DIAGNOSING MICRONUTRIENT DEFICIENCIES

Both soil testing and plant analysis can be useful in diagnosing micronutrient deficiencies. Testing the soil for micronutrients has become a widely-accepted practice in recent years. However one must recognize that micronutrient soil tests are not as reliable as the tests for soil acidity (pH) or for phosphorus (P) and potassium (K) content.

The major limitation of current micronutrient soil testing is that it measures only the quantity of nutrients present, not their availability. Much work is still needed in correlating soil micronutrient content with availability to the plant.

For this reason plant analysis is also very important in deficiency diagnosis. By combining plant analysis with soil tests more accurate assessment of the micronutrient status of soils and crops can be attained.

There are two ways to use plant analysis. One is to monitor a crop's nutrient status; the other is to diagnose any problem situations that might occur.

As a monitoring tool plant analysis can point out existing or potential problems before visual symptoms develop. Table 3 is a guide for sampling corn, soybeans small grain and forage legumes including when to sample what plant parts to collect and how much material is needed. While analyses can be run on other plant parts and on samples collected at other times the growth stages and plant parts listed in the table allow for the most reliable readings.

Practice good Field management

Neither the diagnosis of micronutrient deficiencies nor the treatment to eliminate them is complex or expensive. Given the current price of fertilizer land labor and other production inputs, it makes sense to do everything possible to ensure that the yield of your crop is not limited by a micronutrient deficiency. Such a deficiency effectively wastes all the dollar-expensive and energy-intensive inputs that we commit to a crop today

Attention to the availability of micronutrients for field crops is just good management!

How Soil Ph affects the availability of nutrients
Mineral Soil
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Effects of soil reaction on availability to plants of soil nutrients. The width of the bar determines the relative availability of each element with a change in soil reaction.
Micronutrient Deficiency Symptoms

Some micronutrients have characteristic deficiency symptoms. However, symptoms can be easily confused with other nutrient deficiencies, salinity, disease, drought, herbicide injury or other physiological problems. Visual symptoms alone are not a reliable method of determining a micronutrient, problem, but they are useful indicators when used with other diagnostic tools.

Boron

This deficiency results in stunted growth of young plants. The youngest leaves are affected first. They will be misshapen, thick, brittle and small. Because boron is not easily transferred from old to young leaves, older leaves usually remain green and appear healthy. Often dark brown, irregular lesions appear, followed by pale yellow chlorosis of young leaves. Stems are short and growing points may die. In canola, the symptoms of a boron deficiency can be confused with a sulphur deficiency. In alfalfa, boron deficiency symptoms include death of the terminal bud, rosetting, yellow top and poor flowering.

When a boron deficiency is moderate, seed yield is often reduced without any evidence of severe deficiency symptoms during vegetative growth.

Chlorine

Chlorine deficiencies are very rare; therefore, symptoms are seldom observed. Symptoms may include stubby roots, some chlorosis of new leaves and plant wilting.

Copper

Copper is not readily transferred from old to young leaves, so older leaves remain darker and relatively healthy and the deficiency symptoms develop on younger leaves. The visual symptoms of a copper deficiency in wheat include yellowing of younger leaves, limpness, wilting, pigtailing (whip tailing or curling) of the upper leaves and kinking of the leaf tips. Other signs include excessive tillering, aborted heads, delayed maturity and poor grain filling resulting in a high straw to grain ratio. On copper deficient soils these symptoms tend to occur in irregular patches. Copper deficiency is often associated with the disease stem or head melanosis and an increased incidence of ergot. For barley, the symptoms of a copper deficiency include yellowing, pigtailing, awn kinking, excessive tillering and weak straw. Oats will also show pigtailing. Copper deficiency symptoms have not been well documented for canola or alfalfa.

Iron

Chlorosis of the younger leaves characterizes an iron deficiency. The tissue between the veins gradually turns yellow, while the veins tend to stay green. The tips and margins of some leaves may turn brown and become dry and brittle.

Manganese

In legumes, deficiency symptoms include pale green young leaves and a pale yellow mottling develops in interveinal areas, while the veins remain green. Oats are an excellent indicator crop. Manganese is partly mobile in oats. White to grey flecks or specks first appear and become more severe on mature leaves about halfway up the shoot. If a deficiency persists, symptoms spread to old leaves then to the youngest leaves. The specked condition is referred to as "grey speck" and will appear in the interveinal area of the lower half of older leaves and extend toward the tip as symptoms develop.

Manganese is not readily transferred from old to young leaves in wheat and barely. In wheat and barley, affected young leaves frequently turn pale green and have a limp or wilted appearance. A mild interveinal chlorosis develops in the mid-section of the leaf and spreads rapidly becoming pale yellow-green. Small white to grey spots, specks or strips appear a short distance from the end of the leaf tip on young leaves.

Molybdenum

Molybdenum deficiency symptoms are similar to those of nitrogen. Since molybdenum deficiencies are very uncommon symptoms are rarely seen.

Zinc

Zinc is partly mobile in wheat and barley. In these crops pale yellow chlorotic areas appear on middle leaves, halfway up the stem. Chlorotic symptoms first develop in the lower half or mid-section of the leaf followed by grey or dark brown necrosis of the leaf. Generally, stems are very short and often fan-shaped with leaves crowded together at the top.

Zinc deficient beans are stunted and older leaves are smaller and narrower. The older leaves may have light blotches between the veins. Younger leaves will have a more normal healthy green color but may be smaller.

In flax, a zinc deficiency can cause grayish-brown spots in the younger leaves with shortened internode spaces and stunted appearance.

In corn, symptoms occur within a few weeks of emergence as light yellow bands on the youngest leaves. The most severe symptoms occur on the youngest leaves from the unfolding bud, referred to as "white-bud". Old leaves remain dark green and appear healthy. In a prolonged case of deficiency the middle leaves develop pale yellow interveinal chlorosis near the tips. A zinc deficiency prevents the elongation of internodes and leaves, which results in short stems with the leaves crowded together at the top in a fan-shaped appearance.

Note that zinc deficiency symptoms are similar to those of manganese and iron in some crops.
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