Abstract
The objective of this paper is to determine spectral bands that are best suited for characterizing agricultural crop biophysical variables. The data for this study comes from ground-level hyperspectral reflectance measure-ments of cotton, potato, soybeans, corn, and sunflower. Reflectance was measured in 490 discrete narrow bands between 350 and 1,050 nm. Observed crop characteris-tics included wet biomass, leaf area index, plant height, and (for cotton only) yield. Three types of hyperspectral predictors were tested: optimum multiple narrow band reflectance (OMNBR), narrow band normalized differ-ence vegetation index (NDVI) involving all possible two-band combinations of 490 channels, and the soil-adjusted vegetation indices. A critical problem with OMNBR models was that of “over fitting” (i.e., using more spectral channels than experimental samples to obtain a highly maximum R2 value). This problem was addressed by comparing the R2 values of crop variables with the R2 values computed for random data of a large sample size. The combinations of two to four narrow bands in OMNBR models explained most (64% to 92%) of the variability in crop biophysical variables.
The second part of the paper describes a rigorous search procedure to identify the best narrow band NDVI predictors of crop biophysical variables. Special narrow band lambda (λ1) versus lambda (λ2) plots of R2 values illustrate the most effective wavelength combinations (λ1 and λ2) and band-widths (Δλ1) and Δλ2) for predicting the biophysical quantities of eachp. The best of these two-band indices were further tested to see if soil adjustment or nonlinear fitting could improve their predictive accuracy. The best of the narrow band NDVI models explained 64% to 88% variability in different crop biophysical variables. A strong relationship with crop characteristics is located in specific narrow bands in the longer wavelength portion of the red (650 nm to 700 nm), with secondary clusters in the shorter wavelength portion of green (500 nm to 550 nm), in one particular section of the near-infrared (900 nm to 940 nm), and in the moisture sensitive near-infrared (centered at 982 nm). This study recommends a 12 narrow band sensor, in the 350 nm to 1,050 nm range of the spectrum, for optimum estimation of agricultural crop biophysical information.

Abstract
The arid zones of the world are all characterized by a large deficit of rainfall in relation to the potential evapotranspiration. Their distribution around the world is governed by the interaction of global atmospheric circulation patterns, the distribution of land and sea, and local topography. Countries that have substantial areas with arid conditions belong to very different groups in terms of resource availability, agricultural productivity, population density and wealth. Most are poorly endowed with good agricultural land. Water resources vary tremendously, both in terms of reserves and consumption. Agriculture is in most cases the main consumer of water. Most arid zone countries have high population growth, and rural population densities are generally much higher than overall population densities. They have a wide disparity in wealth. The arid zones have a surprising diversity of agroecological niches, with edaphic conditions that can deviate substantially from those of surrounding areas. These niches often have a higher biomass or agricultural productivity. At the same time, they are vulnerable to natural processes such as primary salinization, wind and water erosion. Increasing pressure of human and livestock population make that these natural environmental stresses lead to accelerated degradation and depletion of soil and water resources. As they exploit the various agroecological niches, the production systems of the arid zones are equally diverse, and cover the full spectrum of land use intensification from pastoral or transhumant livestock systems to rainfed or irrigated cropping systems.
These production systems show rapid change under the pressures of environmental degradation, increasing land and water shortage, and the needs of expanding populations. With the exception of irrigation management, the agrometeorological needs of the arid zones have been insufficiently addressed in the past. Perceptions of homogeneity, low agricultural potential, low population density and the logistical problems of providing maintenance and collecting data from remote stations are largely to blame for this situation.
Agrometeorological research can have a positive impact on the productivity, resource-efficiency and environmental sustainability of the arid zones by supporting a better characterization of the agricultural environments. Research targeted towards data spatialization and integration of meteorological and remote sensing information will help to alleviate the handicap of sparse meteorological data networks. At the same time these networks will need to be improved through installation of automatic stations and by establishing new partnerships with land users.

Abstract
Whereas previously farmers have had the option of abandoning their land temporarily or permanently, increasing population pressure now dictates that farmers must manage soil acidity in order to maintain productivity and their livelihood. Here we examine the management options available to farmers, from the wealthy to the subsistence farmer and identify those that are effective for the different types of farmers in different environments. We also consider the strategies that apply when governments and other land users intervene in land use matters. The paper draws partly on the authors’ experience and on the wealth of previously published information from around the world. It endeavours to steer a course between fundamental soil/crop science and socio-economic considerations, although it cannot be divorced from either. Techniques considered range from those that, to a degree, avoid the problem (alternative land use, alternative crop plants), to those which treat the problem (correcting the soil via inputs of organic materials, lime, mineral fertilisers, etc.). Techniques that aim at modifying the plant or the soil should rely on correct diagnosis of the major limitations since yields on different acid soils are low for different reasons. Converting such techniques into successful strategies must consider external factors. That is, alternative plants must depend on availability of markets; soil inputs must depend either on a local source of the inputs and/or sufficient cash flow in order that they be feasible. Other feasibility issues include whether subsoil acidity can be managed, whether the level of management required is compatible with the farming system practiced, or whether the acidity management is short-term or sustainable over a longer period. We conclude that it is technically feasible to make acid soils productive. However, successful implementation depends on integration of technical factors with social and economic factors, that is, on the development of suitable strategies.

Acid soils cover more than a third of Subsaharan Africa. Due to their poor fertility, aluminium toxicity and fragile structure, their agricultural productivity is low and declines rapidly under cultivation. The sustainable use of these soils requires adequate inputs of nutrients and amendments in the form of fertilizers, organic matter and sometimes lime. In view of population pressure the gradual intensification of the cultivation of acid soils, often the last reserve of underutilized land, is essential for Africa’s economic growth. Sustainable increases in productivity can only be achieved by a gradual transformation of traditional farming systems through development pathways that take full account of socioeconomic and agroecological diversity.