Potassium responses in winter crops and pastures
In collaboration with NSW DPI and the GRDC, the response of wheat and barley (Warndoo 2015) and wheat, triticale and canola (Breadalbane 2015) are being investigated. Responses of wheat and canola to K are assessed at Glenthompson (2016) with Southern Farming Systems and the reponse of pasture yield and quality to N, K and Mg is being addressed at Bessiebelle. Additional demonstrations have been undertaken on pastures in coilaboration with Meridian Ag P/L. Support of these trials is provided by Canpotex P/L and IPNI ANZ.
IPNI-2015-AUS-24
26 Feb 2016
K responses in fodder crops 2015
These trials were facilitated by Mr Andrew Speirs (MSA P/L – now Meridian Agriculture). The objective was to demonstrate the response of hay or silage crops to K. These were not replicated experiments but purely demonstrations, and it was planned to link the demonstrations to field walks with Mr Speirs and Dr Norton. The region selected was south-western Victoria which is adjacent to the large south-west dairy industry which has a high demand for fodder, both imported and home-grown. Soil test data from Incitec Pivot showed that about 30% of 590 samples showed low available K levels.
Table 2.1 Soil tests and site characteristics for sites selected for K demonstrations in southwestern Victoria 2015.
| Analyte | Hamilton | Condah | Mepungah |
| Texture | Sand | Sand | Sandy Loam |
| pH (CaCl2) | 6.1 | 5.7 | 4.9 |
| EC (1:5) | 0.08 | 0.09 | 0.08 |
| Olsen P mg/kg | 18 | 25 | 16 |
| Colwell K mg/kg | 170 | 53 | 160 |
| CEC cmol(+)/lkg | 7.3 | 5.7 | 7.7 |
| Avail. K mg/kg | 120 | 44 | 110 |
Three field sites were selected, and a brief summary is shown in Table 2.1. Based on the Better Fertilizer Decisions soil test critical values for pastures, the critical Colwell K range for sands is 109-142 for 95% of maximum pasture production. Two of these sites are marginally above this critical value, but even so, a response to added K is expected under good conditions.
Half of each paddock was spread with 50 kg K/ha as MOP in the late winter. Seasonal conditions in the spring were very hot and dry, with 71 mm falling in September to November (Hamilton), compared to long term average of 168 mm for the same period. As a consequence none of these paddocks were cut for hay or silage. An assessment of plant K status was taken at the Hamilton and Condah sites, and those results are shown in Table 2.2. The untreated tissue values indicated that both sites showed low K supply, and there were increases in tissue K were added K was supplied. As has been widely reported, the tissue concentrations of Mg declines with added K. although the values are still within the normal range for plant nutrition. However, Mg concentration in conserved fodders and silages, is more a problem with animal health as low values can result in grass tetany in cattle.
Table 2.2. Pasture tissue sample nutrient concentrations as taken in October. Indicative critical values are given for Perennial Ryegrass (PRG).
| Nutrient | Adequate levels (PRG) | Hamilton | Condah | ||
Plus K | No K | Plus K | No K | ||
| K (%) | 2.0-4.8 | 1.88 | 1.33 | 2.49 | 1.54 |
| Mg (%) | 0.2-0.4 | 0.27 | 0.33 | 0.30 | 0.34 |
| N (%) | 3.5-4.5 | 3.62 | 3.33 | 3.51 | 3.45 |
| P (%) | 0.25-0.55 | 0.23 | 0.24 | 0.36 | 0.37 |
Conclusions
• Undertaking field demonstrations on-farm has seasonal and operational risks
• Current soil tests are not reliable indicators of K response – need to consider subsoil K as well as K in topsoil. Really low numbers are easily picked up, there is a grey area above say 140 – 160 on heavier soils especially. Sampling time is critical.
• Maybe 50 kg K/ha is not enough especially on the Hamilton site. On K depleted heavy textured soils, the fixation of K may take preference over root access and uptake. There is a need for rate response data.