Trace Element Essentials
This study focuses on trace elements. Iron, zinc, copper, manganese, boron, molybdenum and cobalt will be covered in depth. The interrelationship between these elements will be considered, as will their respective functions, the conditions creating deficiencies and the symptoms of these deficiencies. The best sources of each trace element will also be listed.
Interrelationships in Mineral Nutrition
There are numerous relationships between nutrients, but the following examples will illustrate the fact that ‘no element is an island’ – They are inextricably intertwined.
The cation quartet: The cations calcium, magnesium, potassium and sodium are prime examples of these interrelationships – both productive and destructive. For example, overliming can induce a magnesium deficiency, while oversupply of potash can also reduce magnesium uptake. High potassium can replace calcium in the plant, creating a myriad of problems, and excess sodium can replace potassium, producing another set of inherent problems. The calcium / magnesium ratio is the most important nutrient ratio in fertility management, as soil structure, nutrient availability and biological activity are all governed by the relative balance between this pair.
The iron / manganese duo: Iron chlorosis often occurs when iron levels on leaf analysis fall below 50 ppm, or when manganese exceeds iron levels by two times or more. In soil analysis terms, iron should always be higher than manganese to avoid likely iron lockups.
The calcium / boron partnership: ‘Calcium is the trucker of all minerals, and boron is the steering wheel.’ Boron can be toxic in the absence of sufficient calcium. The synergy between this pair is such that deficiencies should ideally be addressed together.
The phosphorus / magnesium surprise package: Phosphorus is often taken up by plants as a magnesium compound, so, in some cases, magnesium may alleviate phosphate deficiency more efficiently than applied phosphate.
The phosphorus / zinc see-saw: There is a very strong relationship between phosphorus and zinc. High phosphorus will invariably reduce zinc uptake, and excess zinc will have the same effect on phosphorus. The ideal phosphorus / zinc ratio is 10:1 in favor of phosphorus.
The molybdenum / nitrogen symbiosis: Nitrogen-fixing bacteria cannot fix atmospheric nitrogen to the soil without molybdenum.
IRON – Poverty in Abudance
Iron is the most abundant element in the known universe, and yet the lack of plant-available iron can be a serious yield-limiter in almost every area of agriculture. Most soils contain between 20 tonnes 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, as iron does not move easily within the plant. Iron is the only element 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 FUNCTIONS OF IRON
1) One of the essential elements required for biological nitrogen fixation.
2) Adequate iron, in plant-available form, is essential for protein synthesis.
3) An indispensable oxygen carrier for chlorophyll production.
4) A central component of respiratory enzyme systems.
5) Increases leaf thickness, which, in turn, enhances nutrient flow, which eventually increases yield.
6) Iron makes the leaf darker, with a greater capacity to absorb solar energy.
CONDITIONS CREATING IRON DEFICIENCIES
1) Excessive phosphate applications or high phosphate levels in the soil.
2) High manganese reduces iron uptake (excessive copper or molybdenum can also cause iron shortages).
3) Cold, wet conditions limit iron uptake, particularly in the early growth stages.
4) Excessive lime applications reduce iron availability.
5) Inadequate soil aeration hinders mobility.
6) High soil pH (7.5 or higher) – A foliar application of iron should always be considered in these situations.
7) Low organic matter is another limiting factor for iron nutrition.
SOME SYMPTOMS OF IRON DEFICIENCY
- In vegetables, orchard crops and maize the symptoms are very similar.
- The youngest leaves develop a light green chlorosis of all the tissues between the veins. The veins remain dark green.
- 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 or iron within the plant.
- In small grains, the leaf blades develop yellow stripes between green veins, and upper leaves can turn completely yellow.
Catalysts – The Spark Plugs for Plant Growth
Healthy plant growth involves numerous chemical reactions, and these reactions are governed by triggering mechanisms called catalysts. Just as a spark plug is required to trigger an engine into life, plant processes will flounder when the correct catalyst is not available to initiate a reaction. The principal role of trace elements is as catalysts for these chemical reactions. These catalysts are not actually used up in the chemical reactions they promote, so they are required in very small quantities – hence the name micro-nutrients. Although mere traces of these minerals are all that is required, maximum production is not possible in their absence.
