Wednesday, November 15, 2006

Soybean Root Growth in Acid Subsoils in Relation to Magnesium Additions and Soil Solution Ionic Composition.

Yohey Hashimoto, T. Jot Smyth, Dean Hesterberg, and Daniel W. Israel. Dept of Soil Science, North Carolina State Univ, Box 7619, Raleigh, NC 27695

Hydroponic studies with soybean [Glycine max (L.) Merr.] have shown that µM additions of Mg2+ were as effective in ameliorating Al rhizotoxicity as additions of Ca2+ in the mM concentration range. The objectives of this study were to assess ameliorative effects of Mg on soybean root growth in acidic subsoils and to relate the soil solution ionic compositions to soybean root growth. Roots of soybean cultivar Plant Introduction 416937 extending from a limed surface soil compartment grew for 28 days into a subsurface compartment containing acid subsoils from the Cecil (oxidic and kaolinitic), Creedmoor (montmorillonitic) and Norfolk (kaolinitic) series. The three Mg treatments consisted of the native equilibrium soil solution concentrations in each soil (50 or 100 μM) and MgCl2 additions to achieve 150 and 300 μM Mg (Mg150, Mg300, respectively) in the soil solutions. Root elongations into Mg-treated subsoils were compared with a CaCO3 treatment limed to achieve a soil pH value of 6. Subsoil root growth responses to the Mg treatments were less than for the lime treatments. Root length relative to the limed treatments for all subsoils (RRL) was poorly related to the activity of soil solution Al species (Al3+ and Al-hydroxyl species) and Mg2+. However, the RRL values were more closely related to the parameters associated with soil solution Ca activity including Ca2+, Al3+/Ca2+, and Al3+/(Ca2+ + Mg2+), suggesting that Ca could be a primary factor ameliorating Al and H+ rhizotoxicity in these subsoils. Increased tolerance to Al rhizotoxicity of soybean by μM Mg additions to hydroponic solutions, inducing citrate secretion from roots to externally complex toxic Al species, may be less important in the acid subsoil with low native Ca levels.