FERTILIZATION OF ARRACACHA IN AN ULTISOL
In a sequence of ten crops on a Torres clay, an Ultisol, variable responses to fertilizer P were obtained. Rice and soybeans did not respond to fertilizer P. In three corn crops out of six, and in two field bean crops, there was response to banded fertilizer P but not to broadcast residual P. In the first corn crop, near maximum yields were obtained with 90 kgjha of banded P and with 359 kg/ha of broadcast P. The residual effect of large applications of broadcast P on successive crops was almost nil. In field experiments, soil tests indicated a sharp P drop in the broadcast P treatments after three years, perhaps due to P fixation. Organic P mineralization appears to be rather important in the highly weathered soils of the humid tropics. Apparently, P is mineralized more rapidly during. the summer months.
An experiment was conducted to determine if the P fertilizer response of corn planted in sunken bottomless drums was comparable to that in the field. Subsoil and surface of an Ultisol (Orthoxic Palehumults) soil low in available P were placed in sunken drums and planted to corn with and without added P. P response experiments also were carried out with the same soil in the field and in pots in the greenhouse. There was a significant yield and P uptake response to added P in the drum and pot experiments but not in the field, indicating that neither sunken drum experiments nor greenhouse pot tests substitute for field tests in determining response to P by corn.
An experiment with soybeans, Glycine max (L .) Merrill, was planted at Cidra, Puerto Rico, on May 23, 1973, on a Torres clay, an Ultisol (Orthoxic Palehumults, clayey, mixed, isohyperthermic). The experiment was one of a series designed to study the relative efficiency of banded vs. broadcast P and of its residual effects on various successive crops. This was the fourth in the series after two corn and one rice experiments cropped in the same field in the seasons immediately preceding its installation. The identity of the plots was maintained throughout the whole series. There was no grain response to the broadcast or banded P, whether limed or unlimed, which had been applied to the previous crops, although soil analyses by both the Olsen and the Bray No.2 method indicated deficiency of available P. However, a field-wide grain average of 3698 kg/ha, equivalent to 55 bu/acre, was obtained. The Bray No.2 extractable P content had fallen to less than 20 p/m 2 years after applying 359 kg/ha or less fertilizer P. This yield is almost twice that of the average commercial 1973 U.S. yield and demonstrates the potential for growing soybeans on Ultisols of the humid tropics.
An 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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