Soil-Yield Characterization 21 Corn yield as an index of soil productivity 21 Effects of soil variables on corn yield 24 Yield Characterization-Statistical Approach 32 MATERIALS AND METHODS 40 General Description 40 Site and Soil Variables 44 Location 45 Slope, configuration,and aspect 46 Erosion class and depth of A horizon 4? Organic carbon 4? Natural internal drainage 49 Soil permeability 52 Soil texture 55 Biosequence 57 Structure 57 Plant available water capacity 57 Bulk density 6l Parent material 62 Soil test variables 66 Yield and Management Variables

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Field studies were conducted in Niger using 15N-labeled fertilizers to assess the fate and efficiency of fertilizer N in pearl millet (Pennisetum glaucum [L.] R.Br.) production. Total plant uptake of fertilizer N was low in all cases (20%-37%), and losses were severe (25%-53%). The majority of N remaining in the soil was found in the 0-to 15-cm layer though some enrichment at lower depths was found when the N fertilizer was calcium ammonium nitrate (CAN). In a comparison of urea placement methods (band, broadcast, or point placement), no significant differences in lSN uptake or yield were noted though point placement did exacerbate 15N loss. The mechanism of N loss is believed to have been ammonia volatilization. Yields were similar whether urea or CAN was used, but 15N uptake from CAN was higher. A statistical model was developed relating millet yield and N response to midseason rainfall. In drought years, no N response was found, whereas in years of good rainfall a response was found of 15 kg grain for each kilogram of N applied (at 30 kg N ha -~ rate).

pounds are water insoluble (Sikora and Giordano, 1995). The European Economic Community, for example, set An increase in the concentrations of water-insoluble iron phosa minimum level of 93% of total available P (water ϩ phate (Fe-P) compounds in acidulated P fertilizers has raised the citrate) as water-soluble P in fully acidulated P fertilizers concern that the agronomic effectiveness of these P fertilizers will marketed in the European community (Anonymous, decrease. This study was conducted to evaluate the agronomic effectiveness of three sources of single superphosphate (SSP) varying in 1975). Several researchers have found that the water-Fe-P impurities for upland and flooded rice (Oryza sativa L.) grown insoluble fractions of acidulated P fertilizers after water for 55 d. A modal chemical analysis and X-ray diffraction were used leaching are lower in agronomic efficiency compared to characterize the SSP samples and their water-leached forms. A with the original fertilizers without leaching or MCP. greenhouse study was conducted where rates of 0, 5, 15, 30, 50, andThe P availability of the isolated water-insoluble frac-100 mg P kg Ϫ1 as total available P (water plus citrate-soluble P) were Int. Fert. Dev. Cent.

Although some mathematical models exist to assist in deciding whether or not to apply phosphate rock (PR) to soils as a source of P, these models focused either on the use of highly soluble PR sources, mainly for pastures, or were too impractical for field use. There was, therefore, a need to develop a Phosphate Rock Decision Support System (PRDSS) with more comprehensive capabilities to predict the relative agronomic effectiveness (RAE) of PR with respect to watersoluble phosphorus (WSP) fertilizers at field and farm level. The PRDSS includes PR sources varying widely in solubility and different crop species grown on variable soil properties and rainfall conditions. In this paper, we describe the development and use of the mathematical model, PRDSS, to estimate RAE of PR sources with respect to WSP fertilizers and to calculate a simple index to be used for preliminary economic comparison of these two types of P sources. The PRDSS is able to function with a minimum input of only soil pH, the name of the mine that is the source of the phosphate rock, and the species of crop to be grown. A more accurate prediction of RAE is obtained from the PRDSS when more detailed data describing the soil, crop, and weather are included as inputs.