Soil texture, structure and water

1. RHS Level 2
Certificate
Year 2 Week 2 – Soil Texture and
Structure and Soil Water

2. Learning outcomes
Explain what is meant by ‘soil texture’.
• Describe the physical characteristics of the soil particles sand, silt
and clay.
• Describe how the characteristics of each of the soil particles listed
in 3.2 affect soils and their suitability for horticultural use.
• Explain what is meant by ‘soil structure’ (soil aggregates) and state
how root establishment and growth are affected by different soil
structures.
• State that a crumb soil is the ideal structure for horticultural use.
Understand the factors that determine the water status in the
soil.
Describe the relationship between air and water content in the
pore space of soils and growing media.
• Explain the importance of an appropriate balance between air and
water for the healthy growth of plants.
• Define the terms: ‘saturation point, ‘field capacity’, ‘permanent
wilting point’ and ‘available water’.

3. Soil Texture - definition
 Soil texture is the percentage of sand,
silt and clay in a particular soil
Soil texture cannot be permanently
changed. 

4. Soil particle sizes

5. Soil Particles
Soil minerals exist as sand, silt and clay particles.
Sand particles are big. Clay particles are
small.

6. Sand: big particles,
big pores.
Clay: little particles,
tiny pores.

7. Small particles,
enormous surface
area.
Big particles,
small surface area.

9. Soil Structure
 Soil structure is the way in which soil
particles are arranged into aggregates
(crumbs or larger blocks) and how the
aggregates relate to each other

10. Soil structure stability
 Clay and humus act like glue in soil aggregates
containing larger particles, making them stable.
 However soils with high clay content may not be
stable. The aggregates will break down if
compressed when wet. This leaves a solid mass.
May form large, blocky or column- like aggregates.
 Sandy soils with low levels of clay or humus are
unstable; the aggregates break down easily into
their component particles under compression or
cultivation.

11. Characteristics of soil types
Clay soils Warm slowly and cool slowly. High CEC so fertile.
Retain high levels of water which can lead to
working and drainage problems.
Silt soils Good water retention. Can be fertile. Easier to
work. Unstable soil structure can lead to capping
and water logging.
Sandy soils Quick to warm and cool. Drain freely, which
makes them easy to work, but also prone to
drought. Organic matter breaks down quickly.
Very sandy soils can have unstable structures.
Nutrients leach out (wash out) as no CEC.
Loam The best of all worlds. Structure is easily worked
into crumb aggregates. Good CEC combined with
good drainage and water holding capacity. Easy
to work.

12. Soil structure impact on
plants
 Sandy soils – large pores (lots of air, little water),
weak aggregates, no nutrient retention (CEC)
which may limit growth if not
supplemented/irrigated. Easy for roots to
penetrate.
 Clay soils – small pores (lots of water, less air),
may form very large aggregates, good CEC.
Fertile so good growth. May be hard for roots to
penetrate but give good anchorage. Lack of air in
soil may lead to root death if waterlogged.
 Loam soils – mix of pore sizes (holding both water
and air), stable small/medium sized aggregates,
some CEC so good growth, good root penetration.

13. How Water is lost from the
soil

14. What water is available to
plants?

15. Learning outcomes
Explain what is meant by ‘soil texture’.
• Describe the physical characteristics of the soil particles sand, silt and
clay.
• Describe how the characteristics of each of the soil particles listed in 3.2
affect soils and their suitability for horticultural use.
• Explain what is meant by ‘soil structure’ (soil aggregates) and state how
root establishment and growth are affected by different soil structures.
• State that a crumb soil is the ideal structure for horticultural use.
Understand the factors that determine the water status in the soil.
Describe the relationship between air and water content in the pore
space of soils and growing media.
• Explain the importance of an appropriate balance between air and
water for the healthy growth of plants.
• Define the terms: ‘saturation point, ‘field capacity’, ‘permanent wilting
point’ and ‘available water’.