ORIGIN OF DESERTS

New Zealand Herald, Volume LXXVI, Issue 23356, 26 May 1939, Page 7

ORIGIN OF DESERTS

FORMERLY RICH LAND EFFECT OF SOIL DEPLETION LOSS OF HUMUS AND MINERALS BY OUIDIRE Professor N. B. Williams, of - the Timiriasov Academy, Moscow, has propounded an hypothesis that the decay of past civilisations was due to a decline in soil fertility consequent on the destruction qf the crumb structure of the soil when more grassland had to be brought under cultivation. Other investigators of this problem who -have studied existing soil conditions in the now desert areas of Northern Africa, Persia, Arabia and Northern Mongolia, which at one time were evidently very fertile and carried dense populations of people practising intensive agriculture, believe that the soil's fertility was chiefly exhausted through the employment of irrigation in a system which did not return humus and soluble minerals to the surface soil. Constant flooding of the land with water, it is thought, carried the soluble minerals and other essential plant iood to depths where they could not be reached by the roots of the crops then grown, and when finally the humus in the surface soil became exhausted there remained no medium in which bacterial life could survive and continue to convert inorganic minerals into soluble plant foods. When the humus was finally exhausted nothing remained to bind the surface soil together. It then bccanie lifeless dust and sand, and when the former was blown away it left the barren sandy wastes now occupying the bulk of these areas. ; Support is lent to this theory by the similar process which is now taking place in the "dust bowl" of Isorth America, in parts of Australia, and in some hitherto fertile regions of Egypt where injudicious irrigation was carried The lessons to be learned from these past disasters is that the fertility ot the soil can only be maintained '"'"here the supplv of humus is not depleted, and that deep-rootinc leguminous crops should bo alternated with shallowrooting cereals to draw on mineral and other supplies at depth, bringing these to the surface and at the same time renewing the supplies of nitrogen. It is also evident that care must be employed in the use of irrigation water, particularly on soils of a sandy and open nature.