Potassium (K) availability is linked to the moisture content of soil, according to Professor Mike McLaughlin from the University of Adelaide.

Prof. McLaughlin examined this key issue on the most recent edition of the Tillage Edge podcast.

“There is a certain proportion of K that diffuses through the soil’s pores to the plant roots, ” he said.

“And when a soil is dry, diffusion through the soil will become quite tortuous. As a consequence, K deficiencies will often show up when there is a moisture limitation in the soil.

“And this has actually happened in the past in Ireland,” Prof. McLaughlin warned.

Research has also shown that K levels in plants will also help them to withstand moisture deficits.

Prof. McLaughlin explained on the podcast exactly what impact stress can have on crops.

“Any stress on a plant is minimised by good nutrition. So crops that are under stress as a result of any nutrient deficiency will be more susceptible to disease, moisture stress and the impact of acidic conditions,” he said.

Prof. McLaughlin has previously researched the impact of phosphorous (P) nutrition on plants’ ability to punch into acidic sub-soils.

“We found that healthy crops are more capable of coping with these conditions,” he commented.

He highlighted that adding P and K on a ‘little and often’ basis to soils that have the ability to bind them up is not the preferred fertiliser management strategy in these instances.

“Split applications of fertiliser work best for elements that are quite mobile through soil. These include nitrogen and sulphur,” he said.

“With P and K, I used to be of the view that it is not feasible to side-dress these elements.

“If they are applied to the soil surface, they tend to get a bit isolated. But recent research carried out in Canada on side-dressed P and K has confirmed a direct response with maize,” he added.

Soil content

But Prof. McLaughlin said that it is important to look at this work in more detail.

“Climatic conditions throughout the growing season may well be an issue here,” he said.

“Rainfall may well be bringing the elements down through the soil and closer to a crop’s roots.

“Some crops may also have feeder roots on the surface. And these may be responsible for capturing P and K that have been applied as a side dressing.

“But, generally, split-dressings work best with fertilisers that are very mobile throughout a soil profile,” he said.

However, where foliar applications of fertiliser are concerned, Prof. McLaughlin said that P uptakes of up to 80% are achievable when this approach is taken.

“Plants are actually quite efficient in taking up P through their leaves,” he said

“This was identified courtesy of a six-year research project that I was involved with in Australia.

“We did not do any work with foliar K, but i would imagine that this element can be transported into the leaf as well. The big issue is the amount of P and K that can be added in a foliar solution,” he outlined.

One key factor that the research project identified was the solubility of the P and K salts. It outlined that any added solutions must be quite dilute, otherwise they can burn the leaves of the crop.

“A very salty solutions containing high levels of P and K will burn off the crop’s foliage,” Prof. McLaughlin stated.