Irrigation and Crops1

Abstract

THE deleterious effect of irrigation on the soil, and therefore on the crops grown when it is not duly balanoa by drainage, are described simply and clearly in an important memorandum drawn up by Dr. B. A. Keen, of Rothamsted, for the Empire Marketing Board. The memorandum is the work of an experienced soil physicist, and deals in summary form with the chemical and physical reasons for the accumulation of soluble salts on the surface of the ground, usually but somewhat erroneously described as ‘alkali.’ Such concentration can, obviously, only take place where there is great evaporation of soil moisture, but little or no rain to wash the salts away, and the parts of the world where these conditions prevail are somewhat clearly defined. Taking the rainfall map as our guide, it can be seen at a glance that the amount of rain falling in the temperate and tropical regions is generally what may be called ample for crop production. But, round about and just outside the tropics of Cancer and Capricorn, belts of little rain or deserts are met with in the Old World, while in the New such tracts are confined more or less to the western sides of the continents. These conditions are due, in the main, to the distribution of the ocean winds and currents. Clear skies are met with in these tracts, which in the local summer are connoted with intense heat and great evaporation of the soil moisture; whereas within the tropics the amount of cloud is much greater, the retarding force of the sun on vegetation is less, and a greater amount of rain falls.