Soil Structure Determination-

Soil Structure

Soil structure is defined as the arrangement of the soil particles. With regard to structure, soil  particles refer not only to sand, silt and clay but also to the aggregate or structural elements, which have been formed by the aggregation of smaller mechanical fractions. The size, shape and character of the soil structure varies, which could be cube, prism and platter likes.

On the basis of 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 the extent to which the finer mechanical separates are aggregated into coarser or larger fractions. The resulting aggregate-size distribution depends on the manner and condition in which the disintegration is brought about.

For the measurements to have practical significance, the disruptive forces causing disintegration should closely compare with the forces expected in the field. The field condition, particularly with respect to soil moisture, should be compared with the moisture condition adopted for soil disintegration in the laboratory.

The sampling of soil and subsequent disintegration of clods in relevance to seed bed 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 (ring is 20-cm diameter and 10- cm height)
  • Rotary sieve shaker
  • Aluminum cans
  • Balance
  • Spade
  • Brush
  • Polyethylene bags
  • Labels

Procedure

A. Field processing

  1. Collect the soil sample with 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.

  2. 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

  1. Each sample is given a laboratory number.

  2. 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 with respect to fine and coarse soil material.

  3. Calculate the oven-dry weight of the soil sub-sample (soil moisture content).

  4. Weigh the soil subsamples to the nearest 0.1 g.

  5. 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, make sure that the O-rings are placed between individual sieves, the bottom sieve and the sieve pan, and the top sieve and the clamping lid.

  1. 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)
  2. Cover the top sieve with the lid and place the nest of sieves on a rotary shaker.

  3. 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.

  4. Analyze the duplicate sample following the same procedure and calculate the percent
    distribution of dry aggregates retained on each sieve.

Calculations

Technical Remarks

  1. If the percentage of dry aggregates on 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 above Steps 5, 6, 7) for the estimation of aggregate size distribution.

  1. Use work gloves to protect hands, and a mask and other personal protective equipment to protect from dust.
  2. The fume hood is the best place to use the rotary sieve shaker.

  3. The number and sizes of aggregate-size classes collected depends on the objective of the experiment.

 

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