Potassium (K) is the second most abundant mineral found in the plant, but unlike its more prolific partner, nitrogen, this mineral can only be sourced from existing soil supplies. Nitrogen can be mined from the atmosphere by organisms on the leaf’s surface and in the soil, but this is not the case with potassium. Large amounts of K are uptaken from soil solution, so when dry conditions prevail, K shortages can surface.
During dry times, there can be great benefit in foliar supplementation with K. In fact, much of the drooping plant stress in low moisture soils can actually be linked to declining potassium availability in the shrinking soil solution. A foliar application of 5 – 10 kg per hectare of potassium sulfate combined with around 200 g of NTS Fulvic Acid Powder™ will often deliver a significant reduction in visible drought stress.
Potassium Provides Vigour, Strength, Size and Sweetness
There are a variety of key benefits associated with managing optimum potassium nutrition, which include the following:
Potassium governs stomatal opening and this is hugely important. There are major implications for production if the uptake of CO2 for photosynthesis is compromised by substandard stomatal opening due to K shortage.
Potassium strongly influences stem strength. K shortage can promote loss to lodging following rough weather, or poor presentation of the solar panel (leaf) due to stem weakness.
Potassium is a critical vegetative mineral promoting early growth. When early vigour is lacking, the first suspect is a K deficiency. If you are using soil conductivity as a measure of the level of mineral salts in the soil, low soil conductivity is often linked to insufficient potassium.
Potassium is also responsible for sugar movement within the plant, which is essential for sizing up fruit, grain and vegetables. In this context, potassium can be the single most important mineral for profit. You will inevitably have disappointingly small produce with insipid flavour, if your crop lacks K. This is graphically evident in a potassium-deficient citrus orchard, where sweetness and size are so obviously compromised.
Potassium balance is a key player in plant resilience, where either too much or too little of this mineral can increase disease and pest pressure. There can be exciting yield improvements and cost savings when we manage this mineral efficiently and the best precision strategy involves regular use of a Plant Sap Potassium Meter (available from NTS).
Five Factors Affecting Uptake
1) Healthy soil-life seriously improves potassium availability. There are specialist bacteria that can slowly solubilise the unavailable potassium found as insoluble rock minerals in the soil. 98% of all potassium in the soil is in this unavailable form. There is also a portion of the total potassium component in your soil that is ‘slowly available’. This involves the trapping of the small potassium ion between clay platelets. Here, K is only available to plant roots when the clay sandwich is full to overflowing.
However, there is one soil-life creature that can help overcome this K-trapping phenomenon. The superb K-miner that allows access to this frozen resource within the clay is mycorrhizal fungi. This wonderful worker attaches to the plant root and creates a massive root extension. The fossicking hyphae can drive between the clay platelets and extract the potassium trapped within. They then supply this payload to their host, the plant, along with the phosphorus, calcium and zinc that they have mined along the way.
2) Much of the readily-available potassium is in soil solution, so dry conditions impact potassium availability. In fact, potassium availability directly increases when soil moisture levels are good. Of course, the other source of readily-available K is that found attached to the clay colloids. Here, there can sometimes be a problem associated with the single positive charge of potassium when competing for space on the clay colloid with double positive charged calcium and magnesium ions. The weakly-held potassium ion is easily elbowed off the colloid when base saturation percentages of calcium and magnesium are higher than they should be (80% when combined).
3) Soil aeration impacts potassium availability. I often teach the importance of gas exchange. In fact, it is arguably the most important responsibility of a grower seeking to enliven and regenerate their soils. If you have successfully managed gas exchange, then your soil breathes, your biology rejoices and your bank account is humming.
Oxygen is the single most important element for crop production. It is the essential requirement for root health and the vitality and performance of the army of aerobic organisms surrounding the roots. Optimum gas exchange requires sufficient calcium to flocculate the soil. Of equal importance in this equation is the soil-life that determines crumb structure and creates the pathways that facilitate easy entry of oxygen and a smooth exit of CO2 when the roots and their tag-on biology ‘breathe out’.
The capacity of a soil to breathe determines everything, and many of our problems relate to our lack of understanding of this simple fact. Fix the Ca/Mg ratio, repopulate your dying soils and feed up your workforce and you will reclaim your fun in farming.
4) Soil temperature also affects potassium uptake and availability. Root activity, plant functions, and metabolic activity all increase as soil temperature increases. This increase in physiological activity leads to increased K uptake. Optimum soil temperature for uptake of K is around 20°C. Potassium uptake is reduced at low soil temperatures.
This might seem like worthless knowledge, because there may appear to be little that can be done to influence this environmental factor, however, this is incorrect. There are three key strategies that can help maximise soil warmth, which include the use of microbial inoculums delivered with a lunchbox, sulfate sulfur, and humates. Humates have been shown to introduce an insulating factor that helps maintain soil warmth. Ammonium sulfate is recognised as a good soil-warming nitrogen option to trigger an earlier Spring response. When trillions of beneficial microbes are brewed and applied, in conjunction with microbe foods like humates, kelp, fish and molasses, the resulting explosion in biological activity can generate its own warmth.
5) Your choice of tillage influences your access to K in the soil. It is now understood that potassium availability suffers with no-till farming. The reason is not fully understood, but my guess is that the nutrient stratification that occurs over time with the no-till approach could be a contributor. Potassium is highly mobile and it seems logical that it could move down the profile in undisturbed soils, reducing access to young roots. There could also be a link to the blanket that is a feature of the no-till approach.
We now know that glyphosate is something of a biocide and it is highly likely that potassium-solubilising bacteria and K-mining mycorrhizal fungi are collateral damage of what is now being called "the new DDT". I am not suggesting that we abandon no-till, but minimum-till may be preferable and there is a screaming need to explore more sustainable weed management technology like blind cultivation and crimp rollers.
In the second installment of this two-part feature, I will highlight the best potassium management strategies and we will look more closely at the nuances of this key determinant of your farming profitability.