- Plant tissue testing is the preferred method for diagnosing trace element toxicities and deficiencies on the farm.
- Soil pH should be tested and corrected before interpreting trace element soil test results.
- Trace elements can be provided to livestock via supplements, but deficiencies in soils can still affect pasture productivity.
- Take care when correcting trace element issues with fertiliser application, as correcting one can sometimes induce an imbalance in others.
for Farming in Tasmania - Bill Cotching
10. TRACE ELEMENTS
Soil tests are a valuable tool for identifying the macronutrient status of paddocks on a farm, but using them to indicate micronutrient levels, especially on acid soils, can be inaccurate. However, soil analysis was found to give reasonable predictions of soil trace element status in Tasmania when crop species and soil order were considered (Salardini and Sparrow 2001). Trace elements should not be routinely applied to vegetables and crops unless soil analysis or plant analyses indicates a deficiency (Salardini and Sparrow 2001). This prevents causing an imbalance with other trace elements. The micronutrient status of Tasmanian soils is generally adequate for growing potatoes, and this is corroborated by petiole Zn, Cu and B concentrations, the vast majority of which lay within the ranges considered adequate for potatoes (Salardini and Sparrow 2001). Plant tissue testing is the preferred method for diagnosing micronutrient toxicities and deficiencies in plants (Botta 2015) and has been helpful for diagnosing trace element deficiencies (apart from molybdenum and boron) in horticultural crops (Dowling, Back Paddock Company, pers. comm). In many cases, it is easier to address concerns for animal health issues directly with nutrient licks or injections but deficiencies in soils can still affect pasture productivity.
Plant tissue or sap testing for trace elements should be part of a regular fertiliser management program as this is the most reliable way of confirming trace element levels. Testing will identify low trace element levels before deficiencies and production losses occur. It is recommended that soil pH is measured before trying to interpret plant tissue micronutrient tests as acid soils need correcting with agricultural lime before adding micronutrients. In horticultural crops, trace elements are routinely applied via foliar applications to correct identified deficiencies.
Extraction procedures for trace elements can vary between laboratories, resulting in different figures. The most widely used trace element test for trace metals iron (Fe), copper (Cu), manganese (Mn) and zinc (Zn) across Australasia is the DTPA test. EDTA extracted trace elements are considered more reliable on acid soils while DTPA is considered more reliable on alkaline soils (DTPA produces generally lower levels than EDTA). The Mehlich 3 extraction can be used over a relatively wide pH range (Blaesing 2017).
Trace elements can be applied by topdressing granular fertilisers that contain trace elements, spraying trace elements onto granular fertiliser to improve placement in the soil, or as foliar sprays. Top dressing is relatively cheap for the trace element component when combined with other nutrients where they are needed. However, distribution is not even and is onto the soil surface resulting in slow uptake. Applying foliar sprays with a boom spray directly onto the plant means that uptake is almost immediate, but this is more expensive than topdressing.
Boron is involved in cell division and development and deficiencies are often seen as stunting and deformation of growing points in plants. Deficiency reduces the uptake of calcium and inhibits the plant’s ability to use it. Pollen development and viability is also closely linked to adequate boron nutrition. Boron is a very mobile nutrient in the soil, and it can be rapidly leached on sandy soils in high rainfall areas. Great care must be taken with boron applications as the band between deficiency and toxicity can be narrow, hence seek qualified advice on which boron product to use and rates of application. While soil analysis can give an indication of boron status of the soil, dried leaf analysis of the crop provides more accurate data for decision making.
Boron should be applied when growing bulb brassicas such as turnips but not rape and hybrids like pacer. There are a range of boron fertilisers that can be applied in granule or foliar form to vegetables, tree crops or poppies that are susceptible to boron deficiency.
There is unlikely to be a boron deficiency in pastures if soil pH is less than 7 but deficiencies may occur if soil pH >7. Soil test values > 0.1 mgB/kg under pastures indicate boron is non-limiting but brassica crops, such as turnips, will respond to applied boron if soil levels are low (< 1.0 mgB/kg).
All ruminants (including sheep, cattle and goats) require cobalt in their diet for the synthesis of vitamin B12 which is essential for energy metabolism and the production of red blood cells. Sheep are more susceptible to cobalt deficiency than cattle and it is more common in young stock due to their increased energy demand for growth. Pasture tests do not routinely include a check for cobalt as it is not an essential nutrient for plants, but it can be done on request.
