SOIL TESTING
REQUIREMENT FERTILIZER RECOMMENDATION
E. Preparation of the Laboratory Sample
1. Drying
The conventional procedure is to air-dry field soil samples at ambient labo-ratory temperature (21 to 27°C; 70 to 80°F) prior to crushing and sieving (Anonymous, 1994a). The drying process should be done as promptly and rapidly as possible to minimize microbial activity (mineralization). The time required to bring a soil sample to an air-dried condition is determined by its moisture, organic matter content, and texture. Soils high in clay and/or organic matter content require a considerably longer time to bring to an air-dried condition than do sandy-textured soils.
Drying can be facilitated by exposing as much surface of the soil to circulating air as possible and by elevating the drying temperature, but not to exceed 38°C (100°F), because significant changes in the physiochemical properties of the soil can occur at elevated drying temperatures. Field soils should not be oven-dried at elevated temperatures or if frozen.
The drying of some types of soils will result in a significant release or fixation of K (Goulding, 1987; Sparks, 1987); therefore, for some determi-nations, the arriving soil sample may be assayed as received without remov-ing field moisture (Gouldremov-ing, 1987; Bates, 1993). In addition, the determination of the micronutrients Cu, Fe, Mn, and Zn can be affected by the drying process (Kahn and Soltanpour, 1978; Shuman, 1980). Since sig-nificant changes do occur when soil is dried (Hanway et al., 1962; Murphy et al., 1983), there was a time when some soil testing laboratories took field soils as received for analysis, using a slurry method of sample preparation.
However, the method proved cumbersome and time-consuming for process-ing large numbers of samples.
The moisture content of an air-dried soil is determined by the physio-chemical properties of the soil and the relative humidity of air surrounding
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the sample. This variability has little effect on most soil analysis procedures, the minimal effect occurring when the soil aliquot is measured by volume rather than by weight.
2. Crushing/Grinding/Sieving
Following drying, the soil sample is crushed, either by hand or by using a mechanical device (Figure 2.2), and then passed through a 10-mesh (2-mm) screen (Anonymous, 1994a). The grinding process can have an effect on AB–DTPA-extractable Fe, Zn, Mn, Cu, P, and K (Soltanpour et al., 1979).
Sieving through a 10-mesh (2-mm) screen removes stones and other extraneous substances, yielding a uniform sample that can be easily handled in the laboratory and stored indefinitely. This preparation procedure can contaminate a soil sample, either from the composition of the contacting surfaces or from deposition of dust and/or previous sample residue. The crushing and sieving devices must be free of elements that might be deter-mined in the analysis. For example, brass sieves should not be used if Cu and Zn are elements to be determined.
Although crushing and sieving can also be a mixing process, sample size reduction may be necessary and care must be exercised to ensure that the sample is thoroughly mixed before dividing.
Figure 2.2
Soil grinding and sieving device. (Courtesy of Custom Laboratory Equipment, Orange City, FL.)
22 Laboratory Guide for Conducting Soil Tests and Plant Analysis
Particle size reduction can have an effect on some elemental determina-tions, as discussed by Kahn (1979) for the determination of Cu, Fe, and Zn and by Houba et al. (1993) for equilibrium extraction reagent procedures.
In general, once the soil sample has been air-dried, crushed, and screened, it can be stored indefinitely in a dry environment without significant changes in soil test values (Bates, 1993; Houba and Novozamsky, 1998;
Houba et al., 2000).
F. Sample Aliquot Determination 1. Weighing vs. Scooping
In most soil testing laboratories, analyte sample aliquots are obtained by scooping rather than by weighing, primarily because of the time required to weigh samples. Normally, scoops are designed to deliver an estimated weight rather than a specific volume of sample. Scoop size will vary depending on the estimated volume-weight (bulk density) for the soil being scooped.
Assumed volume-weights range from a low of 1.18 to a high of 1.33 (a 1-cm3 volume of soil would weigh from 1.18 to 1.33 g). The volume-weight is determined in part by texture and organic matter content; sandy, low-organic-matter content soils have a higher volume-weight than soils high in clay and organic matter content. Peck (1980), in a study of volume-weight determinations for soils from the north-central region of the United States, defines a “typical” soil as a medial silt loam texture with 2.5% organic matter content crushed to pass a 10-mesh screen. The volume-weight (bulk density) was found to be 1.18 for this “typical” soil as compared with a volume–
weight of 1.32 for “undisturbed” soil. This compares with the estimated volume-weight of 1.25 for the sandy soils found mostly in the southeastern coastal plain area of the United States.
The design of the scoop itself is an important factor that can affect the ability of the scoop to deliver the same “estimated” weight of sample each time. In general, a scoop whose radius is equal to its height is more consistent in its delivery than a scoop whose height is greater than its radius. Peck (1980) describes the best scoop design for use with prepared (dried and passed through a 10-mesh screen) soils that have an approximate volume-weight of 1.18 as those whose height and radius are approximately equal.
