Introductory Soil Science Lab 6 Soil Water NRES 201 Page 1 of 8 RGD 1/07 LABORATORY 6. SOIL WATER AND PLANT-AVAILABLE WATER. Plants and other organisms are composed primarily of water. Most actively growing plants contain between 75 and 90% water. In addition to the water in the plant cells large quantities of water are lost into the atmosphere through open stomates as part of the transpiration stream. Transpiration occurs because the stomates must be open to facilitate the gas exchange required for photosynthesis. Plants growing on well-watered soils lose large amounts of water to the atmosphere. Plants growing on dry soils control water loss by wilting. Wilting prevents the desiccation and death of the plant, but also prevents the exchange of gases necessary for photosynthesis and generally results in lower crop yields. The following table indicates the approximate amount of water required to produce one pound of aboveground dry matter (stems, leaves, and seeds) in a humid region: Crop lb. Water/lb. Dry Matter Sorghum 271 Corn 372 Wheat 505 Barley 521 Cotton 562 Oats 635 Alfalfa 858 Source: Modern Crop Production, Aldrich, Scott, and Leng. Using the above data a 4 ton/acre crop of alfalfa requires approximately 6,800,000 lbs of water. The source of all of this water is water stored in the soil between matric potentials of 1/3 and 15 bars. Soil is the reservoir for water storage and through periodic recharging (rainfall or irrigation) soil is generally capable of supplying the needs of growing plants. Even so, water is usually the most critical factor controlling the growth of terrestrial plants. 6.1 STORAGE OF WATER IN SOILS. Water is held in soil against the pull of gravity by adhesive and cohesive forces. The adhesive forces are the result of water being chemically bonded to exchangeable cations associated with soil colloids and bonded to polar groups associated with soil minerals and soil organic matter (humus). The cohesive forces are due to hydrogen bonding of additional water molecules to the chemically bonded water resulting in films of water being attracted to soil solids. Following a heavy rain, when the soil is saturated (all the pores are filled with water), the water in large pores drains relatively quickly due to gravitational forces while water in smaller pores, due to capillary interactions with the soil matrix, is more much more resistant to drainage. After much of the soil has drained in response to the pull of gravity only the latter water is retained in the soil. Plants can only utilize water that is not strongly held to the soil solids, hence only a portion of the water stored in the soil is available to plants. If a small amount of water is added to a dry soil that water will be strongly bonded to the soil solids. If additional water is added to the soil, the bonding energy will be spread out over a greaterIntroductory Soil Science Lab 6 Soil Water NRES 201 Page 2 of 8 RGD 1/07 mass of water and the water will be less strongly held to the soil solids. This leads to an important concept about soil water, that is as the amount of water in the soil increases the energy with which the water is held decreases, and conversely as the amount of water in the soil decreases the strength of bonding of water to the soil solids increases. There is a continuum of energies involved. From very strong bonding (high energies) when only a single molecular layer of water is bonded to the soil solids, decreasing as more and more water is added to the soil down to the point that free water exists in the soil that is not bonded to the soil solids. Classification of soil water. Although there is a continuum of energies involved in the bonding of water in soils, there are certain energies that correspond to observable phenomena. The energy of bonding of water to the soil solids is usually expressed as the atmospheres (atm) or bars. This corresponds to the energy required to remove water from the soil. Saturation: The point where all the soil pores are completely filled with water. Water films associated with soil particles are at maximum thickness. Both bonded and free water exist. -- 0 atm matric potential. Field capacity: The point where water ceases to drain out of the soil in response to gravity. Represents the maximum amount of water that can be stored in a soil. -- approximately -1/3 atm matric potential. Permanent wilting coefficient: The point where all water that is energetically available to the plant has been removed. For most plants this corresponds to a matric potential of -15 atm. Hygroscopic coefficient: The water content corresponding to air-dryness, the point where soil water is in equilibrium with atmospheric moisture. Although this point is affected by the relative humidity of the atmosphere is corresponds to approximately a matric potential of -31 atms. Oven-dry: The point where all cohesively bonded water has been removed only chemically (adhesively) bonded water remains ~ -10,000 atm matric potential. Water held between these observable points (potentials) can also be classified. Free or Gravitational water Nonbonded water that drains in response to gravity. Capillary water: Water held between field capacity and the hygroscopic coefficient, between matric potentials of -1/3 and -31 atms. Functions as the soil solution. Plant available water: Water held between field capacity and the wilting point. Water held at low enough matric potentials that it can be utilized by plants and other soil organisms. Hygroscopic coefficient: Water held in the soil between the hygroscopic coefficient and oven dryness. Water occurring as thin tightly bonded films of water on solid surfaces. Plant unavailable water: Water held at matric potentials that are too large for plants to overcome. Corresponds to matric potentials of -15 atms for most higher plants. Includes a portion of capillary water and hygroscopic water.Introductory Soil Science Lab 6 Soil Water NRES 201 Page 3 of 8 RGD 1/07 Calculation of soil water content. The amount of water in a soil is most commonly expressed as a percentage of the oven-dry weight of the soil (W%). Unless otherwise stated, water contents in the soils literature and in this class are always expressed by this method. W% = (wet weight of soil - OD wt. of soil)/OD wt. of soil W% = weight of water/OD wt. of soil Example: Calculate the percent water (W%) of a soil that has a wet weight of 120 grams and