Zinc – The Leaf Enlarger
Zinc is the second largest mineral deficiency amongst Australians (just behind magnesium), but this shortage is not all linked to soil deficiencies. It can also be related to food processing, toxicity (the detox and immune systems are zinc dependent) and through over-consumption of cereal grains (which contain a natural acid called phytic acid which can make zinc insoluble). Zinc is commonly deficient in broadacre soils and foliar sprays are often used to compensate for soil deficiencies. However, most growers involved in intensive horticulture are aware of the importance of this trace element. The biggest zinc problem in these soils is usually related to excess phosphate which antagonises zinc uptake or through overuse of nitrogen which also increases zinc requirements. In some cases hybridisation has compromised the plant’s capacity to uptake zinc and so more zinc is required than would normally be needed. A shortage of zinc will normally be more costly in terms of associated yield reduction than any other trace mineral and it can even be more important than some of the majors. The reason for the massive cost to benefit ratio from applied zinc is linked to zinc’s principle role within the plant. Zinc governs the production of natural hormones called auxins within the plant. Auxins are responsible for leaf size, and the size of these solar panels determines photosynthetic potential. 95% of crop production comes from photosynthesis so it is easy to understand why zinc is often called the “major minor”.
The Functions of Zinc
Zinc is also called the “energy micro nutrient” because it is needed for phosphorus uptake and is therefore critical for the production of adenosine tri phosphate (ATP). ATP provides the energy for every enzymic reaction in microbes, plants, animals and humans and is one of the single most important of all nutrient factors. Zinc regulates plant sugar, transforms carbohydrates and it is critical for the uptake of moisture. This latter function makes zinc exceptionally important in drought conditions. Zinc is also one of the most important nutrients for micro-organisms. We tend to think of micro-nutrients solely in terms of their benefits for plants when they are also critically important for the massive, invisible, microbe workforce in the soil. Zinc is particularly important for nitrogen-fixing organisms, including Azotobacter, because they are so reliant on ATP to fuel the nitrogenase enzyme that converts atmospheric nitrogen to ammonium nitrogen in the soil. However, as already mentioned, it is the role of zinc in facilitating the leaf-enlarging capacity of auxins that is of most importance from a production perspective.
Different Strokes for Different Folks
There is a distinct variation between plants in terms of the scale of their response to a zinc deficiency. Orchard and vine crops are the most sensitive to a zinc shortage and these crops are closely followed by beans, garlic, corn and onions. Cotton, sorghum and tomatoes are reasonably sensitive to a zinc shortage. Cereal crops, potatoes, carrots, lettuce and lucerne are less sensitive, but still need zinc. Citrus is probably the most sensitive crop in my experience and it is almost the norm to see citrus leaf tests with zinc deficiencies. However, this can also be linked to the overuse of copper in this industry, as copper can lock up zinc.
Ten Factors Limiting Zinc Uptake
There are several factors that can impact the plant’s capacity to access zinc and they include the following:
Decimation of Mychorrizal fungi - these creatures, that are so often missing in our soils, are key players in the delivery of zinc from the soil to the plant. The other reason that citrus suffers so much from zinc deficiency is because the toxic levels of copper in those soils kill these zinc-delivering fungi. Zinc is not very mobile in the soil. It does not float freely in soil solution like many other minerals. The massive network of fine Mychorrizal filaments, which can extend the plant’s original root system by 1000%, gives much greater access to the immobile zinc ion.
Alkaline soils - as soil pH moves beyond the ideal of 6.4, the availability of zinc to the plant declines.
Over liming - excess calcium is antagonistic to zinc and six other minerals. In fact, in some cases it can be more harmful to have too much calcium than too little. As always it is all about balance. Magnesium carbonate can actually limit zinc uptake, more than lime if over supplied, but this is because magnesium has 1.4 times more impact on pH than calcium. However, magnesium in itself does not antagonise zinc like calcium can.
High phosphorus - the phosphorus to zinc ratio should ideally be 10:1 in favour of phosphorus and any significant variation in that ratio can tie up either of these two minerals, depending upon which one is excessive.
Cool wet conditions - as with phosphorus and iron, zinc is less available in cold soils. It is common, particularly when zinc levels are borderline, for zinc deficiencies to be present when the soils are cool. However, the crop often grows out of it as things warm up.
The previous crop - the preceding crop can impact zinc availability in the current crop. Brassicas for example do not have a Mychorrizal relationship and can have a vague fumigation effect that can limit the biological delivery of zinc.
Excesses of other metallic minerals - high levels of iron, manganese and copper can antagonise zinc uptake. High soil levels of copper, from fungicide residues, commonly induces zinc deficiencies.
Heavy manure applications - large applications of poultry manure can create zinc shortages in the plant. This is probably due to the influx of phosphate associated with this manure.
Laser levelling - zinc levels decline with soil depth, so land levelling can create zinc deficits.
Sunlight intensity - constant bright sunlight affects auxin activity, which in turn impacts zinc uptake.