Soil aliquot transfer to a saturation or extracting vessel is commonly done by weighing. The use of volume as the measurement for aliquot amount has been recommended by Mehlich (1972; 1973). Bates (1993) has discussed weight vs. volume measurement considerations and van Lierop (1989) has compared weight vs. volume measurement of soil aliquots on accuracy of the assay result.
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In this laboratory guide, both volume and estimated-weight scoops are used to obtain the soil aliquot for many determinations as well as determi-nations based on weighed samples. In most instances, the method most commonly associated with that procedure is specified.
2. Estimated Weight Scoops
Scoop size is based on an assumed “average” volume-weight of prepared sample, air-dried, 10-mesh-sieved (2-mm) soil. The typical soil prepared for analysis, as described in this instruction guide, has an assumed weight-to-volume ratio of 1.18 forsilt loam and clay-textured soils, and 1.25 for sandy soils. Therefore, those soil test procedures adapted to a soil with a particular texture will designate scoop volumes that match the assumed weight-to-volume ratio:
Scoops are of a fixed volume and do not necessarily yield an estimated or assumed weight. However, when the volume-weight of asoil sample is known, a specific volume of that soil can be scooped to give an estimated weight.
In most instances, a dual system of weighed and/or volume-measured samples is presented. This rationale is necessary in cases in which the original method specified a weight of sample or a volume of known or assumed specific weight. The reader may refer to Mehlich (1972; 1973) and van Lierop (1981; 1989) for additional information on volume-weight consider-ations and to Peck (1998) for more details on scoop design and use.
Another scoop is designed with a rounded or “cup-shaped” bottom to avoid the possibility of unfilled cavities in the base of the scoop. Tucker (1984) describes a technique for making scoops with 1-, 2.5-, 5.0-, and 10.0-cm capacities, as well as a technique for calibrating prepared scoops.
Some have recommended the use of a round surface, such as a glass rod, as the leveling tool, which allows the soil particles at the edge of the leveling tool to roll under the moving edge, thus reducing the possibility of creating small cavities in the planed surface after leveling.
To the purist, scooping is anathema, introducing error into the analysis as a result of variations in sample densities (Glenn, 1983). However, experience
Silt loam and clay-textured soils Sandy soils
Weight, g Scoop size, cm3 Weight, g Scoop size, cm3
2.5 1.70 5.0 4.0
5.0 4.25
10.0 8.50
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has shown that scooping, if properly done, can be an adequate substitute for weighing, producing equivalent analytical results. The major sources for error are in the design of the scoop and its improper use.
Mehlich (1973) has proposed a system of soil testing based entirely on scooped samples, a volume method of analysis and interpretation that will be discussed in greater detail later. Similarly, Wolf (1982) has a soil testing methodology based entirely on a scooped sample for laboratory analysis. In addition, the Adams–Evans Lime Buffer Test (Adams and Evans, 1962) is performed with a volume (scooped) sample.
Although scooping does have some unique advantages, convention has dictated that the laboratory aliquot be measured by weight unless the test itself or operational conditions dictate otherwise.
3. NCR-13 Scoops
The design specifications of the NCR-13 scoops commonly used by soil testing laboratories in the north-central region of the United States, described by Peck (1998), are as follows:
The NCR-13 standard soil scoop is shown in Figure 2.3.
4. Procedure for Using a Soil Scoop
• Stir the crushed and screened sample with a spatula to loosen soil prior to measuring.
• Dip into the center of the soil sample with the soil scoop, filling it heaping full without pushing against the side of the soil container.
NCR-13 Standard Soil Scoop Specifications (manufactured from stainless steel) Scoop
sizea, g
Scoop capacity, cc
Outside diameter, in.
Inside diameter, in.
Inside diameter, in.
1 0.85 ⁵⁄₈ ¹⁄₂ ¹⁷⁄₆₄
2 1.70 ³⁄₄ ⁵⁄₈ ²²⁄₆₄
5 4.25 1 ⁷⁄₈ ²⁸⁄₆₄
10 8.50 1¹⁄₄ 1¹⁄₈ ³⁴⁄₆₄
a Grams of soil in terms of the “typical” soil (defined as a medial silt loam texture with 1.25% organic matter crushed to pass a 10-mesh screen, bulk density of crushed “typical” soil approximates 1.18 compared with 1.32 for
“undisturbed” soil) weighing 2,000,000 lb/acre in the top 6²⁄₃-in. layer.
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• Hold the scoop firmly and tap the handle three times with a spatula from a distance of 2 or 3 in. from the soil-filled scoop.
• Hold the spatula blade perpendicular to the top of the scoop and strike off excess soil. A flat spatula blade may be replaced by a round rod, which protects against scarring the leveled surface.
• Empty the scoop into an appropriate extraction vessel.
Since an accurate measure for a scooped sample is essential, scoop design is a very important factor. The diameter of the scoop should be twice its height to ensure the most efficient packing density in the scoop.
Variance among repeated scoopings of a soil sample will be within 2 to 3% of the same volume or estimated weight. In general, scooping of soil samples has been found to yield results comparable to weighed samples in repeated analyses of the same soil sample.