Undiagnosed metal phytotoxicity in soils: Measurement of soil pH micro-variability under Manawatu pastures, and assessment of an alternative means of amelioration.


  • P. Bishop
  • B.F. Quin




Although not always recognised as such, the major reason for the recommended application of sufficient lime to achieve a soil pH of 5.5 or more on New Zealand pastures is to ameliorate phytotoxic effects of water soluble cations of aluminium (Al), manganese (Mn) and iron (Fe). However, evidence that increasing numbers of farms have sub-optimum soil pH levels suggests that traditional treatment, viz. infrequent heavy applications of agricultural lime, is not considered cost-effective by many farmers, probably due largely to increasing application costs. Application of the carboxylate copolymer AlpHa® to eliminate phytotoxic levels of Al, Mn and Fe was found to be comparable in effectiveness to typical rates of lime application, but with the advantage of minimal application cost, as the low application rate required (2 L/ha) can be incorporated into fertiliser or (reduced) lime applications. Standard soil testing of commercial farms involves combining 15-20 soil cores from a given paddock to determine the "average" pH. As well as underestimating the mean pH, this method gives no information regarding micro-variability of soil pH in the pasture root zone. Over 25% of the pasture root zone was found to have pH levels 0.3 to 0.6 lower than the true average, the therefore susceptible to metal toxicity. Commercial laboratory testing of individual cores would be uneconomic. The in-field direct measurement of soil (damp) pH using the antimony (Sb) electrode was found to be a very convenient and robust technique for assessing variability. The benefits of more accurate identification of metal toxicity risk coupled with the potential of a more cost-effective method of amelioration are discussed. Keywords: metal toxicities, Al, Mn, Fe toxicity treatment, alternatives to liming, acid soils under pastures, pH variability in pastures, urine patch pH effects, AlpHa®, carboxylate co-polymers