)1

The result is that there exist great depths of weathered soil, 80 ft. to 100 ft. at times, very porous, honeycombed, in fact, but highly stable and resistant to erosion. All fine and all soluble material has been removed, therefore this mineral framework is singularly devoid of mineral nutrients for plants which must depend entirely, therefore, on the products of disintegration of the native vegetation for their food.

In the torrid zone the destruction and humification of dead and dying plants are very rapid, and plant nutrients pass almost directly from the decaying vegetation to the roots of the growing plants, using the mineral framework as a passage-way, not as a storehouse of fertility at all. Clearing of the forest in such cases means the removal of the sole source of plant food, and is followed by serious deterioration of the soil, the bright red colour of which in the extreme case," Laterite ", gives a false impression of fertility, but is due, actually, to intense chemical weathering.

large herds of cattle which constitute for the natives, not only a source of food, but a currency and an index of rank. For these reasons, cattle are kept numerically in excess of their economic value, and with the increase of native population under the civilized rule of the white man, pastures have become overstocked, forest cleared (especially on the watersheds in the native reserves of Basutoland and Swaziland), and soil structure degenerated so that wind and water have borne the soil away!

"In soil erosion, Africa deserves first place, for here water-shortage, the sinking of the water-table, and the advance of the desert are most obvious, and these are not due to changes of climate but to soil erosion ",1 and to the consequent grave increase of destructive run-off waters. They have appeared only since the white man occupied the country. Not only white but native cultivators now grow cultivated crops on a commercial scale on cleared forestparkland and savanna, exposing the soil to the great evaporating power of the air of those low latitudes, with consequent loss of moisture, organic matter, and soil structure, and resulting in severe soil erosion, while the original fertility of the land is exported as commercial

crops.

In South Africa where the ill-effects of soil erosion seem to reach a climax, mineral wealth is the chief source of income; but as the mines are already showing signs of decreased output, a somewhat narrow time-limit is indicated in which to undertake the costly reclamation measures necessary to put South African agriculture on a more or less stable footing.

The inefficient native methods of shifting cultivation in the forest and nomadism on the grasslands were the human response, in its unsophisticated stage, to the geographical environment, but it was safe. Now, after bringing agriculture to the brink of disaster by his ingenuity, civilized man admits that it is unwise to allow land to depart too far from its natural condition of vegetation ". Yet soil must be used by man, and the way out of the difficulty seems to be to find out what type of culture stabilizes the soil-hitherto man's only interest seemed to be to get a commercial crop out of the soil at any cost!

The remedies suggested for the ills of the African soils are many; but one is outstanding; it is the regional control of production, so that all the farming within the region shall be such as to conserve the soil. This implies a rigorous control of the watersheds which must not be de-forested. It also implies a rigorously controlled, practically a feudal system of land tenure.

It is said by those who know them that the African natives will never adopt soil-conserving methods while there is more land to explore, and to which the nomadic habit urges them to wander. An example of a very different mentality is provided by a tribe, the Bokara, living on an island in Lake Victoria Nyanza, who on their sharply limited area have evolved a conservative agriculture calculated to maintain both a fair output and soil fertility. It seems that the great instructor, Necessity, working on the very foundations of all life, the soil, can teach even primitive man, and hasten his social and intellectual progress by comparison with men of the same race, who have not had the spur of need so strongly pressed home.

Under white domination, the colour-bar' operates yet further to restrict the area available for native settlement and expansion, while the civilization' of the white man encourages that expansion, by eliminating inter-tribal wars, forbidding infanticide, and attacking the problem of diseases and pests. Conspicuous among the latter is the tsetse fly, to exterminate which large tracts of infested forest have been cleared, thereby increasing the liability to soil erosion, and thus aiding and abetting the ignorant native in the destruction of the natural wealth of the land. The present outlook, however, is more promising, as it is considered that by selective felling, leaving trees of a certain height and (Continued on page 88)

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type, it will be possible both to control the fly, and retain the protective forest cover."

So, a little knowledge leads to activities-these, to mistakes; these, to corrections and more knowledge, in an endless chain of cause and effect-all forging links of human responsibility and solidarity.

This article has attempted to outline work on the study of the soil, and its importance as a foundation of human geography.

Admitting at the outset the complexity and subtlety of the subject, a somewhat obvious method of approach, ignoring all such difficulties to begin with, was undertaken, using British maps showing the geology, relief, rainfall, and vegetation, in order to introduce local soils, and give the students concrete ideas as a foundation for their work. This opened the way for the use of a similar set of world maps to approach the study of world distribution of prevailing soil types, and the principles underlying that distribution.

