Effects of soil acidity on the growth of sericea lespedeza

Field experiments were conducted in 1992 and 1993 to examine effects of soil acidity on growth and N₂ fixation by “Serala”; sericea lespedeza Lespedeza juncea (L.F.) var. sericea (Mig.)]. Effects of acidified soil on N₂ fixation could not be determined because nodulation was suppressed, apparently by sufficient availability of N. Apparently‐suppressive, mean 1993 levels of KCl‐extractable NH₄ and NO₃ in zero nitrogen (N) control treatments were 20 and 13 mg‐kg^‐1, respectively. In soil acidified with sulfur (S), growth of sericiea lespedeza was significantly reduced (PO.05) when the concentration of water‐extractable Mn exceeded 1.3 mM or calculated Mn²⁺ activity exceeded 0.4 mM. This occurred at pH values of 4.1 to 4.3 depending on S treatment. At a given value of pH, shoot dry weight production was greater in S‐amended soil than in Al₂SO₄‐amended soil. Reduced growth in the latter did not appear to be directly related to higher measured levels of toxic Al but may have been caused by a combination of aluminum (Al), hydrogen (H), manganese (Mn), and phosphorus (P) effects. Lespedeza growth was lowest in nonacidified soil with pH values near 6.0, indicating a preference for acid soils by the variety “Serala.”; The demonstrated tolerance of sericea lespedeza to acid soils make it a valuable reclamation species. However, Mn may inhibit growth in acidic soils when the activity of water‐extractable Mn²⁺ exceeds 0.4 mM, and it may not fix appreciable N₂ unless available soil N is quite low.