In the last article we looked at the functions of the three major elements in fertilisers. That is Nitrogen, Phosphorus and Potassium. In that article we also looked at the symptoms of both deficiencies and over-abundance.

In this article we will look at the roles of the minor and trace elements. Some of these are only required in minute amounts but still have a vital role to play in plant nutrition and health. Minor elements appear in 'complete' fertilisers and some of these fertilisers will also contain trace elements. Most Australian native soils lack trace elements and these deficiencies will often display as poor plant vitality or fruit production in exotic plants.

Let's now examine these elements and their function in plant biology.

Plants use sulphur in the production of various enzymes and as a component of the photo­synthetic process. Fertiliser strategies for amending sulphur deficiency vary with soil type and location. Gypsum is often the cheapest source of sulphur and one application can supply many years' sulphur requirement. However, on soils where excess sulphate can readily be leached out of the root zone, regular, strategic applications of sulphur in compound fertiliser products might be more cost effective.

Calcium is used in the formation of the cell walls of plants. This includes the fruit of those plants. A lack of calcium may cause a plant to produce fruit that is soft and unable to be harvested at maturity. Tomatoes are one fruiting plant that suffers badly from a lack of calcium. The difficulty is to deliver this calcium requirement without radically changing the soil pH.

Calcium deficiency symptoms are most pronounced in young plant tissue. The emerging leaves produce brownish-black (dead tissue) areas along the leaf margins. The newer or bud leaves are distorted at their tips or bases. This causes later growth to have a cut-out appearance at these points. As the leaf expands, the surfaces can become puckered because of the injury to the leaf tips. With more severe calcium deficiency, the new bud tissue or entire growing point may blacken and die.

Magnesium is a vital component of a chlorophyll molecule. At the centre of the molecule is a single atom of magnesium surrounded by a nitrogen-containing group of atoms. A long chain of carbon and hydrogen atoms proceeds from this central core and attaches the chlorophyll molecule to the inner membrane of the chloroplast, the cell organelle in which photosynthesis takes place.

A magnesium deficiency will have symptoms such as old leaves and spur leaves surrounding the fruit turn yellow at leaf tips in mid-summer and then fall from the tree. The top half of older, basal leaves may become yellow but veins remain green. Magnesium deficiency may occur where too much potassium and not enough nitrogen is used.

The symptoms of Boron deficiency are plant stems become thickened, tough and brittle and internode length is reduced, when a plant is deficient in boron. Other symptoms include the young leaves at the terminal bud becoming light green at the base, which breaks down and causes leaves to become twisted in later growth. Fruit that lacks Boron can be 'woody'. This is particularly obvious in citrus.

Zinc is a vital component in the growth hormone, auxin. When auxin levels are low, the plant will suffer from small leaf syndrome. That is where some leaves will achieve full size but a majority of the plant's leaves will be dramatically smaller and often misshapen. Leaves may also be mottled. Reduced leaf size decreases the plant's ability to carry out photosynthesis.

At the same time it is not difficult to raise Zinc levels to a point where it is toxic. A number of small applications of Zinc until the problem is rectified is the recommended method. This application rate from a DPI note on plant nutrition will give some idea of the amount of Zinc a plant needs.

"A rate of 2.5 kg of soil-applied elemental zinc a hectare (equivalent to 10 kg/ha of zinc sulphate) is needed to overcome a severe zinc deficiency and its benefit should persist for three to 10 years, depending on soil type."

Manganese is a component of enzymes used in controlling the speed of the photosynthetic process. Since these enzymes are not consumed by the plant, the requirement for manganese is quite low.

The symptoms of Manganese deficiency include older leaves turn pale green except for veins, which remain dark green. Manganese deficiency occurs on alkaline, lime-rich soils.

Copper is another component used in the production of enzymes in plants. Copper deficiency occurs mainly on sandy soils, often when too much nitrogen and phosphate fertilisers are used. A nutritional copper spray should only be needed every 2-3 years.

Plants suffering from severe copper deficiency may be stunted, with exceedingly small terminal leaves (often only one fifth or less of normal size). Terminal leaves may eventually die and multiple budding occurs immediately below the dead terminal. These buds may also die.

Molybdenum is involved in photosynthesis. It plays a part in the conversion of nitrites, taken in by the root system, to nitrate.

The availability of Molybdenum to the plants is related to soil pH and soil type (e.g. gravel content). The more acid the soil, the lower the availability of molybdenum for plant uptake. A lack of molybdenum causes yellowing between the leaf veins; the older or mid­stem leaves are the first to be affected.

Iron is needed by all plants. It is an essential precursor for the formation of chlorophyll, which gives plant foliage its green colour. Chlorophyll is an active element in photosynthesis, a process which allows the plant to grow, mature and produce flowers. Iron is also important for normal activity of enzymes involved in plant respiration.

Plant growth and vigour are reduced when the iron supply is limited. Since iron is not easily translocated within the plant, the symptoms of reduced green colour, due to less chlorophyll production, appear on the new leaves. The visual symptom on these new leaves is interveinal chlorosis, or yellowing of the plant between the leaf veins. In severe cases the entire leaf may be white, exhibited on the older leaves. Symptoms of iron deficiency tend to be more pronounced during winter.

In the next article we will discuss the ways of delivering the major and minor trace elements to plants and their effectiveness.

Nutrient deficiency symptoms of plants. Revised by David Beardsell, Knoxfield, April, 1995.

Apples nutrition, minor elements, S Tancred and S Middleton, Horticulture Division.

Tree Crops - Nutrition, Jim Wait, Sept, 1996


DATE: February 1999

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