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 www.econewfarmers.eu 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 Clay loam Silty clay Silty clay loam Sand Sandy Sandy silt loam 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 Sand Silt Clay Ability to retain nutrients very poor poor good and supply to plant Feel 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. Mineral skeleton Secundary soil products eg. humus 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 Living < 5% Non-living > 90% roots and other plant organs biomass 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.