Precise fertilization is necessary to ensure grain yield, increase profitability, and reduce environmental pollution. A pre-sowing bioassay for soil tests of N and P availability are usually made to determine the recommended fertilizer applications in the Northern Negev (Amir and Ephrat, 1971; Amir et al., 1994; Bonfil et al., 2001). This bioassay can be used to characterize long term soil fertilizer changes and to test fertilizer efficiency in the short term. Data of residual N and P, available from the Northern Negev farms for the past 24 and 9 years respectively, were used to find patterns in residual pools. Pools show enormous variation, from 2 to 720 kg N per ha, and 1.3 to 87 kg P per ha of available nutrients in the top soil layer. Fallow usually affects the residual quantity, but variation between fields was too high to allow prediction of mineral levels after a fallow without doing the bioassay. Nitrogen content in the soils can be divided into 3 different periods (1981-1989, 1990-1998, and 1999-2005) with each exhibiting a stepwise increase in soil N content. This can be partly explained by new high yielding cultivars growing in fields, which require more nutrients; hence farmers applied more nitrogen, which is partially retained in the soil. The year 1999 was very dry and crops died in most commercial fields, leaving the applied nutrients in the soil for the next season. After the highest level of N applied in 1999, a continuous decline in soil N content was observed. Residual P exhibited a very wide range within each year, but no trend is seen over the years. Therefore it appears that the soil P content is reflecting the variability in soil management methods used by specific farmers. Since 1994 more irrigated wheat fields and fields receiving sewage water, sludge or city composts have been tested. These practices might explain the increase in the extreme residual nutrient levels and suggest that excessive nutrients are accumulating in these soils. The rising residual nutrient levels in the soil should raise environmental as well as agronomic concerns. Many P fertilizers are on the market and are usually applied according their price and application equipment requirement. During the last two years we tested the availability of nine P fertilizers: Superphosphate; Triple-superphosphate; Ammonium polyphosphate; Ortho ammonium phosphate; Phosphoric acid-agr. Grade; Phosphoric acid-conc.; Diluted phosphoric acid; Suspension; and Mono ammonium phosphate, by the bioassay platform. The bioassay mimics a corn field that was fertilized by different fertilizer forms, solid and liquid, natural and acidic, and others. It was found that if soil was not extremely P-deficient the fertilizer form had only minor effect. However, in P-deficient soil that was taken from the non P fertilized continues wheat treatment of the Gilat long term permanents plot experiment (since 1975; experiment factors: irrigation, crop management, 8 N&P fertilization combinations; soil type sandy loam loess – Calcic Xerosol) the Ammonium polyphosphate had the highest efficiency, as shown by higher corn dry matter and NPK accumulation, while other fertilizers differ in their supply ability P to corn during the first month of growth. These results emphasize the importance to adapt the fertilizer form to the crop absorption time scale and soil condition. Consequently, long-term experiment, the bioassay procedure and data can be used to understand long term alteration, as well as for testing several other hypotheses.