Potassium responses in winter crops and pastures

In collaboration with New South Wales DPI and GRDC, two grain response experiments were in high rainfall (>550 mm) regions that are in transition from pastures to cropping and this work aims to provide K responses that growers can use to interpret soil test results. At Breadalbane, the exchangeable K soil test concentration was 34 mg/kg (0 to 10 cm depth), and 15 mg/kg (10 to 30 cm depth). Crops received 0, 12.5, 25, 50, 100, or 150 kg K/ha as KCl applied just after crop emergence. Test crops were canola, wheat and triticale.

Wheat yields increased by 113% following the addition of 100 kg K/ha and the triticale crop showed a 54% yield increase to 100 kg K/ha. Canola yield responses were more modest, with a 28% increase with 100 kg K/ha. There were several field walks undertaken at this site with local agronomists. A site at Wandoo was selected for K response trials, based on better crop growth in prior crop windrows, compared to between the windrows, and this was confirmed by tissue tests. The experiment had liming treatments across the windrows and topdressed K at four rates (0, 25, 50, 100 kg K/ha as KCl) was applied with and without lime and on and between the windrows. Wheat and barley will be sown as the test crops, giving 32 treatments in total. Seasonal conditions reduced yields from what should have been 4 to 5 t/ha to less than 2 t/ha at this site, similar to the whole district. There appears an interaction among lime, K rate and position within or between the prior windrows. These data also indicate that barley is less responsive to K than wheat and may suffer yield depression under high K rates. Three half paddock demonstration sites were established in the southwestern Victorian (Australia) dairying district to assess the impact of improved K nutrition on fodder production. Paddocks selected were to be cut for hay or silage and soil and plant testing indicated that responses could be expected. Half paddocks were spread with 50 kg/ha as KCl in late winter, but seasonal conditions from then onwards were very poor. During the spring, only 71 mm of rainfall was recorded compared to a long term average of 168 mm, and so the paddock growth was not sufficient to conserve fodder.

It is planned to undertake replicated field experiments in this region focusing on the interaction of K, N and Mg on fodder quality and yield. Part of the reluctance of growers to use K is the potentially lower Mg contents of fertilized fodders, and this work will seek to quantify these effects.