Site specific management on field level: high and low tech approaches
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Spatial variability of soil conditions and potato growth were studied in a 6 ha farmers field in a Dutch polder. Potato yields, measured in 65 small plots varied between 30 and 45 tons ha-1, while yields of commercially attractive large potatoes varied between 3 and 15 tons ha-1. An experiment in Niger indicated a major effect of spatial variability on growth of millet, with yields from different plots within a 0.3 ha field varying by as much as a factor 3.6, for the same treatment. Such differences are economically significant in both areas. A high-tech system for site-specific management is discussed for Dutch conditions including site specific sampling for soil fertility and use of dynamic simulation modeling to characterize soil water regimes and nutrient fluxes, e.g. of nitrate. A low-tech system for Niger includes field sampling and site specific interpretation of data obtained. In both cases, uniform fertilization rates based on one mixed sample for the entire field, are bound to result in local over- and underfertilization, implying inefficient use of natural resources. Modeling can be used to balance production and environmental aspects of soil fertilization, as was demonstrated for the Dutch study. Data needs of the WAVE model, used for simulation of yields and nitrate fluxes, are discussed including distinction of only four 'functional layers' for the 6 ha field, which define all variability in basic hydraulic characteristics. Fine-tuning of management practices, including fertilization, taking into account natural variability patterns appears to be an attractive and practical procedure to increase the use-efficiency of natural resources. The Niger example illustrates use of possible low-tech procedures involving field experimentation and improved advisory practices.
Key wordsglobal positioning systems modelling nitrate leaching potato growth pedotranfer functions spatial variability soil fertility
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