Colwell potassium (K) is the go-to K test in many growing regions. Colwell-K measures extractable K and is used to estimate plant-available K.
But interpreting Colwell-K values gets tricky on heavier soils. This is partly because research on critical values has been dominated by Western Australian sandy soils. There is some anecdotal evidence where soils thought to have adequate K are responding to applied K.
Why does soil texture matter?
There are three forms of plant-available K in the soil. Exchangeable K is bound to the surface of clay and organic colloids. Solution K is in the soil water. Fixed or ‘interlayer’ K is trapped between the layers of some clay minerals, and is only slowly available to plants. Cowell-K measures the solution and exchangeable K.
Heavy soils have more fixed (slowly-available) K and this is what makes the Colwell test less reliable. Over time this K can diffuse out and plants can use it. However, the Colwell test calibrations developed for sands may not predict how much will become available, and when. Plus, heavier soils with more illite clay are prone to absorbing K into the interlayer space, making it unavailable to crops.
Researchers are looking into it
Through the GRDC investment on NPK, researchers are looking into K responses on loamy soils in the Western grains region. Here, researchers have identified loamy soils in the Avon Valley where:
- Colwell-K levels are falling but still above critical levels for sandy-surface soils.
- K fertiliser is not routine.
- Better growth in residue windrows has been observed and symptoms of K deficiency occur between windrows.
This research is currently at the pilot stage, aiming to move to field trials next year.
On the heavier soils in north-eastern Australia, seasonal K deficiency is an emerging trend, with widespread K deficiency in drier years but no obvious deficiency in wetter years. Continually depleted and stratified K stocks are an extra challenge for crops in drier years. If the topsoil is moist and roots are active, K supply is adequate. If the topsoil is dry, depleted subsoils often can’t supply enough K to meet crop demand.
So what do you do if you’re on heavy soils?
There are a few ways to refine your K budget:
First step – soil test. If you think your paddock could do with some K, look closely at soil K levels down to 30 cm. Good topsoil K stocks can mask subsoil deficiency. K recycled from crop residues can keep topsoil K at an adequate level, but crops also take K from deeper in the profile and subsoil stocks will eventually run down if removal exceeds inputs from fertiliser.
2. Use a test strip
Test strips are invaluable. A K test strip will show if you need to re-think the K critical value on a site. Apply strips of K-rich fertiliser at rates of 50 to 100 kg/ha of K at right angles to the line of harvest to take account of any windrow effect. While there is little benefit to late in-crop K applications, the results will help refine the fertiliser budget for the next crop.
3. Look out for windrow effects
Better growth in windrows compared to between windrows can be caused by the concentration of K in crop residues into a narrow strip. Windrow effects are an opportunity to diagnose which nutrients are limiting growth. Sample on and off the windrow in parts of the paddock where the growth difference is greatest.
4. Check exchangeable sodium (Na) levels
Excess Na can interfere with K uptake. This is more likely if exchangeable sodium is above 6% in the topsoil and 15% in the subsoil. In this situation, which is more likely on heavier textured soils, K requirements may be higher to maintain plant K levels. Again, a test strip will help refine crop K needs.
5. Choose your placement strategy and product mix carefully
In clay soils, K does not leach into deeper soil layers and the highest soil K is typically in the topsoil. Further enrichment of that topsoil may not help if the layer dries out during the season. Deeper placement (e.g. in bands) can meet some of the crop demand, but plant roots need encouragement to multiply in and around a band of K fertilizer. Applying a compound fertilizer product that supplies N and P, as well as K, can help to improve crop K acquisition.