Written by George Hepburn, Crop Nutrition Consultant with AIVA.
The time has come to think about crop nutrition and the questions that I hear year on year are: should I be doing tissue tests? Or sap analysis? Which crops? At what stage? How many fields? Is it even worth it?
If you are looking to reduce your chemical input, then there is much more emphasis on building the crops own natural defences. A healthy plant, photosynthesising well, will be much better at fighting off pests and diseases versus a plant high in nitrate with imbalanced nutrition. Which would you say is the more likely to succumb to pressure?
Each nutrient plays their own role and has their own story. Previously in the Direct Driller, I have written about Phosphorus. We are now entering a period where Potassium is a key nutrient.
All soils contain good amounts of Potassium (K+), generally the more clay in your soil the more Potassium you have. Some types of clay are very rich and can have up to 20000 kg/ha of Potassium. Medium soils can have 10000 -15000 kg/ha (in the top 6”- 15cm). The same old issue crops up though. The amount in the soil is not the problem, it’s accessing the soil reserve especially at fast growing times like stem extension and grain fill.
Using a past analysis as an example of a wheat crop during its growing period, we can see what the crop has taken up by mid-June. Nitrogen is at 400 kg/ha, which is reasonable, however Potassium is clocks in at 300 kg/ha, which is low. This is important because a crop of wheat needs as much Potassium as Nitrogen in the next 15 weeks (up until mid-June), somewhere around the 400 kg /ha mark. Depending on your soil type, there could be ample stocks of stored Potassium, but only if you can access it. Rooting and root structure is key in this scenario (the benefits of phosphorus availability). The better the root system of your crop, the more soil they will explore, and the more potential Potassium it will find. Potassium (K+) is held on to the negatively charged clay colloids in the soil and is easily exchanged with the plant for H+ (which the plant expels) due to Potassium’s relatively weak charge.
https://digfir-published.macmillanusa.com/life11e/life11e_ch35_14.html
Biology plays a key role here too. Graeme Sait says, ‘For every mineral there is a microbe’, and we know that there are species of Potassium solubilising bacteria that feed from the exudates pumped out by the plant roots. They create weak acids that also contributes to making Potassium and other nutrients available for the plant to take up.
You might apply a soil applied Potassium fertiliser. An application of 100 kg/ha of MOP provides 60 kg/ha of Potassium, which delivers 6g per metre squared. This is not much when we potentially already have over 1000g in the soil naturally. This can however make a big difference, so it shows how a modest amount can make a real impact.
The question is then, are we doing a good job as an industry with our Potassium fertiliser strategies? NRM, one of the top UK laboratories, released this statement recently:
The trends for Potassium and Magnesium were contrary to phosphorus and tended to be below the target levels. This could imply that nutrient supplies are not optimal, or factors such as weather or soil conditions and/or imbalance of nutrients within the soil might have influenced the results and affected the concentrations.
At GS30-31, nearly three-quarters of the samples analysed were found to be deficient in Potassium and only 30% of samples were within the target range. This trend continued at GS32-37 for both Potassium and Magnesium, where more samples continued to contain below-target concentrations.
Following that statement, it seems that we aren’t doing as well as we had hoped with these Potassium fertiliser strategies! This rings true to me, in my 20 years in the advisory side of the industry, many of the tissue and SAP analysis that I have interpreted over the years have commonly shown deficiencies in Potassium, Magnesium, Calcium and Boron.
This is when we turn our focus on to foliar nutrition. Utilising analysis that provides a snapshot of where the crop is at, enabling us to then target key timings, whilst using the right partner products. Informed decisions!
My old mentor, Mr Plumb, always said that a small amount of Potassium applied as a foliar acted as a catalyst to take up more Potassium from the soil. I have applied this over the years, especially after hearing it reinforced at the Acres Conference in the USA. To echo these teachings, here’s some more data that I’ve dug up:
This paper, published in 2022, studied the use of foliar Potassium in winter wheat and maize and drew some interesting conclusions:
- Foliar-applied Potassium increases both the uptake of soil K and overall plant growth in wheat and maize.
- This application significantly improves plant growth traits, including root biomass, root-to-shoot ratios, and water content, by 13-56%.
- Photosynthetic activity and chlorophyll content in plants also improved, with increases of 10-14%.
- The study found that foliar-applied K enhanced root K uptake by 23% in wheat and 55% in maize.
- After foliar K application, the soil had 10% less biologically available K, suggesting that the plants absorbed more from the soil.
Taking this into account, foliar Potassium does not only work in providing more uptake into the plant, it is also actively encouraging the biology to release more from the soil. You are priming the plant to pump out the exudates, bettering the soils microbiome, stimulating the right fraction of biology. Root mass, photosynthetic ability and chlorophyll levels also increase, all very useful traits!
When doing tissue tests, remember that plants can still be low in applied nutrients between tests because of an increase in biomass stimulated with foliar applications. When testing, it’s important to sample both old and new leaves. This is because when the plants extend, this can show the movement of the nutrients up and through the plant.
At AIVA, we like to use foliar Potassium because of all the benefits that it brings. At these current key stages of the growing season, a modest amount of a quality foliar product can deliver big benefits to the crop. We use tissue and SAP analysis (this can be organised for you, should you wish) to reinforce the decisions made to use such solutions. As always, at AIVA we follow a systems approach practice and would look to partner this foliar application with a source of carbon, a quality fulvic acid, and other required nutrition to facilitate a balanced application.
AIVA Potassium is 14% Potassium, supported with an amide complex. Apply at 5-10 l/ha with your T1 and T2 sprays.
https://aivafertiliser.co.uk/product/potassium
