Fertility

Plant Nutrition and Soil Fertility

L. A, Dean.

At least sixteen elements are considered necessary for the growth of green plants carbon (C), hydrogen (H), oxygen (O), nitrogen (N), phosphorus (P), sulfur (S), potassium (K), calcium (Ca), magnesium (Mg), iron (Fe), manganese (Mn), zinc (Zn), copper (Cu), molybdenum (Mo), boron (B), and chlorine (Q.

We commonly refer to these elements as the plant nutrients and as essential elements. An element is said to be essential if the plant cannot complete its life cycle without it and if the malady that develops in its absence is curable only by that element.

Plants obtain carbon, hydrogen, and oxygen from water and carbon dioxide, and the other nutrients from the soil.

We classify plant nutrients as the macronutrients and micronutrients (or to use an older term minor elements). This classification is based on the relative amounts that are found normally in plants and does not imply relative importance.

The micronutrients are iron, manganese, zinc, copper, molybdenum, boron, and chlorine.

Green plants contain sodium, iodine, and cobalt, which are essential to animals but have not been proved to be essential to green plants. Although sodium is not considered as an indispensable element, it does enhance the growth of some plant species. Silicon and aluminum occur almost universally in plants, but they perform no recognized function.

The essential elements originate and are distributed within the soil in several ways. A study of these matters gives an insight into the nutrient-supplying power and reserves of soils.

Overall chemical analyses indicate that the total supply of nutrients in soils is usually high in comparison with the requirements of crop plants. Much of this potential nutrient supply, however, is tightly bound in forms that are not released to crops fast enough to produce satisfactory growth. Thus interest has developed in measuring the available nutrient supply as contrasted to the total quantities involved.

SOILS ARE COMPOSED of three physically separable phases: Solid, liquid, and gaseous. The nutrient supply originates with the solid phase. The usual path to the plant is from the solid particles to the surrounding liquid and thence into the plant root.

The actual transfer involves the movement of ions. The positively charged ions, called cations, include K+, Mg++, Ca++, Fe+++, Mn++, Zn++, and Cu++. The anions, those of negative charge, include NO₃-, H₂PO₄-, SO₄__, Cl-, HB₄O₇, and HMoO₄-. The liquid phase, which contains these ions, frequently is referred to as the soil solution. Chemical and biological processes occurring in the soil solution and at the interfaces with solid soil particles create the ions necessary for plant nutrition.

MATERIALS OF ORGANIC ORIGIN one of the two broad classes of materials that constitute the solid phase of soils consist of a large series of products ranging from fresh plant and animal tissue to the more or less stable black or brown degradation product, which is humus, formed by biological decay.

The organic matter of soils is a potential source of nitrogen, phosphorus, and sulfur. It contains more than 95 percent of the total nitrogen, 5 to 60 percent of the total phosphorus, and 10 to 80 percent of the total sulfur.

Biological processes are required to convert these organic sources to an ionic state that is available to plants.

Within the biosphere a continuous turnover of these elements takes place among soils, plants, animals, and sometimes atmosphere. Thus there is a nitrogen cycle, a phosphorus cycle, and a sulfur cycle.

We can consider soil organic matter to be one of the storage points in these cycles. Energy must be expended before the elements again reenter the cycle. For the growth of nonlegumes in the absence of fertilizer or manures, the soil organic matter is the origin of the nitrogen supply of crops. But only a part of the phosphorus and sulfur supplied to crops is derived from this organic matter.

The inorganic or mineral fraction the other broad solid phase of soils comprises the bulk of most soils. It derives from rocks and their degradation products. The composition, mineralogy, and nutrient-supplying power of the larger particles that is, the sand and silt are quite different from those of the fine particles or clay fraction. The minerals that comprise the sand and silt fractions contain most of the elements essential for plant growth as a part of their structure.

Mineral decomposition is necessary before the nutrients in this form are available to plants. These minerals decompose very slowly in the soil. For example, experiments have been conducted with finely ground feldspar and apatite, common primary minerals of soils bearing potassium and phosphorus, respectively. When they were applied in quantities comparable with which they usually are found in soils, the rates of supply of the nutrients were insufficient for the good growth of plants.

