Soil Water and Soil pH Lab Exercise 2024 PDF

LAB EXERCISE 3 Soil Water and Soil pH Soil Water Soil water is probably the most limiting component in agricultural production on a world-wide basis. Very few areas of the world do not suffer at one time or another from a deficiency of water. Yet at othe...

LAB EXERCISE 3 Soil Water and Soil pH Soil Water Soil water is probably the most limiting component in agricultural production on a world-wide basis. Very few areas of the world do not suffer at one time or another from a deficiency of water. Yet at other times, there may be a water surplus. Soil water management requires knowledge of properties of soils that control water retention and water movement into and out of the soil. In this exercise, we will look at water movement and retention in soil. Two measurements of importance in the management of soils are the field capacity and permanent wilting point. Field Capacity (FC) Field capacity is the amount of water held in the soil two or three days after it has been saturated by rainfall. The two to three days allow water to move down into drier soil from the saturated soil through capillary action (suction) and gravitational pull. Eventually the movement due to gravitational pull becomes negligible and the movement due to suction becomes slow. Water continues to move through the soil but at a very slow rate and the moist soil can be seen to have an abrupt contact with drier soil below. At this stage, a given soil will tend to have fairly constant moisture content in the upper moist layers. This water content is called field capacity. Theoretically it is impossible to wet a soil above the field capacity for any length of time unless there is impeded drainage of water. Thus, the field capacity is the upper limit of moisture content in the soil that can be attained under normal agricultural conditions. As you can see from the definition, field capacity is a term related to conditions occurring in natural soils. Therefore, it is somewhat difficult to duplicate in the laboratory. In this exercise we will measure field capacity by placing soil in a plastic column or in a tall glass container, adding enough water to wet the soil approximately half-way down, covering the container to prevent evaporation, leaving the soil for approximately one week, and then sampling for or estimating moisture content. Permanent Wilting Point (PWP) A second important constant in soil is the permanent wilting point. This is defined as the amount of water remaining in a soil when a plant wilts in a vapour-saturated atmosphere. In other 1 words, water in the soil at or below PWP is not available to the plant as the plant cannot absorb it. Therefore, the plant ceases to retain turgidity. This is not the same as temporary wilting which can occur in a moist soil when conditions are such that the plant is transpiring faster than its roots can absorb moisture. Temporary wilting can be observed during hot, windy days when the relative humidity of the atmosphere is low. Under these conditions, a plant loses more water than it can absorb and wilts. However, when the atmosphere becomes more saturated or when the wind or the temperature drop, these plants will regain their turgidity. Available Water Soil water that is held between field capacity (0.1 bar suction - -10 kPa) and permanent wilting point (15 bar suction - -1500 kPa) can be used by plants for transpiration (Fig. 3.1). This is called available water. To express the available water as a volume, multiply percent gravimetric moisture by the bulk density. In Fig.3.2, available water is indicated by the distance between the two values of field capacity and wilting point. The amount of available water retained by a soil depends largely on its texture. It is lowest in coarse textured soils and maximum in loams to clay loams (Table 3.1). Growers should never allow the soil moisture content to reach the PWP. At PWP the plants are severely stressed due to low moisture content and yields are reduced. Even when plants recover as a result of timely irrigation or rainfall they will have had their growth retarded and this will result in reduced yields later on in the season. Generally, crops should be irrigated when the soil's moisture content is between FC and PWP (Fig. 3.2). Hygroscopic Coefficient Another water constant is the hygroscopic water content or coefficient. This has no importance to biological life as hygroscopic water is held so tightly by soil particles that the only way it will move in the soil is in the vapour phase. Thus, soils that are air dried still retain hygroscopic water. The amount of water held by a soil in hygroscopic form depends on the surface area of the particles in the soil. The larger the surface area, the more hygroscopic moisture the soil will hold in an air-dry condition. Rooting Depth in the Field Examine Table 3.2. The rooting depths of the various crops represent general averages which will change with different management techniques and climate. 