Because of complex chemical reactions within the soil, nutrient availability is ultimately controlled by the equilibrium between the soil type, moisture, soil organic matter, cation exchange sites, and insoluble compounds of nutrients.
Soil acidity or alkalinity has a large effect on the tie-up of micronutrients and their availability to plants. Micronutrients are most commonly available in acid soils and often unavailable at high pH (Figure 1).
This acts as a reservoir for essential plant nutrients, continuously supplying these nutrients to the crop as it decomposes over time. This reservoir is especially important for anions such as boron, which does not bind to soil particles and is therefore subject to loss. Soils that receive regular additions of organic residues such as manures rarely show micronutrient deficiencies. An exception is deficiencies caused by nutrient imbalances, such as a deficiency of manganese caused by an excess of phosphorus in overly manured soils. Another exception is soils of extremely high organic matter such as muck or peat soils. In these soils, strong, natural chelation (the combination of a micronutrient with an organic molecule) can make some micronutrients unavailable, particularly copper, manganese and zinc.
Nutrient removal by crops
Crop yields are continually increasing due to genetic improvements in stress tolerance, disease
resistance, incorporation of insect resistance traits, seed treatments and a better (or more complete)
selection of available nutrients (applied via granular or liquid fertiliser).
This increase in crop yield also implies that micronutrients are removed from the soil at a rate proportionate to the increase in crop yield.
If the replacement of nutrients at the same rate of removal is neglected, this will mitigate efforts to sustainably increase crop quality & yield.
At Crommelin AgriCoatings® crop trial site, a noticeable yield improvement was achieved by the
addition of the more obscure trace minerals molybdenum, boron and cobalt to typical mid-wheatbelt
80% of the soils tested across most countries are lacking molybdenum. We only require 0.5 parts
per million, but most soils have less than half of that. In many cases it has been mined from our
soils through continuous cropping and rarely replaced. It is also a leachable anion, a little bit like
boron. Similar to boron, it is only stored on the humus colloid, and we have lost 2/3 of our humus
with the chemical, extractive model. It is also used in the conversion of nitrates to protein. If
nitrates are oversupplied, more molybdenum is required.
The first major role of Molybdenum relates to free access to the atmosphere’s available nitrogen gas. We were supposed to have access to those 5000 truckloads of urea (equivalent). In fact, it is essential, if we are seeking to maximise resilience.
In the soil, we are seeking equal amounts of ammonium nitrogen and nitrate nitrogen, while ensuring that we never exceed 20 ppm of each. It is very difficult to ever achieve the desirable ratio of 1:1 between these two minerals as they are constantly swapping around in the soil. The important thing is to have enough of this most abundant mineral in the plant but to avoid oversupply when we decide something is good by applying even more.
Interestingly the desirable ratio between these two forms of nitrogen in the plant is different to the soil. Here, we want three parts ammonium nitrogen, to one-part nitrate nitrogen.
You would assume that the leaf would parallel the soil, as the plant has equal access to both forms
of N in the soil?
As with many things in the soil and plant, it often comes back to microbes. The zone immediately
surrounding the roots that is receiving the glucose exudates from the plant, is called the rhizosphere. This is where most of the soil life is based.
Consequently, if we are seeking to monitor soil biology, we take soil samples from near the roots.
There is also considerable life on the leaf surface. The plant pumps out carbon exudates from the
leaf to sustain another community of diverse organisms. This feeding zone is called the phyllosphere. Amongst the leaf dwellers are free living nitrogen-fixing bacteria, like Azotobacter. This
direct fixation of nitrogen into the leaf partially accounts for the higher percentage of ammonium nitrogen within the plant, compared to the soil. There are also armies of nitrifying bacteria in the soil, converting ammonium nitrogen to nitrate N, whenever it is moist and warm. This warmth and
moisture factor explain why there is often higher nitrate levels in sub-tropical soils, but does not explain the mode of action molybdenum plays. Both soil and leaf dwelling nitrogen-fixers create an
enzyme called nitrogenase, which converts the nitrogen gas in the atmosphere into ammonium nitrogen in the soil, or on the leaf. The key building block for nitrogenase is molybdenum. If you are
one of the 80% of farmers missing this mineral, you cannot access the free gift. You would then have to invest more into nitrogen from a bag, and you will never achieve this desirable resilience ratio.
There is one other hugely important role. Nitrate nitrogen is housed in the leaf, awaiting an energy intensive conversion to protein. It is protein that drives the immunity of plants, animals and
humans, so this conversion is important. It is the nitrate reductase enzyme that catalyses this
conversion of nitrates to protein. This enzyme requires molybdenum. If you are one of the 8 out of
10 farmers missing this mineral in your soil, you are producing low brix, nitrate-dominant plants
that are more attractive to insects and disease.
