Iron and Soil Fertility

R. S. Holmes and J. C. Brown.

Most soils contain an abundance of total iron, which all plants need, but many interacting factors affect and limit the iron that plants can use.

An accumulation of the heavy metals copper, manganese, zinc, and nickel in relation to available iron may induce iron deficiency in plants in acid soils.

Lime-containing soils, however, are most apt to contain too little iron; plants on such calcareous soils may have an abnormal growth, which is called lime-induced chlorosis. Its symptoms are yellow foliage, lack of vigor, and unproductiveness. It is common in the and Intermountain and Southwestern States.

A deficiency of iron exists in almost every major fruit-growing area.

It is difficult to supply iron in a form available to plants. Such soil amendments as ammonium sulfate have been used to furnish plant nutrients and at the same time increase the soil acidity, which affects iron solubility. Some new compounds, called iron chelates, have been found to correct many deficiencies of iron.

One of the functions of iron is to be a catalyst in the production of chlorophyll, the green pigment in plants.

You can recognize iron deficiency by looking at the new, growing leaves. The tissue between the veins becomes lighter in color than the veins. The areas between the veins become yellow as the deficiency advances. Only a branch of a tree may be affected, or perhaps only a few trees in an orchard are chlorotic. The entire tree in severe cases is affected, plants lose part of their leaves, and dieback results. If the condition is not corrected, the plant becomes unproductive and dies. Young peach trees in some places cannot be kept alive longer than 2 years because of iron chlorosis.

The iron that is proportional to chlorophyll in the plant is called active iron. Enough active iron for normal chlorophyll development can be present only after a certain minimal amount of residual iron is in the leaf. Active iron affects the iron-porphyrin protein complex, which acts as an oxygen carrier, transporter of electrons, activator of oxygen, and decomposer of hydrogen peroxide (H₂O₂). Iron becomes active biologically only when it becomes a part of these complex organic compounds. Factors that limit the supply of iron to a plant limit the life of the plant.

THE IRON IN SOILS of humid sections comes from the weathering of many iron-containing minerals of the parent material of the soil.

Among the more common iron-bearing primary minerals are hornblende, biotite, and chlorite. The iron in those minerals is largely in the ferrous state (that is, a combining value of 2, such as Fe++ O--). When they and other minerals are weathered during soil formation, most of the iron is changed to the ferric state (that is, the combining value has been increased to 3--e. g., Fe₂O₃) and forms different iron compounds. The kind of parent material and the conditions under which it is weathered influence the distribution and kinds of iron compounds formed in the soil.

We have two groups of conditions under which the iron of the parent material is most altered in the process of soil formation.

One is when rainfall is high, temperature is low, and an organic cover has accumulated. In such conditions, which prevail in much of the Northeastern States, a large part of the sodium, potassium, calcium, and magnesium of the parent material is dissolved and leached away and a more acid residue is left in the top horizons of the newly formed soil. A great deal of the iron, in turn, becomes reduced and dissolved and is transported to the lower, less acid horizon, where it is precipitated and forms new iron compounds, such as iron hydrates, oxides, and some organic iron complexes. Those compounds vary in color and ease of solubility. The more hydrated iron compounds are the most easily dissolved and reduced (that is, oxygen is removed) of all the inorganic iron minerals. The organic iron in acid soils is somewhat soluble in water, and the iron in this form tends to be protected from oxidation-precipitation.

The other set of conditions occurs when little organic cover has accumulated, but rain is abundant and temperatures are high. Such conditions prevail in many tropical areas. Here the more active base elements are also removed, as in the first set of conditions, and an acid material is formed, accompanied by oxidation. The iron is converted largely to hydrated oxides, which are retained in place. Some of the hydrated iron subsequently becomes dehydrated to Fe₂O₃.

Sizable amounts of the iron compounds form coatings on clay, silt, and sand particles in many soils. The coating may give them their color. The colors of other soils are due to the presence of the iron compounds as such. For example, some soils have various shades of red and reddish brown or yellow and yellowish brown because of the presence of hydrated iron oxides.

Thus all manner of modifications affect soil-forming processes. It is true, however, that in almost all soils that have developed enough to have clay and nonclay components, the clay fraction is several times higher in iron than the nonclay part.

We can get a general idea of the total iron content of many soils from an analysis of the soils and colloidal material of 30 alluvial soil profiles in the Mississippi River lowlands and 15 of its major tributaries.

The percentages of Fe₂O₃ in the whole soil of the A horizons of these profiles ranged from 1.8 to 6.5, with an average of 3.9 percent. The percentages of Fe₂O₃ in the colloidal or clay material of the same soils ranged from 7.7 to 12.6, with an average of 9.9 percent. The estimated iron content in the nonclay part of the soils was 0.7 to 2 percent. These soils may be considered as composite samples of the various soil areas drained by the Mississippi River. Their iron content was influenced by a yearly rainfall of 30 to 60 inches.

Iron deficiency also occurs in acid soils. Oftener than we think, iron may be a limiting factor in the growing of plants that prefer an acid soil blueberry, cranberry, rhododendron, azalea, and many others. Many of the deficiencies can be corrected by increasing the acidity in the soil by using soil amendments, such as sulfur or ammonium sulfate, or by supplying a soluble iron chelate.

Iron chelates (which we define later) have been used successfully in the culture of blueberry, azalea, and some other acid-preferring plants on soils not acid enough to grow them well.

But asparagus, spinach, cucumbers, squash, and many other plants do not grow well on acid soils. We do not know definitely whether such plants require a particular soil reaction or other soil conditions that are affected by the reaction. Until more is known about this important question, the selection of plants adapted to the soil or the use of soil amendments to control the soil reaction is recommended.

Because the soils of the and and semiarid regions are less weathered and leached than those formed under heavier rainfall, many of them are calcareous and alkaline in reaction. The iron is less altered and is more uniformly distributed in the soil profile. Rainwater in many places had little to do in soil development or formation of profiles, except that it leached the soluble salts from the surface soil.

Extensive areas of the arable soils of the intermountain districts of the United States, however, are alluvial fans, lake terraces, stream terraces, and bottom lands. Many of them show some weathering in place and profile development, but much of this material, before its deposition, already had been partially disintegrated by frost, wind, and water erosion. The soils contain variable (but usually small) amounts of organic matter. Most of them are well supplied with the inorganic elements, but because of their alkaline nature the soluble iron is low. Iron chlorosis often occurs in some plant species. It is most serious in the culture of fruits and ornamentals. It is less prevalent in certain field crops. In fact, almost all the soils west of 100 longitude are and and have problems of iron chlorosis when some plants are grown.