Soil Plant Water Relationship (SPWR) – Short Notes

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Soil Plant Water Relationship (SPWR) – Short Notes From Past Papers

Follow this link to get complete notes of SPWR (written by Naeem Javid Muhammad Hassani)

Surface tension 

  • The result of intermolecular forces exerting an unbalanced inward pull on the individual surface molecules; this is reflected in the considerable curvature at those edges where the liquid is in contact with the wall of a vessel.
  • More specifically, the tension is the force per unit length of any straight line on the liquid surface that the surface layers on the opposite sides of the line exert upon each other.
  • The tendency of any liquid surface is to become as small as possible as a result of this tension, as in the case of mercury, which forms an almost round ball when a small quantity is placed on a horizontal surface. The near-perfect spherical shape of a soap bubble, which is the result of the distribution of tension on the thin film of soap, is another example of this force; surface tension alone can support a needle placed horizontally on a water surface.
  • Surface tension is important at zero gravity, as in space flight: Liquids cannot be stored in open containers because they run up the vessel walls.


  • Substance maintaining pH: a substance that minimizes a change in pH of a solution by neutralizing added acids and bases, or a solution containing such a substance.


  • Absorption of fluid by colloid or the absorption or adsorption of something such as liquid or heat by a mixture colloid such as a gel.


  • Diffusion is the flow of energy or matter from a higher concentration to a lower concentration, resulting in a homogeneous distribution.
  • If one end of a rod is heated or electrically charged, the heat or electricity will diffuse from the hot or charged portion to the cool or uncharged portion. If the bar is made of metal, this diffusion will be rapid for heat and almost instantaneous for electricity; if the bar is made of asbestos, the diffusion will be slow for heat and extremely slow for electricity.
  • Diffusion of matter occurs most rapidly in gases, more slowly in liquids, and most slowly in solids.
  • Diffusion is the natural tendency of molecules to flow from higher concentrations to lower concentrations. When the barrier between two substances is removed (as shown in fig), the molecules will diffuse throughout the entire container. While the number of molecules in the container is the same as it was before the barrier was removed, the substances are now at lower concentrations. The rate of diffusion depends on the weight of the molecules—heavy molecules diffuse more slowly than light molecules.

Root pressure

  • If the top of a plant using osmotic water absorption is detached, and a pressure gage such as a mercury manometer attached to the root stump, pressure may be measured. This is called root pressure, and it is a plant expression of osmotic pressure.  Root pressure may reach as much 0.15 MPa under unusual circumstances.  Root pressure has not been detected in many tree species.  However, root pressure may be demonstrated by tying a portion of a child’s balloon on the stump of grape in the spring, and watching the balloon swell with exuded sap until it bursts.


  • Humus refers to organic matter that has decomposed to a point where it is resistant to further breakdown or alteration.
  • Humic acids and fulvic acids are important constituents of humus and typically form from plant residues like foliage, stems, and roots.
  • After death, these plant residues begin to decay, starting the formation of humus. Humus formation involves changes within the soil and plant residue, there is a reduction of water-soluble constituents including cellulose and hemicellulose; as the residues are deposited and break down, humin, lignin, and lignin complexes accumulate within the soil; as microorganisms live and feed on the decaying plant matter, an increase in proteins occurs.
  • Humus formation is a process dependent on the amount of plant material added each year and the type of base soil; both are affected by climate and the type of organisms present. Soils with hummus can vary in nitrogen content but have 3 to 6 percent nitrogen typically; humus as a reserve of nitrogen and phosphorus is a vital component affecting soil fertility.
  • Humus also absorbs water, acting as a moisture reserve that plants can utilize; it also expands and shrinks between dry and wet states, providing pore
  • Humus is less stable than other soil constituents, because it is affected by microbial decomposition, and over time its concentration decreases without the addition of new organic matter.
  • However, some forms of humus are highly stable and may persist over centuries if not millennia: they are issued from the slow oxidation of charcoal, also called black carbon, like in Amazonian Terra preta or Black Earths


  • Shrinking of protoplasm from cell wall: the shrinking of the protoplasm in a plant or bacterial cell away from the cell wall, caused by loss of water through osmosis.

Turgor Pressure

  • The rigidity of living cells or the normal rigid state of plant cells, caused by the outward pressure of the water content of each cell on its membrane
  • Some scientists classify plant vacuoles as a type of lysosome. These membrane-bound structures are much larger than other lysosomes, measuring up to 20 micrometers in diameter. Vacuoles maintain water pressure within plant cells, called turgor, preventing wilting. Vacuoles may also provide long-term storage of polysaccharides, lipids, proteins, pigments, and harmful materials such as rubber or opium that may deter predators.


  • In addition to the stomata, many kinds of leaves have large specialized water pores in their epidermis. These pores, called hydathodes, permit guttation, the process by which a plant loses liquid water. Unlike the stomata, hydathodes remain open all the time.

Root hair

  • The main root of many plants divides as it grows downward. The branches, called lateral roots, further divide to form a network that anchors the plant in the ground. New growth takes place at the ends of the smallest roots. Tiny root hairs absorb water and nutrients from the soil, channeling them up to the stem and leaves of the plant through the xylem tissue at the center of the root.


  • A symbiotic relationship between fungus and plant. A mutually beneficial association of a fungus and the roots of a plant such as a conifer or an orchid, in which the plant’s mineral absorption is enhanced and the fungus obtains nutrients.

Infiltration and Percolation

Infiltration: –   The passage of water through the soil from surface soil to the sub-surface soil is termed as infiltration
Percolation: –   The passage of water from the surface soil, sub-surface soil up to parent material of the soil profile is called percolation.


  • The basic International System unit of amount of a substance equal to the amount containing the same number of elementary units as the number of atoms in 12 grams of carbon-12. Symbol mol


  • In addition to the stomata, many kinds of leaves have large specialized water pores in their epidermis. These pores, called hydathodes, permit guttation, the process by which a plant loses liquid water. Unlike the stomata, hydathodes remain open all the time (See pic).
  • Guttation takes place only when water is being rapidly absorbed by the roots, such as after a heavy rainfall, and when transpiration slows down, as on cool, humid nights. When these conditions occur together, droplets of water can be seen on the leaf early in the morning before they evaporate in the heat of the day. Unlike dew, which condenses on leaves from water vapor in the air and covers the entire leaf surface, guttation droplets form only on the edges and tips of leaves. Generally, the droplets are noticeable only on the leaves of strawberries and a few other kinds of plants.

For correction and improvements please use the comments section below. Plz, help us update the list by adding new/latest terminologies from SPWR recent papers. Write all these in comments below.

Naeem Javid Muhammad Hassani is working as Conservator of Forests in Balochistan Forest & Wildlife Department (BFWD). He is the CEO of Tech Urdu ( Forestrypedia (, All Pak Notifications (, Essayspedia, etc & their YouTube Channels). He is an Environmentalist, Blogger, YouTuber, Developer & Vlogger.

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