Nitrates can also be carcinogens when oversupplied in our diet. They restrict the oxygen carrying
capacity of our blood. Professor Otto Warburg won his Nobel prize for identifying the root cause of
Boron is shown as deficient in most of the leaf analysis. We like to see luxury levels of this mineral in the plant and it is one of the four minerals critical to maximum production.
The other members of this productivity group include calcium, magnesium and phosphorus. It is important to have optimal levels of this group to achieve a good outcome. Boron is the surprise member in this exclusive group, but when we look more deeply at the key roles of this trace
mineral, you come to understand its importance.
This element is calcium’s indispensable sidekick. Many do not realise that you will never see the full
benefits of liming, in a boron deficient soil. Gary Zimmer said it well when he claimed that, “Calcium is the trucker of all minerals, and boron is the steering wheel”. That’s exactly how it works. Calcium
sponsors the cell division required for a healthy spring flush, for example, but it does so much less
effectively, in the absence of boron.
Boron has several important roles apart from synergising calcium. It is hugely important before
flowering and most crops we monitor have insufficient boron in the plant at this time. Boron
improves the fruit to flower ratio by lengthening the male pollen tube to improve reproductive
efficiency. One of the single most productive strategies any grower can embrace involves a simple
foliar spray of boron before flowering.
Typically, the recommended rate for a foliar spray would be one kilogram of sodium borate (soluble
boron) per hectare on orchard, vine and vegetable crops, but it might be 700 grams on a broadacre
crop, due to the lower water rate used. Sodium borate should always be combined with humic-acid
to stabilise, buffer and magnify the boron. A boron humate cannot leach, and it is up taken 30%
more efficiently than the standalone product. Our trials in fertilizer coating indeed have confirmed
Avocados seem to have a huge hunger for this mineral. Growers who understand this thirst will
often fertigate several kilos of soluble boron before flowering and then foliar spray double rates (2
kg per hectare), as well. Avocados have many flowers, but a very low fruit to flower ratio. We have
seen yields double when the male pollen tube is lengthened, in this manner, to enhance pollination.
Boron does play an important role in our health. Boron deficiency in humans is strongly linked to
osteoarthritis and sore joints in general. This shortage is also linked to osteoporosis and hormonal
deficiency, particularly testosterone. There is evidence to suggest that boron works synergistically
with magnesium, so it would be a good strategy to supplement magnesium in conjuncture.
Cobalt (Co) is an element which is not commonly thought to play as crucial of a role in the health of a plant. However, cobalt is one of the elements which is classified as an essential micronutrient. Cobalt is intimately involved in many different processes of growth which a plant undergoes to reach its full potential. One of these processes is the process by which the plant stem grows and the coleoptile elongates. These two functions both assist in the overall growth of the plant as well as the availability for CO2 absorption by the plant. The plant also needs sufficient levels of cobalt in the system in order to properly expand the leaf discs. This becomes vital for the plant to reach full maturity, in addition to the curvature of slit stems and ultimately full, healthy bud development.
In addition to the functions which help to assist in the outward appearance of plants, by promoting
the development of plant buds, leaf discs, plant stem and coleoptile, cobalt is very important to many
of the chemical and biological reactions which a plant must undergo in order to survive. Co is a
primary constituent of Vitamin B12 as well as Propionate. Propionate actually serves as one of the
main sources of energy which a plant uses to continue to grow. Vitamin B12 is needed for cell
division, which plays a major role in the growth of a plant because it provides more cells for the plant
to continue to grow. One other function which is aided or promoted by cobalt levels is the nitrogen
fixing ability of legumes in the plant. It also helps to improve the efficiency of ruminal digestion.
There are a few different signs and symptoms that point toward a cobalt deficiency and if you notice
any of these, we suggest that you urgently treat and work to correct these deficiencies. One of these potential symptoms could be that of small root nodules on legume species of plants. However, if you begin to notice an overall pale green colour on the leaves that is fairly uniform, this can be fairly specific to a cobalt deficiency, or on old leaves this discoloration can be more pronounced and actually be more of a yellow colour. However, it could also be presented in a reddening of the leaves, stems or petioles. But discoloration is not the only form of cobalt deficiency as there can also be stunted growth with the tops of the plants producing less than what would be normal leaf production. There is also typically a retardation of the grain or seed production. The retardation and slowing of the reproduction for plants can lead to significantly decreased crop
production from one generation of plants to the next.
This means that the presence of cobalt is very important for the continued production of crops
regardless of the plant type. However, it is true that some plants are more resistant to a Co deficiency than others. Regardless of the case however, there is one commonality. That is when these symptoms begin to present themselves, the key is to begin treating them as soon as possible.
Cobalt availability is dependent on the manganese content of the soil. Application on low humus soils (i.e. less than 1,000 ppm) will not be of any benefit – providing only 1 year of availability!