Residual Effects of Labeled N in Field Trials
1980
Broadbent, F. E.
Field trials utilizing ¹⁵N-depleted fertilizer were conducted over a 6-year period at two locations for the purpose of providing as complete an accounting as possible of N in the soil-plant-water system under conditions of irrigated agriculture. These trials permitted measurement of residual effects while continuing a constant rate of N application, and also after applications were discontinued. On Yolo fine sandy loam (Typic xerorthent) residual value of the previous year's fertilizer varied from negligible to 23% of the total crop uptake depending on N rate and amount of irrigation water applied. These values were equivalent to 5 to 91 kg N/ha of a current year's N application. In Hanford sandy loam (Typic xerorthent) 51 to 60% of the N in a corn (Zea mays L.) crop was labeled N in the year following discontinuance of N application and about 32% in the second year. Most of the residual N subsequently utilized by a crop was in the inorganic form. There was little evidence of remineralization of labeled organic N. At fertilizer rates that were adequate, but not excessive, for maximum grain yield coupled with adequate irrigation water the residual N was equivalent to 3 to 24 kg/ha. It is concluded that with efficient management which avoids overfertilization, the value of residual N is not sufficient to affect application rates. Substantial quantities of residual N reflected overfertilization and/or insufficient irrigation water in previous years.
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