The effect of salinity on growth and nutrition of Protea obtusifolia

Abstract

Adequate information on tolerance to salinity is lacking for P. obtusifolia. Irrigation with chlorinated water, the main salts being NaCl and CaCl2 (normal ratio 2:1), was used to study the effect of Na+ and Cl- concentration on the yield of fresh and dry matter of P. obtusifolia. Five water salinity levels (0.4, 1.5, 2.5, 4.2 and 8.2 dS m-1) were imposed on a mixture of peat and volcanic ash (1:1 in volume), in pots. Soil, roots and foliar nutrients, visual rating of foliar salt lesions, and yield of fresh and dry weight of the plants were determined. Plants were not highly sensitive to Na and Cl. A decrease in the yield of fresh and dry weights from the October sampling onwards was observed at concentrations in leaves above 213.0 mmol per kg of Na and 295.8 mmol per kg of Cl. The lowest production of fresh and dry material observed in the high-salinity treatment coincided with the greatest foliar values for the Na/K ratio, the greatest percentage of Na and the lowest percentage of K and Ca in the sum of cations, as well as the highest Cl values. At the final stage of the experiment, the thresholds, expressed as electrical conductivity of irrigation water (ECi) and saturated soil extract (ECe), for fresh and dry weight were 3.1 and 2.7 dS m-1 (ECi) and 6.1 and 6.0 dS m-1 (ECe), respectively. At that stage, vegetative growth was reduced by 21.2% for fresh weight and by 21.7% for dry weight for each unit of increase in the ECe beyond the threshold values. P. obtusifolia was moderately tolerant to salinity. Various correlations between some soil factors and the studied plant nutrients were tabulated.