Technology has been developed which permits continuous production of annual crops in some of the acid, infertile soils of the Amazon Basin. Studies in Yurimaguas, Peru, show that three grain crops can be produced annually with appropriate fertilizer inputs. Twenty-one crops have been harvested during the past 8(1/2) years in the same field, with an average annual production of 7.8 tons of grain per hectare. Soil properties are improving with continuous cultivation. The technology has been validated by local farmers, who normally practice shifting cultivation. Economic interpretations indicate large increases in annual family farm income and a high return on the investment of chemical inputs. Other promising land use alternatives include low-input crop production systems, paddy rice production in fertile alluvial soils, and pastures or agroforestry in rolling areas. Stable, continuous food crop production is an attractive alternative to shifting cultivation in humid tropical regions experiencing severe demographic pressures. For each hectare of land managed in a highly productive manner, there may be less need for clearing additional tropical forests to meet food demands.

A method is proposed for developing fertilizer recommendations based upon a statistical estimate of the amount of “plant‐available” nutrient in the soil (d), and determining the appropriate mathematical form of the relationship of this estimate to soil test values (T). A function of soil test is then substituted into the fertilizer response model for d. Some comparisons of the effects of three different models (Mitscherlich, quadratic and square root) upon d estimates and on the form of their relationships to soil test values were made. Regressions of d estimates obtained using the three models upon soil test P values were carried out to determine empirically the appropriate mathematical form [called f(T)] for a set of Irish potato data from Maine and North Carolina. The f(T) was found to be linear for that particular set of data for all three models, i.e., d = b0 + b1T. From these data, it was concluded that environmental conditions between states did not affect f(T) but did affect the level of maximum response (A, in the Mitscherlich model) and a factor related to curve shape (c, in the Mitscherlich model). Once f(T) is known for a model, it may be substituted into the model for d and this equation then used for fertilizer recommendations based upon site‐specific soil test information. Equations for estimating optimal P fertilizer rates assuming different cost‐price ratios (p) and marginal rates of return (R) were given for this data set. Similar equations may be derived for other specific crop‐soil conditions using the techniques described in this paper.

Although soils vary considerably in their P sorption characteristics, this factor is often not considered in a soil test interpretation. One soil property closely related to P sorption is clay content. Residual P studies were conducted for 4 yr on three tropical soils of similar clay mineralogy, two Oxisols of 63 and 27% clay, and a Quartzipsamment of 12% clay. The Mehlich 1 (1:10) extractable P level was described as a function of time, initial soil P level, and P fertilization rate. One crop of soybeans [Glycine max (L.) Merr.] was grown each year and the yield related to extractable P, which, in turn, was related to the initial P soil level and P fertilization rate for a period of 1 yr. Based on a soybean price of $0.23/kg and a fertilizer P price of $1.23/kg, rates of P were calculated for each of the three soils that would maximize net returns for various initial levels of extractable P. As these rates differed markedly with clay content, a soil test interpretation was created by multiple regression based upon both Mehlich 1 extractable P concentration and clay percentage to predict a recommended rate of fertilizer phosphorus (R) as follows: R = 80 − 2.57(soil P) + 0.01386(clay)2 − 0.003281(soil P)(clay2). This function should be applicable to many soils with similar clay characteristics.

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Limited literature is available to provide recommendations of K source and rate and P rate for sweet potato [Ipomoea batatas (L.) Lam.] production. Many growers, therefore, continue to use the more expensive sulfate (SO4) source of K rather than chloride (Cl) and higher than recommended rates of K and P. Accordingly, on‐farm experiments were conducted during 3 years on North Carolina Paleudults to determine effects of K source and rate and Prate on sweet potato yield, grade, and quality. Potassium sources were KCl and K2S04. Fertilization rates of K varied over the five K experiments and depended on initial soil test levels which ranged from 0.04 to 0.12 cmol K L−1 by Mehlich‐I extractant. In the three P experiments, the various P rates were also dependent upon the initial soil test levels which ranged from 11 to 30 mg P L−1 also by Mehlich‐I extractant. As K source had no effect on yield, grade, or quality, it was concluded that the higher Cl concentrations, up to 22.8 g kg−1, in vegetative tissue with increasing KCl rates had no detrimental effect on sweet potato yield, grade, or quality. Total yield response to K applications was obtained where soil test K levels were ≤ 0.08 cmol L−1, although no. 1 yields increased only where soil test K levels were ≤ 0.05 cmol L−1. Phosphorus applications had no effect on yield, grade, or quality of sweet potato.