Potassium is the most expensive and among the most productive of all fertiliser inputs, so mastering the management of this mineral will inevitably reap rewards. Those rewards will often include the savings and stress relief that come from a reduced need for chemical intervention, because potassium balance hugely impacts pest pressure.
Potassium and Sap pH
Many of you will be familiar with the concept of sap pH as a monitoring tool. This discovery was one of many contributions from the brilliant U.S. researcher, Bruce Tainio. Bruce observed that healthy, resilient plants had a sap pH of 6.4. Interestingly, this is the same level as a healthy soil, a robust animal and a healthy human (according to urine pH). A cell is a cell and this guideline seems relevant across these different mediums.
A sap pH below 6.4 is indicative of likely fungal pressure, while a high sap pH can spell the arrival of unwanted insects. The potassium link to this story is profound because this super mobile, alkalising mineral can be a major player in the disease department. Potassium is one of the most mobile minerals in the plant and it commonly moves from the lower leaves up to the fruit or growing tips where it is required. This first sign of potassium deficiency is rarely recognised, because we are usually monitoring the plant by tissue testing the first fully-developed leaf. In this context, leaf tests are not always a reliable indicator of potassium status.
When potassium vacates the lower leaves, the sap pH drops in those leaves and this becomes the most likely site for disease attack. Think of that tomato plant in your garden. Where do the little brown spots that eventually decimate your plant first appear? It shows up first on the lower leaves and very commonly this is due to an unrecognised potassium deficiency. Tomatoes require much more potassium than that provided by many home gardens, but if you keep a sack of potassium sulfate in your shed you will more likely enjoy problem-free tomatoes.
The invaluable technique for fingertip control of K involves the use of a Plant Sap Potassium Meter. On a regular basis, you simply test the older leaves and then the younger leaves. Ideally, the potassium levels of the leaves sourced from both sites should be the same. If they differ by more than 10%, then you have discovered a K deficiency and the sooner you act the better. K-Rich™ is a popular choice for a foliar correction of K shortage. This concentrated formula features fully buffered liquid potassium (35%) that provides a particularly impressive potassium response.
Symptoms of a K Shortage
When potassium levels decline in the soil, you will often notice a lack of plant vigour. Fruit drop may occur as weak stems struggle to retain their load. Lodging in cereal crops is common and the flavour of food crops suffers. There may also be evidence on the plant leaf, beginning with the lower leaves. Chlorosis or scorching of the leaf margins is the most likely outcome. In corn crops, for example, the margins of the lower leaves turn brown. This burnt edge is often accompanied by a striped appearance on the remainder of the leaf. There is a visible and distinct light green colour to these leaves, which is easily visible from a distance.
In soybean crops, the margins of the lower leaves turn yellow – but in crops from the potassium-hungry nightshade family (tomatoes, capsicums, potatoes and eggplants), the lower leaves develop distinctly burnt edges, rather than yellowing.
The bottom line here is that a potassium shortage will be costly, as this is the fruit and seed filler and yield loss is inevitable.
Ten Potassium Pointers
Potassium sulfate is almost always a better choice than potassium chloride. It has a lower salt index, it is more microbe friendly and most crops need sulfur but do not require chloride. Part of the chloride becomes chlorine and it is not a great strategy to treat your soil like a sterile swimming pool. Potassium sulfate is more expensive, but I have argued for years that even if you spend the same dollar value and get less potassium, you will never be unhappy with your choice. I have yet to meet a grower who regrets his or her choice of this more bio-friendly option.
The application of excessive compost can create an excess of potassium. This is not a good situation as it can limit the uptake of all-important, cell-strengthening calcium, along with the chlorophyll-building magnesium and the boron steering wheel. Green waste compost is high in potassium. We know of cases where growers have used up to 100 tonnes of compost per hectare and created serious balance issues.
Don't waste your fire ash as it is a great potassium source. The very best outcome is to add it to a compost to stabilise this highly leachable mineral.
Fulvic acid is a great additive to liquid potassium as it magnifies K uptake and stabilises this mineral through the creation of a much less leachable, potassium fulvate. Fulvic acid is also a great foliar fertiliser in its own right.
NTS Soluble Humate Granules™ are a great additive to dry-applied, granulated potassium sulfate at 5%, as they stabilise both the potassium and the sulfate to reduce their leaching. Always remember that humus is the sulfur storehouse and humic acid is basically a humus concentrate.
Never allow base saturation levels of sodium to exceed base saturation levels of potassium. When sodium base saturation exceeds that of potassium, sodium is invariably taken up by the plant instead of potassium. In this manner, the man-made imbalance can be costly, because sodium does not offer the same range of benefits as potassium. In fact, the sodium that has been mistakenly uptaken can expand and burst cell walls in hot conditions. Not only do you miss out on size, flavour and resilience, but you have a bunch of burnt leaves to boot.
There is great benefit in trying to achieve equal ppm of potassium and magnesium on a soil test. This important ratio can make a big difference to your bottom line. When the K:Mg ratio is perfected, you will be delivering optimum levels of both minerals to your crop – but you will also notice something very special. When comparing thousands of soil tests to associated leaf tests, over many years, I noted that whenever we achieved equal parts per million of magnesium and potassium, we increased the uptake of both minerals into the leaf. Not only did we maximise uptake of these minerals, but there was also an associated positive impact upon the uptake of phosphorus.
Potassium is a major mineral with multiple roles and getting K nutrition right can be very productive. Of all of the tips I have shared in this two-part article, the most important relates to closely monitoring this mineral so you don't miss yield. The technique of monitoring the top and bottom leaves and striving to maintain similar levels across the plant is huge. If you can't afford a potassium meter, you can sometimes gain some insight by using simple pH strips at both sites. If you can maintain the same sap pH on the top and bottom leaves of the plant, then it is highly likely you will not suffer potassium deficiency and associated loss of quality and yield.