Eco new farmers
Module 2 – Soil and Nutrient Cycling
Section 1 – Soils and soil
fertility
Module 2 – Soil and Nutrient Cycling
Section 1 - Soils and soil fertility
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1. Introduction
You will remember from the introductory module that organic farming rely on the management of
soil to enhance the chemical, biological, and physical properties of the soil, in order to produce
crops with minimal synthetic inputs. Increased interest in organic crop production has been
prompted by both consumer demand and the desire to sustain or improve the soil resource.
In this session you will learn about the meaning of soil fertility and how it can be managed in
organic systems. Soil fertility is usually defined as the ability of a soil to supply nutrients for crop
production. This includes biological, chemical and physical properties.
2. Composition of soil
Soil, suppler of air, water, nutrients, and mechanical support for plants, is normally considered as
the solid material on the Earth’s surface as a result of constant physical and chemical breakdown
of rock and the action and turnover of living organisms. Soil is a complex medium made up of
around 45% mineral particles, 20-30% air, 20-30% water and around 5% organic matter (Figure
1). The organic component consists of dead organisms, plant matter and other organic materials
various stages of decomposition.
Mineral particles 45%
Air: 20-30%
Water: 20-30%
Organic matter: 5%
Fig.1. Soil composition
In reality, the percentages of these four main components vary depending on several factors such
as soil type, climate, cultivation practices and water supply.
Soil minerals are derived from two principal mineral types. Primary minerals, changed little in
chemical composition during weathering such as those found in sand and silt, are those soil
materials that are similar to the parent material from which they formed. Sand particles are the
largest, ranging in size from 0.05 mm (very fine sand) to 2 mm (very coarse sand). Sand particles
have a low capacity to hold water and nutrients. Secondary minerals, on the other hand, formed
by the breakdown and weathering of less resistant minerals, which releases important ions and
more stable mineral forms such as silicate clay. Secondary minerals are small in size and dominate
the clay fraction of soil. Clay particles are the smallest mineral particles and are less than 0.002
mm in size. They have the largest surface area which facilitates the adsorption of water and
nutrients to the clay particles.
The relative abundance of sand, silt, and clay particles determines soil texture. Texture greatly
influences soil physical properties, water storage, and heat transfer.
Air is the next basic component of soil. Because air can occupy the same spaces as water, it can
make up approximately 2% to 50% of the soil volume. Soil aeration influences the availability of
many nutrients. Oxygen is essential for root and microbe respiration, which helps support plant
growth. Carbon dioxide and nitrogen also are important for belowground plant functions such as
for nitrogen-fixing bacteria. If soils remain waterlogged (an increase in soil water content often
causes a reduction in soil aeration), it can prevent root gas exchange leading to plant death.
Therefore, maintaining the balance between root and aeration and soil water availability is a
critical aspect of managing crop plants.
Water is important for transporting nutrients to growing plants and for facilitating both biological
and chemical decomposition. Soil water availability is the capacity of a particular soil to hold
water that is available for plant growth and development and depends on soil texture. The more
small particles in soils, the more water the soil can retain. Thus, clay soils having the greatest
water-holding capacity and sands the least. Additionally, organic matter also influences the waterholding capacity of soils because of organic matter's high affinity for water. The higher the
percentage of organic material in soil, the higher the soil's water-holding capacity.
Organic matter, carbon-containing compounds, is the next basic component that is found in soils
at levels of approximately 1% to 5%. The organic component consists of dead organisms, plant
matter and other organic materials in various stages of decomposition. The organic component is
important for nutrition because it serves as a reservoir of nitrogen, phosphorus and sulfur for
plants and provides nutrients and habitats for a diversity of soil-borne organisms. The largest of
these organisms are earthworms and nematodes and the smallest are bacteria, actinomycetes,
algae, and fungi that reduce everything into carbon dioxide, water, nitrates, sulfates, phosphates,
and other inorganic compounds that are common in the soil.
When the organic matter is stable and no longer undergoing decomposition it is called humus,
which can persist in the soil for fairly long times. Humus is much richer in nitrogen, phosphorus,
and sulfur that the original plant residues.
3. Soil texture
The term soil texture is used to describe the proportion of different sized particles in a soil. There
are 3 groups of particles - sand, silt and clay. The texture triangle classifies the soil into different
textures (Figure 2). For example a sandy loam contains 30-50% sand, 0-20% silt and 15-20% clay.
