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Forested soils are being increasingly transformed to agricultural fields in response to growing demands for food crop. This modification of the land use is known to result in deterioration of soil properties, in particular its fertility. To reduce the impact of the human activities and mitigate their effects on the soil, it is important to understand the factors responsible for the modification of soil properties. In this paper we reviewed the principal processes affecting soil quality during land use changes, focusing in particular on the effect of soil moisture dynamics on soil carbon (C) and nitrogen (N) cycles. Both physical and biological processes, including degradation of litter and humus, and soil moisture evolution at the diurnal and seasonal time scales were considered, highlighting the impact of hydroclimatic variability on nutrient turnover along with the consequences of land use changes from forest to agricultural soil and vice-versa. In order to identify to what extent different models are suitable for long-term predictions of soil turn-over, and to understand whether some simulators are more suited to specific environmental conditions or ecosystems, we enumerated the principal features of the most popular existing models dealing with C and N turnover. Among these models, we considered in detail a mechanistic compartment-based model. To show the capabilities of the model and to demonstrate how it can be used as a predictive tool to forecast the effects of land use changes on C and N dynamics, four different scenarios were studied, intertwining two different climate conditions (with and without seasonality) with two con-trasting soils having physical properties that are representative of forest and agricultural soils. The model incorporates synthetic time series of stochastic precipitation, and therefore soil moisture evolu-tion through time. Our main findings in simulating these scenarios are that 1) forest soils have higher concentrations of C and N than agricultural soils as a result of higher litter decomposition; 2) high frequency changes in water saturations under seasonal climate scenarios are commensurate with C and N concentrations in agricultural soils; and 3) due to their different physical properties, forest soils attenuate the seasonal climate-induced frequency changes in water saturation, with accompanying changes in C and N concentrations. The model was shown to be capable of simulating the long term effects of modified physical properties of agricultural soils, being thus a promising tool to predict future consequences of practices affecting sustainable agriculture, such as tillage (leading to erosion), ploughing, harvesting, irrigation and fertilization, leading to C and N turnover changes and in conse-quence, in terms of agriculture production.

Nutrient management is the key issue in sustainable soil fertility. N, P, K fertilization aims not only for a high economic return of the investment through optimized yield and quality, but also for minimum environmental hazards. The basic concept underlying integrated plant nutrition systems, is the maintenance and possible increase of soil fertility for sustaining enhanced crop productivity through optimal use of all sources of plant nutrients, particularly inorganic fertilizer, in an integrated manner and as appropriate to each specific ecological, social and economic situation. Much research has established the importance of fertilizers in increasing the fertility of soil and in influencing its productivity. It has been observed that applying fertilizers causes many changes in the soil, including chemical changes, that can positively or negatively influence its productiveness. Only a fraction of the fertilizer applied to the soil is taken up by the crop, the rest either remains in the soil or is lost through leaching, physical wash-off, fixation by the soil, or release to the atmosphere through chemical and microbiological processes. The critical information on the relative merits of different fertilization practices such as method of fertilizer placement, time and rate of application and type of fertilizers, is essential. Results from different field and laboratory experiments which helped to achieve maximum efficiency, in the most economical and sustainable way of fertilizer use to reduce the nutrient losses and production costs to the farmers and prevent environmental pollution are presented in the paper.

This paper focuses on pollution of soil by heavy metals. Methods for assessing the risk of increased metal concentrations in soils are discussed in relation to the physico-chemical and biological factors influencing bioavailability. More fundamental understanding is needed before we can predict the likely uptake of metals by plants on any given soil. Few ecotoxicological studies of the effects of metals on soil organisms have been made under realistic conditions and almost none have associated measurements of appropriate soil physico-chemical factors. Extrapolation from such small amounts of toxicological data seems to give critical levels for metals which are too low. Some of the problems with this methodology are discussed and this approach is contrasted with one example of the way arguments for protecting an agriculturally important organism are progressing. The two approaches are shown to be far apart.

Despite a century of research, irrigation water in arid and semi-arid regions is not used as efficiently as it might be. Even in humid regions there is often less than optimum management of soil water, though the manager in such regions has less latitude in that the timing of water applications is not optional. Many inefficiencies are due to economic or social factors, which are beyond the scope of this paper. Of more direct concern here are those inefficiencies which arise from a failure to integrate and combine all the physical and biological factors that effect efficiency into an integrated and quantitative concept.

About 20 years ago the emphasis in soil chemistry research switched from studies of problems related to scarcities of plant nutrients to those arising from soil pollutants. The new problems have come about because of the excessive uses of fertilizers, the inputs from farm and industrial wastes, the widespread applications of anthropogenie xenobiotic chemicals, and the deterioration of soil structure resulting from certain modern agriculture practises. The International Society of Soil Science (ISSS) recognized these problems and challenges. A provisional Working Group was set up in 1978 to focus attention on soil colloids with a view to understanding better the interactions wh ich take place at their surfaces. It was recognized that these interactions are fundamental to problems of soil fertility, as weIl as to those of soil pollution. After the group had received the official support of ISSS at its 12th International Congress in New Delhi in 1982 it set as its priority the assembling and evaluation of information, relevant to the soil and environmental sciences, concerning the composition and structure of soil colloids. Prior to that aseries of Position Papers were published in the Bulletin of the International Society of Soil Science (Vol. 61, 1981) outlining the state of knowledge about the composition and properties of soil colloids.