Soil Water Status: Content and Potential by Jim Bilskie, Ph.D
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Soil water status: content and potential By Jim Bilskie, Ph.D. The fundamental forces acting on soil Campbell Scientific, Inc. mwet 94 g water are gravitational, matric, and osmot- ic. Water molecules have energy by mdry 78 g he state of water in soil is described virtue of position in the gravitational force T in terms of the amount of water and sample volume 60 cm3 field just as all matter has potential ener- the energy associated with the forces gy. This energy component is described which hold the water in the soil. The by the gravitational potential component amount of water is defined by water con- of the total water potential. The influence mwater 94gg− 78 tent and the energy state of the water is θ == = −1 of gravitational potential is easily seen g 0. 205 gg the water potential. Plant growth, soil msoil 78g when attractive forces between water and temperature, chemical transport, and soil are less than the gravitational forces ground water recharge are all dependent acting on the water molecule and water m on the state of water in the soil. While dry 78 g − moves downward. ρ ===13. gcm 3 there is a unique relationship between bulk volume 60 cm3 The matrix arrangement of soil solid water content and water potential for a particles results in capillary and electro- particular soil, these physical properties static forces and determines the soil water describe the state of the water in soil in θρ∗ matric potential. The magnitude of the g soil − distinctly different manners. It is impor- θ = = 0. 267 cm33 cm forces depends on texture and the physi- v ρ tant to understand the distinction when water cal-chemical properties of the soil solid choosing a soil water measuring matter. Most methods for measuring soil instrument. water potential are sensitive only to the ρ −3 bulk 13. gcm matric potential. ε =−11 =− =050. Water content ρ −3 Soil water is a solution. The polar solid 26. gcm Soil water content is expressed on a nature of the water molecule results in gravimetric or volumetric basis. interaction with other electrostatic poles Gravimetric water content (θg) is the The porosity of 0.50 defines the maxi- present in the solution as free ions. This mass of water per mass of dry soil. It is mum possible volumetric water content. component of the energy status is the measured by weighing a soil sample The measured θv value of 0.267 indicates osmotic potential. (mwet), drying the sample to remove the the pore space is just over half-full of Methods for measuring soil water water, then weighing the dried soil water. If the sample is from a 30-cm matric potential include tensiometry, ther- (mdry). depth profile, there are 8 cm of water in mocouple psychrometry, electrical con- mm− the profile. duction (generically Buoyucous blocks), θ ==mwater wet dry g Water content measurement methods and heat dissipation such as the Campbell msoil mdry include gravimetric, as described above, Scientific sensor model 229. Volumetric water content (θv) is the neutron probe, time-domain reflectometry There is a unique relationship between volume of liquid water per volume of soil. (TDR) and other dielectric permittivity water content and water potential for each Volume is the ratio of mass to density (ρ) sensitive devices such as the CS615. soil. The soil water characteristic curves which gives: Water content indicates how much for three soils are depicted below. For a m water is present in the soil. It can be used given water potential, the finer the soil water to estimate the amount of stored water in ρ θρ∗ texture the more water held in the soil. θ ===volumewater water g soil a profile or how much irrigation is Coarse texture soils like sand consists v ρ volumesoil msoil water required to reach a desired amount of mostly of large pores which empty of ρ water. water when a relatively small force is soil applied. Fine texture soils have a broader Soil bulk density (ρbulk) is used for Water potential pore size distribution and larger particle ρsoil and is the ratio of soil dry mass to Water flux—the movement of water— surface area. Consequently, a larger sample volume. The density of water is occurs within the soil profile, between the change in water potential is required to close to 1 and often ignored. Another soil and plant roots, and between the soil remove the same amount of water. useful property, soil porosity (ε), is related and the atmosphere. As in all natural sys- Greater surface area means more water is to soil bulk density as shown by the fol- tems, movement of a material is depend- adsorbed via electrostatic forces. lowing expression. ent on energy gradients. Soil water potential is an expression of the energy Soil Water Characteristic Curves ρ state of water in soil and must be known ε =−1 bulk ρ or estimated to describe water flux. solid Water molecules in a soil matrix are subject to numerous forces. If no adhe- loam clay The term ρsolid is the density of the sive forces were present, the water mole- soil solid fraction and is approximated by cules would move through the soil at the the value 2.6 g cm-3. same velocity as in free air minus delays sand A simple data set is given at the top of from collisions with the solid matter— matric potential the following column as an example. A much like sand through a sieve. Soil sample of known volume was weighed water potential accounts for adhesive and cohesive forces and describes the energy before and after oven drying at 105°C for volumetric water content 24 hours. status of soil water. Copyright © 2001 Campbell Scientific, Inc. 1 815 W. 1800 N., Logan, UT 84321-1784 (435) 753-2342 App. Note: 2S-I.