A few weeks ago, I had an interesting chat with a man called Charlie Bannister, who works in the nutrition area with Headland Agrochemicals Ltd. Charlie has a passion for and deep knowledge of nutrition and the importance of trace elements. The following questions and answers should help growers to better understand what trace or micronutrients do, but Charlie will always emphasise that nutrition is about all elements and also about soil health.
What are micronutrients?
Micro-nutrients include copper, manganese, zinc, iron, boron, molybdenum, chlorine and nickel, each of which is considered to be an essential element. An essential element is one which the plant needs to complete its lifecycle from seed to seed. Some such as molybdenum and nickel are only needed in very small amounts but are still essential. Molybdenum is necessary for the assimilation of nitrogen so, although it is only needed in small amounts, a deficiency can compromise the utilisation of nitrogen.
Compared with macronutrients, micronutrients are only need in small amounts by the plant but without them, or in the case of a deficiency, plants fail to function and grow to their optimum. So, they are just as important as macronutrients – it’s just that the amount needed is far lower.
What are the critical availability times?
All plants go through an exponential growth phase when they accumulate large amounts of dry matter. This is even true for perennial plants, although it is less evident. This is a critical period for the availability of all nutrients, so it is where deficiencies can be most damaging. However, there are other critical periods also. Availability of all nutrients is important during autumn growth to enable winter crops to put down roots. Manganese is critical in autumn to give plants winter hardiness as manganese deficiency can reduce the amount of leaf wax. Research has shown that boron deficiency is associated with reduced winter hardiness in oilseed rape.
There are other critical periods which can relate to the specific functions of a particular element. Copper provides one example of this as it is important for the production of lignin in plants. There are four copper atoms present in an enzyme which is responsible for lignin production.
Copper deficiency can also have an effect during the flowering process by reducing pollen viability and by compromising the natural opening (dehiscence) process, which is important for splitting open the pollen sacs for fertilisation.
Copper is important at GS29-31 when the flowering structures are being formed and probably at GS39 when pollen production begins.
What are the main plant functions limited by the different nutrients?
While plants only need 17 essential elements, although they can take in many more (I have seen a list of 60 different elements mentioned as being found in plants), there must be millions of different processes that occur in the plant and even more different compounds produced that enable these processes to continue. Therefore, essential elements have many functions.
However, I like to focus on photosynthesis, which is the most important process in the planet and the one on which every living thing depends.
It is the ultimate “free energy process” and it is what the farmer uses to turn plants into food. You can see where most of these elements fit in photosynthesis in the chart.
If manganese is not available in the plant and the splitting of water and release of oxygen is stopped, this compromises the whole photosynthesis process in the plant. Manganese is vital to the process of photosynthesis.
Formulation
Does micronutrient
formulation matter?
Yes, absolutely. However, there is no absolute consensus about how these plant nutrients get into the plant through the leaf. And when I say “no consensus”, I mean within the relevant scientific research community.
I have spoken to professors in Germany, Spain and America about this topic. I asked if entry is directly through the cuticle, through the stomata, or through the guard cells around the stomata. All may have a role but the general feeling is that the elements enter the leaf in solution. However, I have seen some recent research results, which suggest that very small particles, rather than solutions, can enter the plant.
Foliar uptake is influenced by such factors as the solubility of the nutrient (different salts have different solubilities) and a characteristic called the point of deliquescence (POD), which relates to how different salts /formulations dry out on the leaf surface at different humidities.
Taking these comments into consideration, formulation is important to enhance spread, maybe to enhance a temporary “melting” of the waxy cuticle and maybe to reduce drying out.
What formulations
are available?
Formulations are many and varied. True chelates (based on EDTA) used some years ago brought the advantage they were compatible in the spray tank but were usually low in element loading and expensive. The chelate protected the element from an incompatible reaction in the tank. But I do not believe that they offered anything in terms of enhancing uptake despite the claims made.
Next were the suspension products, based largely on relatively insoluble forms (see previous comment about solubility being a factor in the efficiency of foliar uptake). They were cheaper than true chelates and were also compatible in the spray tank. They may or may not have been formulated with adjuvants/stickers/spreaders to enhance uptake.
Suspensions tend to be heavy-loaded products, ie they contain high levels of element in the formulation which is a key advantage.
Then there are the true solutions based, for example, on manganese nitrate or magnesium nitrate. These are very soluble forms of the element. These also have low PODs, which means they dry out less quickly than other forms. They give a quick supply of element (seen in experiments with tissue tests after application) compared with other forms.
