MICRONUTRIENTS - THE ROLE OF THE SOIL - AVAILABILITY OF MICRONUTRIENTS
It has already been pointed out that the total amount of a micronutrient in the soil is often only a relative indication of a much lesser amount that is available.
The availability of the micronutrients to the plant is related to their solubility, particularly of the respective oxides and silicates. These, in turn, are dependent upon the acidity or alkalinity of the soil solution and, in some cases, the state of oxidation of the element; there is, the number of oxygen atoms with which the element has combined.
In order to understand effects of soil conditions on the availability of micronutrients we should understand the term "pH", which is a measure of acidity or alkalinity. The pH scale ranges from 0 to 14 with 7 being neutral, neither acid nor basic. At a soil pH of 5.0, for instance, it is considered to be quite acid, and pH of 8.0 is quite alkaline.
The pH scale is logarithmic; that is to say, as you move from pH 7 to pH 6 the acidity increases 10 times. As you move from pH 7 to pH 5 the acidity increases 100 times.
pH is actually a method of expressing the concentration of hydrogen ions in a solution. Pure water can be written in the following form:
The pH scale is based on water breaking down into its components of hydrogen ions (H+) and hydroxyl ions (OH-). At pH 7 equal quantities of these ions are present in water. Any chemical present in water which increases the amount of H+ increases the acidity of the water. Elements which increase the hydroxide (OH-) content of water are considered to be alkaline.
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Availability of Manganese
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Manganese is most available between pH 5.0 and 6.5. Below pH 5.0, manganese is absorbed by iron and aluminum oxides and silicates to form insoluble complexes. Since under normal soil conditions there is plenty of oxygen, we find that increasing pH favors both chemical and biological oxidation of manganese. The availability of manganese is directly related to its state of oxidation. Another way of stating this is to point out that the more oxidized, or more positively charged, the manganese becomes, the more inactive it gets.
Thus, the activity of manganese decreases with increasing pH, and this tells us that we may have to consider manganese fertilizer applications on alkaline soils.
In some acid soils we can encounter toxic levels of manganese in plant tissues. Liming helps to shift the pH towards the neutral area where the manganese is less soluble and, therefore, less available.
Zinc, like manganese, is most available at about pH 5.0. On the extremely acid side, zinc is absorbed by iron and aluminum oxides and silicates. On the alkaline side zinc is absorbed on the clay complex and also may react to form insoluble carbonates and slightly soluble hydroxide complexes. The availability of the zinc is not directly pH-dependent as is manganese since its state of oxidation doesn't change over the pH range where plants normally grow. Colloidal organic matter, like clay, absorbs zinc. In plant roots, phosphorus may tie up some zinc.
Note, then, that although the oxidation state of zinc remains the same throughout the normal cropping pH range, its availability decreases slowly with increasing pH.
Now you see why you may need more zinc in an alkaline soil condition than you do with acid soils and why we carefully consider the phosphorus status of the soil before making a zinc application.
Iron is most soluble in the lowest pH ranges suitable for plant growth. Increasing pH favors both chemical and microbial oxidation of this element, and its ionic activity drops as with manganese. Above pH 6.5, insoluble iron oxides predominate. The uptake of iron has also been shown that phosphates will inhibit iron uptake by plants, perhaps by forming some insoluble complex.
Copper is most available at about pH 5.0. Below this value, copper is aborbed by iron and aluminum oxides and silicates. The reduction in availability of copper at high pH values is related to the lowered activity of the iron. In other words, copper becomes somewhat inert at higher pH levels. Copper also forms strong complexes with organic matter, but it does not change its state of oxidation (charge) over the range of common plant growth. Mineral imbalance can seriously disturb the root's ability to take up copper.
Since we lime our soils for good crop growth, we must realize that we may seriously affect the limited copper supply. Fortunately, the plant needs only a very small amount of this element.