The use of rainfall maps as keys to vegetation maps is justified by general usage, but needs qualification-we must remember that one of the chief functions of the soil is "to stabilize climatic moisture", and that a soil retentive of moisture in any locality may maintain a vegetation cover there characteristic of a heavier rainfall than actually exists; while conversely, a porous soil may introduce prematurely a vegetation indicative of a deficient rainfall.

The sections are related mainly to rainfall, though the factor of temperature, in promoting chemical weathering and increasing evaporation, has been borne in mind.

The whole subject is so vast and so intricate, that the simile of the human mind exploring a zone of contact of many influences, and responding selectively to the subtle interplay of such as come within its apprehension, was immediately suggested. For it is in this way that man has responded to the stimulus of that amazing zone of innumerable contacts that is covered by the term the Soil.

We usually think of the soil as a derivative of the country rock, and a function of the geological formation; and such,

AT

so far as the original mineral framework is concerned, it is. But it is much more than this.

From below upwards, inorganic Nature provides a liaison zone with organic Nature through plants and their derivative, the organic matter of the soil, which enters into something more than mere physical contact with the most finely divided mineral matter to form the colloidal complex, a region of intense chemical and biological activity.

On the maintenance of the activities of this zone, keeping going aeration, percolation, the Carbon and Nitrogen cycles, with the resultant growth, decay and humification of plants, all living creatures depend absolutely for their food. Here, such animals as earthworms and rodents, ants and termites, help in the work; but man, in his early life, merely rummaged, clumsily taking what he needed, like any other predatory animal, without thought of the result! Later, having learnt some tricks that gave him good results at home, he went about the Earth, like a mischievous monkey, using his knowledge merely for his own gain. At last, he is emerging from this irresponsible stage to one of greater apprehension, and is now exploring the vital zone of contacts in the soil to see what potentialities it has beyond those hitherto manifested. Up to the present time, crumbstructure, depending on a certain aggregation of particles only to be maintained by a fairly high percentage of organic matter in relation to certain indispensable climatic factors (or to most careful and continuous cultivation), has been the criterion of success in soil treatment. Must it remain so? Or will the evolving mind find other and so far unsuspected potentialities in that yet little-explored region of obscure influences, the soil?

1 The Rape of the Earth. By G. V. JACKS and R. O. WHYTE. (Faber & Faber.)

* Mother Earth. By G. W. ROBINSON.

3 Presidential Address to the Agricultural Section of the British Association; Cambridge, 1938.

4 The Distribution of Vegetation in Relation to Climatic Conditions in the United States of America. By BURTON E. LIVINGSTONE and FOREST SHREVE. Carnegie Institute Publications,

1921.

T the end of the summer term a group of undergraduates started a labour camp a few miles from Oxford, and some account of this experiment may be of general interest. The idea behind the scheme was that a period of training in a communal life of service with hard work on the farms would fit the men for the army, physically and morally, and that this training would stand them in good stead both during and after the war.

It has been the rule that every one going to the camp takes his turn at all the various jobs and each man is in turn agricultural worker, gardener, cook, accountant, orderly, &c., thus by very practical training acquiring a good working knowledge of how to look after himself and others; and the time spent at this camp has been a very real help to many young men now in the army.

The scheme was started as the result of a suggestion by a local landowner, who offered a range of disused farm buildings. There was a considerable amount of work involved in making the buildings habitable, but this was in itself a valuable part of the scheme as all sorts of problems of sanitation, heating, ventilation, &c., were involved and had to be solved with the simplest of resources. The original party was ten undergraduates, but their number rapidly increased, and between twenty and thirty men of every walk in life have been in residence most of the time, and just under one hundred men in all have spent varying periods in the camp. To cover possible periods of idleness

it was originally proposed that every man who could afford it should pay 17s. 6d. per week, but it was soon apparent that the camp would be self-supporting and the accumulated funds were used to pay railway fares for men who could not afford them, and to purchase pigs, hens, tools, &c. to form a small reserve. Work has been done for farmers in the district, hay, harvest, root-hoeing, hedging, threshing, &c., and a good deal of work was done for the neighbouring aerodrome in the way of hedging and clearing bushes. From agricultural work done a matter of £350 has been earned by the Camp in the nineteen weeks ending October 25. From an agricultural point of view the Camp has been an unqualified success, as it has dealt successfully with all the demand for casual and seasonal work in the area, and the farmers express the warmest appreciation of the work done. Most of the men at the camp had no experience of manual work or of life under primitive conditions, and, as the camp is completely isolated and there is no going round to Woolworths for it', great ingenuity has had to be developed in making use of the materials at hand.

The begetters of this scheme were all members of the 'Oxford Group' and the camp has been run as a Group' camp, although not all the men attending have been members of the group. The strong moral background thus provided has given a common spiritual bond which has been of very great value in running the camp and has simplified (Continued on page 90)