ZINC – The Hormone Catalysts
The application of zinc, when it is needed, gives a more obvious response than any other micro-nutrient, and a lack of zinc also produces more dramatic symptoms than other trace element deficits. Zinc governs production of the natural growth hormone auxin. When the production of this hormone is compromised, plants become obviously stunted and distorted. From a human nutrition perspective, zinc is the second most abundant trace element found in our body (after iron) and should be supplemented on a daily basis.
THE FUNCTIONS OF ZINC IN SOIL AND PLANT NUTRITION
1) An essential component in many enzymes.
2) Linked to the growth hormone auxin – low auxin levels cause stunting of leaves and shoots.
3) Plays an important role in the formation and activity of chlorophyll.
4) Involved in protein synthesis.
5) Important for carbohydrate metabolism.
6) Zinc plays a major role in the absorption of moisture (plants with adequate zinc nutrition have enhanced drought-handling capacity).
CONDITIONS CREATING ZINC DEFICIENCIES
1) High pH soils – solubility increases 100-fold for each pH unit lowered.
2) Soils lacking Mycorrhizal fungi.
3) Calcareous soils.
4) Over-limed soils.
5) Light, sandy soils.
6) High phosphorus levels – phosphate ties up zinc.
7) Cold, wet soils.
8) Soils featuring anaerobic decomposition, ie zinc bonds with sulphides produced in these conditions and becomes insoluble.
SOME SYMPTOMS OF ZINC DEFICIENCY
- Small crops: Shortened shoots produce a cluster of small, distorted leaves near the growing tip. Interveinal yellowing is often combined with overall paleness. Flowers and pods drop off and yields are dramatically reduced.
- Fruit crops: Interveinal chlorosis is present in small, narrow, often distorted leaves arranged at the ends of seriously shortened shoots. The degree of chlorosis varies with the crop, ie there are very few visual symptoms with apples, but severe symptoms with citrus, stonefruit and grapes. Blossoming and fruiting declines rapidly as a zinc deficiency develops.
- Cereal crops: Symptoms appear within two weeks of emergence and feature a broad stripe of chlorosis, more pronounced towards the base of the leaf. Young leaves are most severely affected. Delayed maturity and reduced yields are the likely outcome.
MANGANESE – The Element of Life
Manganese is critically important in the reproductive stage of plant growth. Not only is it essential for seed formation in all crops, but it also plays an important role in the germination of that seed and the early establishment of the seedling. Early maturity in all crops is also linked to manganese. Conventional agronomists are aware of the relationship between manganese and germination, but it is the Carey Ream influenced consultants who have explained this link most convincingly. The Reams team believes that manganese, the element of life, electrically charges the seed, enabling the subsequent magnetic attraction of other elements into the seed.
THE FUNCTIONS OF MANGANESE
1) Hastens the fruiting and ripening of crops.
2) Accelerates and improves germination.
3) Required for chlorophyll production.
4) It is a critical enzyme activator.
5) Essential for carbohydrate and nitrogen metabolism.
6) Required for the assimilation of carbon dioxide in photosynthesis.
7) Directly involved in plant uptake of iron, carotene and Vitamin C.
8) Necessary for optimal seed formation in all crops.
CONDITIONS CREATING MANGANESE DEFICIENCIES
1) High soil pH – manganese solubility increases 100-fold per unit drop in pH. Manganese can be toxic in low-pH soils.
2) Soils with very high levels of organic matter.
3) Cool, wet conditions.
4) Excessive calcium can tie up manganese. Overliming can be a problem. In fact, even moderate applications of lime will magnify a manganese deficiency.
5) High phosphorus and iron can limit manganese uptake.
6) Light, sandy soils.
7) Heavy cuts on graded paddocks can often create manganese deficiency, as can heavy erosion.
8) An overuse of potassium and magnesium can reduce manganese uptake because of soil pH increases.
9) When sodium and potassium base saturation percentages, combined, total over 10%, then manganese uptake will be limited, regardless of soil test results. This problem occurs most often in lighter soils.
