Experiment Details

Understanding the formation of soil structure

About This Activity

Soil structure depicts the arrangement of solid particles of the soil and of the pore space between them. Apart from the natural aggregation of soil particles the soil structure also affected by its natural formation and also by the soil management practices. Each individual unit of soil structure is called a ped (aggregate of soil particles as a result of pedogenic process). The natural organisation of particles in the form of discrete units (peds) is separated by pores or voids. 

 Based on the aggregation of soil particles the soil structures are categorized as follows - 

Granular - Granular structure is consists of the aggregates having equate and just about spherical shapes. Water circulates very easily through such soils. They are commonly found in the A-horizon of the soil profile.

                                                                                                                                                                                                                                                                                 Figure: Granular Soil         

Blocky - Blocky and subangular blocky structures are formed when soil particles cling together in nearly square or angular blocks having more or less sharp edges. Relatively large blocks indicate that the soil resists penetration and movement of water. They are commonly found in the B-horizon where clay gets accumulated.                               

                                   

                                            Figure: Blocky Soil 

Prismatic and Columnar - Prismatic and columnar structures are formed by arrangement of soil particles into vertical columns or pillars separated by miniature but definite vertical cracks. Water circulates with greater difficulty and hence drainage is poor. They are commonly found in the B-horizon where clay gets accumulated. 

                                     

                                  

                                      Figure: Columnar Soil 

Platy - Platy structure is made up of soil particles aggregated in thin plates or sheets, piled horizontally on one another. Plates often overlap, greatly impairing water circulation. Such soils are found in forest in the A- horizon, and in clayey soils.

                     

Material Required

*  Soil Structure leaflet

* magnifying glass

* shovel or trowel 

Method

Stage One - Soil Ball Test: 

Collect the soil sample from different horizons of a soil profile. Mark the samples with the profile code and horizon code, depth, date & time.    

From this take a handful of soil (about 25-50 grams) and spread the soil onto a tray and remove any pebbles and visible plant material like roots etc. 

There might be lots of lumps and clumps (aggregates) of soil particles, these needs to be broken apart so that you can examine the soil texture conveniently. 

Take a handful of soil and wet it slowly and gradually until it becomes like moist putty. Now shape this wet soil into a ball (see picture). 

If the wet soil does not get shape of a ball that means it is very sandy soil. 

If the soil sample can be shaped as ball then go for next stage test.   

Stage Two - Ribbon Shape Test:

Take the wet soil ball and gently push it into a ribbon between your thumb and forefinger (see picture for help)  

If the soil does not make ribbon and get broken down or fall apart that means it is Loamy Sand or Silt. 

If the soil does form a ribbon you just keep pushing the soil out until the ribbon breaks from its own weight. 

If the ribbon breaks into pieces less than 2.5 cm length then it is ‘weak ribbon’, if it breaks into 2.5 to 5.0 cm pieces then it is ‘medium ribbon’ and if it breaks into more than 5 cm pieces then it is ‘strong ribbon’.   

Stage Three - Rubbing Test:

Take a small amount of the soil sample in the palm and add some water into it for making a paste like mixture. 

Using your finger rub the soil paste against your palm. 

Find out how it feels by rubbing the soil paste – gritty or very smooth or sticky?    

 Derive inferences of the soil texture types based on the respective length of ribbon formed and rubbing test - 

For ribbon size less than 2.5 cm, test by rubbing  - 

* Whether the soil feels very gritty? If yes then it is ‘Sandy Loam’;

* If the soil feels very smooth then it is ‘Silt Loam’,

* If the soil neither feels gritty nor very smooth then it is ‘Loam’.

For ribbon size between 2.5 to 5.0 cm, test by rubbing -   

* Whether the soil feels very gritty? If yes then it is ‘Sandy Clay Loam’;

* If the soil feels very smooth then it is ‘Silty Clay Loam’,

* If the soil neither feels gritty nor very smooth then it is ‘Clay Loam’.

For ribbon size more than 5.0 cm, test by rubbing -   

* Whether the soil feels very gritty? If yes then it is ‘Sandy Clay’;

* If the soil feels very smooth then it is ‘Silty Clay’,

* If the soil neither feels gritty nor very smooth then it is ‘Clay’.  

Data Collection and Analysis

Record the soil texture test results in the designed table or data format. A sample data table is given for example –

- Record the soil structure of different samples in a data sheet. An example of data table is given below. You can modify or redesign the data table according to the need.

Table to record the soil structure categories of different horizons –

Open Ended Question

* Observe seed germination of different plant species in relation with soil texture classes.

* Observe and experiment to understand the relation between soil texture and water movement (Drainage, percolation, water holding capacity).

* Observe the conditions of soil erosion in your area in reference to existing soil texture types and try to find remedial measures for it. 

* Try to find out interrelationship between different soil textures and associated plant species growing in the area.

Result

Present your results in the form of numerical data in the form of quantitative and/or qualitative characteristics. Various interpretations can be done through mathematical and/or statistical calculations. 

Photographic evidences of the landscape surrounding the sampling site will be useful for further reference.  

Key points learned

* Texture classes of soil

* Classification / Categorization of soil based on texture

Intersting Facts

Hyperlink for USDA-NRCS: https://www.nrcs.usda.gov/conservation-basics/natural-resource-concerns/soil