Search

Additions of micromolar concentrations of magnesium (Mg) to hydroponics enhance aluminum (Al) tolerance of soybean by increasing citrate secretion from roots and external complexation of toxic Al species. The objective of this study was to assess the ameliorative effect of Mg additions on soybean root elongation into mineralogically different acid soils. Roots of soybean seedlings grew for 28 days into acid soils treated with three Mg levels in their soil solution (Control, 150 and 300 μM) and lime. Root growth in the acid soils and aboveground dry matter responses to the Mg treatments were less than for the lime treatments. Citrate fate in the acid soils revealed that 66-99% of added citrate was either adsorbed or biodegraded, suggesting that root secreting citrate in the soil abundant with Al and iron (Fe) hydroxides potentially reduces the availability to complex rhizotoxic Al. A calcium (Ca) deficiency may have constrained root growth response to the Mg-treated soils. [ABSTRACT FROM AUTHOR]

Dissolution of phosphate rocks (PR) in soils requires an adequate supply of acid (H+) and the removal of the dissolved products [calcium (Ca2 +) and dihydrogen phosphate (H2PO4-)]. Plant roots may excrete H+ or OH- in quantities that are stoichiometrically equal to excess cation or anion uptake in order to maintain internal electroneutrality. Extrusion of H+ or OH- may affect rhizosphere pH and PR dissolution. Differences in rhizosphere acidity and solubilization of three PRs were compared with triple superphosphate between a grass (Brachiaria decumbens) and a legume (Stylosanthes guianensis) forage species at two pH levels (4.9 and 5.8) in a phosphorus (P)-deficient Ultisol with low Ca content. The experiment was performed in a growth chamber with pots designed to isolate rhizosphere and non-rhizosphere soil. Assessment of P solubility with chemical extractants led to ranking the PRs investigated as either low (Monte Fresco) or high solubility (Riecito and North Carolina). Solubilization of the PRs was influenced by both forage species and mineral composition of the PR. The low solubility PR had a higher content of calcite than the high solubility PRs, which led to increased soil pH values (> 7.0) and exchangeable Ca, and relatively little change in bicarbonate-extractable soil P. Rhizosphere soil pH decreased under Stylosanthes but increased under Brachiaria. The greater ability of Stylosanthes to acidify rhizosphere soil and solubilize PR relative to Brachiaria is attributed to differences between species in net ion uptake. Stylosanthes had an excess cation uptake, defined by a large Ca uptake and its dependence on N2 fixation, which induced a significant H+ extrusion from roots to maintain cell electroneutrality. Brachiaria had an excess of anion uptake, with nitrate (NO3-) comprising 92% of total anion uptake. Nitrate and sulfate (SO42 -) reduction in Brachiaria root cells may have generated a significant amount of cytoplasmic hydroxide (OH-), which could have increased cytoplasmic pH and induced synthesis of organic acids and OH- extrusion from roots. [ABSTRACT FROM AUTHOR]