SOME SYMPTOMS OF MANGANESE DEFICIENCY
- Small crops and soybeans: Interveinal yellowing of recently matured leaves is a feature in most crops. The development of the deficiency progresses from pale green to yellow, rather than the whitish cream colour associated with severe iron deficiency. In contrast to iron deficiency, the veins remain darker.
- Fruit crops: Bands of dark green surround the main veins, and a light green mottle develops on the area between the veins. Symptoms are most common in early summer growth on recently mature leaves (as opposed to very young leaves with iron deficiency). Leaf shape, size and shoot length remain normal, and chlorosis symptoms are more pronounced on the shady side of the tree.
- Maize and grain sorghum: Interveinal chlorosis with general stunting.
- Small grains: Marginal gray and brown necrotic spots and streaks appearing on the basal portion of the leaves. On older affected leaves the spots are oval and gray brown.
COPPER – The Stem Strengthening Fungus-Fighter
Copper is involved in the formation of lignin, which is the key to strong shoots and stems. It also inhibits the growth of many fungal species. This feature can be a problem where copper levels in the soil have become too high (above 15 ppm), as this fungicidal quality then becomes detrimental to beneficial fungi in the soil. Excessive copper can also affect phosphate, zinc and iron uptake.
THE FUNCTIONS OF COPPER
1) Essential in many enzyme systems, particularly those associated with grain, seed and fruit formation.
2) Important for water movement within the plant.
3) Copper is a key component in many proteins.
4) Essential for chlorophyll formation and associated photosynthesis.
5) Regulates elasticity, ie it has a marked effect on the formation and composition of cell walls. Important for strong, flexible stems.
6) Vitally important for root metabolism.
7) Helps prevent development of chlorosis, rosetting and die-back.
8) Provides a natural fungicidal effect.
CONDITIONS CREATING COPPER DEFICIENCIES
1) Light, sandy, coastal soils are invariably deficient.
2) Peaty, high-organic matter soils tend to hold copper, strongly reducing plant-availability.
3) Excessive phosphate and nitrogen can limit copper availability.
4) Over-liming can create deficiencies.
5) High zinc levels can reduce copper uptake.
6) High soil pH.
7) Drought conditions can intensify any copper problems.
SOME SYMPTOMS OF COPPER DEFICIENCY
- Note: Copper is relatively immobile within plants, so deficiency symptoms normally occur on new growth.
- Small Crops: Symptoms vary considerably between crops, but there are some common symptoms. Copper-deficient crops are usually patchy, stunted and yield poorly. The youngest leaves are worst affected. The plant is often wilted and lacks firmness. Leaf rolling, bending or crinkling is common.
- Fruit Crops: Young shoots may be vigorous, but they are weak and often become S-shaped as they bend and continue to grow. The leaves on these shoots are usually large, but pockets of browned gum form on the stems, and affected twigs often die back. Fruit peel on citrus often develops gum-impregnated browning.
- Maize and Small Grains: Leaf tips die and curl like pigs tails.
- Lucerne: Youngest tissue turns faded green with a grayish cast. Plants appear bushy and drought-stricken.
BORON – The Calcium Synergist
American author / consultant, Gary Zimmer, summarises the unique relationship between boron and calcium with the following quote: “Calcium is the trucker of all minerals and boron is the steering wheel.”The fact is that boron is associated with several of the functions of calcium. Boron improves calcium efficiency and vice-versa, but there is a reverse side to this kinship. If calcium levels are low, then boron can become toxic. Calcium and boron deficits should always be addressed together. Maintaining ideal boron availability for the full crop cycle can prove problematic for several reasons: Boron is the most leachable of trace elements, and maintaining good levels in wet conditions is a major challenge, especially in light soils. However, it doesn’t stop there, because boron availability declines just as rapidly in very dry conditions. Apart from these potential problem areas, boron management is further complicated by the fact that this element is not readily mobile within the plant.
THE FUNCTIONS OF BORON
1) Boron increases nitrogen availability to the plant.
2) It is involved in the synthesis of cell wall components.
3) This element increases calcium efficiency within the plant.
4) It has a central role in pollen viability and good seed set.
5) Boron influences cell development and elongation of cells.
6) It increases the elongation growth of primary and lateral roots.
