Iron and manganese powerfully impact each other and the ratio between these two minerals is one of the six key mineral ratios we strive to perfect. Trace mineral deficiencies are characterised by chlorosis, a loss of the green pigment housed within the sugar factories that determine crop production. In our principal role as chlorophyll managers, we are always seeking to avoid chlorosis. I am hopeful that the following paragraphs will help improve your understanding of two key players in your trace mineral team.
Iron – Reluctance in Abundance
Iron is one of the most abundant minerals in the world, but there are times when the release of this trace mineral is compromised. It is important to understand these limitations and to minimise their triggers.
Iron (along with molybdenum) is an essential element required for biological nitrogen fixation.
This mineral is a key player in photosynthesis. It is required for the synthesis of chlorophyll and for maintenance of the structure and functioning of the chloroplasts (the sugar factories).
Many metabolic pathways are activated by iron, including several that are directly related to plant immunity.
Adequate iron, in plant-available form, is essential for protein synthesis.
Iron is strongly linked to all-important oxygen. It is an indispensable oxygen carrier for chlorophyll production and it is a central component of respiratory enzyme systems.
Iron increases leaf thickness and darkens the leaf, so that the greener, more robust solar panel can absorb more solar energy.
Most soils contain between 20 and 200 tonnes of iron per hectare, but very little of this reserve is in plant-available form. The potential for problems is also magnified by the fact that iron does not move easily within the plant.
Iron is one of the few minerals where deficiencies are not reliably detected with leaf analysis data. If the test figures are low, then there will definitely be a deficiency, but there can often be a deficiency present that is not reflected in the data.
The best strategy to counter the unreliability of leaf tests is to learn to visually identify an iron deficiency in your crop.
In vegetables, orchard crops and cereals, the symptoms are very similar. The youngest leaves develop a light green chlorosis of all the tissues between the veins. This is a more complete loss of chlorophyll than with any other trace mineral shortage. It is a total paling, with the exception of very thin, dark green veins. This contrasts with the much thicker green veins and mottled chlorosis that is a feature of other trace mineral deficiencies.
In severe cases, the chlorosis becomes yellow or even white. Older leaves can remain green, while emerging leaves become increasingly chlorotic, as a result of the poor mobility of iron within the plant.
In small grains, the leaf blades develop yellow stripes between green veins, and upper leaves can turn completely yellow.
In our Soil Therapy™ guidelines we look for iron levels of between 40 ppm and 200 ppm. However, there are a variety of conditions that impact iron availability, regardless of soil levels. These include:
Excessive phosphate applications or high phosphate levels in the soil.
High manganese, which reduces iron uptake (excessive copper or molybdenum can also cause iron shortages).
Cold, wet conditions limit iron uptake, particularly in the early growth stages.
Excessive lime applications reduce iron availability.
Inadequate soil aeration hinders mobility.
High soil pH (7.5 or higher) – A foliar application of iron should always be considered in these situations.
Low organic matter is another limiting factor for iron nutrition.
Always be aware that if you are overapplying chicken manure in a soil that does not require more phosphorus, you will be inevitably creating an iron deficiency in your crop.
Similarly, if you are overdoing copper fungicides, the copper will accumulate in your soil and reduced iron availability is just one of the consequences.
The greatest tool to increase the solubility of iron in all soils is humic acid. There are now several studies demonstrating this phenomenon. The effect is so pronounced that we often just fertigate humic acid to address an iron shortage.
Manganese – The Seed Energiser
Interestingly, manganese is associated with energy production in human cells, just as it energises seed germination and seed production. It becomes particularly important in this context because many of us are effectively growing seed. Even the saleable weight of fruit will be impacted in the absence of sufficient manganese.
1) Photosynthesis – Manganese is hugely involved in photosynthesis, to the point that a deficiency can contribute to low brix levels and associated insect and disease pressure. This impact upon photosynthesis is not just the chlorosis (lack of chlorophyll) that comes with the green-veined anemia found on the young leaves of manganese deficient plants. Manganese is required for the process of photolysis of water (where photons split water molecules into hydrogen and oxygen). This process effectively kickstarts the chemistry of photosynthesis and the associated production of glucose, the building block of all life.
