WithTheGrain: Tissue test for accurate micronutrient diagnosis
Author: Rachael Oxborrow | Date: 12 Jul 2018
Farmers suspecting their crops have a trace element deficiency should rely on timely plant tissue testing rather than making unnecessary general micronutrient applications or waiting on visual signs.
This advice from South Australian Research and Development Institute (SARDI*) farming systems leader – Dr Nigel Wilhelm comes at a critical crop growth period in July and August as many crops reach the optimal growth stage for tissue testing.
Dr Wilhelm says visual signs of trace element deficiencies indicate a severe deficiency but yield penalties of up to 10 or 20 per cent for multiple seasons can still occur without any symptoms of a trace element shortage being obvious. Toxicity can also be present well before the crop shows visible signs in the paddock (Figure 1).
“Once symptoms become visible, it can be difficult to turn the crop around from a severe deficiency,” Dr Wilhelm says.
“This can be a costly discovery, but so can unnecessarily applying treatments which were not needed. It is always good practice to undertake tissue tests to better inform decisions before taking action.”
Dr Wilhelm also warns the visual symptoms of many micronutrient deficiencies can be mistaken for other crop problems. For example, copper deficiency can also resemble frost damage or drought stress.
The GRDC Crop Nutrition Fact Sheet states that the likelihood of micronutrient deficiencies can be estimated by considering region, soil type, season, crop and past fertiliser management.
However, Dr Wilhelm says if a farmer has decided to test their crop for deficiencies, tissue testing is the most accurate option rather than soil testing as micronutrient levels in a soil sample rarely reflect their availability to the crop for the whole season.
He warns the biggest issue with tissue testing is risk of contamination and encourages care is taken in handling, storage and to use correct sampling techniques (Table 1) to ensure accurate results.
Samples vary by crop type but conducting the test during earlier growth stages provides the opportunity to apply a trace element before visual symptoms appear and minimises crop yield penalties.
“Growers really have to be aware of contamination and must avoid letting the crop sample come into contact with any metal surfaces,” he says.
“The sampling procedures can take some time so making yourself familiar with the sampling procedures for different crops is really important.”
|Crop type||Growth stage||Plant part||Number|
|Cereals||Early to late tillering||Youngest expanded leaf blade||100 leaves|
|Lupins||Early vegetative stage and before flowering||Youngest open leaves||100 leaves|
|Faba beans||Early vegetative stage and before flowering||Youngest open leaves||50-100 leaves|
|Field peas||Early vegetative stage and before flowering||Youngest open leaves||100 leaves|
|Oilseeds||Early vegetative stage and before flowering||Youngest open leaves||100 leaves|
*Source: Nutrient Advantage Lab Services (http://www.nutrientadvantage.com.au/)
Some crops can be more susceptible to certain micronutrient deficiencies than others (Table 2). Soil type can also assist in identifying potential deficiency risks because trace element deficiencies are more likely in some soils than others.
In the south, calcarosols, sodosols and vertosols can be zinc and manganese deficient, especially if these soils contain more than 60 per cent free calcium carbonate.
|Copper||Wheat, barley, sunflower, lucerne (canola tolerant)|
|Iron||Some legumes (cereals tolerant)|
|Zinc||Oats, wheat, barley (canola tolerant)|
Dr Wilhelm says the small investment to conduct tissue testing could result in hundreds of dollars per hectare in extra yield if a deficiency is identified and corrected.
“If growers are able to get the trace elements right before crop quality is permanently affected they can increase grain price and profit,” he says.
“Conversely, if tests show the crop has adequate trace element levels then they are saving themselves from needlessly applying treatments.”
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