Stunted Roots Need Help, Says Plant Nutrition Expert

Drought conditions aren’t necessarily a precursor to stunted growth and reduced yields, a European plant nutrition expert told farmers attending an Agri-Trends field day recently.

Volker Rmheld, a plant nutritionist from Hohenheim University in Stuttgart, Germany, said that more intelligent use of fertilizer, such as placement, banding or foliar application, could be used to salvage crop yields in the event of a drought.

“Under normal, good root growth conditions, only 20 per cent of chemically available nutrients are taken up. Under drought, it’s less than five per cent,” said Rmheld.

The most important nutrients that a plant needs to cope with drought stress are potassium (K), magnesium (Mg) and zinc (Zn). Even though soil test results may indicate that all are abundant in the root zone, the reduced root growth due to lack of water prevents the plants from accessing them in sufficient quantities.

“Under drought, an elevated supply of these three nutrients is vital for adaptation,” he said.

In Germany, farmers in previous decades applied five times the amount of K than was needed by the crop. Now, their soil is very fertile. And with the skyrocketing price of crop nutrients, simply continuing the practice of piling it on isn’t economically feasible.

In Western Canada, nature placed enough nutrients in the soil to supply crop needs under normal conditions. But under drought stress, the reduced root growth means crops may suffer even if soil tests indicate the nutrient is available in suffi cient amounts.

Rmheld, speaking at a seminar in Brandon hosted by Agri-Trend, added that magnesium, which is critical for photosynthesis, is often a “forgotten nutrient.”

Deficiency often shows up after flowering, when it’s already too late. In cases where K and Mg shortages occur, the translocation of sugars from the leaves to the fruit-bearing parts of the plant is affected.

Photosynthesis continues, but no movement of sugars out of the leaves results in photo-oxidative stress, damage to cell membranes and proteins, and other problems.

Studies in the field and in a light chamber have shown that shaded plants are better able to cope with zinc deficiencies.

“In your western part of Canada, you have severe problems with high light in some summers,” he added. “Plants grown under high light require more potassium, magnesium and zinc (Zn) than plants grown under low light conditions.”

That means that K, Mg, and Zn are “greatly needed” for the efficient use of high light intensity, as well as under drought, heat and salinity pressures.

Rmheld noted that studies have found that orange groves managed by farmers using organic production methods seemed to suffer less photo-oxidative stress symptoms than those where glyphosate was the main means of weed control.

“What is the main difference? Again, plant nutrition. If you analyze the leaves of the trees, you find very low magnesium and very low zinc. Due to the very high use of glyphosate in that system, you induce zinc deficiency which is the cause of photo-oxidative damage under high light,” he said. “So there is a link to these agri-chemicals.”

One possible coping strategy that Prairie farmers might try is a late foliar application post-flowering of magnesium and potassium to improve the crop’s ability to move sugars from the leaves into the grain, said Rmheld.

Although Canada is home to some of the world’s largest potash mines, ironically, in some regions the replacement rate via fertilization is only 21 per cent of the annual crop removal rate, he added.

“You are saying that your soils are rich in potassium. Maybe. But this rich potassium in the soil cannot be used under a drought situation. So, maybe you need a foliar application,” he said.

“If you look at your data in Canada – where you never apply potassium fertilizer and you have such a severe drought situation – you have to really consider potassium application.”

daniel. winters

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