Unlock the Secrets of Soil Structure! Discover Effective Methods for Determining Soil Structure & Boost Your Gardening Success.
Soil structure is defined as the arrangement of soil particles. Concerning structure, soil particles refer not only to sand, silt, and clay but also to the aggregate or structural elements, which the aggregation of smaller mechanical fractions has formed. The soil structure’s size, shape, and character varies, including a cube, prism, or platter.
Based on size, the soil structure is classified as follows: very coarse (>10 mm), coarse (5-10 mm); medium (2-5 mm); fine (1-2 mm); and very fine (<1 mm). Depending upon the stability of the aggregate and the ease of separation, the structure is characterized as follows:
Poorly developed, weekly developed, moderately developed, well developed, and highly developed.
1. Dry Aggregate Analysis
An aggregate analysis aims to measure the percentage of water-stable secondary particles in the soil and how much the finer mechanical separates are aggregated into coarser or larger fractions. The resulting aggregate-size distribution depends on how and condition the disintegration is brought about.
For the measurements to have practical significance, the disruptive forces causing disintegration should closely compare with those expected in the field. The field condition, particularly soil moisture, should be compared with the moisture condition adopted for soil disintegration in the laboratory.
The soil sampling and subsequent disintegration of clods relevant to seedbed preparation for upland crops should be carried out under air-dry conditions for dry sieve analysis. A rotary sieve shaker would be ideal for dry sieving.
Apparatus
- Nest of sieves having different diameter round openings with a pan and a lid
- Metal ring (the ring is 20 cm in diameter and 10- cm in height)
- Rotary sieve shaker
- Aluminum cans
- Balance
- Spade
- Brush
- Polyethylene bags
- Labels
Procedure
A. Field processing
- Collect the soil sample with a metal ring by pressing on the bulk soil sample (better when moist) until level with the surface. Avoid excessive compaction or fragmentation of soil. Remove the loose soil within the ring and collect it in a polyethylene bag.
- Record all the information about samples (depth and profile), put one label inside the bag, and tie the other label to the bag. Then bring the soil samples to the soil preparation laboratory.
B. Laboratory processing
- Each sample is given a laboratory number.
- Spread samples uniformly over a plastic or paper sheet, and let them air dry. Prepare the sub-samples by ‘quartering.’ The mixed soil material is coned in the center of the mixing sheet, with care to make it symmetrical for fine and coarse soil material.
- Calculate the oven-dry weight of the soil sub-sample (soil moisture content).
- Weigh the soil subsamples to the nearest 0.1 g.
- Prepare the sieving tower and stack sieves starting with the smallest sieve opening, and add a pan at the bottom of the set.
Note
To avoid leakage, ensure that the O-rings are placed between individual sieves, the bottom sieve and the sieve pan, the top sieve, and the clamping lid.
- Gently pour the soil aggregates collected on the top of the nest of sieves (having different diameter round openings, usually 5.0, 2.0, 1.0, 0.5, and 0.25 mm)
- Cover the top sieve with the lid and place the nest of sieves on a rotary shaker.
- Switch on the shaker for 10 minutes, remove the sieves, collect the soil retained on each sieve in the preweighed aluminum cans with the help of a small brush, and weigh the cans with the soil.
- Analyze the duplicate sample following the same procedure and calculate the percent
distribution of dry aggregates retained on each sieve.
Calculations
Technical Remarks
- If the percentage of dry aggregates on a 5-mm sieve exceeds 25 %, transfer these aggregates to a nest of sieves with 25, 10, and 5-mm sieves along with a pan. Then, cover the top sieve containing the aggregates with a lid and place the nest of sieves on the rotary sieve shaker.
Switch on the motor for 10 minutes and proceed (as in Steps 5, 6, and 7) to estimate aggregate size distribution.
- Use work gloves to protect hands, a mask, and other personal protective equipment from dust.
