The agronomic potential of 11 South American phosphate rocks was estimated by four laboratory methods (neutral ammonium citrate with two extractions, acid ammonium citrate [pH 3], 2% citric acid, and 2% formic acid) and by crop response data obtained from a greenhouse experiment with guinea grass (Panicum maximum Jacq.) on an acid (pH 4.9) Tropeptic Haplustox (Las Gaviotas) from Colombia. Standards for comparison included triple superphosphate (TSP) and phosphate rocks from the USA (North Carolina, central Florida, and Tennessee), Tunisia (Gafsa), and Israel (Arad). The relationship between laboratory reactivity values and crop response was evaluated following three cuttings of guinea grass at four rates of P application. Almost no yield was obtained without added P, and significant differences were observed between response curves of the P sources. Based on both solubility as measured by the four methods identified and crop response data, the 11 phosphate rocks were found to be segregated into four groups representing high (Bayovar), medium (Huila and Pesca), low (Sardinata, Patos de Minas, Lobatera, Araxá, and Abaeté), and very low (Catalão, Jacupiranga, and Tapira) reactivity. Solubility in neutral NH+4 citrate was found to be the most accurate of the chemical procedures tested. The relationship between yield and source solubility in neutral NH+4 citate showed that all of the sedimentary sources were more reactive than the igneous or metamorphic sources.

Latin America is the center of domestication and diversity of maize, the second most cultivated crop worldwide. In this region, maize landraces are fundamental for food security, livelihoods, and culture. Nevertheless, genetic erosion (i.e., the loss of genetic diversity and variation in a crop) threatens the continued cultivation and in situ conservation of landrace diversity that is crucial to climate change adaptation and diverse uses of maize. We provide an overview of maize diversity in Latin America before discussing factors associated with persistence of large in situ maize diversity, causes for maize landrace abandonment by farmers, and strategies to enhance the cultivation of landraces. Among other factors, maize diversity is linked with: (1) small-holder farming, (2) the production of traditional food products, (3) traditional cropping systems, (4) cultivation in marginal areas, and (5) retention of control over the production system by the farmers. On the other hand, genetic erosion is associated with substitution of landraces with hybrid varieties or cash crops, and partial (off-farm labor) or complete migration to urban areas. Continued cultivation, and therefore on-farm conservation of genetic diversity held in maize landraces, can be encouraged by creating or strengthening market opportunities that make the cultivation of landraces and open pollinated varieties (OPVs) more profitable for farmers, supporting breeding programs that prioritize improvement of landraces and their special traits, and increasing the access to quality germplasm of landraces and landrace-derived OPVs.

Evaluating maize (Zea mays L.) in the field for tolerance to soil acidity is expensive and provides variable results. To identify alternative screening techniques for maize, the present study adapted a pot bioassay already used for other crops. Two experiments were conducted. One included a set of diallel crosses from eight parents and the other a set of 10 open‐pollinated cultivars. These materials were evaluated under field conditions, in nutrient solution, and in pots containing 2 kg of soil. The best results were obtained after growing plants for 14 d in pots containing soils having intermediate stress (45–65% AI saturation). Fresh root weight, total length (or lateral root length), and visual scoring provided the best separation between tolerant and susceptible genotypes, lacked a significant genotype ✕ soil interaction, and were efficient, simple, and rapid measurements. Significant phenotypic correlations (≈ 0.55) were observed between these variables and yield in the field. While efficiently distinguishing tolerant from susceptible genotypes, the pot bioassay could not clearly detect differences in levels of tolerance. This technique should be useful mainly in the early stages of maize breeding. Results suggest that maize possesses several mechanisms for tolerance to soil acidity.

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The effects of lime (applied in the field) on the amounts of total and isotopically-exchangeable phosphate adsorbed from solutions were measured in five soils. The total amount of phosphate adsorbed without lime was in the range 200 to 1700 pg P per g of soil at 0.05 pg P cm-3 of solution. Lime diminished the amount of phosphate adsorbed at all concentrations of solution in an oxisol and a dystropept; in an ultisol and another dystropept, lime tended to increase sorption at small concentrations and diminish it at large concentrations; in a dystrandept that contained spheroidal allophane and a great deal of organic matter, lime increased adsorption at all concentrations up to I pg P ~n -~. Lime increased the proportion of added phosphate that was isotopically exchangeable in the oxisol and one dystropept, had no effect in the other dystropept, and diminished the proportion in the ultisol and dystrandept.Adsorbed phosphate was subsequently desorbed by suspending the soils in solutions without phosphate. After desorption the quantity of exchangeable phosphate in all soils was closely correlated with aluminium extracted by ammonium oxalate; buffer power was correlated in all except the dystrandept, in which it was larger per unit of aluminium than in the other soils; possibly the cause was aluminium associated with organic matter. In all soils lime diminished buffer power allowing a specific amount of exchangeable phosphate to maintain a larger concentration in solution. The beneficial effects of lime on exchangeable phosphate after desorption were consistent among soils, despite inconsistent results when the phosphate was adsorbed.