Study Notes on Soil | Plant Physiology

The below mentioned article provides a study note on soil.

Soils constitute complex natural formations on the earth surface and are made up of five components, soil water, minerals, organic material, air and living organisms. The chief function of the soil is to give physical support, help the plants to attach and supply them different inorganic substances.

Soil influences the growth and development of plants in several ways. Its various components interact with the plant roots and influence the chemical and physical properties of the plant. Soils have developed over several thousand years due to various geological processes including withering of rocks and minerals, exertion of several pressures, accumulation of decayed living materials, etc.

The texture of soil is made up of several particles which vary in size. Based on textures, the soils may be coarse, sandy with varying levels of organic matter. Sometimes the soil particles are extremely small and attain the properties of colloids. These particles constitute the clay part of the soil.

Based on their size soil particles are classified as follow:

A soil is loamy if all the three particles are present in equal amounts. Sand particles are small parts of a rock which do not interact strongly with water or minerals. The surfaces of silt particles are smooth or uneven, coated with clay. Their properties are in between sand and clay.

Sandy soils allow the water to percolate and evaporate quickly whereas clay soils retain water tenaciously. The amount and presence of organic material affects the water retention capacity of the soil.

Soil structure comprises aggregation of soil particles into clumps; sandy soils have small clumps whereas clay soils form clumps. Good soils are made up of clump structures which provide them a large surface area, aeration and high water retention capacity.

Mechanical means tend to damage the soil structure. Heavy clay soils are best suited for agricultural practices. However, these clods must be broken and organic material may be suitably added. A good agriculture soil contains 5-18% of organic matter which retains the soil structure and increases water retaining capacity.

Organic matter also holds the nutrients, provides substrate for the soil organism during their metabolism. Soil microorganism increases the soil fertility, help in nitrogen fixation and participate in several divergent symbiotic relations with the plant roots.

Water is retained in the soil in several ways and is open to various types of stresses. Hydrostatic pressure or absorptive forces hold the water in soil. Several factors like evaporation, gravity or root absorption tend to decrease the level of water in the soil. Water in soil diffuses through water potential.

Its components are the same as those of a cell. Furthermore, the materic potential and osmotic potential interact strongly in soils. The tendency of a soil to absorb water is called water tension. There is a considerable variation in the water potential of soils. A field capacity of soil is when a soil is wetted and then allowed to drain till capillarity movement’s stop.

Clay soils dry out slowly but its water potential is low. Soil contains water in different forms, e.g., as water of hydration of colloids, free water or water vapour. The size of spaces between soil particles and the firmness with which water is absorbed on colloidal particles determine the movement of water in the soil. Evidently water moves very fast in sandy and slowly in clay soils.

Plants fail to absorb enough water and replace the one lost through transpiration. When there is a fall in the water potential sufficiently, the leaves begin to wilt. However, if the leaves are placed in a saturated atmosphere they begin to recover and are said to be in a state of incipient wilt.

If water in the soil continues to be low then leaves wilt to a point of no recovery even though enclosed by water vapors. At this point the water content of the soil is referred to as the permanent wilting percentage.

Mineral Nutrients:

The nutrients and other substances in the soil are present in a dynamic state. Soil fertility depends upon the level, removal and relative rate of addition of these nutrients which are added or removed by divergent pathways.

The potentiality of roots to absorb these nutrients contributes to the nutrient status of the soil. Ions which are dissolved in soil water are freely absorbed by the roots. The concentration of soil nutrient solution also determines the soil fertility.

pH, oxygen and capacity of ion exchange of the soil play important roles in determining the soil fertility. Ion exchange capacity of soils is also dependent on the concentration of H⁺ and OH^– ions. Soil microorganisms also affect the soil fertility. The type and level of microorganisms also affect the exchange of ions by altering soil pH, or changing the availability of other ions.

Sometimes microorganisms enhance the availability of several nutrients like iron, boron or molybdenum. Ions may exist as soil solution or exchangeable nutrients in the exchange complex or rarely they exist as non-exchangeable ions. Sulphate and chloride ions have weak bonding energy and exist in high level in the soil solution.

On the other hand, calcium and magnesium are present in variable amounts depending upon the nature of soil particles. Soil is amphoteric i.e., it has both anions and cations. The exchange capacity of cation is more than that of anion and in general exchange capacity varies among different soils. In general, clay soils have highest exchange capacity. The measure of tightness with which ions are held is called bonding energy.

The elements in the exchange complex from the soil solution to the plant depend upon bonding energy. It may be stated that aluminium, barium, phosphorus are present in low concentration in the soil solution and have high bonding energies. Sodium, chloride and sulphate have weak bonding energies Calcium, magnesium and potassium have intermediate bonding energies (H⁺) and Hydroxyl (OH^–) ions are tightly bound.

Several factors affect the amount of ions in the soil solution and these are pH, abundance of ions, its exchange capacity, etc. Clearly when roots remove ions from the soil, then more ions are liberated from the exchange complex and equilibrium is attained between the soil water and exchange complex. In this way more nutrients are made available to the root.