2  If a crop is growing on a deep, well-drained soil with good pore- space distribution, then the depth of rooting is very dependent upon water supply. Most of the available plant nutrients exist in the upper soil horizons and this is where plant roots tend to concentrate. This shallow rooting condition is encouraged by frequent light additions of water (either from irrigation or rain) and the presence of a surface mulch which prevents evaporation. It is important to remember that most plants will only exhibit their ability to root deeply if soil properties allow easy root penetration. If a farm is in an area that commonly experiences a mid-summer dry spell, shallow rooted crops will suffer if frequent additions of water are stopped. In such an area, deeper roots can be encouraged by irrigating with more water, less often (Russell, p. 533). Oven-dry (10000bar - -106 kPa) PWP (15 bar suction- -1500 kPa) FC (0.1 to 0.2 bar suction- -10-20 kPa) Zero suction Space occupied by air or gravitational Capillary water water Unavailable water Available water Excess water Figure 3.1 The physical and biological classification of soil water 3  Figure 3.2 The influence of soil texture and moisture on available water Table 3.1 The effect of soil texture and organic matter content on soil available water Available Field Texture Sand Silt Clay OM PWP water capacity % By weight 88 8 4 1.0 6.3 2.8 3.5 Sand 84 11 5 4.1 11.1 4.2 6.9 Sandy loam 54 36 10 3.2 14.8 5.9 8.9 33 44 23 3.2 21.6 10.5 11.1 Loam 41 32 27 8.4 37.6 24.2 13.4 Clay 23 31 46 4.0 23.7 13.1 10.6 (Adapted from OMAF) 4 Table 3.2 Average rooting depths for selected plants Root depth (cm) Crop Grass pastures and sorghums 45 - 60 Carrots and peas 45 - 75 Potatoes and beans 60 - 90 Corn, tomatoes, cantaloupe, sugar, beets 75 - 105 Small grains, clovers, grapes 90 - 120 Alfalfa 140 - 180 Broccoli, spinach, radishes 45 - 91 Turf grass 30 - 45 Black spruce, organic soil- almost all lateral growth 20 - 30 Sugar maple, St Bernard soil, lateral growth 50 - 80 White Cedar 20 - 35 Experiment 1: Capillary Action in Soils If a soil is saturated with water and the excess water is allowed to drain by the force of gravity, then the quantity remaining in the soil is near field capacity. This amount of water retained by the soil is the main reservoir from which plants satisfy their moisture requirements and, as such, the capacity of different soils to retain different quantities of water against the pull of gravity attains a practical significance. The force that retains water in soil is made up of two related components: adhesion and cohesion. Adhesion is the attraction between soil and water, while cohesion is the attraction of water molecules toward each other. Everyday evidences of the forces of cohesion and adhesion forces as a drop of water is held between fingers. 5 PROCEDURE (Capillary Action in Soils): 1. Cover the bottom of the column with gauze and secure with an elastic band. 2. Fill 2/3 of a column with your sieved oven dried soil. 3. Immerse the column into an Erlenmeyer flask containing exact 250 ml of dist. water. 4. Measure the heights of the wetting fronts at the indicated times (Table 3.3). 5. Obtain the result for the other soil series to record in the table below (use for the graph). 6. Measure the final height of the wetting front. 7. Transfer the un-wetted soil (dry) to the soil container. 8. Plot the rise in cm versus time for all 2-soil series using the graph paper. Table 3.3 Results of capillary-rise experiment Time Your soil: ---------- Other soil----------- minutes cm cm 5 10 15 20 30 40 70 Experiment 2: Field Capacity Field-capacity determinations are most valid when determined in the field. However, this method is not practical considering the time of year and time available for the determination, so you will make your determination on a disturbed sample in the lab. 6 PROCEDURE 1. Obtain a plastic tube and cover one end with a double layer of cheese cloth, held in place with an elastic band. 2. In a tin can weigh 100-g sample of your crushed, oven dried, sieved soil. 3. Transfer the soil to the tube using a funnel. Tap the base of the tube on the palm of your hand until no further settling of the soil occurs. 4. Calculate the volume of the soil in your tube. Volume = 3.14 r2 h Where, r = internal radius of your tube in cm, and h = height of the soil in your tube in cm. 5. Calculate the bulk density of your soil. Soil weight Bulk density (g/cm3 ) = ---------------- Soil volume 6. Add 15 ml of dist. water, note the position of the wetting front of your soil. 7. Calculate the weight of dry soil which was wetted (after 1 week). i.e., volume of wet soil x bulk density 8. Calculate the percent water at field capacity. Weight of water added % Water = ------------------------------------------x 100 Weight of soil wetted Weight of soil wetted = B.D. X Volume of soil wetted 7 Experiment 5: Hygroscopic Water 1. Weigh a labeled aluminum container. 2. Obtain air-dry soil sample of a soil. 3. Place a sample of your air-dried soil (about 20.0 g) into the container. 4. Weigh the soil in the container. 5. Dry the soil at 105 C. 6. In your next lab, re-weigh the soil and calculate the moisture content. Hygroscopic water calculations Weight of aluminum container 31.62 g Weight of air-dry soil and container 41.32 g Weight of oven dry soil and container 40.96 g Mass of air-dry soil - Mass of oven dry soil % Hygroscopic Moisture = ---------------------------------------------------------- x 100 Mass of oven dry soil 8 Soil Water Content Several methods can be used to determine the water content of a soil. Four of the more common procedures are outlined below. 