There are also products based on soluble salts with added humic acid, eg Verdi-Crop Magnor, which contains magnesium nitrate and magnesium chloride (both salts are very soluble) and is formulated with humic acid. Humic acid has been shown to act as a bio-stimulant in its own right and to enhance the uptake of plant elements. It also acts as a complexer, which means the element is protected to an extent from incompatibility in the spray tank
Quantity v formulation
The optimum situation is to supply a good amount of element in a well formulated product. Again, taking Magnor, we have two available salts, which are very soluble. Then we enhance that product with good formulation technology, which includes the use of humic acids in the formulation. But a product containing magnesium nitrate alone without the humic acid would be very acceptable in terms of meeting the needs of the plant because it is very soluble.
If you take an insoluble source of magnesium and use excellent co-formulants with known spreading and penetrant activity (we have done work at University of Amsterdam looking at spread patterns of all our co-formulants), plant availability is restricted because the magnesium source is insoluble. In summary, you need a good formulation with a good loading of nutrients.
Interactions
Interactions between micronutrients
There are known interactions between individual micronutrients, but I think they are of far less important than the interactions between macro and micronutrients. To get an extreme antagonism between micros you need a significant oversupply of one or the other and this would be rare.
Interactions between micro and macronutrients?
Such antagonisms are becoming increasingly accepted. These have long been accepted as being soil-based but they are now being considered as existing within the plant. The big potential antagonist is calcium, which affects pH and element availability, eg manganese. Calcium can occlude or attach to other elements such as zinc to its surface or it can react with specific elements such as phosphate to make them less available.
Phosphate has a particular antagonistic relationship with zinc. It seems that it goes out of its way to antagonise zinc and high P reduces zinc uptake. Potassium can affect the uptake of magnesium and sulphur can reduce the uptake of molybdenum and probably boron as well. This is something that must be borne in mind for oilseed rape, in particular.
Does soil type play a role in these interactions?
Yes, it can have an effect. For example, in calcareous soils with high levels of calcium, it can antagonise element availability. That, I believe, is why we have had seen yield benefits from the application of foliar boron on oilseed rape even with high levels of soil boron.
Similarly, high calcium soils can antagonise the uptake of iron, though it would appear that this is facilitated indirectly by bicarbonate rather than calcium. Calcium can antagonise the uptake of a number of elements in a number of ways.
Clay soils tend to have good inherent levels of potassium and this can antagonise the uptake of magnesium (although magnesium is a macroelement).
Poor soil structure can affect element availability by reducing root penetration and restricting the movement of elements through the soil profile. However, this may not be an antagonism as such.
Other factors
There are many factors which can affect element availability eg soil type/soil structure/weather/rainfall etc. However, these relate more to availability rather than interactions between elements or antagonisms.
Deficiencies
Timing of treatments
My preferred timings on autumn cereals would be GS21 in autumn, followed by GS31 in spring. If I had only one shot, I would go for GS31 but for the best consistent yield benefit opt for the double application at GS21 and GS31. The next best timing would be GS39-45, specifically for elements like magnesium and potassium.
On spring cereals, I would opt for one shot at GS29-31. However, problems such as manganese and magnesium can need earlier treatment in dry springs or deficient sites.
Repetition of treatments
My view is that a programmed approach is best, maybe as a drip feed of elements using a product that covers a number of elements. Where you have a known deficiency of certain elements, then target that with the specific products.
Can we rebuild soil
micronutrient levels?
The only way I can see to do this is by building up soil organic matter (SOM) levels but even this has its drawbacks.
Applying large quantities of element to the soil may not provide a solution. If you apply lots of manganese to the soil as granules (these are commercially available) this does not guarantee that manganese will be crop available. Manganese availability is tied up with many soil reactions which depend on such factors as organic matter levels (high OM can lock up manganese as seen on peaty land), pH and what is called redox or reduction oxidation reactions.
Manganese is taken up by the plant in one form but, in the soil, it can easily change form to one which is unavailable depending on the balance between air and water.
Manganese is immobile in the soil and any factor which restricts mobility restricts plant availability as well. So factors such as these can over-ride the actual level of nutrient in the soil.
You could apply boron to build up the inherent levels in soil. However, in a calcareous soil, the availability of boron could be antagonised by the presence of high calcium levels.
Rebuilding SOM levels
Personally, I think the case for building soil organic matter is strong and for many good reasons. However, building SOM as a vehicle to rebuild micronutrient levels is not jhigh on the list of good reasons for me. That said, in times of drought, SOM holds more water so, logically, crops would be less drought affected and better able to access all nutrients.
Also, SOM, and humus in particular, increases the cation exchange capacity (CEC) of soils, so they are better able to act as a reservoir for all positively charged cations.