2) Pathogen protection – This is a hugely important role of this mineral and it is part of the reason that crops become so much more susceptible to disease when manganese availability is compromised by glyphosate. There are four components of this protection: hydrogen peroxide, phytoalexins, lignin synthesis and the shikimate pathway.
Hydrogen peroxide is produced naturally in plants, animals and humans and the process involves the peroxidase enzyme. This enzyme is one of 35 enzymes that are manganese-dependent. Hydrogen peroxide helps to stabilise and strengthen the cell wall (the barrier) and it also serves as a natural fungicide.
Phytoalexins are the plant equivalent of antibodies in our immune system. They are markers of plant immune competency. An exciting, emerging strategy for proactive pest management involves the use of immune elicitors to boost the plant production of phytoalexins and associated resilience. Manganese is part of an enzyme that is an essential requirement for the biosynthesis of phytoalexins. Again, we begin to understand the serious issue of increased disease susceptibility linked to glyphosate and other "mistakes", like the oversupply of copper, which limit manganese availability.
Lignin is a cell wall strengthener that is also critically important for the conducting of water in plant stems. If lignin synthesis is compromised there will always be less resilience in terms of disease and drought tolerance. Several manganese-dependent enzymes are involved in the formation of lignin. Manganese deficient plants are particularly susceptible to root rot diseases due to weakened, lignin-deficient roots. Other common diseases like take-all in wheat, powdery mildew in grapes and potato scab are also linked to manganese deficiency.
The shikimate pathway is also manganese-dependent. This is the fatal flaw in the glyphosate story because this pathway governs immune competency in plants and most microbes. This herbicide has effectively compromised immunity in all other living things. Plants die from glyphosate because they now have no protection against opportunistic pathogens. You can not kill a plant with glyphosate if the soil has been sterilised (i.e., there are no pathogens to kill the plant). Unfortunately, humans are not immune to the effects of the world's most widely used chemical, because the 100 trillion organisms that live in our digestive tract do have a shikimate pathway. When these creatures are compromised it impacts every aspect of our health.
3) Insect resistance – Insects feast upon two reducing sugars called glucose and fructose. A manganese-dependent enzyme called sucrose synthase converts these sugars to sucrose, which is much less attractive to insects. We are measuring sucrose, rather than these reducing sugars, when we use a refractometer to measure brix levels and associated insect resistance. There is also a link to nitrate nitrogen. Drought conditions shut down the nitrate reductase enzyme, which leads to an over-concentration of nitrate nitrogen in the leaf (this is an even bigger problem if you are also lacking molybdenum). The excess nitrates shut down manganese, and then the insects party amidst the associated sugar imbalance. In one sugar cane study, conducted by Professor Don Huber, manganese in the plant tissue was tied up, chelated and immobilised within 4 – 6 hours of a glyphosate application. There was a rapid reduction in sucrose and an increase in glucose and fructose in this crop and, sure enough, the insect pressure began shortly thereafter.
4) Stress resistance – We have covered the powerful link between manganese and biotic stress (insects and disease), but there is an equally important connection to abiotic stress, which is environmental pressure associated with weather extremes. In this context, it is fascinating to note the parallels between plants and humans. Our bodies also make hydrogen peroxide for protection. Our most important defensive enzyme system is called glutathione peroxidase, which is selenium-dependent. It turns out that plants require selenium for similar purposes. We depend upon a free radical scavenging system called Super Oxide Dismutase (SOD) to counter oxidative stress. The plant does the same. There is a manganese dependent form of SOD responsible for increasing plant tolerance to drought stress, salinity, and other environmental stressors. Winter hardiness, for example, is directly linked to manganese availability.
The wax layer on the leaf and stem surface is an important, physical player in abiotic stress resistance. This layer limits non-stomatal water loss and reduces the impact of excess heat on the leaves. Manganese is required to produce the fatty acid building blocks for this waxy shield. Weakening of this protective coating has been shown to increase susceptibility to heat waves, and there are no shortage of these extremes in the brave new world of global warming. In a 2009 study by Hebbern and associates, they found that barley suffered a 40% loss of this wax layer in the absence of sufficient manganese. The crop suffered increased water loss and much less water use efficiency.
