ASSESSMENT OF ERRORS ASSOCIATED WITH PLOT SIZE AND LATERAL MOVEMENT OF NITROGEN-15 WHEN STUDYING FERTILIZER RECOVERY UNDER FIELD CONDITIONS1

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Abstract
The high cost of 15N-labeled fertilizers encourages the use of field plots having minimum size. If plot size is reduced too much, however, lateral movement of N near the plots by mass flow or diffusion within the soil or by translocation through plant roots can become a significant source of error in determinations of fertilizer N recovery. This study was initiated to assess the importance of lateral movement of labeled fertilizer when unconfined plots are used to determine recovery of fertilizer. Corn grain samples were collected at various positions inside and outside 15N plots, and the 15N contents of these samples were determined. The data were fit to mathematical models to estimate the extent to which lateral movement of fertilizer N caused errors in determined values of fertilizer recovery for the first, second, and third crops following fertilization. These models also were used to predict the plot size needed for similar 15N-tracer studies in the future. The results of these studies indicate that 15N plots having a size of 2 by 2 m are sufficiently large for determining recovery of fertilizer N for corn crops under most conditions. Where lateral movement of fertilizer N in soils is suspected to be a problem, we recommend collection of a few plant samples outside the 15N plots as insurance against misleading conclusions concerning fertilizer N recovery.