Measurement of Soil Water / Soil Moisture

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MEASUREMENT OF SOIL WATER:

Water Content:

  • Soil water content is measured by drying a known volume or weight of soil at 105 degrees, then re-weighing in order to determine the amount of water evaporated.
  • The water content of soils in the field can be determined by a neutron probe. This technique is especially useful in watershed management. A thin aluminum pipe is driven into the ground (neutrons pass through aluminum) and a probe containing a fast neutron source is lowered down the pipe. When fast neutrons collide with hydrogen atoms they are deflected, slowed, and measured when they rebound back toward the probe which also contains a slow neutron detector. Since water is the primary source of hydrogen atoms, measurement of the slow neutrons created by fast neutron collisions with hydrogen can be used to estimate water content. The probe allows measurements to be made at several soil depths.
  • Soil water content can be measured by lysimeters. Soil and plants are placed in containers that can be weighed. Precipitation, evapotranspiration, and soil water can be measured by periodic weight measurements of the lysimeter.



Field Capacity (FC):

  • The ability of the soil to retain water against the force of gravity can be measured by weighing a saturated soil that has been allowed to drain for 24 hours.
  • FC is more commonly estimated in soils laboratories by placing a sample in a pressure membrane apparatus and attracting all the water held in a saturated soil at a tension less than 0.03 MPa. This approximates the force of gravity. The soil is then weighed, dried, and re-weighed in order to determine its water content.

Permanent Wilting Percentage (PWP):

  • The classic method of determining PWP is to place a plant, generally a sunflower, in a pot of soil, allow it to grow so that its roots thoroughly penetrate the soil. The plant is then allowed to withdraw water from the soil until it wilts.
  • Permanent wilting is determined when the plant does not recover turgor overnight when placed in a humid environment (e.g. under a bell jar). The soil is then removed, weighed, dried, and reweighed to determine the amount of water left after plant extraction.
  • Since the classic method requires several days, a more convenient laboratory method is used, again with the pressure membrane apparatus. In this case, water is removed at 1.5 MPa pressures, the approximate value that most crop plants can extract soil. The water remaining after pressure extraction at 1.5 MPa is determined by weighing, drying, and reweighing the soil, and this is expressed as the PWP.

Available water:

  • Available water is computed as the difference between FC and PWP. It also may be crudely estimated on the basis of the texture of the soil.

Soil Moisture Tension or Water Potential ψ:

  • Soil ψ may be measured by a tensiometer. These have a porous clay or ceramic water-filled bulb attached to a water-filled stem connected to a mercury manometer or other means of measuring tension. Water is withdrawn from the porous bulb into the soil under increased tension as mercury rises in the manometer. Measurement is made at equilibrium and expressed in MPa.
  • Soil ψ may also be measured by porous (often gypsum) blocks in which electrodes have been inserted. The blocks are placed into the soil, and the moisture in them allowed coming into equilibrium with the soil moisture. The more water absorbed by the block, the less resistance to the flow of electricity from one electrode to the other. The blocks are calibrated to relate resistance units to soil ψ.
  • Neither of the above methods of measuring soil ψ takes into account of contribution solutes in the soil solution. These are usually negligible in soils, but may be substantial if soils approach saline conditions. In such cases, soil ψ can be measured using colligative properties of water; the change in vapor pressure with change in ψ.
  • In this method, the soil sample is placed in a temperature controlled chamber with a thermocouple. Current is applied to the thermocouple, which causes it to cool (Pelter effect). When it cools to the dew point of the atmosphere in the chamber, which is controlled by the soil ψ, a droplet condenses on the thermocouple, liberating heat of condensation, which is measured. Soil ψ is computed from the vapor pressure of water in the sample chamber caused by water in the soil.

Soil Salinity:

  • The most common method of measurement for salinity is conductivity, using the relationship that the greater soil solute concentration, the greater soil conductivity since salts are charged ions.

For correction and improvements please use the comments section below.




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Naeem Javid Muhammad Hassani

NJMH is working as Deputy Conservator of Forests in Balochistan Forest & Wildlife Department (BFWD). He is the CEO of Tech Urdu (techurdu.net) Forestrypedia (forestrypedia.com), Majestic Pakistan (majesticpakistan.pk), All Pak Notifications (allpaknotifications.com), Essayspedia, etc & their YouTube Channels). He is an Environmentalist, Blogger, YouTuber, Developer & Vlogger.

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