7) It is involved in the nodulation of legumes.
8) Boron is important for fruit set. Avocadoes, notorious for their poor fruit to flower ratio, will often set more fruit with a pre-bloom boron foliar.
9) Boron carries the starch from the leaf to the grain or fruit.
This trace element is involved in so many stages in the production of the salable product, it is virtually inevitable that production will suffer when boron runs low.
*THE CONDITIONS ASSOCIATED WITH BORON DEFICIENCIES
*Note: Sensitivity to boron deficiency varies greatly between different plant species.
1) Leached, acidic soils.
2) Calcareous or overlimed soils.
3) Light, sandy soils.
4) Excessive usage of potassium and nitrogen.
5) Drought conditions.
6) Soils low in organic matter (humus is the boron storehouse).
7) Soils with high pH.
8) Boron performance can be negatively affected by low phosphate levels in some crops, eg corn.
SOME SYMPTOMS OF BORON DEFICIENCY
- Small Crops: Brittle tissue may crack or split. The surface of petiole stems and leaves develop many transverse cracks or corkiness. Storage roots split, and stems develop hollow sections. The growing point may die, creating multiple shoots. In all, the dramatic nature of the symptoms is illustrated by some of the names given to boron deficiency, ie ‘beetroot cancer’, ‘cauliflower hollow stem’, ‘water core’ of turnip, etc.
- Fruit Crops: Symptoms can be found in fruit shoots and leaf growth, but fruit are usually the first victim. ‘Internal cork’ is a boron deficiency found in pomefruit. Trees with severe boron deficiency can suffer from die-back in spring, because buds fail to develop. Fruit may be misshapen, with irregular depressions developing as it ripens. The most common symptom in grapes is the uneven development of berries and the presence of seedless berries within the bunch. Strawberries exhibit symptoms that include burning and crinkling of the edge of young leaves, stunting of the growing part and often deformed fruit.
- Cereals: Stunting of young plants, shortened, bent ears, tip kernels aborted and leaves fail to emerge and unfurl properly.
- Lucerne: Upper leaves become rosetted and turn yellow.
- Peanuts: Dark depressions in the centre of the nut.
- Potatoes: Shortened internodes and the death of the growing bud. Tuber stem-end browning.
MOLYBDENUM – The Nitrogen Catalyst
Molybdenum is the least abundant of all the recognised micro-nutrients in the soil, but it plays a critical role in one of the most significant soil-life functions – the fixation of nitrogen from the atmosphere to the soil. Both free-living nitrogen-fixing bacteria, like Azotobacter, and symbiotic species like Rhizobium cannot fix nitrogen in the absence of molybdenum. Molybdenum is the only trace element where availability increases as pH rises.
THE FUNCTIONS OF MOLYBDENUM
1) Essential for nitrogen fixation.
2) Required for the synthesis and activity of the enzyme nitrate reductase (reduces nitrates to ammonium in the plant).
3) Involved in electron transport in plant metabolism.
4) Linked to organically-bound phosphorus uptake in the plant.
CONDITIONS ASSOCIATED WITH MOLYBDENUM DEFICIENCIES
1) Acidic soils that are highly leached.
2) Timber soils.
3) Acidic, sandy soils.
4) Soils that are high in other metal oxides.
SYMPTOMS OF DEFICIENCY
Few symptoms are ever obviously apparent in fruit crops, but in vegetable crops and legumes the nitrogen connection results in deficiencies, which look very much like nitrogen deficiency (paleness and stunting). The leaf edges also tend to burn because of the accumulation of unused nitrates.
COBALT – The Soil-Life Supporter
Cobalt is rarely measured in soil tests, but it plays a significant role in the support of Rhizobium and other soil bacteria. 0.5 ppm is considered ideal.
FUNCTIONS OF COBALT
1) Involved with atmospheric nitrogen fixation by Rhizobium bacteria on legume plants.
2) Appears to promote a variety of soil bacteria.
3) Plays a little-understood role in pest resistance.
SYMPTOMS OF DEFICIENCY
1) Limited Rhizobium colonisation on legume roots.
2) Nitrogen deficiencies in legumes, ie chlorosis and poor growth.