Also, remember that most of these micronutrients are complexed by the organic acids of decaying organic matter. Under normal organic matter levels of 2 percent to 6 percent this complexing is often beneficial and retains the micronutrient in the soil for the plant's use. But with higher organic matter levels, such as in peat soils, or when large amounts of manure are applied, the complexing activity of the over-supply of organic acids may keep the micronutrients away from the growing crop.
Boron is most available above pH 5.0. Below pH 5.0 boron forms insoluble borosilicates containing iron and aluminum. On the alkaline side, the relative insolubility of calcium borate accounts for the decrease in boron availability. Above pH 8.5 the soil solution is dominated by sodium, which forms a more soluble borate product.
For each element that we have examined, the root is feeding more efficiently on the available supply between the pH 5 and 6 range. We should particularly call attention to the fact that root hairs create their own localized acid environment and the roots of some crops, and even some varieties within crops, are better at developing the acid conditions which keep the micronutrients in their most soluble and available forms. This is part of the reason why you can see two varieties of a crop growing side by side with one showing a deficiency and the other not. This can be true of soybeans and manganese, corn and zinc and other combinations.
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Availability of Molybdenum
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In general, molybdenum solubility increases with pH. Note that this is just the reverse of manganese and iron. Below pH 5.0 molybdenum is inactivated by the formation of insoluble iron and aluminum molybdates. In the middle range the increasing alkalinity favors oxidation to the soluble Mo04 salt.
Remember that vary often we can improve molybdenum availability by liming the soil, but that in a soil that is over-supplied with molybdenum too much lime may cause high levels of this element to be present in the plant.
Well, all that has really played me out … so now let's take it easy. Here's your NFSA Agronomist's summary of what we've just gone over.
The availability of all of the micronutrients except molybdenum decreases as you increase the pH from 5 to the alkaline range. In general we note that at the most soluble point of pH 5 the micronutrients can be in too great a supply and may very well be at toxic levels for the plant. In liming your acid soils consider their micronutrient levels. Generally, liming to a pH of 6.8 is to be recommended in soils where the micronutrients are abundant. But on the other hand, we should consider liming only to pH 6.5 if micronutrient shortages could be created by over- liming. In the case of certain limitations of iron or manganese we may well consider the balance between costly micronutrient supplementation or farming with a less than desirable pH of about 6.2. Every care should be taken not to over-lime.
Please consider also that almost any natural condition or farming practice that slows down root growth and development can induce micronutrient deficiencies. Also watch out for deficiencies when the following conditions occur in the field:
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High organic matter (6 percent or more)
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High soil phosphates with low micronutrient levels
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High sand content of soil
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Drought
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Compaction
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High pH (above 7.0)
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Land leveling and forming
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Major Causes of Micronutrient Deficiencies
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Micronutrients
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Soil Conditions
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Crops
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Boron (B)
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Sands, overlimed acid soils, organic soils
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Legumes requiring high lime, cotton, tomatoes, citrus, sweet potatoes, leafy vegetables, tree fruits
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Copper (Cu)
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Sands, organic soils, high pH
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Small grains, vegetables, tree fruits
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Iron (Fe)
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High pH soils, high phosphate
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Blueberries, ornamentals, corn, sorghum, soybeans
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Manganese (Mn)
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Sands, overlimed soils, high phosphate
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Soybeans, small grains, tree fruits, cotton, sweet potatoes, leafy vegetables
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Zinc (Zn)
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Sands, high pH soils, high phosphate
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Soybeans, corn, citrus, sorghum, tree fruits, pecans, some vegetables
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Molybdenum (Mo)
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Highly weathered acid soils
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Legumes, citrus, cauliflower
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DON'T PULL IN YOUR LINE YET! JUST REMEMBER THESE THINGS:
Sandy soils and soils low in organic matter are usually low in micornutrient content. However, crops grown on soils very high in organic matter often need more of the micronutrients thanb low-organic soils.
Listed above are some soil conditions under which micronutrient deficiencies often occur... and the major crops where we see them.
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