Relationship of Soil, Water and Fertilizer with Plants

After reading this article you will learn about the relationship of soil, water and fertilizer with plants.

Plants require a large amount of water for their growth and production. A good crop of wheat requires about 1000 kg of water to produce 1 kg of wheat. But only a fraction of this water is retained by the plants. Plant requires essential elements that comes from soils.

These chemical elements move from soil to plants as ions and molecules in a water solution. Carbohydrates, the products of photosynthesis, are conveyed in water solution to storage organs such as seeds, roots or tubers.

Nutrient Absorption by Plants:

Plants require only 16 of the 88 elements occurring in nature. These are carbon, oxygen, hydrogen, nitrogen, phosphorus, potassium, magnesium, calcium, sulphur, chlorine, copper, boron, iron, manganese, molybdenum and zinc.

The plants obtain their carbon from the carbon dioxide of the air, oxygen from atmospheric and soil air and hydrogen from soil water. The other 13 essential nutrients are absorbed in ionic form primarily through plant roots growing in the soil.

Nutrient uptake includes three mechanisms:

(1) Mass flow

(2) Diffusion, and

(3) Root interception.

1. Mass Flow:

Essential elements in the soil solution around the plant roots are absorbed as cations and anions. For example, nitrogen and sulphur. Transpiration and water uptake by plants results in mass flow of nutrients present in soil solution to the roots. Mass flow supplies a abundance of Ca⁺⁺ and Mg⁺⁺ as well as most of mobile nutrients including NO₃ and SO₄.

The movement of nutrient ions by mass flow is reduced in winters (at low temperature) because the transpiration demands of plants are considerably less in winter than the summer. Moreover, low evaporation of water at soil surface in winter also diminishes flow of nutrient ions.

2. Diffusion:

(a) Passive Absorption:

In passive absorption, ions move independently. A nutrient cation or anion moves from high concentration to an area of low concentration. Ions in soil solution enter the root tissue through diffusion and ions exchange processes.

The outer free space, which facilitates diffusion and exchange of ions is located in the walls of epidermal and cortical cells of the roots. Diffusion of ions occurs along concentration gradient since the concentration of ions is less in the apparent free space than is the bulk solution concentration i.e. from high to low concentration.

Interior surfaces of the cortical cells being negatively charged attract cations. This uptake into the outer space by diffusion and ion exchange is termed passive uptake as the process is nonselective and require no metabolically produced energy rather it is governed by ion concentration and electrical gradients. Uptake of cations are higher than anions because the interior surfaces of cells in the cortex are negatively charged, they attracts cations.

(b) Active Absorption:

In active absorption, ions move into cells against a concentration gradient using energy from respiration. For example, phosphorus, potassium etc. The transport of ions from outer space to inner space (cytoplasm) require expenditure of energy derived from cell metabolism because the ion concentrations are greater in inner space than in the outer space i.e. is strictly against an electrochemical gradient. This is referred to as the active ion uptake.

Active transport is a highly selective process as different ions are carried across by specific carrier. Ion-carrier complex is able to cross plasma lemma barrier of cell which is otherwise not permeable to free ions. Thus ion is released into the inner space of the cell.

3. Root Interception:

The growth of plant roots into new soil areas were untapped supplies of nutrients are available. Nutrients are absorbed by mass flow or diffusion. For example, calcium, magnesium, sulphur, molybdenum etc. Plant root hairs absorb nutrients from clay particles and soil solution by releasing hydrogen cations for equivalent amounts of other cations i.e., calcium (Ca⁺⁺), magnesium (Mg⁺⁺), potassium (K⁺), ammonium (NH₄⁺), etc.

Soil, Plants and Fertilizers:

The soil supplies 13 out of 16 essential elements. These essential elements must be available in balanced form through fertilizer application. Availability of fertilizers depend upon the presence of water in the soil. In dry regions, soils originally have large reserves of potassium and too much lesser degree of phosphorus and nitrogen.

When soil moisture is definitely inadequate, crops may fail to respond to fertilizers, or the response may be too small to be economic. The minimum amount of rainfall needed to ensure a fairly satisfactory yield-level of grain crops which in turn makes possible an economic return on the fertilizers applied, depends on factors such as the efficiency of rainfall, moisture reserve in the soil etc.

In irrigated areas, high yields are obtained due to appropriate water and fertilizer regime. Increased needs for fertilizers under irrigation are due to a more intensive cropping schedule, higher yields produced and leaching of nutrients due to irrigation. Application of 100-200 kg of nitrogen per hectare can give economic yields in cereals. The high rate of nitrogen applied calls almost automatically for an increase in phosphatic fertilizers (50-100 kg of phosphorus per hectare).

In submerged soil, or in rice fields, water is retained throughout the growing season. The important changes due to surface flooding is pan formation (hard layer). Pan formation is due to chemical cementation, physical induration and formation of textural B-horizon. In pan formation, ferric, manganese, silicic or other compounds may accumulate in layer under the reduced puddled top soil is also known as chemical cementation.

Puddling breaks down the soil structure and permits rapid reduction of nutrients and provide poor environment for root penetration. Water-logging creates anaerobic (absence of air) environment where de-nitrification (reduction of nitrate to gaseous state). Reduction to ammonia is insignificant.

Most of the nitrogen obtained by low land rice is taken up in the ammonium form (pH and iron status of most water­logged soils favour the uptake of ammonium by rice plant). Ammonium nitrogen or ammonium forming sources of nitrogen (urea) should be used for low land rice. To check the ammonium nitrogen from being nitrified, it should be applied several inches deep on the reduced zone (absence of oxygen) of a water-logged soil.

In general, lowland rice shown considerably less responses to phosphate than do upland crops. Ca⁺⁺, Mg⁺⁺, K⁺ and Na⁺ are not involved in the reduction process. The increases in their concentration is a secondary effect of submergence.

The reduction in submerged soil of these elements occur due to solvent action of CO₂ and cation exchange reactions. In submerged , soil, iron (reduced to ferrous forms), manganese (reduced to unavailable forms), zinc (concentration of water-soluble zinc is decreased) become less available to plants.

Movement of Fertilizer Salts in the Soil:

The movement of fertilizer salts in the soil is of practical importance. Nitrate nitrogen and potassium movement is largely vertical. The salts up or down movement depends upon the direction of water movement. The vertical movement of nitrate nitrogen is of greater extent while potassium salt is of lesser degree.

These translocations greatly influence the time and method of applying nitrogen and potassium. For example, it is undesirable to supply nitrogen in one annual application because of leaching hazard. Downward movement of nitrogen (nitrate) makes available the salt to the roots of crops when fertilizer applied on the surface of top dressing.

The movement of nitrogen and to a lesser degree, potassium must be considered in the placement of the fertilizer with respect to the seed. If the fertilizer salts are located in a band directly under the seeds, the upward movement of nitrates and some of the potassium salts by capillary water often results in injury to the plants.

Phosphorus compounds move very little, except in sandy soils. In some organic soils, movement of phosphorus salts has been recorded. Therefore, this element should be placed in the root zone for greater effectiveness. Surface applications do not supply the deeper roots with phosphorus. Leaching loss of phosphorus is minimal in mineral soils because of its immobility.

Prevention from Fertilizer Contamination:

(i) Soil application of fertilizers should be reduced.

(ii) Timing of fertilizers application should coincide with plant need.

(iii) The fertilizers should be mixed with some soil to reduce surface run off of the fertilizer compounds.