Soil, Pedology(an introduction)

Aim: To become aware of how soil is formed and various soil properties.

Learning outcomes:

(C) To sequence a soil profile with a brief description of each horizon.  (B) To recognise how differing soil properties affect vegetation and agriculture. (A/ A*) assess soils in the field to determine pH, texture and profile, and relate this to theory.

What is soil?

Soil: is a natural body consisting of layers (soil horizons) of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics. Joffe (1949)

Simple: Soil results from interrelationships between, and interactions of , several physical, chemical and biological processes, all of which vary according to different natural environments.

Why is the study of soil important?

“If you live in the SE and have an unfeasible fetish for broccoli then you may just die.”

“If you are going to eat soil then make sure it’s the top fraction and bake it well.”

“Indeed I once accosted a man of Asian origin in the street and asked him to secure me some soil, it was of course an under the counter job”

How is soil formed?

To form soil you first need Regolith, which can be derived from either the in situ parent material or from elsewhere, so deposited alluvium, glacial deposits or loess.

With the addition of air, water, living organisms and decayed organic matter a true topsoil starts to form.

Soil formation = f (parent material + climate + topography + organisms + time)

How do we display what soil looks like in profile?

Idealised soil profile in the UK

Properties that define soils

There are many different properties that can be used to define soils and studied for various reasons.

They include:

Minerals, texture, structure, biota, moisture, air, pH, organic matter, nutrients and temperature.

We are going to look at: texture, moisture and acidity

Soils are made up of three parts: sand silt and clay. The proportion of sand silt and clays determine how rough the soil is otherwise known as its texture.

Texture is important because:

Controls the size an spacing of pores.

Controls water flow

Controls retention of nutrients

The wonders of clays

Texture of soil.

Why is texture important for farming?

Sandy

Silt

Clay

•Good for roots

•Bad for moisture retention.

•Not much organic matter

•Lack mineral content and organics.

•Small pores so more water retained

but sometimes cements

•Large nutrient content.

•Water logging can become

gleyed.

•Difficult penetration

The amount of water in a soil can be expressed as a formula.

W∝ R – (E+T+D)

Moisture

W= water in soil, R = rain / proportional E = evaporation, T = transportation D = Drainage.

Points to consider.

•Macro and micro pores and retention.

•High water holding capacity.

Acidity (pH)Its all about Hydrogen cations, depending on how many H cations there are changes the pH of the soil.

•Leaching calcium lime.

•To acid then Fe Al toxic.

1. The A horizon is at the bottom of a profile and contains weathered bed rock.

2. You can die from geophagy.

3. Soil is made up of three parts sand, clay and silt.

4. Typically in the UK plants like to grown in slightly alkali soils

5. I love soils and cant wait to go and touch some next week!

True or False

a

a

a

a