Agriculture is becoming more intensive in central Brazil, but there is little information available on the fertility status of most of the soils. To determine this status a total of 518 topsoil samples were collected from a 600,000‐km2 area in central Brazil. Laboratory characterization included pH, exchangeable Al, cation exchange capacity, nutrient levels (including micronutrients), organic matter, texture, and color. The results, presented as frequency distributions, indicate the following general conditions: a pH of 4.8 to 5.2, an organic matter level of 1.5 to 3.0%, extremely low levels of effective CEC and extractable Ca, Mg, P, and Zn, a high level of Fe, a high degree of Al saturation, and a broad range in textural class. Organic matter was the most important soil fraction in relation to effective CEC in these soils. The data suggest that problems associated with fertilizer inputs needed to bring these soils under more intensive agriculture are generally similar, but may vary in degree.
Dissolved phosphorus (P) in runoff increases with an increase in soil test P, but slopes found for this relationship vary by an order of magnitude. A difference in clay content contributes to this variation. Experiments were conducted on Ultisols of 5 and 32% clay by incorporating poultry (Gallus gallus domesticus) litter and triple superphosphate (TSP) to create a wide range in Mehlich 3‐extractable phosphorus (M3P). Mehlich 3 P increased similarly for the two sources with rate of P applied, but the increase was greater as clay content decreased. Total P in runoff was measured and it was related primarily to the quantity of sediment. Dissolved P (mg L−1) was measured with simulated rainfall and it increased with an increase in M3P (mg dm−3 in the surface 15–20 cm of soil). The slope of dissolved P versus M3P was 0.0040 for the 5% clay soil and 0.0014 for the 32% clay soil. With these relationships, M3P would need to be 250 mg dm−3 at 5% clay and 714 mg dm−3 at 32% clay to result in 1.0 mg L−1 dissolved P in runoff. Dissolved P in samples of runoff collected from natural rainfall with automatic samplers was substantially less than that with simulated rainfall. Thus, there was a marked difference due to method of measurement as well as the difference noted due to day content. Both factors should be considered if attempting to predict the level of dissolved P in runoff from soil test data.
Two studies were conducted in growth chambers to evaluate the effects of temperature on the vegetative and reproductive growth phases ofpeanuts (Arachis hupogaea L., cv. Florigiant). Temperature regimes in the first study ranged from 34/30 to 18/14°C day/night and in the second from 34/30 to 22/ISOC. The experiments were conducted for 61 and 110 days, respectively. In the first study, the plants were kept vegetative by clipping the pegs. Dry weights of leaflets, petioles and stems, leaf area, and number of flowers were measured. The same measurements plus fruit weights were made in the second study. Early growth, as determined by accumulation of top dry weight, was optimum at a weighted mean temperature of 27.5°C and essentially no growth occurred at 15.5°C. When plants were grown at the optimum temperature for four weeks and then subjected to different temperatures, the treatments had much less effect on top weight during subsequent growth and the optimum temperature decreased somewhat relative to that for early growth. Total fruit weight and individual pod weight were greatest at 23.5°C. Also, the rate of increase in pod weight was greatest at 23.5°C. During early growth the optimum temperatures for development of leaf area and for rate of flowering were above 2SOC while in later stages the optimum for rate of flowering decreased to about 26°C.
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