Cobalt has been found to be moderate to high (4 – 94 mg/kg) on most Tasmanian soils but low (< 3 mg/kg) on some sandy duplex soils (Nicholls and Honeysett 1964). Levels in dry plant tissue < 0.10 mg/kg are deficient. Cobalt deficiency in livestock can be corrected with drenches, vaccines or bullets.
Copper is necessary for chlorophyll formation in plants and is a catalyst for several key reactions within the plant for normal growth. Plants with copper deficiency are often weaker in the cell walls, lower in proteins, fail to flower and may be more prone to fungal attack. Copper deficiency in cattle and sheep most commonly occurs on acid sandy soils, particularly in areas of high rainfall on Tasmania’s northwest coast, with cattle being more susceptible to copper deficiency than sheep. Copper deficiency in cattle is associated with harsh dry coats and a failure to fatten. High concentrations of molybdenum and/or sulphur in forage that may occur on peat soils or be due to over liming, can induce copper deficiency in grazing animals. Conversely, high dietary copper, low molybdenum and low sulphur may cause copper poisoning (Incitec Pivot 2022a).
Soil test levels (DPTA method) of Cu: < 0.5 mg/kg = deficient; 0.5 – 1.0 mg/kg = marginal to adequate. When soil test levels are > 1.0 mg/kg Cu, any response to Cu application is unlikely and may induce deficiencies in Zn and other trace minerals (Salardini and Sparrow 2001).
If soil copper levels are low and there is known deficiency in livestock, then feed supplements containing copper are an efficient means of administering it to animals. If a pasture response to a field trial of copper is found, apply copper in fertiliser prior to, or at planting of new pastures to ensure the emerging plant roots are able to access available copper for uptake. Copper responsive crops include oats, wheat and lucerne, whilst potatoes and soybeans are less likely to respond.
Copper Sulphate (CuSO4; 25% Cu) or Copper Oxysulphate (CuO10S2) can be added to fertiliser and applied directly to pasture to elevate herbage copper concentrations. For sheep and cattle, rates are 5-10 kg/ha copper sulphate annually in autumn. Copper takes at least a month to get into herbage via fertiliser, so to ensure adequate levels have been reached in the herbage, test the herbage 4-6 weeks after application.
Manganese functions primarily as a part of enzyme systems in the plant. It has a direct role in several important metabolic processes including chlorophyll production. It has an important role in seed germination and accelerates plant maturity. Phosphorus and calcium availability are increased via adequate manganese nutrition. Manganese is immobile in the plant and deficiency is first seen in the younger leaves. Manganese deficiency occurs in plants grown in alkaline soils but is not common elsewhere. Soils with high organic matter content (peat soils) and/or pH levels (greater than 7.0) often require additional manganese inputs. In poorly drained water-logged soils insoluble manganese oxide can be reduced by soil bacteria to Mn2+. This may result in temporary toxicity. At pHwater < 5.5, manganese becomes very soluble, and toxicity may occur (> 50 mg/kg). Toxicity is usually associated with other acid soil infertility problems such as aluminium toxicity and deficiencies of calcium, magnesium and molybdenum.
Soil analysis is not regarded as a satisfactory diagnostic approach to assessment of the manganese nutrition of plants or animals, due to the complexity of the soil chemistry of manganese.
Calcareous soils with high pH do not occur on mainland Tasmania and so soil applications of Mn are not appropriate. Where Mn deficiency has been identified in horticultural crops, foliar Mn fertiliser can be applied but Mn will not need to be applied in fertiliser programs where manganese-based fungicides are applied on a routine basis in horticultural crops.
Molybdenum is vital in nitrate metabolism in a wide range of crops particularly in pastures, cucurbits, legumes and brassicas. Molybdenum is vital for Rhizobia bacteria to fix soil atmospheric nitrogen into nitrate in legume root nodules. In acid soils (pHwater < 5.5), molybdenum availability is reduced often resulting in crop deficiency. A molybdenum content of less than 0.1 mg/kg Mo in dried plant tissue (usually leaves) indicates molybdenum is deficient. Liming usually increases molybdenum availability to pasture, and if this is combined with low copper availability, it may lead to copper deficiency in livestock. If the soil copper level is close to marginal (0.5 mg/kg), it is advisable to apply copper with the molybdenum as high molybdenum levels in pasture can induce a copper deficiency in livestock.
Clovers and Lucerne – Apply every 4-5 years as sodium molybdate (50g/ha) or molybdenum trioxide (75g/ha) mixed with superphosphate. Water soluble sodium molybdate or ammonium molybdite are applied as foliar sprays in vegetable crops.
Selenium is an essential element for animals, but not plants. Selenium plays an important role in immune function, growth and muscle function, fertility and for prevention of white muscle disease. White muscle disease in lambs and calves in spring is most prevalent in years when there is good autumn rainfall and abundant clover growth in spring.
