How Iron and soil microbes influence plant growth

Iron is the most abundant element in the earth but is not always plant available. Humic acid may be a key to make it available. Iron draws heat into the leaf, promoting growth. It is essential to the function of chlorophyll, fixing magnesium to the chloroplast. Iron absorption is hindered in cold soils (as is P), and in small root systems.

Adequate sulphur reduces the chances of iron deficiency.

In animals, iron is concentrated in haemoglobin and is used by the liver and immune system. It is found in many enzymes, proteins, and in DNA synthesis. Ruminants require 100-200 ppm and humans 20 mcgs per day. Symptoms of deficiencies include nutritional anaemia and pale mucus membranes. Beef, liver and tuna are very good sources of iron.

Always ensure iron exceeds manganese at all times. There are at least 500,000 hectares of iron deficient hill soils in NZ. Maintain at least 200 ppm in the soil. Kiwi Fertiliser consultamts will always prove the need for iron before recommending it. Since soil probes are shallow and iron may be present in deeper profiles, the addition of iron sulphate may be unnecessary. Proving a need may involve a test plot, leaf test or a deeper soil test.

Plants are only organisms that can directly utilise the sun’s energy. They transfer 30% of their energy from the leaves to the rhizosphere and in doing so stimulate soil microbes. If this process is interrupted, (e.g. girdling), diseases are more likely.

Plant exudates feed microbes that in turn can deplete soil oxygen. Ethylene is then produced at the anaerobic micro-sites at 1-2ppm to inactivate the microbes. Oxygen demand reduces and oxygen diffuses back into the soil.

Presence of ethylene in undisturbed soil indicates the soil is healthy. Agricultural soil has little or no ethylene. Plant pathogens are unimportant in the former, but prevalent in the latter where inorganic fertilisers and pesticides are used, increasing production costs.

Agricultural soils fail to produce ethylene because of the changes in soil nitrogen. Virtually all nitrogen in undisturbed soils is in the ammonium form. Modern farming techniques encourage a specific group of bacteria that convert most nitrogen to the nitrate form. Nitrate nitrogen inhibits ethylene production. Ammonium nitrogen does not.

When oxygen is consumed at a rhizosphere micro-site, a series of complex chemical changes occur. In adequately aerated soil, virtually all the iron is ferric (oxidized) and immobile. If oxygen is consumed, the minute iron crystals break down into the highly mobile ferrous form. Ferrous iron is a specific trigger for ethylene production. When oxygen is depleted and nitrate nitrogen is present, the production of ferrous iron is inhibited or prevented.

However, a precursor produced in senescent vegetation is required to react with the ferrous iron before ethylene can be produced. In intensive agriculture most of the older leaves are removed by grazing animals, by harvesting or by burning, so little is left to decay.

A major limitation to plant growth in most agricultural soils is an inadequate supply of plant nutrients, regardless of the supply in the soil. Iron crystals have a large surface area and are highly charged. As a result, nutrients such as phosphate, sulphate and trace elements are tightly bound to the crystals and unavailable to plants. If anaerobic micro-sites are able to develop, the crystals break down, releasing the nutrients for plant uptake. Ferrous iron is released into the soil. Other nutrients including calcium, magnesium, potassium and ammonium, are held on the surface of clay and organic matter. The release of ferrous iron displaces these nutrients into the soil solution where they are available for uptake by plants.

Modern farming techniques encourage the aeration and oxidation of the soil and give a short-term increase in plant growth. Unfortunately, those practises also rapidly create long-term problems of nutrient depletion and increased plant diseases. Treatments that stimulate rates of nitrification (conversion of ammonium to nitrates,) such as excessive use of nitrogenous fertiliser, or excessive removal of plants by overgrazing, or forestry operations require moderation and re-examination.

Based on research by Dr. M.A. Smith, Principle Research Scientist, NSW Dept of Agricultural, Biological and Chemical Research Institute. PMB 10, Rydalmere 2116, Australia. Published in the Australian Plants Periodical Vol. 9 Dec. 1977.

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