AbstractAn experiment with Irish potatoes was started at Cidra, Puerto Rico, March 29, 1976, in a Torres clay, an Ultisol (Orthoxic Palehumults, clayey, mixed isohyperthermic), with the purpose of studying the effect of N levels and time of N application on tuber yields. The pooled tuber yields of two cultivars, i.e., Chieftain and Kennebec, showed that there was a strong response to N when increased from 0 to 56 kg/ha, all at planting. Tuber yield was drastically reduced when N at the highest level was applied all at planting. The highest yield of potatoes was obtained with one application of 56 kg/ha. There appears to be a better utilization of N when split in two applications, but only at high levels. A mean yield increase of 23% over the preplant treatments was obtained when N was split; however, at the 56 kg N/ha level no benefit resulted from splitting applications. When yield of cultivar Kennebec was considered by itself, again depressed yields were observed with the highest N level applied all at planting. In contrast, no beneficial effect was observed when N was increased from 0 of 56 kg/ha, as was the case when yield of the two cultivars was pooled together. There was no benefit by splitting 56 kg N/ha in two applications, although the highest yield was recorded with this treatment. A mean yield increase of 19% over the preplant treatments was evidenced.
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