As mentioned previously, SOM can tie up some elements. On the one hand, higher SOM will be a good source of micronutrients, but, on the other hand, it can tie up certain elements. Boron is held well in organic matter. In mineral soils, negatively charged ions such as sulphur and chlorine are subject to leaching.
Organic matter
Can added organic matter supply trace elements?
It will help but, as mentioned previously, availability can be affected by processes that over-ride the supply from the by SOM. And it is also difficult to indicate how long it might take to achieve a worthwhile increase in the supply of micronutrients from the soil via added organic matter.
Micronutrients are among the 17 nutrients that are essential for plant growth. Solubility and formulation matter to the efficacy of element uptake.Supplying micronutrients to the soil is not a guarantee of adequate availability from that source. Micronutrients can be held and supplied through the soil in organic matter, but other factors can impinge on their availability.What is the role of crop nutrition in photosynthesis?
Plants need 17 essential elements to complete their lifecycle from seed to seed.Most of these have a direct role in photosynthesis. Photosynthesis underpins every other process in the plant. Crop nutrition is fundamental to photosynthesis.MAGNESIUM is the central atom surrounded by NITROGEN atoms in chlorophyll (PS11).
MANGANESE is the main atom in the oxygen-evolving complex, but CALCIUM and CHLORINE are also part of the structure.
COPPER is the central atom in plastocyanin, a protein involved in electron transfer between the two photosystems. Also present are SULPHUR and NITROGEN containing amino acids
Ferredoxin is an IRON and SULPHUR-containing protein involved in electron transfer in PS1.
PHOSPHORUS is an integral part of ATP (adenosine tri-phosphate).
POTASSIUM plays both an indirect and direct role in photosynthesis. It influences stomatal opening, hence affecting carbon dioxide concentration in the leaf. It regulates the balance of ions within cells and has an effect on the synthesis of ATP.
ZINC is part of an enzyme, carbonic anhydrase (CA), involved in the transport and utilisation of carbon dioxide. Good levels of CA are important for crops like maize.
BORON: The primary role is related to cell wall structure and function but a deficiency can reduce the efficiency of PS11. Boron has an indirect and secondary, rather than primary, effect on photosynthesis.
MOLYBDENUM has an indirect role but involved in the assimilation and transport of nitrogen.
Note: Inadequate crop nutrition = inefficient photosynthesis = reduced yields
A few weeks ago, I had an interesting chat with a man called Charlie Bannister, who works in the nutrition area with Headland Agrochemicals Ltd. Charlie has a passion for and deep knowledge of nutrition and the importance of trace elements. The following questions and answers should help growers to better understand what trace or micronutrients do, but Charlie will always emphasise that nutrition is about all elements and also about soil health.
What are micronutrients?
Micro-nutrients include copper, manganese, zinc, iron, boron, molybdenum, chlorine and nickel, each of which is considered to be an essential element. An essential element is one which the plant needs to complete its lifecycle from seed to seed. Some such as molybdenum and nickel are only needed in very small amounts but are still essential. Molybdenum is necessary for the assimilation of nitrogen so, although it is only needed in small amounts, a deficiency can compromise the utilisation of nitrogen.
Compared with macronutrients, micronutrients are only need in small amounts by the plant but without them, or in the case of a deficiency, plants fail to function and grow to their optimum. So, they are just as important as macronutrients – it’s just that the amount needed is far lower.
What are the critical availability times?
All plants go through an exponential growth phase when they accumulate large amounts of dry matter. This is even true for perennial plants, although it is less evident. This is a critical period for the availability of all nutrients, so it is where deficiencies can be most damaging. However, there are other critical periods also. Availability of all nutrients is important during autumn growth to enable winter crops to put down roots. Manganese is critical in autumn to give plants winter hardiness as manganese deficiency can reduce the amount of leaf wax. Research has shown that boron deficiency is associated with reduced winter hardiness in oilseed rape.
There are other critical periods which can relate to the specific functions of a particular element. Copper provides one example of this as it is important for the production of lignin in plants. There are four copper atoms present in an enzyme which is responsible for lignin production.
Copper deficiency can also have an effect during the flowering process by reducing pollen viability and by compromising the natural opening (dehiscence) process, which is important for splitting open the pollen sacs for fertilisation.
Copper is important at GS29-31 when the flowering structures are being formed and probably at GS39 when pollen production begins.
What are the main plant functions limited by the different nutrients?
While plants only need 17 essential elements, although they can take in many more (I have seen a list of 60 different elements mentioned as being found in plants), there must be millions of different processes that occur in the plant and even more different compounds produced that enable these processes to continue. Therefore, essential elements have many functions.
