Plant and Soil 205: 155–162, 1998. © 1998 Kluwer Academic Publishers. Printed in the Netherlands. 155 Phosphorus supply and the growth of frequently defoliated white clover (Trifolium repens L.) in dry soil Dhananjay K. Singh 1,2 and Peter W. G. Sale 1 1 Department of Agricultural Sciences, La Trobe University, Bundoora, Victoria 3083, Australia and 2 Frank Wise Institute, Kununurra, Western Australia 6743, Australia Received 29 July 1997. Accepted in revised form 7 September 1998 Key words: defoliation frequency, phosphorus deficiency, soil water stress, water and P uptake, white clover growth Abstract A previous study found that increased phosphorus (P) supply to frequently defoliated white clover plants, growing in a low-P, dry soil, alleviated water stress symptoms and increased plant recovery on rewatering. In this study we determined how these stresses influence white clover growth. Measurements were made of the leaf canopy, stolon infrastructure and root system of the white clover plants growing in a low-P soil. Treatments included the factorial combination of four levels of P supply, two defoliation frequencies and two soil water treatments. White clover growth declined markedly when P-deficient plants were exposed to frequent defoliation and dry soil conditions. Leaf area was more affected than other parameters, in that the combination of stresses reduced leaf area to 2% of maximum observed for infrequently defoliated plants growing in high-P soil, with adequate water. Increased P supply generally increased the growth of all plant parts. Frequently defoliated plants growing in dry soil produced similar or greater leaf mass and leaf area as plants from similar treatments growing in wet soil, when the P supply increased to 50 mg P kg −1 soil. Higher P rates were able to negate the effect of dry soil on these frequently defoliated plants, as a result of larger water and P uptake. Also, the frequently defoliated plants with restricted root growth did not respond to a small increase in P supply (17 mg P kg −1 soil) for the leaf growth, irrespective of whether they were growing in wet or dry soil. Infrequently defoliated plants with greater root growth, compared to frequently defoliated plants, more than doubled their leaf mass with this P treatment. Introduction Even in the higher rainfall regions of south east Aus- tralia, it is not uncommon for pastures to be subjected to dry soil conditions. The restriction in forage supply during these periods can result in frequent grazing by livestock. Recent findings suggest that the persistence of white clover (Trifolium repens) under these condi- tions can be improved by increasing P supply to the plants. For example, Singh et al. (1997) found that fre- quently defoliated, low-P plants in dry soil displayed severe water stress symptoms, and many plants died following the desiccation and senescence of most of the leaves. On the other hand, high-P plants not only survived and recovered well on rewatering, but they ∗ FAX No: 3 9471 0224. E-mail: agrds@lube.latrobe.edu.au were also better able to maintain a higher leaf water potential and extract more water from a drier soil. Increased P supply was found to maintain shoot growth of white clover under frequent defoliation regimes, although overall shoot growth was reduced by frequent defoliation (Caradus and Snaydon, 1988; Singh and Sale, 1997). The higher P supply resulted in increased numbers of smaller leaves and an increase in stolon branching in these plants (Singh and Sale, 1997). Other studies show that an increased P sup- ply improves clover growth in dry soil (Simpson and Pinkerton, 1989 for subterranean clover; Dodd et al., 1992 for white clover). However, the effect of P sup- ply on the growth of white clover that is subjected to both frequent defoliation and dry soil conditions to- gether has not been studied. The question is whether increased P supply can negate the effect of dry soil on the plant growth, given that a high-P supply alleviated