We have covered the major elements and some key trace minerals in previous postings, but here the focus will be upon the minor minerals, zinc and copper. The concept of major and minor minerals can be misleading, in that we might assume that the majors are more important. This is not the case. The major minerals are simply more abundant, as they are required in larger amounts for their structural and metabolic roles within the plant. The minor minerals typically serve as catalysts or spark plugs, and hence they are required in much smaller amounts. A lack of zinc can actually be as costly as a lack of magnesium, as you will come to understand.
Zinc (Zn) – energy, leaf size and soil-life essential
A deficiency of this trace mineral can be amongst your most costly oversights, because it will always impact yield. Let’s look at why zinc can wield such a punch.
There are five key roles of zinc, which include the following:
1) Zinc is often called the "energy trace mineral" because it is required for phosphorus to successfully build "the battery of life", ATP (adenosine tri-phosphate). ATP is required to transport energy to where it is needed.
2) Zinc has also been called "the drought mineral" because it is required for efficient plant uptake of moisture, particularly in dry conditions.
3) Zinc is of particular importance for Azotobacter, the free-living, nitrogen-fixing organisms that source nitrogen from the atmosphere. In fact, these creatures struggle in the absence of zinc.
4) This trace mineral is also a major player in the transformation of plant sugars into complex carbohydrates.
5) Most importantly, zinc is required to make plant hormones called auxins and these critically important substances, in turn, govern leaf size. A zinc deficiency will always mean smaller solar panels (leaves), reduced photosynthesis potential and less yield. This is why a zinc deficiency can be so costly.
Zinc deficiency is characterised by interveinal chlorosis, which means the colour drains from between the deep green veins, and this loss of sugar factories (chloroplasts) reduces production. This loss is further exacerbated by a smaller leaf size and less glucose production. This is why a zinc deficiency can often be as costly as the loss of a major mineral.
It is always wise to avoid inducing deficiencies when addressing any trace mineral shortage. If another trace mineral is marginal, it is so easy to overstep the mark with your target mineral and affect the uptake of another. Zinc, for example, antagonises the uptake of iron, sulfur and copper and, as mentioned, an excess can also impact phosphorus uptake. Ideally, these minerals should also be considered when applying zinc.
Ideally zinc should be present in your soil, according to soil test data, at 5 – 10 ppm. The "ideal" is dependent upon phosphorus levels, because we are trying to achieve a phosphorus to zinc ratio of 10:1.
In this context, there is an issue If you have oversupplied phosphorus, because this excess will limit your zinc uptake. This is a common scenario when chicken manure has been abused and misused. In this instance, it is essential to forget about the ideal phosphorus to zinc ratio of 10:1 – if you shoot for this numbers game, you will come unstuck. Lifting zinc to try to counter high phosphorus levels will only induce deficiencies of iron, sulfur and copper. If you have overdone phosphorus (P), you will be locked into regular foliar sprays of zinc to compensate for your mismanagement of P.
A zinc deficiency will always be expensive because you will have a smaller than optimum leaf size and production will suffer with that smaller solar panel. Leaf analysis will help you diagnose a shortage and perfect zinc nutrition.
Zinc sulfate can be simply chelated with fulvic acid to create an inexpensive zinc fulvate that is very well absorbed.
If you can't afford to correct your soil levels of zinc, an inexpensive liquid inject with zinc sulfate and fulvic acid will provide some early zinc to help get you through the season. There is also the option to foliar spray Farm Saver® Zinc Fulvate (broadacre), Nutri-Key Zinc Shuttle™ (horticulture) or Zinc Essentials™ (Australian Organic Registered Farm Input) to provide in-season zinc nutrition.
Copper fungicides can induce zinc deficiencies. These inputs are amongst the most destructive of all farm chemicals, because copper is a biocide that does not leach. Copper levels build and build with the fungicides until they become a serious liability, impacting beneficial fungi, bacteria, protozoa and earthworms.
Copper (Cu) – natural protection and protein
Copper, when oversupplied, can become a biological liability, as it can kill most creatures in the soil foodweb. However, if it is undersupplied, you will have low protein in your produce and much less resilience.
A copper deficiency will present as reduced chlorophyll density and less photosynthetic potential.
Copper is also important for stem strength, so you may see lodging in field crops, or branch snapping in tree crops, when this mineral is missing.
Copper is often called the "protein mineral", because it is required to lift protein levels (particularly in wheat crops).
Another important role of copper, however, relates to its link to fungal protection. We drench our crop leaves with copper hydroxide and copper oxychloride to combat fungal pathogens on their surfaces, and we can poison our soils in the process. Copper is much more effective when delivered efficiently into the plant rather than onto the plant. The NTS favorite, Nutri-Key Copper Shuttle™, for example, is widely used to address this issue, and it can yield remarkable results.
There are usually better alternatives to copper fungicides. It is a big price to pay if you have overstepped the mark with these chemicals, because their impact upon your soil workforce can be long term. Copper stays in the soil like a heavy metal. It accumulates and does not leach.
We work with citrus growers, who have used 100 g of copper sulfate per 100 L to control black spot for decades. They apply as much as 3500 L of water per hectare each treatment, and this equates to 3.5 kg of copper sulfate per hectare, per application. When your soil test reveals copper levels beyond 100 ppm, you are effectively beginning to grow your crop hydroponically.
Beneficial soil life is seriously impacted in the presence of so much of this biocidal mineral.
Our horticulture soils need between 5 ppm and 8 ppm of copper, but broadacre soils can still be productive with as little as 2 ppm.
There are some cautions with addressing copper deficiencies. Broadcast rates should never involve more than 15 kg of copper sulfate per hectare. Higher rates can be biocidal, or they can induce deficiencies of other minerals, like zinc.
If you are fertigating copper sulfate to address a deficiency, you should never apply more than 5 kg per hectare, and even that amount should always be buffered with fulvic acid.
High copper in your soil shuts down phosphorus, zinc and iron. We work with citrus growers where every leaf test reveals serious shortages of these three minerals, regardless of their respective levels in the soil. The grower might have high levels of all three of these important minerals, but the high copper limits their uptake. These growers have created a rod for their own back. They are forced into regular foliar regimes to maintain sufficient phosphorus, zinc and iron within the plant.
We have discovered a productive strategy when copper levels are this extreme. We have found that high applications of fertigated humic acid can immobilise high copper and the associated negatives. 10 – 12 kg per hectare of NTS Soluble Humate Granules™, when solubilised (i.e., DIY liquid humic acid) and fertigated, can immobilise and buffer the copper excess. This high rate of humates also stimulates mycorrhizal fungi. Interestingly, these creatures will, in turn, stimulate improved delivery of immobile minerals like phosphate and zinc. Humic acid also increases plant availability of iron, so this practice can effectively counter all of the three mineral antagonisms associated with excess copper. Humic acid offers multiple other benefits, so it is a genuine win/win scenario. Try it and you will be impressed!
Next week we will look at iron and manganese, in our ongoing exploration of more productive mineral management. I trust you are all enjoying the journey.
To read Part 7 of this feature, please click here.
To go back to Part 1 of this feature, please click here.