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Trace elements, which comprise of Boron, Copper, Iron, Manganese, Molybdenum and Zinc are required by plants in very small but regular amounts. Their role in plant health is absolutely critical due to the part they play in enzyme systems.
Relative to many parts of the world that have more arid climates, ancient, well-weathered soils and generally high soil pH, New Zealand soils do not experience many problems with trace element deficiency.
Boron and Manganese are the most commonly occurring trace element deficiencies in New Zealand orchards. With movement away from zinc containing fungicides, we can also expect zinc deficiency to become more common.
Diagnosis of Trace Element Problems
Particular trace element deficiencies tend to be associated with soil type. When crops known to be sensitive to a particular trace element deficiency are grown in such soils, it is usually prudent to apply the trace element as part of the general fertilizer programme.
Boron deficiency is widely associated with soils derived from strongly weathered grey wake rock such as are found in Nelson, Marlborough, Canterbury, parts of Hawke's Bay and Central Otago.
Manganese deficiency is associated with high soil pH and is widespread in the drier East Coast districts and Central Otago. Manganese toxicity is also quite common in New Zealand. It is associated with low soil pH or soils which suffer anaerobic conditions through waterlogging. Zinc deficiency is often found where manganese deficiency also occurs. Light sandy high pH soils.
Copper deficiency is rare in orchards, but can occur in peats and other organic soils, or strongly weathered heavily leached soils.
Molybdenum deficiency while widespread in pasture and arable cropping in New Zealand, is rare in deciduous fruit.
Soil analysis is generally considered an unsuitable diagnostic tool for finding trace element deficiency. This is because of the very small amounts of material involved and difficulty with interpretation.
Tissue analysis and identification through visual deficiency symptoms are the main ways of determining if trace element deficiencies are present.
For major crops, internationally recognized standard concentrations for foliar nutrients have been published together with sampling protocols, details of which may be found in publications such as "Fertiliser Recommendations for Horticulture Crops", edited by C J Clarke, G S Smith, M Prasad and J S Cornforth and published by MAF. These days, most analytical laboratories supply this data.
Visual deficiency symptoms are often used as a diagnostic tool, but this method has some drawbacks, such as:
- Considerable decline in performance occurs before symptoms appear.
- Toxicity symptoms can be similar to deficiency symptoms.
- Other stress factors, particularly cold or chill can produce symptoms which look similar to a nutrient deficiency.
- Often several deficiencies occur simultaneously and these are difficult to diagnose from visual symptoms.
Correction of Trace Element Problems
As a general rule, foliar rather than soil applications are used to correct trace element deficiency.
The following are suggestions for correcting trace element deficiencies:
Boron
Boron is very mobile in the plant and readily taken up through the leaf.
There is good published experimental data to show that post-harvest, pre-leaf fall Boron sprays of either Borax (sodium borate) or boracic acid at 100 to 200 grams/100 litres are an effective way to supply Boron to fruit trees, except where soil levels are very low.
Where soils are deficient, a capital dressing of 50 kg/ha of Borax, followed by an annual maintenance of 10 kg/ha is necessary for crops such as apple which have a relatively high boron requirement.
At high rates, Boron is very toxic to plants, particularly highly sensitive species such as peach and nectarine, so care must be taken not to over-fertilise with Boron.
With very sensitive species, foliar application is preferred to soil application.
Copper
The normal spray programme should supply sufficient.
Iron
Although leaf analysis often shows low levels of iron, typical iron deficiency symptoms are seldom seen here. Correction is usually by iron chelate sprays, ferrous sulphate at 200 g/100 litres or "Shotgun" nutrient sprays which usually contain some iron chelate.
Manganese
Manganese deficiency is best addressed by foliar sprays of manganese sulphate at 100 g/100 litres or a proprietary manganese foliar fertilizer at label rates in the late spring/early summer after danger of fruit russet has passed.
The fungicide, mancozeb, is a good source of manganese.
Experimental data is available to show little carry over effect of manganese foliar sprays from one season to the next, so post-harvest/pre-leaf fall is not a good time to apply manganese sprays for the next season's manganese supply.
Manganese toxicity is best tackled by either liming to raise soil pH or where poor drainage is a factor in its expression, improving drainage and soil aeration.
Molybdenum
Normal apple leaf levels tend to be in the range of 0.09 to 0.27 ppm. Deficiency occurs when leaf levels fall below 0.05 ppm.
Foliar sprays of sodium molybdate at 25 to 50 grams per 100 litres of water will cure deficiency in the unlikely event of it occurring.
Zinc
The traditional method of applying zinc has been to apply dormant sprays of zinc sulphate at 1 to 2 kg per 100 litres.
There is published scientific data to show that zinc is not withdrawn from leaves prior to leaf fall, so the effectiveness of post-harvest, pre-leaf fall zinc sprays are questionable unless these sprays make good contact with the bark and buds. On the other hand, there is published scientific data from China to show that zinc sulphate at 10 to 20% plus 3% urea just prior to leaf fall are effective in supplying zinc requirements for at least two years. This is a very high zinc rate and would defoliate the trees, so probably does a similar job to the dormant spray.
Zinc ions in solution are very phytotoxic to green leaves and tissue, and will cause russet if they come in contact with open buds or developing fruitlets. For this reason, it is not advisable to apply zinc salts after green tip, or where high rates are being used even immediately prior to green tip.
For growing season zinc application chelated forms or fungicides containing zinc such as Ziram are the safest zinc formulations to use.
Fruit Russett
Because of the risk of fruit russet and the association between spray application and the inducement of russet, it is prudent to avoid uneccesary sprays over the russet sensitive period between tight to open cluster and about six weeks after full bloom. Effective nutrient sprays can be applied at times outside of the critical russet period and therefore should not be applied at times when fruit russet risk is present.
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