However, I like to focus on photosynthesis, which is the most important process in the planet and the one on which every living thing depends.
It is the ultimate “free energy process” and it is what the farmer uses to turn plants into food. You can see where most of these elements fit in photosynthesis in the chart.
If manganese is not available in the plant and the splitting of water and release of oxygen is stopped, this compromises the whole photosynthesis process in the plant. Manganese is vital to the process of photosynthesis.
Formulation
Does micronutrient
formulation matter?
Yes, absolutely. However, there is no absolute consensus about how these plant nutrients get into the plant through the leaf. And when I say “no consensus”, I mean within the relevant scientific research community.
I have spoken to professors in Germany, Spain and America about this topic. I asked if entry is directly through the cuticle, through the stomata, or through the guard cells around the stomata. All may have a role but the general feeling is that the elements enter the leaf in solution. However, I have seen some recent research results, which suggest that very small particles, rather than solutions, can enter the plant.
Foliar uptake is influenced by such factors as the solubility of the nutrient (different salts have different solubilities) and a characteristic called the point of deliquescence (POD), which relates to how different salts /formulations dry out on the leaf surface at different humidities.
Taking these comments into consideration, formulation is important to enhance spread, maybe to enhance a temporary “melting” of the waxy cuticle and maybe to reduce drying out.
What formulations
are available?
Formulations are many and varied. True chelates (based on EDTA) used some years ago brought the advantage they were compatible in the spray tank but were usually low in element loading and expensive. The chelate protected the element from an incompatible reaction in the tank. But I do not believe that they offered anything in terms of enhancing uptake despite the claims made.
Next were the suspension products, based largely on relatively insoluble forms (see previous comment about solubility being a factor in the efficiency of foliar uptake). They were cheaper than true chelates and were also compatible in the spray tank. They may or may not have been formulated with adjuvants/stickers/spreaders to enhance uptake.
Suspensions tend to be heavy-loaded products, ie they contain high levels of element in the formulation which is a key advantage.
Then there are the true solutions based, for example, on manganese nitrate or magnesium nitrate. These are very soluble forms of the element. These also have low PODs, which means they dry out less quickly than other forms. They give a quick supply of element (seen in experiments with tissue tests after application) compared with other forms.
There are also products based on soluble salts with added humic acid, eg Verdi-Crop Magnor, which contains magnesium nitrate and magnesium chloride (both salts are very soluble) and is formulated with humic acid. Humic acid has been shown to act as a bio-stimulant in its own right and to enhance the uptake of plant elements. It also acts as a complexer, which means the element is protected to an extent from incompatibility in the spray tank
Quantity v formulation
The optimum situation is to supply a good amount of element in a well formulated product. Again, taking Magnor, we have two available salts, which are very soluble. Then we enhance that product with good formulation technology, which includes the use of humic acids in the formulation. But a product containing magnesium nitrate alone without the humic acid would be very acceptable in terms of meeting the needs of the plant because it is very soluble.
If you take an insoluble source of magnesium and use excellent co-formulants with known spreading and penetrant activity (we have done work at University of Amsterdam looking at spread patterns of all our co-formulants), plant availability is restricted because the magnesium source is insoluble. In summary, you need a good formulation with a good loading of nutrients.
Interactions
Interactions between micronutrients
There are known interactions between individual micronutrients, but I think they are of far less important than the interactions between macro and micronutrients. To get an extreme antagonism between micros you need a significant oversupply of one or the other and this would be rare.
Interactions between micro and macronutrients?
Such antagonisms are becoming increasingly accepted. These have long been accepted as being soil-based but they are now being considered as existing within the plant. The big potential antagonist is calcium, which affects pH and element availability, eg manganese. Calcium can occlude or attach to other elements such as zinc to its surface or it can react with specific elements such as phosphate to make them less available.
Phosphate has a particular antagonistic relationship with zinc. It seems that it goes out of its way to antagonise zinc and high P reduces zinc uptake. Potassium can affect the uptake of magnesium and sulphur can reduce the uptake of molybdenum and probably boron as well. This is something that must be borne in mind for oilseed rape, in particular.
Does soil type play a role in these interactions?
Yes, it can have an effect. For example, in calcareous soils with high levels of calcium, it can antagonise element availability. That, I believe, is why we have had seen yield benefits from the application of foliar boron on oilseed rape even with high levels of soil boron.
Similarly, high calcium soils can antagonise the uptake of iron, though it would appear that this is facilitated indirectly by bicarbonate rather than calcium. Calcium can antagonise the uptake of a number of elements in a number of ways.
Clay soils tend to have good inherent levels of potassium and this can antagonise the uptake of magnesium (although magnesium is a macroelement).
