Trace elements back in focus

SARDI researcher Nigel Wilhelm has been investigating the implications of no-till systems for the management of trace elements

After quietly fading as an issue for many years, the role of trace elements is coming back into the spotlight as part of total nutrient management in no-till systems. Many of the nutrients needed to make Australian agricultural soils productive move slowly through the soil when applied as solid fertiliser - and trace elements move especially slowly.

In the past, many nutrients needed a good even mixing into the soil - something that is now less certain under no-till.

Approaches to address this have included applying solid fertiliser down a tube with the seed, or broadcasting on to the surface prior to seeding.

The drawback with these methods is the nutrients become concentrated near the surface, where the soil is prone to drying out. Since plants only absorb nutrients from moist soil, trace elements are subsequently not available to the crop.

Alternative strategies to ensure crops and pastures are well supplied with nutrients now include deep-banding fertilisers, seed enrichment or seed coating, and foliar applications.

Deep-banding fertiliser below the seed helps keep nutrients in moist soil for longer periods of time and places them where roots proliferate.

Using seed high in nutrients increases seedling vigour and consequently increases the crop"s ability to scavenge nutrients. This can give the crop such a kick-start that this practice alone can increase yields.

However, it can be difficult on-farm to produce seed imbued with high trace-element levels unless dramatically different soil types are available on the property. The alternative is to buy outside seed, but this can carry some risk of introducing weed or disease infestation.

Coating seed with a nutrient preparation can achieve similar benefits to using nutrientrich seed, but this can be inconvenient and also cause toxicity problems if application rates are too high. SARDI trials have achieved success with seed dressings and this seems to be a useful option, particularly when supplying trace elements to young crops under difficult conditions such as drought or when the topsoil is dry.

On the other hand, foliar application avoids the problem of uneven distribution of nutrients in the topsoil by supplying nutrients directly to the crop canopy.

This approach is slightly more attractive in conservation farming systems, given that the greater reliance on herbicides provides more opportunities for application without extra operations.

Overall, with continuous cropping systems there is more need to inspect crops for symptoms of trace element deficiency.

The widespread use of high-analysis fertilisers in broadacre cereal crops has been linked to a resurgence in zinc deficiency and in its severity. Zinc deficiency is equally severe in both high- and low-rainfall areas and has been identified on many soil types: acid sandy soils, sandy duplex soils, red-brown earths, "mallee" soils and calcareous grey and red heavy soils.

Copper deficiency in crops has been largely restricted to sandy soils.

The availability of manganese in soil is strongly related to soil pH: the higher (more alkaline) the pH, the lower the availability. Response to manganese has also been recorded in impoverished, acid-to-neutral, sandy soils.

Iron deficiency is most likely to occur in very wet or waterlogged sites or on highly calcareous heavy soils.

For more information: Nigel Wilhelm, 08 8303 9353,

Region North, South, West