Chapter Nutritional requirements Soilplant relationships

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Plants in their natural environment have lived, with almost no exception, in association with soil, an association known as the soil-plant relationship. Soil provides four basic needs of plants: water, nutrients, oxygen, and support. With the advancement of science and technology, humans have provided for these needs in an artificial way and have successfully grown plants without soil. All the various methods and techniques developed for growing plants without soil are collectively called soilless methods of plant culture. These methods include a great diversity of systems, from the purely hydroponic, which are based on water and nutrients only (e.g., nutrient film technique, or NFT), to those based on artificial mixes that contain various rates of soil. In between these extremes lie a great number of soilless methods that make use of some sort of growing medium, either inert (e.g., rockwool slabs, polyurethane chunks, and perlite) or not inert (e.g., gravel culture, sand culture, and peat bags).

1 metre per second

Soil as a growth medium

Soil consists of mineral matter, organic matter, water, and air. An average soil in optimum condition for plant growth might consist of 45% mineral matter, 5% organic matter, 25% water, and 25% air space. The mineral matter is made up of a great diversity of small rock fragments. The organic matter of a soil is derived from plant and animal remains and is a mixture of these materials at various stages of decomposition. In the process of decomposition, some of the organic entities are oxidized to their end-products and others to an intermediate product called humus. Both the type and the relative quantity of the mineral and organic constituents of a soil determine its chemical properties. Chemical properties of a soil are the amounts of the various essential elements present and their forms of combination, as well as the degree of acidity or alkalinity, known as pH.2 The extent of nutrient availability to the plants depends not only on the chemical properties of the soil but also on its physical properties.

Soil structure and texture

The physical properties of a soil describe its texture, i.e., the size distribution of its mineral constituents, expressed as a percentage of content of sand, silt, and clay (Fig. 8), and its structure, i.e., the type and extent of formation of the various mineral and organic constituents into crumb-like soil aggregates. The organic matter of a soil plays an important role in soil structure because of the diversity in the size of its components but, even more importantly, because of the role of humus in cementing together the various soil constituents into crumb-like aggregates.

Soil structure in turn plays an important role in soil fertility (the ability of soil to sustain good plant growth and high yields) because it determines, to a great extent, the water-holding capacity and aeration of a soil. The water held within the soil pores, together with the salts dissolved in it, make up the soil solution that is so important as a medium for supplying nutrients and water to growing plants. The air located in the soil pores supplies oxygen for the respiration of root and soil microorganisms and removes the carbon dioxide and other gases produced by them. Plant nutrients exist in soil as either complex (organic or inorganic). compounds that are unavailable to plants or in simple forms that are usually soluble in water and are therefore readily available to plants. The complex forms, which are too numerous to mention, must first be broken down through decomposition to simple soluble forms to be available and therefore useful to plants (Fig. 9). The available forms of all essential nutrients for plant growth are summarized in Table 1.

2 The pH value of a solution Is the negative logarithm of its hydrogen ion concentration (pH =-log[H + ]).ApH of 7 indicates neutral conditions; values lower than 7 indicate an acid environment; and values higher than 7 indicate an alkaline environment.

Fig. 8 Classification of soils according to texture.

Soil reaction (pH)

The reaction of the soil solution (pH) also affects the solubility of the various nutrients and thus their availability to plants; this process is illustrated in Fig. 10.

In acid soils (pH < 7) the nutrients calcium and molybdenum are less available, whereas in alkaline soils (pH>7) the nutrients iron, manganese, and zinc are less available, and excessive amounts of bicarbonate (HOO^) may interfere with the normal uptake of certain nutrients. Most nutrients are available when the pH range is between 6 and 7, which explains why most plants grow best in soils of that reaction.

The cation exchange capacity of the soil

When small quantities of inorganic salts, such as the soluble mineral matter of soil and commercial fertilizers, are added to water they dissociate into electrically charged units called ions. The positively charged ions (cations) such as hydrogen (H+), potassium (K"1"), calcium (Ca2+), magnesium (Mg2+), ammonium (NH+), iron (Fe2+), manganese (Mn2+), and zinc (Zn2+) are absorbed mostly on the negatively charged surfaces of the soil colloids (microscopic clay and humus particles) and exist only in small quantities in the soil solution. Thus, the humus-clay colloids serve as


Relationship Between Cec And Clay


Fig. 9 The processes of mineralization, soluhilization, cation exchange, and nutrient absorption.



Fig. 9 The processes of mineralization, soluhilization, cation exchange, and nutrient absorption.

a storehouse for certain essential ions (cations). The negatively charged ions (anions), such as nitrates (NO^), phosphates (HPOj*), sulfates (SO;p, and chlorides (CI"), are found almost exclusively in the soil solution and can therefore be leached away easily with overwatering. The roots and root hairs are in intimate contact with the soil colloidal surfaces, which are bathed in the soil solution, and therefore nutrient uptake can take place either from the soil solution or directly from the colloidal surfaces (cation exchange).

The soil solution is the most important source of nutrients, but since it is very dilute its nutrients are easily depleted and must be replenished from soil particles. The solid phase of the soil, acting as a reservoir of nutrients, slowly releases them into the soil solution by the solubilization of soil minerals and organics, by the solution of soluble salts, and by cation exchange. A more dramatic increase in the nutrient content of the soil solution takes place with the addition of commercial fertilizers.

As plants absorb nutrients (ions) they exchange them for other ions. For example, for the uptake of one potassium (K+) ion or one ammonium

Table 1 Essential elements for the growth of most cultivated plants

Atomic Available

Element Symbol weight form

Organic elements (obtained from the air and water)

Hydrogen H 1.00 H20

Carbon C 12.00 C02

Oxygen 0 16.00 02, H20

Macronutrients (needed in large quantities)

Potassium K 39.10 K+

Calcium Ca 40.08 Ca2+

Magnesium Mg 24.32 Mg2+

Phosphorus P 30.92 H2PO;,HPO;;

Sulfur S 32.07 SOj"

Micronutrients (needed in small quantities)

Manganese Mn 54.94 Mn2+

Zinc Zn 65.38 Zn2+

Molybdenum Mo 95.95 MoO^ +

(NH4) ion, one hydrogen (H+) ion is released into the soil solution or directly into the soil colloids by the process of cation exchange. Similarly, for the uptake of one calcium (Ca2+) or one magnesium (Mg2+) ion, two hydrogen (H+) ions are released by the root. Thus, as the plant absorbs these essential cations, the soil solution and the colloidal particles contain more and more hydrogen (H+) ions, which explains why the removal of cations (ammonium (NH 4) nitrogen is a good example) by crops tends to make soils acidic, i.e., having a low pH. Also, as the plant absorbs essential anions such as nitrates (NO 3) and phosphates (HPOI), the soil solution is enriched with more and more hydroxyl groups (OH") and bicarbonates (HCO~), which explains why the removal of anions (nitrate (NO 3) nitrogen is a good example! by crops tends to make soils alkaline, i.e., having a high pH.

Hfl 2

strongly acid

How soil pH affects availability of plant nutrients

Mineral soils

Soilless mix medium acid slightly acid strongly acid medium acid slightly acid

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