Test soil and plant tissue before adding zinc

A nutrition specialist is encouraging grain growers considering applying zinc to use a range of tools to ensure crops are likely to respond to the added nutrient.

Dr Robert Norton, of the International Plant Nutrition Institute, told growers and advisers at the 2014 New South Wales GRDC Grains Research Update, at Temora, that a deficiency was generally likely where the DTPA soil test result was less than 0.5 milligrams of zinc per kilogram of soil.

However, taken alone, he said, soil test values were not a reliable predictor of a likely zinc response in crops.

To guide diagnosis, he encouraged the use of tissue tests by sampling the youngest expanded blade of wheat. A crop is likely to give a yield response to extra zinc, he explained, where the youngest leaf tissue concentration of zinc in wheat was less than 14mg/kg.

A man at a conference

“Using zinc-fortified seed can also reduce the need for added zinc. Under a moderate deficiency, crops are able to take up and respond to foliar zinc before stem elongation.”

 – Dr Robert Norton, International Plant Nutrition Institute.

PHOTO: Nicole Baxter

He said that typically, the zinc supply can be low in many soil types, but zinc deficient soils:

  • have a pH of more than 7.5;
  • have a high sand content;
  • are cold, wet and compacted;
  • occur in some red and acidic soils;
  • have had lime recently applied;
  • have high phosphorus levels;
  • are low in organic matter; and
  • have been recently eroded or land-levelled.

Deficiency symptoms

Dr Norton said zinc deficiencies tended to occur early in the growing season when soils were cold and wet. In these conditions, root growth is slow compared with rapid shoot growth, and the slow growing root system is unable to take up enough zinc to supply the shoot. Root damage and low zinc uptake can also occur with some root diseases or residual herbicides.

“Plants sometimes appear to outgrow this deficiency,” he said. “But often the damage has already been done and yields can be significantly reduced.”

Deficiency diagnosis

To detect a deficiency using soil tests, Dr Norton said zinc levels (using the DTPA soil testing method) are generally less than 0.5mg/kg, but the results were strongly influenced by soil pH, soil texture and soil organic carbon.

While soil tests provide a guide, Dr Norton said research literature supported the use of tissue tests as a more reliable diagnostic tool.

“It is critical to take the correct tissue at the correct time because uptake and redistribution differs with time and tissues,” he said.

“Zinc has low mobility so the usual tissues to sample are the youngest fully expanded leaves (also known as the youngest expanded blade or YEB).”

Dr Norton explained that younger, more rapidly growing tissues were more responsive to changes in zinc supply so are better indicators of deficiency than older leaves or whole plants.

He said the timing of sampling was also critical. Early sampling allows action to be taken to correct the deficiency. As the plant matures, zinc is redistributed and diluted and critical levels decline with plant age.

Dr Norton said tests strips were also useful to check if there was a zinc deficiency in the soil.

Addressing a deficiency

If soil and tissue tests indicate a zinc response is likely, Dr Norton said there were several options available to address the deficiency.

He said roots move to zinc, which means the distribution of drilled zinc needed to be even. If using a granular product, he said it was important to use a product that had a reliable particle size and an even concentration of zinc in each granule.

“Soil mixing via cultivation can dilute the zinc concentration, but can help plants access zinc,” he said. “If placed too shallow, zinc can be stranded in dry soil.”

Dr Norton said crops differed in their response to zinc. As a consequence, his view is that it is more important to apply zinc ahead or onto responsive crops.

In general, he explained, research had shown that canola was more efficient than cereals at accessing soil zinc, while lupins, faba beans and chickpeas have lower demands than wheat, and lentils have a higher demand.

In contrast, he said, maize and sorghum had higher zinc demands than wheat or barley. So zinc supplements are not needed for all crops.

“In a crop rotation, address the zinc demand in the cereal phase rather than in the pulse or oilseed phase,” he said.

While foliar zinc can be used for rescue operations, Dr Norton explained that it has little residual value.

In comparison, soil-applied zinc (with macronutrient) has a residual value of two to five crops, depending on soil texture and pH.

Application time

In Dr Norton’s opinion, the best time to apply zinc is at seeding, mixed or blended with fluid or granular fertilisers.

“Using zinc-fortified seed can also reduce the need for added zinc. Under a moderate deficiency, crops are able to take up and respond to foliar zinc before stem elongation,” he said.

“Later applications up to flowering can increase grain zinc content but will do little in terms of yield response. Using soil-applied zinc ahead of the most responsive crops seems a good strategy that balances cost and risk.”

More information

Dr Robert Norton
0428 877 119
rnorton@ipni.net

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