Giving attention to potassium pays!
Droughts are not uncommon in Southern Africa and farmers have developed a number of strategies to mitigate their effects and increase the chance of harvesting an economic yield. Techniques like no-till to conserve soil moisture and adapting plant populations are such examples but nutrition has a role to play too. Research findings have shown the importance of potassium (K) nutrition and the related crop response during drought conditions.
Response to K in drought years is higher
Research from the Corn Belt in the USA has demonstrated the importance of potassium in drought years. A long-term trial in Iowa where good seasons were followed by drought clearly shows the benefit of applying potassium to maize and soybeans; yields and profits from adding K were highest in the years of stress (Table 1). While increases in good years were negligible in soils with moderate levels of potassium, when drought was experienced, maize receiving 46.5 kg K/ha had a 40% increase in yield and soybeans an increase of 26%.
Results over 18 years from Indiana confirm the soybean response. Figure 1 shows that in years of low rainfall in the first 12 weeks after planting, a higher yield increase was obtained with K applications.
The role of K in alleviating drought stress
Potassium works in multiple ways to maintain photosynthesis during periods of moisture stress. This allows plants to continue growing, albeit at a slower rate, but at a rate that ultimately explains the improved yields as compared to crops with less potassium. For photosynthesis to continue, the plant needs to be able to maintain and increase water uptake and retention as well as maintain the cellular membranes that protect the chlorophyll structures. This is achieved by increasing the osmotic (salt) concentration of the cells and through the rapid closure of the stomata (pores in the leaf surface). Increasing the osmotic concentration of the cells has the effect of increasing the activity of aquaporins (channels in the membranes of the plant cells) which facilitate the uptake of water. The increase in osmotic concentration also assists the plant in extracting water from the diminishing supply in the soil and helps to keep cells turgid. Potassium plays a crucial role in changing the turgor of the guard cells needed to close the stomata and prevent water loss. When potassium is in short supply, the time taken by the guard cells to close increases from a few minutes to over an hour. As a result the plants continue to lose moisture and are more susceptible to drought. Plants that experience moisture stress produce more oxygen radicals which are very reactive chemical compounds that, if left, unchecked will destroy the cellular membranes thereby disrupting photosynthesis. To counteract this, plants produce NADPH oxidase, an enzyme that reacts with the oxygen radicals and reduces them to neutral compounds. NADPH oxidase activity is regulated by potassium.
Maintaining K uptake in times of drought stress
As soils start to dry, the rate at which potassium dissolves into the soil solution declines. This can lead to a shortage at the root surface when potassium is critical for growth. Improving the soil’s potassium status increases the concentration of potassium in the soil solution which allows plants to continue to absorb potassium at the rate at which it is required. Research from the UK (Figure 2) shows that in dry soils the concentration of potassium needs to be four times that which is required for wet soils, if plants an uptake of 5 kg K/ha/day.
Strategies to manage soil K
In profitable seasons one needs to maintain and, if necessary, build the potassium status of the soil, taking into account the potassium removed through the harvesting of the grain. Potassium incorporated into the grain varies according to the crop. A tonne of maize contains 3.9 kg K and soybeans 25-40 kg K per tonne of grain. By the time it reaches flowering, maize needs to have accumulated 66% of the potassium it requires as at harvest, 30% of the potassium is held in the grain. In soybeans, 50% of the potassium is held in the grain with much of the potassium being remobilised from leaf and stem tissue to fulfill this need. In drier seasons, where farmers should apply additional potassium, Omnia is ideally positioned to help the farmer in times of drought. We have a range of top dressing products that contain nitrate nitrogen (also shown to be beneficial in times of drought stress) in combination with potassium – 1:0:1, 3:0:1 or 5:0:1 from which to choose, depending on the amount of potassium required. Investing in maintaining the soil’s potassium status and supplementing with top dressings pays dividends and helps address the risk of poor yields in dry seasons.
- K reduces stress from drought. Better Crops 1998-3 p34.
- Johnston, A.E., Barraclough, P.B., Poulton, P.R. and Dawson, C.J. (1998): Assessment of some spatial variable soil factors limiting crop yield. Proceedings No. 419, The International Fertiliser Society, York, UK, 48 pages.
By Megan A’Bear
Manager: Agronomic Research and Development