LO 3.3 - Texture

Question 1

Question

The particle sizes in different mineral soils influence the physical characteristics of soils.

How are the mineral particles in soil grouped?

Soil Texture

Answer 1

The mineral particles in soils can be grouped into classes by size.

• Sand
• Silt
• Clay

Soil Texture

Question 2

Question

Soil texture particle sizes are measured with an international classification.
What sizes are used for Pebbles, Sand, Silt and Clay?

Soil Texture

Answer 2

Slightly different systems are used in the UK and the US.

This is the international classification.

Soil Texture

Question 3

Question

Soil texture is the term used to describe the proportion of different particle sizes (sands, silts and clays) in a soil.

How big in mm, is the largest particle size considered?

Soil Texture

Answer 3

The largest is 2mm
Any particles over 2 mm in diameter are stones/pebbles and are not considered to be part of the ‘fine earth’ component of the soil.

Soil Texture

Question 4

Question

Laboratory assessment of soil texture.

What are 4 things you need to do?

Soil Texture

Answer 4

• Remove the stones/pebbles with a 2 mm sieve
• Remove the organic matter.
• Put a measured amount in a litre cylinder with a dispersant solution.
• Shake it.

The particles will settle according to size – sand first, then silt, then clay.

Soil Texture

Question 5

Question

What is a Hydrometer?

Soil Texture

Answer 5

A hydrometer is an instrument for measuring the density of liquids.

Soil Texture

Question 6

Question

How do you measure the density of soil using a Hydrometer?

Soil Texture

Answer 6

• Use a hydrometer to measure the density of the liquid at 40 seconds and 2 hours.
• This allows us to work out what percentage of the original sample was in each particle size group
• We can plot those % figures on a textural triangle to determine the textural class of the soil.

Soil Texture

Question 7

Question

Using this textural triangle, what are the textural classes of the following soils?

1. 15% sand, 70% silt, 15% clay.
2. 65% sand, 20% silt, 15% clay.
3. 15% sand, 15% silt, 70% clay.
4. 40% sand, 40% silt, 20% clay.
5. 33% sand, 33% silt, 33% clay

Soil Texture

Answer 7

Textural class of the soils using the textural triangle.
1. Silt Loam
2. Sandy Loam
3. Clay
4. Loam
5. Clay Loam

Soil Texture

Question 8

Question

What’s so important about Soil Texture?

Soil Texture

Answer 8

Soil texture is important because it affects:
* Drainage and water holding.
* Soil temperature.
* Fertility & leaching.
* Workability.
* Soil structure.

Soil Texture

Question 9

Question

Describe what drainage and water holding is in Soil Texture?

Soil Texture

Answer 9

• Sand particles are much bigger than clay particles.
• This means that the gaps between particles are much larger in sand than in clay.
• Water moves more easily through soils that have large pores, so sandy soils are free draining and clay soils are poorly drained.

Soil Texture

Question 10

Question

How does texture affect soil temperature?

Soil Texture

Answer 10

Clay soils, which tend to be wetter, are slower to warm up in spring than lighter, sandy soils.

Soil Texture

Question 11

Question

How does Soil texture affect fertility/leaching?

Soil Texture

Answer 11

• Clay particles are negatively charged so they attract positively charged ions in the soil such as ammonium (NH4+), magnesium (Mg++), potassium (K+) and calcium (Ca++).
• Note: the elements are ‘adsorbed’ by the clay particles.
• Relatively large sand particles have no charge.

Soil Texture

Question 12

Question

What is meant by Workability in Soil texture?

Soil Texture

Answer 12

Clay soils are often referred to as ‘heavy’ and sandy soils as ‘light’.
* This refers to the power required to cultivate them, not to the actual weight of the soil.
* Clay soils are naturally sticky and tend to form clods, so they are more difficult to cultivate than sandy soils which are not sticky.

Soil Texture