The clay fraction of soils is composed of secondary minerals and amorphous materials that differ typically from the components of the sand and silt. The clays are products of weathering and are not found in unaltered rocks. They also are somewhat more stable toward decomposition by weathering processes. The soil clay is composed mostly of two groups of substances the clay minerals and the hydrous oxides.

The clay minerals are composed mainly of three mineralogical types--kaolinite, montmorillonite, and illite, which are important to soil chemistry and the mineral nutrition of plants because of the cation-exchange properties they exhibit.

The hydrous oxides are predominantly compounds of iron and aluminum. They have a part in the fixation of phosphates and so influence the availability of phosphorus in soils.

THE CATION-EXCHANGE properties of soils arise in the clay mineral fraction and the organic matter fraction. This exchange is the reversible process by which cations are exchanged between the solid and liquid phases.

The exchangeable cations are held at the surface of the solid phase because of the negative charges of unbalanced forces. When these particles are bathed in water, some of the cations enter the water until a steady state between the numbers of cations associated with each phase is set up.

Conversely, if a soluble salt (potassium chloride, for example) is introduced into the liquid phase, cations of potassium are formed as the salt dissolves and the steady state is disrupted. In this instance, potassium ions will exchange for other ion species held as exchange cations, and a new equilibrium is established. The net result is that a part of the soluble potassium is converted to an exchangeable cation status and as such is a part of the solid phase of the soil. The amounts held are low in sandy soils and large in clay and organic soils.

This capacity of soil to hold exchangeable cations is termed the cation-exchange capacity and usually ranges between 2 and 50 milliequivalents per 100 grams of soil. One milliequivalent of calcium per 100 grams of soil is equal to 400 pounds per acre of calcium the amount of calcium in 1,000 pounds of pure limestone.

The important exchangeable cations are hydrogen, calcium, magnesium, potassium, and sodium. Other cations that may appear, but in smaller amounts, are ammonium, manganese, zinc, copper, and aluminum. These different ion species are held to the solid phase with different binding energies. Calcium is the nutrient most tightly held, and potassium the least. The ease with which ions are exchanged one for another is related to the binding energies.

The base-exchange properties of soils influence plant nutrition and the desirability of the soil as a growth medium in a number of ways. Nutrient cations held as exchangeable bases are in a readily available state, but are not readily leached from soils. In fact, other things being equal, leaching losses decline with an increase in the cation-exchange capacity. Cation exchange acts as a buffer, which hinders rapid changes in nutrient level or cation balance.

The balance between amounts of exchangeable hydrogen and exchangeable bases governs the acidity of soils. In an acid soil, a large number of the exchange sites are occupied by hydrogen. Soil acidity is usually corrected by replacing these hydrogen ions with calcium ions by liming the soil.

Some nutrient elements form anions and enter into the reactions involving soil and plants in that form. The most prevalent nutrient anions in soils are NO₃-, Cl-, SO₄- and H2PO₄-.

When the soils are bathed in solutions containing NO₃- and Cl- ions, there are no chemical reactions of importance involving these ions and the solid soil phase. Nearly always is that also true of the SO₄ ions.

The phosphate fixation properties of soil are responsible for the normally low concentration of phosphate ions in the soil solution and for the restricted movement of phosphate in soils. In comparison with the exchange cations, the binding energies holding the fixed phosphate ions are high. These fixed phosphate ions, however, do contribute to the reservoir that supplies phosphorus to plants.

BECAUSE THE LIQUID PHASE (or water) in soils contains dissolved soluble salts, it is referred to often as the soil solution. The assumption is that nutrient ions present in the soil solution are immediately available for plant nutrition.

Studies of the soil solution and the water-soluble materials of soils date back to the early attempts to relate soil composition to nutrient uptake:

The soluble materials in soils range from tiny amounts in some acid soils of the humid regions to the saline conditions sometimes encountered in the soils of the and regions.

Crops usually require greater amounts of nutrients than the soil solution contains at any given time.

The soil solution is highly dynamic. Ions are continuously being removed by plant roots.

Simultaneously other ions are renewing this solution through cation exchange and the slow breakdown of soil minerals.