1. Gravimetric Method The simplest direct determination involves weighing a moist soil sample before and after oven-drying at 105 C. This method requires that a sample of the soil be brought in from the field. 2. Resistance Blocks Resistance blocks are made of gypsum (CaSO4), nylon, or fiber glass in which two wires are encased. The electrical resistance between the two wires in the block is related to the amount of water contained in the pores of the block and this in turn is related to the water suction in the soil. With calibration, the moisture content range from 1 to 15 bar suction can be measured. Therefore, the accuracy is good only at low moisture contents. Resistance blocks are useful in dry-land areas (prairie, rangeland) or where irrigation is infrequent. 3. Tensiometers Tensiometers are field soil moisture probes, made of plastic with a porous ceramic tip. The probe is hollow and is filled with water; a suction gauge is attached to the tube near the top. When the probe is inserted into the soil, water moves through the ceramic tip into the soil until equilibrium is established. As the water moves out of the sealed probe, it creates a vacuum which is measured by the gauge. Tensiometers are reliable over soil moisture suctions of 0 to 0.85 bars and are thus not helpful in dry soil. Tensiometers are used in orchards, nurseries, and turf farms. 4. Time Domain Reflectometry (TDR) (Anna Dabros, 2007) The principles of TDR involve measuring the travel time of electromagnetic waves along two (or more) metal rods. The traveling wave reaches the tip of the rod and is partially reflected back to the sensing electronic where it is converted to volumetric water content. The speed of the wave is governed by a property called bulk dielectric permittivity (i.e., a measure of the behavior of electric field in a certain medium). Since water has a much higher dielectric constant than air or soil solids, the speed of the electromagnetic waves traveling along the metal rods will be affected by soil water content, including the pore water as well as water present in tissues of plant roots and organic matter. The rods may be of different lengths (e.g., 12 cm or 20 cm) and the measurement is an average water content along the entire length of the rods, and not at the tip of the rod. In terms of the radius of the soil sampled, the measurements are pretty localized, extending only several centimeters out from the point where the rods are inserted in the soil. 9 Each of these four devices is useful for estimating field water content in order to determine when and how much to irrigate. Most plants obtain most of their nutrients and moisture from the upper volume of the rooting zone. Most plants acquire 70 % of their total moisture requirements from the upper half of the rooting zone. For most crops (except alfalfa) the upper half of the rooting zone extends down into the soil only about 60 to 75 cm. Therefore, for practical purposes, soil moisture probes should not be placed any deeper than 1 m. Water Infiltration into Soils https://www.youtube.com/watch?v=ego2FkuQwxc This video will help you to understand Water Infiltration into Soils 1. Unsaturated Flow in a Homogeneous Soil Water was added to the centre of this dry, homogeneous soil. Note that water moves out almost equally in all directions. Gravity has a very small effect. This illustrates water movement under unsaturated conditions where there is no free water in the soil. 2. Retardation of Unsaturated Flow by a Sand Layer The next demonstration shows how water moves in soil that is not homogeneous. In stratified soil--soil in which layers or horizons of different texture exist--water flow is greatly affected by the size of pores in the strata encountered by an advancing wetting front. If coarse materials are encountered, water movement stops until the soil above becomes nearly saturated. 3. Influence of Aggregation on Infiltration Rates This demonstration compares a well-aggregated soil with a homogeneous sample of the same soil when not aggregated. When water is applied to both soils the infiltration rates and processes are different. 10  Table 3.4 Wetting depth of 5.0 cm of irrigation water in soils of similar organic matter content Soil texture Wetting depth Clay 0.20 m Loam 0.40 m Sandy loam 0.60 m Sand 1.80 m (From Butler, 1979) IMPORTANT 1. Make sure you perform (or record the results) of all the required experiments. (i.e., field capacity, infiltration, saturation., hygroscopic water). 11 Soil pH Soil Reaction Soils may be acid, neutral, or alkaline in pH. The pH level in a soil is a result of the chemical conditions which exist in the soil and pH can influence many other properties. For example, plant growth is affected by pH in several ways, owing to either depressed solubilities of some elements (deficiency) or increased solubility of others (toxicity)(Fig. 3.3 and 3.4). Certain pH conditions may be unfavourable to the growth of some crops, while some species may be affected only slightly or not at all (Fig. 3.5). Soil reaction is expressed on the basis of the activity of dissociated H+ ions in the soil solution. The term used to express this is pH which is the negative logarithm of the H+ ion activity. Soil pH is related to exchangeable hydrogen and aluminium by the following reaction: Clay Al --- solution Al Al + 3 H2O --- Al (OH)3 + 3 H+ +++ Clay colloid H+ --- H+ + Soil solution Reserve or Active acidity buffer acidity Soil pH is a measure of the active acidity only and is usually measured colorimetrically or electrometrically. 