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This chapter discusses some of the testing methods for residual inorganic nitrogen (N) and how the test results are used to aid growers in N fertilization. N-availability indexes are a measure of the potential of a soil to supply N to plants when conditions are ideal for mineralization. Residual inorganic N soil tests are most appropriate shortly before planting to early in the growing season. The number of soil samples that need to be taken per field, time of sampling, and how the samples are handled are all important factors in a successful soil testing program. Knowledge of the amount of N removed by a crop is necessary for the interpretation of a N soil test. When 20 or more cores are compo sited for separate samples of the surface and subsoil of relatively uniform fields in monoculture, then tests results, with time, can become a useful guide to determining N fertilizer rates.