Nutrition of irrigated crops on an alkaline brown clay soil at Trangie, New South Wales. 1. Lucerne

Abstract

The elements nitrogen, phosphorus, potassium, sulfur and zinc were sequentially omitted from a 'complete' fertiliser applied to plots on an alkaline soil, and lucerne (Medicago sativa) was sown immediately afterwards. The dry matter production of lucerne was measured in repeated cuts over 2 years; its mineral composition was determined periodically. In a second experiment 4 rates of phosphorus were applied to a l -year-old stand of lucerne and dry matter production recorded for 1 year. Lucerne yields in the first year were reduced from 17-1 8 t/ha to less than 14 t/ha by omitting phosphorus. Yields universally decreased in the second year as the 50 kg/ha P applied at sowing was depleted through the removal of 40 kg/ha P in forage. Following this depletion a linear yield response up to 80 kg/ha P was found (experiment 2). Omitting phosphorus fertiliser reduced the plant phosphorus concentration from 0.23 to 0.21% when sampled 15 months after sowing. The plant phosphorus concentration decreased with time in all treatments. The available soil phosphorus level decreased from an initial 12 to 6-7 ¦g/g after 6 months and further to 2 ¦g/g after 30 months. Fertiliser phosphorus raised the soil phosphorus level but this also was depleted to 2-3 ¦g/g in 30 months. Omitting zinc reduced the plant zinc concentration. However, neither the omission of nitrogen, potassium, sulfur nor zinc from the fertiliser had any effect on lucerne yields.