Clay
Sandy
clay
Sandy
clay loam
Sandy
Silty
clay
Clay loam
Silty clay
loam
Sandy silt loam
Sand
Silt
loam
%Sand
Fig.2. The texture triangle: it classifies the soil into different textures
So, how does texture affect nutrient supply? Sandy soils, which feel gritty, are not very good at
holding on to nutrients. More powdery, silty soils are a bit better at retaining nutrients. Clay soils
are the best for retaining nutrients and supplying them to plants.
Texture has important effects on fertility (Table 1). The texture of a soil also controls drainage and
water storage, as well as its suitability for different crops. Texture is also important in
determining soil structure.
Table 1. Relation between soil texture and fertility
Ability to retain nutrients
and supply to plant
Feel
Sand
Silt
Clay
very poor
poor
good
gritty
smooth & powdery
sticky or slimy
4. Soil structure
Soil structure is another term commonly used to describe soil (Figure 3). It describes the
arrangement of clay, sand and silt particles into aggregates or peds. The spaces between
aggregates are called pores and hold air and water. This structure is created by a combination of
natural processes which include wetting and drying, freezing and thawing, root growth, activity
of microbes and soil animals.
Secundary soil products
eg. humus
Mineral
skeleton
Macropore
Micropore
Fig.3. Soil Structure - the arrangement of soil: minerals (mineral skeleton),
organic matter (humus), air and water occupying macro and micropores
5. Soil compaction
Why is soil structure important? Soil structure is important in several ways. It controls the very
important processes of water movement and root growth. Good structure has a stable system of
pores of different sizes (Figure 4).
Soil compaction causes poor structure which is very dense and has discontinuous pores.
Compaction can result from cultivation in wet conditions, or from grazing animals. Compaction
caused by livestock is usually called 'poaching'.
(a)
(b)
Fig.4. Good (a) and poor (compacted) soil structure
The effects of soil compaction on root growth are severe. The condition can seriously limit
nutrient uptake and crop yield.
Cultivation is very important for providing good soil conditions for seed emergence and root
development (Figure 5). In organic systems, cultivation is often used for weed control as well.
Cultivation also releases nutrients into plant available forms. As an alternative to machinery, nonmechanical methods, such as pigs, can also be used as a method of cultivation as they turn over
the soil.
Fig. 5. Cultivation (mechanical and non mechanical) is used to provide good soil
conditions (structure) for seed emergence and root development
6. Soil organic matter
Improving and maintaining soil fertility and soil structure are primary aims of organic farming.
Building the soil organic matter content of soil helps to achieve these aims. Soil organic matter
consists of living material, decomposing residues and residual organic matter or humus (Figure 6).
Soil organic
matter
Non-living
> 90%
Living
< 5%
biomass
roots and other
plant organs
micro-organisms
+ animals
Fig. 6. Soil organic matter composition: living and non-living materials, decomposing
residues and residual organic matter or humus
In an average grassland there is around 8 tonnes per hectare of living biomass and a further 20
tonnes of root material. If you imagine an area the size of a football pitch, then the weight of
living biomass is equivalent to the weight of 9 dairy cows, and the root material represent the
weight of another 23 cows.
Why is soil organic matter so important? It provides a source of nutrients. It provides food and
thus energy for soil microbes (stimulates microbial activity), and holds water and helps to prevent
structural degradation.
Crop residues are an important source of soil organic matter and they include roots and stubble as
well as straw. Some crops add more root materials than others. For example potatoes add around
300 kilogrammes per hectare of root material to the top soil, compared with a massive 10,000 kg
per hectare from a 3 year old grass ley. Manures, slurries and other organic wastes like municipal
compost are valuable sources of organic matter.
7. Summary
To enhance and maintain soil fertility in organic farming systems, the fertilization plan can rely on
manures and several other cultural practices: cover crops, green manures, crop rotations,
mulching, composting and other organic fertilizers. Some of these techniques will be explained in
futures sessions and modules.
This completes the section on managing soil fertility. We have seen that soils are made up of
mineral particles, organic matter, living biomass, air and water. We have also noted that
increasing the organic matter content of soils is very important for improving soil fertility.