P.R. SPIES¹, N.J. BRANDON², R.A. DATE², L.M. BAHNISCH¹ and D. GEORGE¹

¹Department of Plant Production, University of Queensland, Gatton, Queensland, Australia
²CSIRO Tropical Agriculture, Cunningham Laboratory, St Lucia, Queensland, Australia

Abstract

The potential for the nutrients P, S, Zn and Mo to limit growth of Desmanthus virgatus in clay soils from Queensland was investigated using nutrient-omission experiments in glasshouses located at Gatton (5 soils) and St Lucia (2 soils). Six of the soils were black earths and were neutral to alkaline (pH 7–8). The seventh soil was a euchrozem and was slightly acid (pH 6). The effects of nutrient deficiencies were determined by measuring top growth 47 and 103 days after planting in the Gatton trial and 73 days after planting in the St Lucia trial, as well as by the observation of deficiency symptoms.
In the first harvest of the Gatton trial, omission of P or Zn significantly (P < 0.05) reduced top dry weight of plants in 3 of the soils by 32–66% relative to plants fertilised with all nutrients. Responses to these nutrients decreased, and were generally not significant, at the second harvest. This was in contrast to the effect of S omission, which increased with time and was significant in 4 of the 5 soils at the second harvest, reducing top growth by 24–75% and drastically reducing pod production in 3 of the soils. A small reduction in growth in one soil also occurred due to the omission of Mo.
In the St Lucia trial, omission of Mo and Zn, as well as S and P, caused significant reductions in plant growth in a slightly acid euchrozem soil. Low pH appeared to reduce the availability of some nutrients in this soil and the application of lime increased growth in both the presence and the absence of added nutrients by 20% and 120%, respectively.
Soil and tissue S concentrations were useful indicators of S deficiency. The greatest effect of S deficiency occurred in soils with KCl-extractable S < 4 mg/kg. Concentrations of P in soil and tissue and of Mo in tissue were less useful in predicting deficiencies of these nutrients. The extent of these nutrient deficiencies needs to be verified by field experimentation.