Potassium (K) deficiency is now common in sandy soils of south-western Australia and the K requirements of most pasture species grown in rotation with crops in the region are not known. In a glasshouse experiment using a K deficient yellow sand, we compared the K requirements of 5 pasture species now commonly grown in the region: French serradella (Ornithopus sativus Brot.) cv. Cadiz, yellow serradella (Ornithopus compressus Brot.) cv. Santorini, balansa clover [Trifolium balansae, classified as T. michelianum Savi var. balansae (Boiss)] cv. Paradana, 2 subterranean clover (Trifolium subterraneum L.) cultivars, cv. Seaton Park (subspecies subterraneum) and cv. Trikkala (subspecies yanninicum), and annual ryegrass (Lolium rigidum Gaud) cv. Wimmera. After 56 days of growth, above-ground growth was harvested and analysed for total K. Mitscherlich curves were fitted to yield of dried shoots, K concentration and K content (K concentration multiplied by yield) of the shoots. These fitted curves were used to determine the amount of K required to produce 75% of the maximum yield of dried shoots, K required to attain a K concentration in dried shoots of 25 g/kg, and K required to achieve a K content in dried shoots of 250 mg K/pot.

Annual ryegrass and yellow serradella more effectively used indigenous soil K and applied K to produce dried shoots than the other species and cultivars, with the other species and cultivars requiring about 55% more applied K to produce 75% of the maximum shoot yield. The K content in dried shoots estimated the total K taken up from the soil, and annual ryegrass took up least K from soil, but it required least applied K to produce 75% of the maximum shoot yield, indicating it used the K it took up very effectively to produce shoots. Yellow serradella took up most K and, after annual ryegrass, it needed least applied K to produce the target yield of dried shoots. After yellow serradella, subterranean clover cv. Trikkala took up most K from soil, but it required nearly the most applied K to produce the target shoot yield so it was inefficient at using K taken up to produce the shoots. For all the other species and cultivars, K requirement was similar as assessed by yield or K content data. The critical concentration of K required for 90% maximum yield of dried shoots was about (g K/kg) 24 for balansa clover, 23 for both subterranean clover cultivars, 21 for yellow and French serradella, and 15 for annual ryegrass.