RHS Level 2 Certificate

Year 2 Week 2 – Soil Texture and Structure and Soil Water

Learning outcomesExplain 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’.

Soil Texture - definition

Soil texture is the percentage of sand, silt and clay in a particular soil

Soil texture cannot be permanently changed.

Soil particle sizes

Soil ParticlesSoil minerals exist as sand, silt and clay particles.

Sand particles are big. Clay particles are small.

Sand: big particles, big pores.

Clay: little particles, tiny

pores.

Small particles, enormous surface

area.

Big particles, small surface area.

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

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.

Characteristics of soil typesClay 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.

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.

How Water is lost from the soil

What water is available to plants?

Learning outcomesExplain 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’.