5) Seed support – All good seed treatments feature manganese because it has such an impact on germination and early vigor. However, this mineral is equally important for seed formation and development. Many of us, including fruit and nut producers, are effectively growing seed and if we are lacking the seed energiser we will suffer. This link to seed production is one of several reasons that manganese has been so often linked to yield increases in many studies.
Deficiency of this mineral is increasingly common and if you are assaulting your soil with copper fungicides and glyphosate, you can pretty much guarantee that you have compromised availability. There are other factors that impact manganese, including:
Alkaline soils seriously impact manganese availability and foliar correctives are an essential strategy.
High iron in the soil will impact manganese, so deficiency of this mineral is common in the red soils (the red colour is iron oxide).
Sandy soils are notoriously deficient as they lack the clay component that houses the cation, manganese.
Glyphosate kills manganese reducing organisms responsible for solubilising and delivering this mineral. Glyphosate breaks down to another toxic substance called AMPA, which persists and accumulates in the soil for 11 years.
Copper is similarly persistent in the soil. As a heavy metal, it does not leach, it builds and builds. This mineral negatively impacts all beneficial soil life, while antagonising the uptake of manganese. It is actually the least sustainable of all fungicides, and yet it is the principal disease management tool in organics.
Tropical soils that are heavily weathered are also prime candidates for manganese deficiency, particularly during the monsoon.
High calcium, from calcitic soils, or those that have been over-limed, will also impact manganese availability.
Calcium, when mismanaged, can have a big impact on this mineral. If the soil is calcium deficient, the problem is reversed. Manganese becomes more available in acidic soils and can become toxic. Manganese toxicity often exhibits as brown or yellow spots or blemishes that can devalue crops dependent upon attractive foliage.
Manganese is poorly translocated around the plant (poor phloem mobility) so that the deficiency symptoms first appear on the new leaves.
Deficiency symptoms include a mottled, interveinal chlorosis on most fruit trees while in cereals manganese can cause pale green or yellow patches on younger leaves. Necrotic spots then form on older leaves, as part of a condition called "grey speck".
Manganese deficiency is best addressed with foliar sprays because this direct injection into the leaf helps bypass the many factors in the soil that can compromise plant uptake.
We like to see between 30 ppm and 100 ppm of manganese on a Soil Therapy™ report but, most importantly, we always consider the iron to manganese ratio. The ideal Fe:Mn ratio in the soil would always see iron levels higher than manganese. More iron should be present, but never more than two parts iron to one part manganese. Higher iron levels will usually impact manganese availability.
Deficiencies can be addressed with fertigation or liquid injection of manganese sulfate but a foliar spray of chelated manganese provides the most effective and rapid corrective. Nutri-Key Manganese Shuttle™, as a foliar is the very best delivery system for manganese. It dramatically outperforms EDTA chelates and it is more cost effective. Farm Saver® Manganese Fulvate is a high performance cost-effective option for broadacre, while organic farmers can use Manganese Essentials™.
If you are using glyphosate, it is critically important to compensate for the associated shutdown of manganese and copper. Glyphosate-induced deficiencies of both minerals will always cost you in terms of reduced yield and increased pest problems. A foliar spray (or two) of Farm Saver® Cop-Man Fulvate is a productive strategy for all crops when glyphosate is involved.
All crops require manganese but some crops have been shown to be particularly responsive in terms of increased yield, structural integrity and disease resistance. These manganese lovers include cereal crops, legumes, stone fruit, potatoes, canola and citrus.
Although there are different leaf level "ideals" for manganese required for different crops, it is important to try to maintain an absolute minimum of 20 ppm according to tissue test (leaf testing) results.
Manganese is critical at the very start of the plant's life to improve germination and kickstart the seedling, but, as mentioned, if you are growing seed or fruit crops you will always benefit by boosting seed formation. In this context, foliar sprays of manganese, before and during the business end of the season, can be of real value.
In the final segment of this series, we will cover cobalt, molybdenum and selenium.
To read Part 8 of this feature, please click here.
To go back to Part 1 of this feature, please click here.