During the 2007 cropping season a survey of seventy paddocks on Upper EP was undertaken as part of the SAGIT funded project looking at Rhizoctonia and disease suppressive soils. As part of the survey, tissue tests were taken and sent for nutrient analysis.  Of the seventy paddocks which were in cereal crop, almost 30% showed deficient levels of zinc.

Within EP farming systems there are three essential trace elements which are important because there can be too little of them available in our cropping soils; - these are manganese (Mn), copper (Cu) and zinc (Zn). Of these three, Zn deficiency is probably the most important because it occurs over the widest area, although it has not been seen to cause the total crop failures that Mn and Cu deficiencies are capable of. However, Zn deficiency can severely limit medic production and reduce cereal grain yields by up 30% and you can also lose further productivity through secondary effects such as increased disease damage. The widespread use of high analysis fertilisers and some herbicides (especially the sulfonylureas (SUs), and some of the group A’s) can make Zn deficiency more severe.

Consider using a foliar spray or seed dressing to correct any trace element deficiencies in 2008 rather than a soil dressing. Foliar or seed dressing treatments are the cheapest option to meet the needs of the 2008 crop but will not provide any residual benefits for subsequent crops or pastures. However, if you opt for a seed dressing you should also budget for a foliar spray because the seed dressing may not be sufficient to meet the needs of the crop if the deficiency is severe. Using seed from a soil with good levels of trace elements will produce similar benefits to a commercial seed dressing so consider sowing seed crops on paddocks with good nutrient and trace element history. If seed comes from another property, be aware of the potential cost of importing weed seed compared to the cost of a commercial trace element seed dressing.

If you can afford the high initial costs of a soil application (which has the long term benefit of substantial carry over for many years in the case of Cu, and for several years in the case of Zn) then banded into or near the seed row, as a fluid is the most effective approach. This technique combines the dual benefits of being able to use the cheapest source of easily soluble trace elements (sulphates) with reduced application rates (compared to “conventional” soil applications).

In cereals, symptoms are usually seen on seedlings early in the growing season. An early symptom of Zn deficiency is a longitudinal pale green stripe on one or both sides of the mid-vein of young leaves. The leaf tissue in this stripe soon dies and the necrotic area turns a pale brown colour.  Severely affected plants have a “diesel-soaked” appearance due to the necrotic areas on leaves, which generally start mid-way down the leaf, causing the leaf to bend or break in the middle.  Plant symptoms appear to be worst early in the season when conditions are cold, wet and light intensity is low. In spring, symptoms often do not appear on new leaves but grain yields will usually be reduced.
It is very difficult to diagnose Zn deficiency in pasture or grain legumes because characteristic leaf markings are rarely produced in the field. In these crops Zn deficiency causes shortening of stems and leaves fail to expand fully. This results in plants which appear healthy but are stunted and have small leaves.

AUTHOR: Amanda Cook,Eyre Peninsula Farming Systems Project Coordinator and Nigel Wilhelm, Leader - Sustainable Farming Systems, SARDI
CONTACT: Amanda or Nigel at Minnipa Agriculture Centre, Telephone 08 86 805 104.