Residual alkalinity as tracer to estimate the changes induced by forage cultivation in a non-saline irrigated sodic soil

Soil alkalinisation generally constitutes a major threat to irrigated agric ulture in the semi-arid regions of west Africa. The improvement of sodic so ils is generally difficult and expensive. However, a recent study in the Ni ger valley in Niger, reveals that a natural de-alkalinisation is possible u nder natural conditions in a semi-arid climate. Transformation of non-salin e sodic soil into brown steppe soil type was recorded. On the same site. th e cultivation of a Sahelian fodder grass, locally known as 'Burgu' was used on the sodic soil/brown steppe soil transition zone to accelerate this nat ural de-alkafinisation and characterise its mechanisms. The geochemical pro perties of both soil types were monitored before cultivation and 1 year aft er continuous crop cultivation. After cultivation and regular irrigation, t he chemical properties of the former sodic soils were close to those of the surrounding brown steppe soils, which are better suited for agriculture. T his modification of the sodic soil properties can be attributed to (i) the large amount of water supplied during cultivation that induced salt leachin g. This is the main phenomenon responsible for the changes observed; (ii) t he root activity that modified the acid-base equilibrium and consumes alkal inity. The residual alkalinity (RA) concept was used to select chemical tracers of the concentration/ dilution of the soil solution. Here, sodium amount and calcite + fluorite residual alkalinity (RA(calcite+fluorite)) were the most adequate ones. These two tracers decreased proportionally under the influe nce of leaching, but the exchanges between cations and protons changed the RA(calcite+fluorite), without modifying the sodium amount. Their combined u se allowed us to separate and quantify the uptake of the plant from the lea ching in the de-alkalinisation process. This study highlighted that reclama tion of this type of sodic soils is feasible. The use of the RA concept is advisable to design a sustainable management system for irrigated sodic or saline soils. (C) 2001 Elsevier Science B.V. All rights reserved.