Components of a fertile soil

What makes up a fertile soil?

• A fertile soil is one that will maintain life, and is made up of minerals, organic matter, water, air and living organisms.

Mineral ComponentsSand

• Formed by physical weathering

• Feels gritty• Does not hold together• Easy to cultivate• Called ‘light soils’• Have large spaces between

the grains (pores) so water moves through quickly

• Dry out quickly

• Warm up quickly in spring• Need irrigation in warm dry

weather

Advantages and Disadvantages of Sandy Soils

Advantages• Warm up earlier• Less damaged by

compaction• Less expensive to

cultivate

Disadvantages• Dry out quickly• Low water storage

capacity so crops growing on them suffer droughts

• Too well aerated, this tends to destroy organic matter

• Tends to be more leached (nutrients leave the soil with the movement of water)

Clay

• Formed by chemical weathering• Particles stick together so they feel sticky• ‘heavy’ to work• Swell when wet and shrink when dry• Absorb water easily• Slow to warm in spring• Clays hold some elements essential to plant growth e.g.

calcium, magnesium, potassium and sodium.• Particles clump together or FLOCCULATE in the

presence of lime – this provides larger spaces for the water to drain through.

Advantages and Disadvantages of Clay soils

Advantages• Good water holding capacity• Rich in plant nutrients

Disadvantages• Water does not drain through

readily• Slow to warm up in spring• Puddle when wet and set like

concrete when dry• Expensive to cultivate• In high rainfall areas may

develop an iron pan or a clay pan which leads to water logging

• Susceptible to damage from stock and implements especially when very wet or very dry

Silt

• Consists of particles in size between sand and clay

• Has a slippery or silky feel• Pure silt soils erode easily as they do not have

good structure• Have properties between sandy and clay soils• Silt soils have the ability to retain large amounts

of moisture• Silt soils are well aerated

Loam Soils

• Contain sand, silt and clay in balance

• Do not feel gritty when rubbed between the fingers

• Have properties between sandy soils and clay soils

• Usually well drained and well aerated

• Have the ability to hold moisture and plant nutrients

Typical characteristics of a sandy loam:

• Drainage and aeration • Free draining due to the relatively large particle

size and the lack of water holding finer particles.• Easily leached (washed away) because water

can move freely through it and it lacks a lot of the nutrient holding clay particles.

• Free gas exchange results in good gaseous environment in the soil promoting good root growth and good mining of resources resulting in good growth

• Water holding capacity• Large pore space allows water to drain freely to lower

levels causing moisture stress in dry weather.• Reduces the rate of photosynthesis and poor structure

due to reduced turgidity therefore less plant growth• Temperature• Warms quickly due to lack of temperature moderating

moisture and free air movement.• Allows the soil to warm quickly resulting increased

chemical reaction rate in roots and earlier plant growth in the spring

Typical characteristics of a clay loam:

• Drainage and aeration • Poor drainage due to the small particle size and micropores. Water

is held tightly so tends to retain nutrients.• Gas exchange is restricted in the soil preventing respiration in the

roots.• Water holding capacity• Small pore spaces prevent water from draining freely preventing

water stress in dry weather.• Temperature• Warms slowly because the temperature is moderated by moisture

and lack of air movement.• Because the soil is slow to warm the sped of chemical reactions in

roots is reduced and crops will be harvested later due to later planting (sowing) in the spring.

Below is a sample of garden loam magnified under the microscope. Copy the diagram and label: air spaces, coarse gravel, water, earthworm, fine sand, and dead organic matter.

Copy out the table below and fill it in correctly

SIZE GROUPING OF MINERAL PARTICLES IN SOIL

NAME PARTICLE DIAMETER

Bigger than 200 mm20 -200 mm2- 20mm0.2 – 2mm0.02 – 0.2mm0.002 -0.02mmLess than 0.002mm

Write in the following names where you think they should go.

CLAY BOULDERS FINE SAND SILT GRAVEL COARSE SAND

STONES

What do the following words mean?• Mineral Matter• Nutrients• Infertile• Light soils• Heavy soils• Leaching• Loam

• SOIL REVISION QUESTIONS• Name the main components in soil.• Explain what the following terms mean• Chemical Weathering• Physical Weathering• Name three of the five factors that affect

the rate of soil formation.• Define the following rock types.• Igneous• Sedimentary• Metamorphic