12 Figure 3.3 Influence of pH on the availability of plant nutrients in organic soils. Widest part of bar indicates maximum nutrient availability which is about 5.5 (Willis, 1975). 13 Figure 3.4 Influence of pH on microbial activity and the availability of plant nutrients in mineral soils. Note that the pH of greatest availability is about 6.5. Where two elements are shown interlocking, they combine at that pH to form insoluble phosphate compounds. (Brady, 1974; Donahue et al., 1983) 14 Figure 3.5 Approximate pH ranges of various higher plants. The soil fertility level will also affect the actual relationship in any one case (Brady, 1974). 15 Electrometric Determination of Soil pH For this determination two electrodes are placed in the solution to be measured. One electrode is hydrogen-sensitive and the other is a standard half-cell. The potential developed by the H electrode is compared to the standard electrode by means of a potentiometer. The pH metre must be standardized against buffer solutions of known pH prior to soil solution measurements. This standardization has been done by your lab demonstrator. In this way, accurate measurements of pH can be obtained. The presence of reserve (exchangeable) and active (water extractable) acidity in an acid soil may be shown very simply. A distilled water extract will give a measure of H + ions in the soil solution (active acidity). When a salt solution is used instead of water, some of the H+ on the exchange complex (reserve acidity) is displaced and is thus measurable using the electrodes. The addition of water or a salt solution can change the H+ concentration from that of the original soil solution. For example, weak acid groups associated with soil organic matter as well as soil minerals may either associate or dissociate upon dilution. The salt content of the diluting solution can result in the release of H+ ions from the exchange complex. Depending upon the nature and type of colloids and weak acid groups present, the effect of dilution may differ from one soil to another. The use of 0.01 M CaCl2 has been recommended to minimize the effects of dilution, since this concentration of salt is generally considered to represent the salinity of a “normal” soil solution in the field. As well CaCl2 is recommended for agricultural fields that tend to have a higher concentration of soluble ions in the soil solution due to fertilizer applications. Procedure A - pH measurement in distilled water and in 0.01 M CaCl 2 1. Use your soil series (dry) and other soil series (dry). 2. Label 4 washed plastic vials, 1 to 4, corresponding with the table below. 3. For each soil series you will test its pH in distilled water and in 0.01 M CaCl2. 4. Weigh out two 10 g soil samples of each soil series and add it to the appropriate 35 ml plastic vial. 16 5. Into each cup add the appropriate solutions. Stir each cup of soil intermittently for 20 minutes (5 or 6 times) then let stand 10 min without stirring and measure the pH. (Total time 30 minutes) Cup Soil series Solution pH value 1 Your series----------------- 20 ml of 0.01 M CaCl2 2 Your series----------------- 20 ml of Distilled H2O 3 Other soil------------------- 20 ml of 0.01 M CaCl2 4 Other soil------------------- 20 ml of Distilled H2O Using the graph paper, construct a bar graph using the 4 pH values obtained above. Label the bars with the soil series and the diluting solution. Put all soil series on the same graph. Example below. Colorimetric Determination of Soil pH This procedure is commonly used because of the ease and low cost of the determination. Many organic dyes change color as pH changes. By bringing such dyes in contact with a soil, which is strongly buffered, the approximate soil pH can be determined by estimating the color of the dye. 17 Two systems are used at present. a) One consists of mixing soil with distilled water and then soaking a pH-sensitized paper in the solution. The change in color is matched against an appropriate color chart which indicates the equivalent pH in the soil. This is a rough estimate of soil pH. b) The other system uses a series of color indicators. We will use soaking pH-sensitized paper in the solution in this laboratory exercise. The pH-sensitized paper method – large pH scale 18 PROCEDURE 1. Use plastic cups labelled 2 and 4 on page 17 (20 ml of Distilled H2O). 2. Dip the indicator strip into the liquid portion of the soil for a few seconds. 3. Remove the strip from the test solution and compare the resultant colour with the colour segments printed on the cover by holding the test strip adjacent to the nearest matching colour to establish the pH of test sample. 4. Record in table below. Cup # Soil Series pH determined by pH paper 2 Your soil 4 Other soil 19