Chemical Analysis:

A desired sample of plant is dried at 10-C in an oven. The water lost from the plant is calculated. This is the amount of water which contributes to the total weight of the plant. In general, plants contain 10-95% of water. The dry matter of a plant comprises both organic and inorganic compounds.

The former fraction represents 90 per cent of the total dry matter and is composed of compounds such as sugars, fat, starch, cellulose, proteins, organic acids, pigments, enzymes, etc. When the dry matter is burnt the organic matter is decomposed and passes off as carbon dioxide, water, and nitrogen, sulphur and phosphorus may also be evolved if the temperature of combustion is maintained between 600 to 700-C in a muffle furnace.

The non-volatile residue that is left out constitutes inorganic compounds. The latter are absorbed from the soil and are found as chlorides, sulphates, phosphates, silicates, carbonates and oxides of sodium, potassium, magnesium, calcium, iron and aluminium and also some other elements in traces. This residue is called ash.

The composition of plant ash varies with the species, their age, sampling part and the environmental conditions. Some minerals like potassium, phosphates, sulphates and other ions accumulate in cells in excess of the amount actually utilized. Further, no discrimination is exercised between essential and non-essential elements in the absorption process.

Solution and Sand Cultures:

Culture solutions are prepared by dissolving salts in a specific ratio in distilled water and used as nutrient solutions for growing plants in liquid culture as well as sand culture. In the sand culture method, the plants instead of growing in the natural soil are grown in well-washed and sterilized pure quartz sand to which an aqueous solution containing various salts in proper proportions is added.

In liquid culture, the plant roots remain dipped in water solution containing all the essential elements required for its development and the effect of different elements on growth and development of the seedling is studied.

Knops and Sachs’ solutions are commonly used (Table 9-1 A).

i. FeEDTA is an iron-chelate complex, specifically sodium ferric ethylenediamine tetraacetate.

ii. Micronutrient stock solution (minus iron) contains per litre: 2.86 g H₃ BO₃, 1.81 g MnCl₂. 4H₂ O, 0.11 g ZnCl₂, 0.05 g CuCl₂. 2H₂ O, 0.025 g Na₂ MoO₄,and 2H₂ O.

The nutrient solution composition proposed by Arnon and Hoagland (1940) is widely used because it contains all the macro-, micro-and trace elements which are needed for the growth of a normal plant. The components of this nutrient solution are given in Table 9-2.

In addition to full culture solution with all the essential elements, solutions are also prepared where each one of the essential elements is omitted individually. Healthy growing seedlings are transplanted in the culture solution and the effect of the missing element on their growth and development is marked after comparing with the seedlings grown in the complete culture solution.

Experiment:

A number of wide mouthed jars are fitted with a split cork (Fig. 9-1). The jars used in the experiments are of Borosill or Corning glass and thoroughly cleaned by treating with a mixture of potassium dichromate and sulphuric acid rinsed repeatedly both in tap water and finally distilled water before use.

The jars are filled with culture solution prepared in double glass distilled water but each one of them is deficient in any of the following nutrients like phosphorus, iron or calcium, etc. The different jars are labelled accordingly. Through the split cork gram seedlings of the same size are suspended in each jar. The jars are wrapped with black papers and the leaves are exposed to sunlight. An arrangement is made for proper aeration of the roots. The culture solutions are renewed fortnightly.

The shifts in pH due to differential absorption of ions occur. When the solution turns alkaline, a few drops of H₃ PO₄ are added, until it is slightly acidic in reaction again.

Essential and Non-Essential Elements:

About ninety one elements have been identified in plants. By water culture experiments Sachs and Knops have established that in addition to carbon, hydrogen and oxygen obtained from the air and water, several other elements are absorbed from the soil and are essential for the normal growth of plants.

They are required by the plant in relatively large quantities, and are called major elements of plant nutrition e.g., calcium, potassium, magnesium, nitrogen, sulphur and phosphorus. On the contrary iron is required in small concentrations than the major or macro-metabolic elements.

Other elements like zinc, manganese, copper, boron, and molybdenum are needed in still lower concentration and along with iron, are referred to as minor or trace or ‘micro-metabolic’ elements of plant nutrition. The non-essential elements present in the plants are sodium, chlorine and silicon.

Criteria for Essentiality of Elements:

The status of a mineral to be an essential nutrient of a plant is based on the following characteristics; the element must be essential for the normal growth and reproduction. It should be indispensable, no other element could replace it; the requirement of this element must be direct and not due to any indirect effect.

These features though used strictly yet in some instances are labile, since the plants grow under variable environmental conditions. It may be stated that it is extremely difficult to show that an element was non-essential since some of the micronutrients are needed in extremely low amounts e.g., silicon is essential for the growth of tomatoes and needed in less than 0.2 µg/g of dry plant tissue.