Red Bay loamy fine sand and Everglades muck were sterilized with gamma radiation, steam, and methyl bromide. Exchangeable NH4+; extractable N, P, and S; moisture equivalent; cation‐exchange capacity; and organic matter were measured. None of the sterilization treatments significantly altered cation‐exchange capacity. Steam significantly increased the amounts of N, P, and S extracted and the organic matter content values, compared to the check and the other two soil treatments; it also significantly decreased the moisture equivalent percentage. Gamma ray and methyl bromide sterilization generally increased the release of N, P, and S above the check. There was also a strong indication, based on the N data, that sterilization by gamma rays disrupted the soil organic materials more than methyl bromide. Finally, the intensity of all these factors is most closely associated with the organic matter content of the soil. Therefore, a sand, for example, would not be altered nearly as much as a loam or an organic soil.

The effects of several levels of gamma radiation ranging from 0 to 2,048 kiloroentgens on the availability of N and P from three fine sands and one peat soil were investigated. Nutrient release was measured chemically and by plant uptake. Irradiation increased the availability of N and P in soils evaluated immediately following irradiation, and after 2 weeks of incubation. This was shown by both chemical and plant uptake measurements. The release of N and P was closely related to soil type and more particularly to its organic matter content. In each case the soil containing the largest amount of organic matter showed the largest increase in extractable N and P after irradiation. An evaluation of the ammonium acetate (pH 4.8) extractable Ca, Mg, and K data indicated that there were no measurable changes in the availability of these elements that could be directly related to irradiation of the soil. The release of N and P, in many soils, will be a significant nutritional factor when plants are grown on irradiated soil.

Arredondo fine sand was exposed to gamma radiation from a cobalt 60 source at doses of 1, 4, 16, 32, 64, 256, 512, 1024, and 2048 kiloroentgens (kr.). Percentage survival of fungi and bacteria decreased • wit h each increase in radiation dose to < 4% at 1024 kr. Algae were not as drastically reduced as bacteria and fungi: some survived at 2048 kr. and greatest reduction occurred above 64 kr. A few nematodes remained in the soil 2 days after irradiation with 1024 kr., but none were recoverable from doses > 256 kr. after 14 and 28 days. The effects of lower doses were not resolvable.Carbon dioxide evolution was initially reduced by ir radiation of the soil. The reduction was generally inversely related to radiation dose except for the 64kr. treatment. Later, all irradiated soils, except those which received doses > 256 kr., essentially equaled or exceeded the con trol in CO 2 production. Soils treated with 512 and 1024 kr. produced only very small amounts of CO 2 throughout the 23day period; soil receiving 256 kr. later recovered completely. The soil irradiated by 64 kr. attained the high est daily production rate. Similar relationships for CO 2 evolution occurred in an earlier study except for some shifts in time and magnitude.Nitrate production in a 14day period was progressively reduced by increasing doses of radiation. Recovery from 32 kr. or less occurred in 28 days; these soils produced at least 87% as much nitrate as the control during this period of time. Average production in 28 days at 64, 256, and 1024 kr. was 55, 7, and 3% of the control, respectively.The production of sulfate from elemental sulfur was also affected by radiation in much the same manner as nitrate production, though radiation damage at doses less than 64 kr. were not readily apparent until the third week. Two populations, one more sensitive to radiation than the other, were probably involved in this reaction.

No abstract

In many tropical soils, excessive weathering of primary minerals confounded by intense agricultural production has resulted in the depletion of organic matter and plant available forms of phosphorus (P). Long-term growth of cover crops in tropical agroforestry systems have been shown to influence nutrient cycling, and soil organic matter pools. The objective of this experiment was to assess the affect of 2 years of cover-crop cultivation on organic matter accumulation and P bioavailability using Mehlich I and sequential fractionation methods. The experiment included six treatments in the understory of a cacao-plantain agroforestry system adjacent to lower montane tropical forests of the San Martin district of Eastern Peru. Cacao and plantain formed the primary canopy on otherwise abandoned agricultural land. The treatments consisted of four perennial leguminous cover crops (Arachis pintoi, Calopogonium mucunoides, Canavalia ensiformis, and Centrosema macrocarpum), a non-legume cover crop (Callisia repens), and a control treatment (no cover crop). After only 2 years of cultivation, results suggest that all cover crop species accessed residual P pools in 0-5 cm soil depths as indicated by a decrease in the 0.5 M HCl extractable P pools when compared to control. Additional use of residual P pools by A. pintoi and C. macrocarpum were indicated by significant reduction in the 6.0 M HCl extractable P pool. Relative to control, there was no treatment effect on soil organic matter content; however significant differences occurred between treatments. The C. ensiformis, C. mucunoides and C. repens treatments in 5-15 cm soil depths contained significantly more organic matter than the A. pintoi treatment. In 15-30 cm soil depths the C. ensiformis treatment contains significantly more organic matter than the A. pintoi treatment. Continued research should focus on monitoring the long-term effects of cover crop cultivation on the bioavailability of soil P pools in surface soil horizons, development of organic matter pools and the productivity of the agroforestry species.