Soil selenium levels are insensitive indicators of animal selenium status, and the selenium nutrition of grazing livestock is assessed from blood and liver selenium levels. The most efficient way of treating stock with selenium is by either drench or vaccine. Concentrations in pasture whole tops less than 0.02 mg Se/kg DM are associated with deficient levels of selenium in blood of grazing animals, and pasture concentrations greater than 0.05 mg Se/kg DM are adequate for grazing livestock. Apply 10 g selenium per hectare in superphosphate mixtures.
Zinc is involved in the synthesis of plant growth regulators responsible for stem elongation and leaf expansion and is essential for promoting certain metabolic reactions. It is necessary to produce chlorophyll and carbohydrates. Zinc is non mobile in the plant and hence deficiency is first seen in the young leaves. Vesicular arbuscular mycorrhiza (VAM) is a beneficial fungus that infects roots of most crops (canola is an exception). The mycelium (fungal threads) act like fine root hairs, which increases plant uptake of immobile nutrients such as phosphorus and zinc. If land is fallowed for a long period, e.g. 12 months, continually cultivated or non-host crops are grown, VAM populations will decline, increasing the likelihood that zinc responses will occur.
Critical levels for zinc are: Soil pH < 7.0 = 0.4 mg/kg; Soil pH > 7.0 = 0.8 mg/kg. Soils with less than 0.55 mg/kg of zinc (DPTA test) are likely to require added zinc for optimum crop production (Salardini and Sparrow 2001). When soil test levels are 1.0 mg/kg Zn or greater, a response to added zinc is unlikely.
As zinc is virtually immobile in the soil, crop requirements are best applied prior to or at planting. The plant roots need to physically intercept zinc in the soil to allow uptake. In crops, zinc can be applied pre-plant during the fallow and incorporated into the soil by cultivation or applied as zinc infused MAP to help with better Zn placement in the furrow when sowing. With a shift to minimum tillage, zinc can be applied annually at lower rates during seeding with the starter fertilisers. In pasture, zinc is normally applied with superphosphate (Incitec Pivot 2022b).
Applying 2.5 kg/ha of elemental zinc (equivalent to 10 kg/ha of zinc sulphate) will correct most deficiencies and the effect will persist for 3-10 years, depending on soil type. Banding zinc with phosphorus at planting is an efficient means of delivering zinc to the plant’s roots. If applied as a starter fertiliser component, the amount should be at least 1 kg of zinc/ha (about 10 kg/ha of product). Foliar sprays are usually used on higher value fruit trees and grape vines and for treating annual field crops.
The most important elements to consider in terms of food-chain contamination include arsenic (As), cadmium (Cd), mercury (Hg), lead (Pb) and selenium (Se) (McLaughlin et al. 1999). Accumulation of these metals in soil and through the food chain may lead to a health risk in humans. Some of these toxic metals have accumulated in soil due to their presence in past application of pest sprays (Cu), in wood preservatives (chromium (Cr), Cu, As) or as fertiliser contaminants (Cd). Cd concentrations in Russet Burbank potato tubers were found to increase with rate of P fertiliser (and hence Cd in fertiliser), but only when those rates of P also increased tuber yield (Sparrow et al. 1992; Sparrow 2000). The response by the Australian fertiliser industry to elevated Cd levels in phosphorus fertilisers has been to make significant reductions in the cadmium contents in fertilisers over the last 20 years. It now uses rock phosphate with lower cadmium concentrations for local manufacture and where fertilisers are imported, low cadmium products are now sourced.
Accumulation of heavy metals in the soil can be considered permanent, except for quantities taken up and accumulated in harvested plant tissue. Toxic levels of heavy metals to animals and humans in the edible portion of plants may occur if crops or pastures accumulate heavy metals in the part of plants that is consumed. There is a tendency for greater concentrations of heavy metals to accumulate in leafy vegetables, such as lettuce and spinach, than roots and tubers, seed or grain (Rayment 2005; Warne et al. 2007). Concentration of heavy metals in the edible portion of crop plants above the maximum permitted level (ML) that can occur before phytotoxicity (Warne et al. 2007), is a food safety risk (Bell et al. 2006) and varies according to the heavy metal and crop (McLaughlin et al. 2006). Metal concentrations in these parts of plants should be monitored if a risk has been identified (e.g. If biosolids have been applied). However, Cotching and Coad (2011) found that increased plant uptake of heavy metals is unlikely under realistic field application rates of biosolids in Tasmania.