Symptoms of A Shortage
Characteristic deficiency symptoms include interveinal chlorosis or mottling of the leaves, small leaves, and reduced terminal growth. In severe cases there can also be dieback and leaf necrosis. Seedlings suffering from a zinc shortage often show no other symptoms other than a general stunting and the deficiency can be hard to diagnose if it is uniform. Because zinc availability can be so impacted by cold soils, the crop may grow out of an early temperature-based stunting but it will never recover its full yield potential due to the poor start. Seed treatment can be the best insurance against this slow start and it is a remarkably low cost strategy. In fact, it should probably be part of most programs where soil levels of zinc are marginal or where there are excesses of phosphorus, iron, copper or manganese. The NTS product Zinc-Life™, for example, can be applied effectively at 5 litres per tonne of wheat seed, which equates to a treatment cost of less than $2 (AUD) per hectare. Zinc-Life™ involves 65% zinc as micronised zinc oxide (at particle sizes of just 0.1 of a micron) combined with a significant percentage of kelp as a seed-start promotant.
Diagnosing and Addressing Soil Deficiencies
In soil tests we are seeking a minimum of 5 ppm of zinc but this optimum minimum is still governed by the amount of phosphorus in the soil. Maintaining the critical 10:1 phosphorus to zinc ratio is actually more important than playing the numbers game with zinc. If, for example, your soil contained just 20 ppm of phosphorus and 2 ppm of zinc, then you have the correct ratio, even though both minerals are deficient. If you were to decide to build your zinc levels to the minimum 5 ppm, because you could not afford the more expensive phosphate correction, then you would be making a mistake! Your perfect 10:1 ratio would now be 4:1 and you would now have a battle getting enough phosphate into the plant due to the inhibitory effect of zinc. The better option, in this example where budgetary constraints are the issue, would be to build both minerals to whatever level you could afford, i.e you could lift phosphorus to 30 ppm and zinc to 3 ppm thereby maintaining the all-important ratio. Soil applications of zinc should always be limited to a maximum of 30 kg per hectare of zinc sulphate monohydrate (34% zinc), as higher rates can be destructive to soil-life and are more likely to induce deficiencies in other metallic minerals sensitive to zinc. If zinc is to be included in a blend, then NTS Soluble Humate Granules™ should also be part of that blend to chelate and magnify zinc performance through the phenomenon of cell sensitisation (where the cell membranes become more permeable allowing a 30% increase in plant uptake). If the zinc is to be applied via fertigation, then the maximum application rate at one time is 15 kg per hectare but this could be repeated 4 to 6 weeks later if necessary. Fertigated zinc should always be combined with fulvic acid as zinc is not compatible (in liquid form) with humic acid. Fulvic acid offers the same chelating and magnifying potential as humic acid but it is more expensive. The most cost-effective way to use fulvic acid is as a soluble powder. Soluble fulvic acid powder should be combined with zinc at 300 to 400 grams per hectare and will offer many additional benefits beyond increasing zinc performance.
Tissue Testing and Foliar Correction
Leaf analysis is a good tool to monitor zinc status in the plant and is particularly useful when used in conjunction with a soil test. However, this test is not infallible. It offers an accurate guideline for absolute deficits and obvious excesses but the grey zone is the wide range (usually between 10 ppm and 25 ppm) that is considered to be within the acceptable range. It is common to see crops within this range which are still showing visual symptoms of zinc deficiency and respond to foliar applications of zinc. Often the best approach is to monitor the crop’s response to zinc and you may be surprised at the positive response when you may have assumed there was no problem. The foliar application of zinc is usually the most rapid and cost-effective way to address a deficiency. Foliar fertilising has been shown to be 12 to 15 times more efficient than ground applications and zinc foliars can be combined with other fertilisers or most chemicals. Chelated zinc is far more effective than simple zinc sulphate and much less likely to burn. You can purchase formulated zinc chelates or chelate your own zinc. Here are some of the options:
Zinc sulphate can be combined with fulvic acid to create a DIY chelate. Typical application rates are 2 to 3 kg per hectare with 250 grams of NTS Soluble Fulvic Acid Powder™ for row crops (or 1 to 2 kg of zinc sulphate for broadacre crops with 125 grams of fulvic powder). The best rate for orchard crops is 5 kg of zinc sulphate with 400 grams of soluble fulvic acid powder per hectare.
NTS offers a budget zinc chelate called Zinc Fulvate™ which includes a total of 17% of both fulvic acid and liquid kelp. Kelp is a chelating agent in its own right, via a long sugar called mannitol but it also offers a host of other benefits. These include broadspectrum mineralisation, plant growth promotion and improved taste and shelf life.
The NTS product, Zinc Shuttle™, offers the ultimate zinc delivery system based on the proprietary Shuttle system. In this unique technology, the zinc ions are chemically bound in a nanocluster, to which a shuttle ligand is attracted. The shuttle ligand picks up its zinc cargo and is then magnetically drawn to the negatively-charged leaf surface. It docks up against the leaf surface until a phenomenon called thermal vibration facilitates the delivery of the zinc into the leaf. The shuttle ligand, without the positively charged zinc ion, is now repulsed from the leaf and is magnetically attracted back to the nano cluster to repeat the process again and again. It can even continue delivering other cations to the plant after completing the zinc delivery, until it is eventually consumed by micro-organisms. Zinc Shuttle™ is used at 5 litres per hectare for most crops and 1 to 2 litres per hectare in broadacre crops. Note: All foliar zinc should ideally be included with a urea foliar spray as urea further increases the uptake of zinc.