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DIVISION S-4—SOIL FERTILITY & PLANT NUTRITION

Simulating Infertile Acid Soils with Nutrient Solutions

The Effects on Brachiaria Species

^a Centro Internacional de Agricultura Tropical (CIAT), A.A. 6713 Cali, Colombia
^b Institut für Ökologie und Naturschutz, Universität Wien, Althanstraße 14, A-1090 Wien, Austria
^c Center for the Application of Molecular Biology to International Agriculture (CAMBIA), GPO Box 3200 Canberra, ACT 2601, Australia

^* Corresponding author (peter{at}cambia.org).

Aluminum toxicity limits plant growth in acid soils. Because of their advanced state of weathering, acid soils of the tropics also tend to be deficient in nutrients. A realistic assessment of plant adaptation to these soils would therefore require Al-toxic conditions under which growth is simultaneously limited by nutrient deficiency. We developed and tested a nutrient solution for this purpose. We analyzed soil solutions of two Oxisols from the Colombian savannas. Nutrient concentrations were extremely low (ionic strength <1.7 mM). Nitrification during incubation of soil samples acidified soil solutions, resulting in a release of cations from the exchange phase, an increase in the activity of Al³⁺, and a decrease in that of H₂PO^-₄. Predicted ion activities were taken as guidelines for designing a nutrient solution that simulates these soil solutions. Growth of well-adapted signalgrass (Brachiaria decumbens cv. Basilisk) and less-adapted ruzigrass (Brachiaria ruziziensis cv. Common) in this solution mirrored the interspecific difference in forage yield that had previously been observed in a field close to where one of the soils originated. This suggests that the designed solution may be a realistic approximation to chemical soil properties that limit forage productivity. The different growth response of the two grasses was apparently due to increased Al sensitivity of less-adapted ruzigrass under low nutrient supply; neither Al toxicity nor nutrient deficiency alone resulted in a comparable growth difference between the grasses. These data highlight the importance of taking into account interactions among stress factors that occur in parallel in infertile acid soils.

Abbreviations: AN, nutrient solution with adequate nutrient levels • AN + Al, nutrient solution with adequate nutrient levels containing Al • Al_inorg, inorganic monomeric Al • LA, leaf area • LN, nutrient solution with low nutrient levels • LN + Al, nutrient solution with low nutrient levels containing Al • RDW, root dry weight • RL, total root length • SDW, shoot dry weight • [ ], concentration of an ion or an ion complex • { }, activity of an ion or an ion complex

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