The effects of soil pH and related acidity factors on the yields of sweetpotatoes, Miguela var., and soybeans, Jupiter var., were determined on typical Oxisols and Ultisols of Puerto Rico. The study revealed no significant effect of pH and acidity factors on the yield of the sweetpotato variety, which was quite tolerant to high soil acidity and exchangeable Al. Yields were similar to those obtained by other researchers who worked with other varieties. Soybeans, although relatively tolerant to high levels of exchangeable Al, were adversely affected when values surpassed 5.5 meq/100 g of soil in a clayey Ultisol. Exchangeable base content was directly related to soybean yield grown on a light textured Oxisol. Yields obtained are considered excellent. Highly significant correlations between soybean leaf N content and yield was found in the clayey Ultisol.
Further attempts were made to explore the N supplied by legume and nonlegume crop residues through crop rotation experiments on a sandy Oxisol and a clayey Ultisol. In the initial crop, soybean yields were only fair, (1,680 to 1,792 kg/ha). Mungbeans, (1,125 to 2,044 kg/ha), winged beans, (1,456 to 2,800 kg/ha), and corn yields were good, (4,480 to 6,123 kg/ha). In the second crop (corn at both sites), grain yields were striking as a result of fertilizer N, regardless of the previous crop. On the Ultisol, corn tended to yield more following the legumes than following corn, but differences were not statistically significant. About 80% of the maximum corn yield was attained when corn followed the legumes and no fertilizer N was applied, especially in the Ultisol.
An attempt was made to evaluate the N supplied by crop residues through crop rotation experiments on Oxisols and Ultisols. Field experiments were conducted on three typical soils following a split-plot design. Main plots were three rotations: soybeans, corn , corn; fallow, corn, corn; and continuous corn. Subplots included two treatments: 0 and 110 kg/ha of fertilizer N. Fair yields of soybeans were obtained while corn yields were good, especially on the Humatas soil (Ultisol), 6240 kg/ha. Substantially higher yields were obtained in the Bayamón (Oxisol) and Humatas soils from the first corn crop following soybeans or corn than when following fallow. Although the second corn crop yield following soybeans was slightly higher than the first, the second corn crop after initial corn and fallow were substantially higher. The effect of applied N at all sites was striking, regardless of the previous crop. There was no apparent relationship between the amount of N returned to the soil and yields of subsequent corn crops.
The effect of soil acidity factors was determined on yields and foliar composition of corn grown on Ultisols and Oxisols typical of the Humid Tropics. Soil pH values increased from about 3.8 with a base saturation of around 20 percent to about pH 5 with a base saturation of around 70 percent based on cation exchange capacities determined with neutral ammonium acetate. The low pH values in relation to exchangeable base contents are explained by the presence of free salts. The level of aluminum saturation of the soil based on exchange capacities as determined with ammonium acetate decreased from 40-percent at about pH 3.9 to 0 at about pH 5.2. The Ultisols had a high content of exchangeable aluminum when acid but the more weathered Oxisols contained little aluminum. A very close relationship exists between exchangeable base (Ca + Mg) and aluminum values based on total exchange capacities determined either with ammonium acetate at pH 7.0 or by the sum of cations at a given pH permitting conversion of one value to another. Corn responded strongly to liming particularly on the Ultisols which had high exchangeable aluminum content when acid. Calcium content of the corn leaves increased with soil base content and with yields but foliar composition was not otherwise affected by liming. Corn yields increased with pH to about 5.2 at which pH level these soils contained essentially no exchangeable aluminum, with exchangeable base content as determined with ammonium acetate to about the 70-percent saturation level, and with decreasing exchangeable aluminum in the soil to essentially 0. Soil pH, exchangeable base, and exchangeable aluminum content were effective criteria for liming these soils.
The effects of lime applied at 20, 40, and 60 cm depth, and of calcium nitrate applied in the top 20 cm, in terms of yield of two corn hybrids and on soil acidity factors in Humatas clay (a typical upland Ultisol of Puerto Rico) were determined. The first corn crop (Funk's G-795 W) revealed a significant linear relationship between corn yield and soil pH and exchangeable bases of the top 60 cm. The second crop (G-795 W), the dry stover yield of which was used as a criterion to evaluate treatments, did not reveal significant effects of soil acidity factors on yield. The third crop (Pioneer X-306 B hybrid) showed a highly significant quadratic effect of pH and soil acidity factors of the 0-60 cm zone on corn yield. The fourth crop (X-306 B) revealed a highly significant linear effect of pH and soil acidity factors on yield. The application of calcium nitrate resulted in low yields approximating those of the unlimed treatments. The inactivation of ionic aluminum , particu larly at lower soi l depths, with concomitant improvement in root development and moisture uptake is considered the main reason for higher yields.
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