Leaving Certificate Agricultural Science Physical Properties of Soil H. Jones, St. Columba’s College

Learning objectives Understand what is meant by soil texture. Explain the properties of sandy soils, loam soils and clay soils. Outline the importance of air in soil. Briefly explain the importance of bacteria in soil. Describe why water is important in soil. Distinguish between the terms saturation point, field capacity, permanent wilting point and available water capacity. Outline how soil texture affects soil temperature. Distinguish between cementation and separation, and explain their relevance to soil structure. Describe the factors affecting soil structure development.

Introduction The physical properties of soil greatly affect how the soil holds water, air and nutrients and is therefore important to consider when growing crops on your farm. In this section we will explore the most important physical properties of soil.

Soil Texture Soil texture refers to the relative amounts of SAND, SILT & CLAY in a soil. Sand, silt and clay particles are rock particles of different sizes, sand being the biggest and clay the smallest (less at 0.002mm). The ideal soil texture is 40% SAND, 40% SILT & 20% CLAY. LOAM SOILS contain equal amounts of sand, silt and clay and are considered the best soils for cultivation of crops. Sandy, loam and clay soils have different properties.

Soil texture & Properties

Sandy Soils Have large air holes. Free Draining soils Is easy to work with (light) Dries out quickly Minerals are easily leached. Poor soil with little or no nutrients. Is a warm soil.

Clay Soils Holds water easily. This protects from leaching of minerals. Is naturally fertile soil. Very poor drainage, which can lead to water logging. Is a cold soil.

Loam Soils Intermediate characteristics of both clay and sandy soils. More advantages and fewer disadvantages than sandy or clay soils. A good mixture is 40 % Sand, 40 % Silt and 20 % clay. Notes: While the nature of soil depends on the particle composition, the amount of humus in the soil is also a major factor

Learning Check What materials are found in soil? How are particles classified as sand, silt and clay? How can you determine the texture of a soil? What are the characteristics of a sandy soil? What are the characteristics of a clay soil? What do you call a soil with intermediate qualities?

Soil Air Soil Air: Soil air contains Nitrogen (78%, Oxygen (21%), Carbon Dioxide (0.03%) and some other gases. Oxygen is the most important component of soil air and is used by plant roots (to uptake mineral nutrients), soil bacteria and other soil animals (e.g. earthworms).

Focus on soil bacteria Bacteria can be AEROBIC (require oxygen to survive) or ANAEROBIC (do not need oxygen to survive) Some bacteria in the soil can turn the nitrogen in the soil (which is not easily absorbed by plants) into NITRATE (which can be easily absorbed by plants). These bacteria are called NITROGEN FIXING BACTERIA. Examples of nitrogen fixing bacteria are Nitrobacter and Rhizobium.

Soil water Soil Water: Soil is important in the soil because: Required for the growth of plants and animals (like earthworms and even bacteria) All chemical reactions that take place in the soil require water. Dissolves minerals from rocks, making them available to plants.

Soil Water - Important Definitions Saturation Point: When a soil has all the space taken up with water but has no water on its surface (also known as water logged). Capillary Water: This is the water held within the small spaces (pores) in the soil – it is available to plants. Hydroscopic Water: This is water bound to the surface of soil particles. It is not available to plants. Gravitational Water: This is water found in large pores after heavy rainfall. Gravity pulls this water down quickly, replacing it with air. Field Capacity (FC): This is the amount of water left in the soil, after all gravitational water was been drained away. Permanent Wilting Point (PWP): Is the moment when a plant can no longer remove any more capillary water from the soil – varies from plant to plant. Available Water Capacity (AWC): Is the amount of water available for plant uptake. Formula: AWC = FC - PWP

Soil temperature Wet soils are more difficult to heat up than dry soils. Therefore, we can say that sandy soils and loam soils warm up more quickly than clay soils, which can affect their suitability for growing crops. Other factors affecting soil temperature are: Aspect – south facing soils warm up more quickly. Altitude – soil temperature drops with increased altitude. Colour – dark soils absorb more heat than lighter soils.

Soil structure While soil is composed of sand, silt and clay particles, these particles often form larger particles when they cluster together. These clusters are called AGGREGATES or PEDS. Soils that contain aggregates have large pore spaces, which aid the soils drainage and aeration. Soils develop their structure over time, forming aggregates or breaking them up again. Factors that affect soil structure can be classified as CEMETATION PROCESSES or SEPARATION PROCESSES. Cementation is the binding together of particles together, forming aggregates. Separation is the breaking up of aggregates into smaller pieces.

Factors affecting soil structure Freeze Thaw Action: The freezing and thawing of soil promotes aggregation (formation of aggregates). Wetting & Drying: Soil volume increases when wet and decreases when dry. As soil dries out, particle cement together into aggregates. Plant Activity: Small roots promote aggregation while large roots can break up soil. Animal Activity: Earthworms promote the formation of aggregates as they burrow through the soil. Cultivation: Ploughing and harrowing break up the soil and promote aggregation. Poaching: This is the compaction of wet soil by animals or machinery, usually occurring near gateways and feeding areas. Poaching reduces soil structure.

Humus 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. Normal sandy soils as mentioned before are free draining and soil water could leach out (including the minerals dissolved in it).

Humus - 2 Humus in the soil absorbs the water and can greatly improve the quality of otherwise poor (sandy) soils. Both clay and humus can hold minerals in the soil. The minerals are in the form of Cations (Positive Ions e.g. N+ or K+ ions. The ability of a soil to maintain mineral ions in the soil is known as Cation Exchange Capacity.

Why Humus is Important to Soils Humus contains minerals (the type and amount depend on the source of the humus). It holds minerals in the soil due to its high cation exchange capacity. It improves and strengthens the crumb structure of heavy soils. It forms clay - humus complexes, which improves soil stability. Its dark colour improves the warming capabilities of the soil (i.e. it is able to absorb more heat from the sun) Can make the soil more acidic (an advantage and disadvantage depending on the use of the soil).