Poor soil structure can affect element availability by reducing root penetration and restricting the movement of elements through the soil profile. However, this may not be an antagonism as such.
Other factors
There are many factors which can affect element availability eg soil type/soil structure/weather/rainfall etc. However, these relate more to availability rather than interactions between elements or antagonisms.
Deficiencies
Timing of treatments
My preferred timings on autumn cereals would be GS21 in autumn, followed by GS31 in spring. If I had only one shot, I would go for GS31 but for the best consistent yield benefit opt for the double application at GS21 and GS31. The next best timing would be GS39-45, specifically for elements like magnesium and potassium.
On spring cereals, I would opt for one shot at GS29-31. However, problems such as manganese and magnesium can need earlier treatment in dry springs or deficient sites.
Repetition of treatments
My view is that a programmed approach is best, maybe as a drip feed of elements using a product that covers a number of elements. Where you have a known deficiency of certain elements, then target that with the specific products.
Can we rebuild soil
micronutrient levels?
The only way I can see to do this is by building up soil organic matter (SOM) levels but even this has its drawbacks.
Applying large quantities of element to the soil may not provide a solution. If you apply lots of manganese to the soil as granules (these are commercially available) this does not guarantee that manganese will be crop available. Manganese availability is tied up with many soil reactions which depend on such factors as organic matter levels (high OM can lock up manganese as seen on peaty land), pH and what is called redox or reduction oxidation reactions.
Manganese is taken up by the plant in one form but, in the soil, it can easily change form to one which is unavailable depending on the balance between air and water.
Manganese is immobile in the soil and any factor which restricts mobility restricts plant availability as well. So factors such as these can over-ride the actual level of nutrient in the soil.
You could apply boron to build up the inherent levels in soil. However, in a calcareous soil, the availability of boron could be antagonised by the presence of high calcium levels.
Rebuilding SOM levels
Personally, I think the case for building soil organic matter is strong and for many good reasons. However, building SOM as a vehicle to rebuild micronutrient levels is not jhigh on the list of good reasons for me. That said, in times of drought, SOM holds more water so, logically, crops would be less drought affected and better able to access all nutrients.
Also, SOM, and humus in particular, increases the cation exchange capacity (CEC) of soils, so they are better able to act as a reservoir for all positively charged cations.
As mentioned previously, SOM can tie up some elements. On the one hand, higher SOM will be a good source of micronutrients, but, on the other hand, it can tie up certain elements. Boron is held well in organic matter. In mineral soils, negatively charged ions such as sulphur and chlorine are subject to leaching.
Organic matter
Can added organic matter supply trace elements?
It will help but, as mentioned previously, availability can be affected by processes that over-ride the supply from the by SOM. And it is also difficult to indicate how long it might take to achieve a worthwhile increase in the supply of micronutrients from the soil via added organic matter.
Micronutrients are among the 17 nutrients that are essential for plant growth. Solubility and formulation matter to the efficacy of element uptake.Supplying micronutrients to the soil is not a guarantee of adequate availability from that source. Micronutrients can be held and supplied through the soil in organic matter, but other factors can impinge on their availability.What is the role of crop nutrition in photosynthesis?
Plants need 17 essential elements to complete their lifecycle from seed to seed.Most of these have a direct role in photosynthesis. Photosynthesis underpins every other process in the plant. Crop nutrition is fundamental to photosynthesis.MAGNESIUM is the central atom surrounded by NITROGEN atoms in chlorophyll (PS11).
MANGANESE is the main atom in the oxygen-evolving complex, but CALCIUM and CHLORINE are also part of the structure.
COPPER is the central atom in plastocyanin, a protein involved in electron transfer between the two photosystems. Also present are SULPHUR and NITROGEN containing amino acids
Ferredoxin is an IRON and SULPHUR-containing protein involved in electron transfer in PS1.
PHOSPHORUS is an integral part of ATP (adenosine tri-phosphate).
POTASSIUM plays both an indirect and direct role in photosynthesis. It influences stomatal opening, hence affecting carbon dioxide concentration in the leaf. It regulates the balance of ions within cells and has an effect on the synthesis of ATP.
ZINC is part of an enzyme, carbonic anhydrase (CA), involved in the transport and utilisation of carbon dioxide. Good levels of CA are important for crops like maize.
BORON: The primary role is related to cell wall structure and function but a deficiency can reduce the efficiency of PS11. Boron has an indirect and secondary, rather than primary, effect on photosynthesis.
MOLYBDENUM has an indirect role but involved in the assimilation and transport of nitrogen.
Note: Inadequate crop nutrition = inefficient photosynthesis = reduced yields 
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