Soil 4 – Soil Properties

Humus is the product of the breakdown of organic matter.

It is the remains of dead animals and plants.

Fully decayed material forms CO2, water and mineral salts.

But the incomplete product is a dark sticky material called humus.

Humus in the soil absorbs the water and can greatly improve the quality of otherwise poor (sandy) soils.

Humus is useful to the soil because: 1. It contains minerals (the type and amount

depend on the source of the humus). 2. It holds minerals in the soil due to its high

Cation exchange capacity. 3. It improves and strengthens the crumb

structure of heavy soils. 4. It forms clay humus complexes, which

improves soil stability. 5. Its dark colour improves the warming

capabilities of the soil (i.e. it is able to absorb more heat from the sun)

6. Can make the soil more acidic (an advantage and disadvantage depending on the use of the soil).

The micro-organisms in the soil include bacteria, fungi and algae but bacteria are the most important group.

The most important function of bacteria is the breakdown of organic matter into humus and play a major role in the Nitrogen & carbon cycle.

The earthworm plays a major role in the structure of the soil.

The number of earthworms is an indication of the fertility of the soil.

In good (fertile) soil there may be as many as 150-200 worms per metre squared.

That is the same as 1.5 – 2 million worms per hectare.

Earthworms improve the soil in the following

ways:• They eat their way through the soil and mix the

ingested material with mucus in their guts. This helps to improve soil crumb structure.

• Depositing soil in different places and mixing horizons.

• Improve drainage of heavy clay soils• Introduces more air into the soil.• When they die the further increase the amount

of organic matter.

 Water is important in the soil for some obvious

reasons:• Required for the growth of plants and animals

(like earthworms and even bacteria)• All chemical reactions that take place in the

soil require water for them to proceed.• But too little or too much water can be harmful

to the soil and its inhabitants.• It is the worlds best known solvent. • It forms salts readily with chemicals found in

parent rock these are then taken up by the plant roots.

There are four extreme conditions to consider

when discussing soil water

1. Flooding – When all available space in the soil is taken up by water and there is excess on the surface of the soil.

2. Water logging – When all available spaces are taken up by water in the soil but there is no excess on the surface of the soil.

• The presence of the water is not the problem here but the resultant absence of air.

• The area where all the air spaces is taken up by water is called the Water Table

 3. Wilting Point – Is the condition of a

particular plant when it has extracted all the available water from the soil.

4. Field Capacity - When a soil has just enough water and is not flooded or water logged.

• Measured in mm of water• Clay soils can hold more water and are said to

have a greater field capacity.

Air is required in the soil for the growth of roots, bacteria and earthworms. It is essentially the same in composition as atmospheric air.• 78 % Nitrogen • 21 % Oxygen • 0.03 % Carbon Dioxide • 0.97 % Other gases

The most important constituent of soil air is oxygen.

This gas is vital for the process of respiration by both plant roots and macro and micro–organisms.

Respiration involves burning up food in the presence of oxygen and releasing energy.

Aerobic – In the presence of Oxygen (O2) Anaerobic – In the absence of Oxygen

(O2)

Food + Oxygen Carbon Dioxide + Water + Energy

C6H12O6 + 6O 6CO2 + 6H2O + Energy

If it happens that the Oxygen content is < 20% of its desired capacity then root growth is impeded (stunted).

This can happen by compaction by heavy machinery, frequent flooding or water logging and in severe cases over use of pesticides killing off helpful bacteria and earthworms.

Plants use O2 for the uptake of minerals from the soil. Most the useful bacteria in the soil are aerobic (i.e. they need Oxygen to live) Anaerobic bacteria do not need oxygen. Certain types of bacteria can use the Nitrogen in the soil and change it into nitrates and other compounds which are useful to the plant.

These are elements or compounds that are either bound chemically to clay particles or are dissolved in water in the soil.

Plants require them as nutrients for growth and repair of their cells and also for certain chemical reactions to take place.

They are classified by their amount of use. The minerals required in large amounts are

called Macro – Nutrients. Examples are N, P, K, Ca and Mg.

Minerals required in small amounts are called Micro – Nutrients. Examples are Fe, Mn, Cu, and B. They are also known as minor elements or trace elements.

Water is very slow to attract heat. This means that soil with a lot of water in

it harder to heat up than dry soil. In fact 1kg of water needs more than ten

times the heat to raise its temperature by 1 degree, as does 1 kg of dry soil.

Clay soils, as mentioned earlier can attract more heat than sandy soils because of their colour but are generally wetter soils; therefore need more heat to raise its temperature.

This is why clay soils are referred to as cold soils and sandy soils are called warm soils.

On average, clay soils need 50 % more heat than sandy soils to raise their temperatures by 1 degree.

Aspect, colour and altitude also affect the heating capabilities of a soil.

This is the direction that land is facing. North facing slopes never get the sun South facing slopes always get the sun Land with a S, SW aspect will be warmer

in spring and therefore more productive.

This is the height above sea level. For every 150ft the soil temperature drops by 10C.

Soil pH is a measure of the acidity of a soil.

pH stands for the power of the Hydrogen ion

Plants require certain pH’s to grow well; if pH changes the plant may not be able to produce as much, or may in fact die as a result. Earthworms and soil bacteria thrive in neutral or near neutral conditions.

In soils, pH ranges mainly from 4 to 9.

< 4.5 = Very Acidic – 5.2 = Strongly acidic 5.3 – 5.9 = Moderately acidic 6.0 – 6.5 = Slightly acidic 6.6 – 6.9 = Near Neutral 7.0 = Neutral 7.1 – 7.5 = Slightly Alkaline 7.6 –8.3 = Moderately alkaline. In Ireland, soils tend to become acidic

with time, and most soils need to be limes (increase the pH) every four years or so.

This refers to dead and partially decayed plant leaves, roots, twigs, and micro and macro organism.

The final degradation or break down of organic matter results in a compound called humus.

Advantages of organic matter• It is dark brown/black in colour. This means it

will absorb heat and warm soils (especially in spring) will have an early germination capacity (Tillage – Barley and Wheat)

• It has a crumb structure and is therefore friable

• It has 4 times the C.E.C. of clay particles. Therefore soils rich in organic matter will be very fertile.

• By promoting flocculation it hugely improves aeration. This increases the population of earth worms and bacteria of decay