- The fume hood is the best place to use the rotary sieve shaker.
- The number and sizes of aggregate-size classes collected depend on the objective of the experiment.
2. Wet Aggregate Analysis
The soil sample is taken when it is moist and friable. It is broken by applying mild stress into smaller
aggregates that can pass through an 8-mm screen. The sieved soil sample is taken on a watch glass for
wetting by either vacuum soaking or immersion.
Aggregates of different sizes can be obtained through several methods, such as samples under vacuum and sieving under water. Among the various procedures adopted, wetting the samples under a vacuum is suggested because the wetting rate influences crumbs’ slaking.
The time of sieving ranges from 10 to 30 minutes depending upon the type of wetting. However, sieving under water compares more closely with the disruptive actions of water and other mechanical forces.
Apparatus
- Yoder -type wet sieve shaker (mechanical oscillator powered by a gear reduction motor)
- Two sets of sieves, i.e., sieves-nest (20-cm diameter. and 5-cm height) with screen openings of 0.5, 0.2-mm diameter.
- Filter paper
- Aluminum metal pot (Al-pot)
- Plastic dishes (small)
- Standard metal ring
- Soil dispersion stirrer: high-speed electric with a cup receptacle
Reagents
- Sodium Hexametaphosphate [(NaPO3)13], 5% Dissolve 50 g Na-hexameter phosphate in DI water, and bring to 1- L volume. This solution deteriorates with time and should not be kept for more than 1 to 2 weeks.
- Sodium Hydroxide (NaOH), 4% Dissolve 40 g NaOH in DI water and bring to 1-L volume.
Procedure
A. Field processing
- Collect the soil sample with a metal ring by pressing on the bulk soil sample (better when moist) until level with the surface. Avoid excessive compaction or fragmentation of soil.
- Remove the loose soil within the ring and collect it in a polyethylene bag.
- Record all the information about samples (depth and profile), put one label inside the bag, and tie the other to the bag. Then bring the soil samples to the soil preparation laboratory.
B. Laboratory processing
Each sample is given a laboratory number.
Spread the samples uniformly over a plastic or paper sheet to air-dry. Field-moist instead of air-dry samples may be used if air-drying is assumed to affect aggregates (“self-mulching soils”).
Prepare the sub-samples by ‘quartering.’ The mixed soil material is coned in the center of the mixing sheet with care to make it symmetrical concerning fine and coarse soil material.
- Calculate the oven-dry weight of the soil sub-sample (soil moisture content).
- Weigh 50 g of air-dry soil (< 2 mm).
- Gently pour the soil aggregates collected into the standard metal ring (place under the metal ring filter paper and a small plastic dish).
- Add 50 ml tap water (allowing the water to soak up underneath) to saturate the soil sample. Leave the wet soil samples for 30 minutes.
- Transfer the set saturated soil sample into the 2 sieves 0.5 mm (above) and 0.2 mm (bottom). Add any larger sieve above the first sieve to reach the tank’s surface water limit.
- Remove the bottom pan, and attach the nest of sieves to the Yoder-type wet sieve shaker.
- Fill the drum (which holds the set of sieves) with salt-free water at 20-25 0C to a level somewhat below the screen in the top sieve of the nest of sieves when the sieves are in the highest position. Then lower the sieves-nest to wet the soil for 10 minutes.
- Bring the nest of sieves to the initial position and adjust the water level so that the top sieve’s screen is covered with water in its highest position.
Then switch on the mechanical oscillator to move the sieves-nest up and down with a frequency of 30-35 cycles/minute and a stroke of 3.8 cm.
- Sieve for 10 minutes.
- Remove the sieves-nest from the water and allow it to drain for some time.
- Transfer the soil on each screen with distilled water and brush into a big Al-metal pot (known weight).
Leave the washed water-stable aggregates in the pot for 4-6 hours to precipitate all the suspended fractions and decant some of the water pot to reduce the drying period.