Zine deficiency in wheat
Zinc
Yield responses to zinc from research and grower experience are common in many areas of Queensland, including the Darling Downs, Western Downs and Maranoa, and through Central Queensland. Zinc plays a vital role in a plant’s ability to use nitrogen and transform it into yield and protein, and should not be overlooked in a balanced crop nutrition program.
The availability of zinc to many crops is increased by the presence of arbuscular mycorrhizal fungi in the soil. Crops grown after long fallows or other events that deplete soil mycorrhiza population will be at most risk of suffering zinc deficiency.
Deficiency symptoms
Deficiency symptoms include pale, stunted plants, with brown lesions developing on severely deficient plants. Zinc deficiency is not easy to identify but responses to zinc fertilisers occur frequently in old cultivation with heavy clay soils and high soil pH levels (>8) or low organic carbon (<0.7%). Soil erosion, soil structural problems (e.g. hard pans), root diseases, and Group B herbicide residues can all increase the likelihood of zinc deficiency. Soil zinc status should be checked by plant tissue testing, through soil tests (0-10 cm) and/or the establishment of fertiliser test strips.
Critical levels for zinc are: Soil pH <7.0 - 0.4 mg/kg; Soil pH >7.0 - 0.8 mg/kg.
On the Western Downs, the deficiency is usually associated with low soil zinc test (<0.3 mg/kg), high soil pH (>8) and low organic carbon (<0.7%).
Zinc can be applied directly to the soil (zinc sulfate monohydrate), as a component of a starter fertiliser, as a foliar spray (zinc sulfate heptahydrate) or as a seed dressing.
- Soil application (pre-plant) - Applying 15-20 kg/ha of zinc sulphate monohydrate 3-4 months before planting should meet the total plant requirement for 5-8 years. Zinc is not mobile in the soil and needs to be evenly distributed over the soil surface, and then thoroughly cultivated into the topsoil. In the first year, a foliar zinc spray may also be required.
- Soil application (at planting) - Banding zinc with phosphorus at planting is an efficient means of delivering zinc to the plants roots. Water injection with either zinc monohydrate or zinc heptahydrate may also be an option for some growers. If applied as a starter fertiliser component, the amount should be at least 1 kg of zinc/ha (about 40 kg/ha of product) which will only provide enough zinc for that crop.
- Seed dressing may provide sufficient Zn to meet crop requirement, but will not build up residual zinc to the soil. May be a cost-effective option where soil P levels are adequate.
- Foliar sprays - Two applications are necessary. Apply at three weeks and again at five weeks after emergence. It is important both sprays be applied before the crop is six weeks old. The most economical form of foliar zinc is a tank-mix of 1 kg zinc sulphate heptahydrate/ha plus 1 kg urea/ha in 100 L water/ha plus surfactant at 100 mL of 1000 g/L product/100 L of spray mixture (or 160 mL of 600 g/L product/100 L of spray mixture).
Water high in carbonate will produce insoluble zinc carbonate. Consider having the water analysed for suitability in crop protection programs and using a commercial buffering agent.
Zinc heptahydrate sprays may not be compatible with herbicides. Several chelate forms of zinc are available which are generally more compatible with herbicides. Always read the label to determine compatibility.
Potassium deficiency in wheat
Potassium
Due to the gradual decline in soil potassium levels with crop removal and historically low fertiliser application rates, some situations (particularly red soils) require K fertiliser applications. However, crops also vary in their response to improved soil K levels. Generally winter cereal responses are low to moderate unless gross deficiencies occur.
Crops generally take up as much potassium as they do nitrogen although this may not be reflected in crop removal. In particular irrigated cotton, grain legumes and hay baling/silage can affect the levels of K reserve in the heavier soil types.
Deficiency symptoms
- Young plants grow very slowly and are often stunted.
- In older plants, the lower leaves exhibit a marginal ´scorch´, with yellow to brow margins towards the leaf tips.
- Potassium deficient plants may also lodge more readily.
Currently, potassium soil tests are reported as exchangeable potassium (meq/100 g or cmol/kg) or, in the case of a Colwell K test, as mg/kg available K. Research is currently underway to better define critical soil test potassium levels but, in the interim, exchangeable K < 0.3-0.4 meq/100 g or 130-160 mg/kg Colwell P would be considered low-marginal, and test strips worth a try.
Remember that, like P, K is effectively immobile in the soil, so profiles are tending to stratify (much higher levels in the top 10 cm, with significant depletion in the 10-30 cm layer below). Testing for soil K in both the 0-10 cm and 10-30 cm layers is advisable, with the deeper K essential when the topsoil is dry.
Potassium fertilisers can be side-banded at planting, drilled in pre-plant, or broadcast and cultivated in fallow or even prior to preceding crop. The residual value of K fertilisers is excellent, so sporadic applications at higher rates can be an effective alternative to lower rates with each crop. However, potassium banded in the seed row can affect germination. Use the safe rates of nitrogen table as an application guide.
Nutrient levels must remain adequate throughout the top 25-30 cm, as K is not highly mobile. Surface applications of K without thorough mixing into the soil profile can lead to problems if deeper layers are also low in K and the surface soil dries out. Similarly, relying solely on deep banding of K (e.g. at 20-30 cm) when surface layers are also K deficient will not allow the crop to obtain enough K to meet plant demands.
Once K fertility of the surface layers has been restored, deep K application is the best way to apply K fertilisers to maintain soil productivity. This deep K helps maintain crop K status when the surface layers are dry. At the same time, a proportion of the deep K taken up by the crop gets returned to the soil surface in the litter and crop stubble, which replenishes the K fertility of these layers.
While soil K reserves are greatest in the heavier alluvial and cracking clay soils, it is important to maintain adequate potassium soil levels by replacing K removed in harvested product as often as possible. Once soil K levels have been depleted in these soils, very heavy fertiliser K applications are required to overcome the problem and this becomes prohibitively expensive.
