New Findings on Foliar Potassium

Potassium (K) is the mineral with the greatest influence upon crop quality parameters and, in this context, foliar-applied potassium during the business end of the season (from flowering onwards), can be particularly productive. Potassium is unique amongst major minerals in that it never becomes part of functional molecules involved in plant structure. Instead, it serves as a spark plug that triggers numerous biochemical and physiological processes related to plant growth, yield and quality.

Extensive research into this mineral by Lester et al in 2005 and 2006 confirmed that adequate potassium nutrition is directly linked to increased yield, fruit size, shelf life, soluble solids and higher levels of ascorbic acid in many horticultural crops. It is also related to improved fruit colour and shipping quality.

An Essential Fruiting Food

In many plant species most of the potassium uptake from the soil occurs during the vegetative stage when root growth is not inhibited by the availability of plant sugars from the above ground plant. The delivery of sugars to the roots is often compromised when this glucose is needed to fuel the reproductive process once fruiting begins. This competition between roots and fruiting organs is often linked to a reduction in root growth and activity, and an associated reduction in potassium uptake.

Potassium is the second most abundant mineral in the plant and there is a substantially increased drawdown during the reproductive period. Disruption in potassium delivery can prove costly at this critical time of the season, particularly if other factors are also impacting K delivery.

Uptake of potassium from the soil depends upon a variety of factors including mineral balance, soil type and plant genetics. Excesses of calcium, magnesium, and phosphorous can negatively impact the uptake of potassium but the mineral with the greatest adverse impact is nitrate nitrogen. In this context, I have always felt that there was a question mark about the popular practice of foliar spaying potassium nitrate as a K source. It seems like giving with one hand and taking with another and now some new research has confirmed my suspicions.

Potassium Nitrate Proves Counter Productive

New, published research by USDA research scientist, Dr Gene Lester and his associate, Dr John Jifon, involved analysis of the benefits of foliar potassium during fruiting and the study also compared various sources of soluble potassium including, potassium sulfate, potassium chloride, potassium nitrate and Mono Potassium Phosphate (MKP). In this comprehensive, multi-year field study (involving rockmelons) the researchers analysed differences in petiole K, fruit K, brix levels and total sugars based upon different potassium inputs and a control that did not receive foliar potassium. In each of the three years of the trial, potassium nitrate performed poorly. In fact, it was the worst performer in each year on every parameter. It was even worse than the control in a couple of instances.

Next the researchers compared nutritional value and fruit firmness with the various imputs. They looked at vitamin C, beta-carotene, and fruit colour. Colour is linked to pigments, which are, in turn, antioxidants of considerable nutritional importance for humans. The more intense the colour, the greater the protective capacity of the fruit or vegetable. Again, potassium nitrate was the dismal performer, although it was edged out for the wooden spoon by potassium chloride on a few occasions during the three-year research project.

Finally, the study looked at yield, fruit size and discarded fruit. Here, the negatives associated with the K nitrate input became most pronounced. The yields on the potassium nitrate treated blocks were actually lower than the control in each of the three years of the study and the discards were substantially higher following K nitrate foliars. In fact, there was an average of three times more throwaway fruit when K nitrate was compared to potassium sulfate and twice the discards in comparison to the controls.

This input proved to be seriously counterproductive and yet tens of thousands of growers around the globe religiously foliar spray potassium nitrate throughout the second half of the season. The authors of the study concluded that “potassium nitrate may not be suitable for late season foliar nutrition” and they are most certainly correct. Nitrate nitrogen is for vegetative growth. It does not provide a reproductive push and it can be antagonistic to potassium uptake. Nitrates are always absorbed with water so there is a nutrient dilution factor that inevitably reduces fruit quality. It is hard to imagine a more inappropriate choice for potassium nutrition!

The Indisputable Benefits of Late Season Foliar Potassium

This comprehensive study highlighted the importance of selecting the correct potassium input but it also demonstrated the value of late season potassium supplementation. Brix levels and total sugars increased by an average of 20% in line with increased potassium in the leaf and fruit. Fruit firmness increased by a similar percentage and key antioxidants (vitamin C and beta carotenes) increased by an average of 15%. Discards were considerably lower than the control in every block (with the exception of the K nitrate treated blocks). Yields were up to 20% higher in the blocks treated with the better potassium sources.

Potassium increases the translocation of sugars and is the major nutrient associated with fruit size and flavour. Small, acid tasting citrus fruit, for example, are a classic sign of potassium deficiency. This mineral can mean more to your bottom line than any other so it is important to get it right. We have achieved impressive potassium response with a specialist potassium foliar that can help maximize yield and quality.

K-Rich™ – pH Neutral, Foliar Potassium

K-Rich™ is a soluble, liquid potassium, based upon potassium citrate. Right back in 1959, Wittwer and Teubner carried out a study to compare plant uptake of potassium from different sources of potassium including potassium nitrate, potassium chloride, potassium sulphate and potassium citrate. They demonstrated that the uptake of K ions was higher with the citrate form of potassium. However, this high analysis concentrate (35% K) features additional agents to further boost the uptake of this important mineral.

Accomplished late American consultant, Bruce Tainio, was a strong proponent of the importance of late season foliar potassium. He believed that excess nitrates often limited potassium uptake at this critical time. He also insisted that the shortage might not be revealed on conventional leaf analysis data because the leaves are tested from the area to which potassium moves.

Potassium is the most mobile of all minerals and it moves from the lower leaves to the upper leaves whenever there is a shortage. This is the area from which leaves are selected for leaf analysis. We have found that the key here is to monitor the lower leaves with a Horiba Potassium Meter. Whenever the lower leaves reveal K levels 10% below the levels in the upper leaves, you have detected a potassium deficiency and should act immediately!

Liquid potassium fertilisers are notoriously caustic or they feature high salts or unwanted tag ons like nitrates or chlorides. K-Rich™ is pH-neutral and delivers a gentle form of this mineral to the leaf, where it is rapidly absorbed with the help of cell sensitising (uptake enhancing) fulvates. This foliar fertiliser is applied at 5 litres per hectare whenever required during the last half of the season.

For more information please contact an NTS Agronomist on +61 7 5472 9900 or email us on sales@nutri-tech.com.au.

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