Soil porosity refers to the amount of open space or voids within the soil. It is an essential characteristic of soil, as it affects the soil’s ability to hold water and air, which are necessary for the growth of plants and other organisms. Understanding soil porosity is critical for farmers and gardeners, as it helps them make informed decisions about soil management practices, such as adding organic matter or avoiding over-tilling the soil.
What Is Soil Porosity?
Porosity refers to the percentage of soil volume occupied by pore spaces. It can be calculated by the following formula.
Porosity = 100- Bulk density Particle density ×100
There are two types of porosity. They are –
- Macrospore (Noncapillary pore): Generally contains air, >30
- Microspore (Capillary pore): Generally contain water, < 30
Calculation of soil porosity (pore space):
% Solid Space + % Pore Space = 100
% pore Space = 100 – % Solid Space
How are Pore spaces formed in soil?
Pore spaces in the soil are formed through various processes and factors. Here are some of the primary mechanisms involved in the formation of pore spaces in soil:
1. Compaction and Settling
Soil particles can be compacted over time due to pressure from above, such as the weight of overlying layers or human activities like farming or construction. As particles become more tightly packed, pore spaces are reduced or eliminated.
2. Organic Matter Decomposition
Organic matter, such as plant roots, leaves, and other organic materials, contribute to pore space formation in soil. As organic matter decomposes, it creates channels and voids within the soil structure.
3. Biological Activities
Soil organisms, including earthworms, insects, and microbes, play a crucial role in creating pore spaces. For example, earthworms burrow through the soil, creating tunnels and channels that improve soil structure and increase pore space.
4. Freezing and Thawing
In regions with seasonal freezing temperatures, repeated freeze-thaw cycles can cause the expansion and contraction of water within the soil. This process helps to create cracks and fractures, leading to the formation of pore spaces.
5. Root Growth and Decay
Plant roots penetrate the soil and create pore spaces as they grow. When roots die and decay, they leave behind root channels and openings that contribute to pore space formation.
Erosion processes, such as water or wind erosion, can remove soil particles and create voids or spaces in the soil profile. This can result in the development of larger pores.
It’s important to note that the size, distribution, and connectivity of pore spaces in soil affect its permeability, water-holding capacity, and aeration, which, in turn, influence its fertility and ability to support plant growth. The soil’s composition, texture, and mineralogy also play significant roles in determining the characteristics of pore spaces.
Factors Affecting Soil Porosity
Soil porosity is influenced by several factors, including the size and arrangement of soil particles, the amount and type of organic matter, and the degree of compaction. Generally, soils with high porosity levels have more space for water and air to move through, resulting in better soil drainage and aeration. The following factors affect the porosity of soil:
The size of soil particles affects porosity. Soils with a mixture of particle sizes, such as sandy loam, tend to have larger pores due to the presence of coarse particles like sand. In contrast, soils with smaller particles, such as clay, have smaller pores and can be less porous.
In sandy soils, pores are quite large. Thus, the total pore space is less. In fine-textured soils, there is the possibility of more granulation and more total space because there are pores between individual particles and within granules. (micropores)
Soil with an aggregate structure has greater pore space than a structure with less or single-grain soil. For the same size of aggregates as individual grains in a single grain soil and for similar arrangement modes, soil with aggregate structures has additional pore space between the primary particles. Granular or crumby type of structure has more porosity than plate-like.
Arrangement of soil particles
The arrangement of soil particles influences porosity. When the sphere particles are arranged in columnar form, it gives the most open packing system. Thus the number of pore spaces will be less. When the particles are arranged in the pyramidal form, it gives the closed packing system. So in this system, porosity would be more. Well-aggregated soils with particles bound together in clumps or aggregates generally have larger pores between the aggregates, allowing for greater porosity. In contrast, poorly aggregated soils may have more compacted particles, resulting in smaller pores and reduced porosity.
Increase in organic matter, comparatively the soil, increase in the percentage of pore space. Organic matter, such as decomposed plant and animal material, significantly affects soil porosity. Organic matter improves soil structure, promoting the formation of aggregates and creating larger pores. It also increases the soil’s ability to retain moisture and nutrients.
Soil organisms, including earthworms, insects, bacteria, and fungi, contribute to soil porosity. For example, earthworms burrow through the soil, creating tunnels that improve aeration and drainage. Microorganisms decompose organic matter, enhancing soil structure and porosity.
Plant roots play a vital role in creating soil porosity. As roots grow and extend, they create channels or fissures in the soil, increasing pore space. Additionally, root exudates and the decomposition of roots contribute to the formation of stable aggregates, further enhancing soil porosity.
Depth of soil
Pore space in the sub-soil has been found to decrease in comparison to soil.
Cropping: Cropping tends to lower the total pore space in comparison to virgin or uncropped soils. This reduction is associated with a decrease in contact. Continuous cropping often results in a reduction of large or macro pore space.
Soil compaction occurs when soil particles are pressed together, reducing pore space. Activities like heavy machinery, foot traffic, or improper agricultural practices can lead to compaction, resulting in decreased soil porosity. Compacted soil restricts the movement of air, water, and roots, negatively impacting plant growth.
Due to pudding under sufficient soil moisture, the soil surface layer is made dense and compact. Eventually, the porosity of this surface soil is reduced by the infiltration of muddy surface materials. % sand = 100 – ( silt + clay )
It’s important to note that soil porosity is dynamic and can change over time due to natural processes, human activities, and management practices. Maintaining a healthy soil structure and porosity is crucial for sustaining productive and resilient ecosystems.
Why Is Soil Porosity Important?
Soil porosity refers to the amount and arrangement of spaces, or pores, between soil particles. These pores can be filled with air or water. Soil porosity is important for several reasons:
1. Water and air movement
Porous soil allows water to infiltrate the ground, reducing surface runoff and promoting groundwater recharge. It also facilitates water movement through the soil, essential for plant roots to access water for growth and nutrient uptake. Adequate soil porosity allows for proper drainage, preventing waterlogging and reducing the risk of soil erosion. The presence of air-filled pores is crucial for root respiration and the survival of soil organisms.
2. Nutrient availability
Soil porosity influences the availability of nutrients to plants. Pores provide a pathway for nutrients to move through the soil, allowing roots to access them. Adequate porosity ensures that nutrients are distributed evenly and can be taken up by plant roots effectively.
3. Root growth and development
Soil porosity affects root growth and development. A well-structured soil with sufficient porosity allows roots to penetrate easily, promoting root branching and proliferation. It provides space for root elongation, enabling plants to anchor themselves securely and absorb water and nutrients efficiently.
4. Soil aeration
Porous soil allows for the exchange of gasses between the soil and the atmosphere. Oxygen is essential for root respiration and the survival of beneficial soil organisms. Adequate soil porosity ensures oxygen can diffuse into the soil, maintaining a favorable aerobic environment for plant roots and soil organisms.
5. Soil temperature regulation
Soil porosity influences soil temperature. Pores in the soil allow for better air circulation, which can help regulate soil temperature. The presence of air-filled pores can buffer temperature extremes, preventing excessive heating or cooling of the soil, which can impact plant growth.
Overall, soil porosity is vital to health, plant growth, and ecosystem functioning. It affects water movement, nutrient availability, root growth, soil aeration, and temperature regulation, all of which contribute to terrestrial ecosystems’ overall productivity and sustainability.
How to Judge the Porosity of Soil?
Judging the porosity of soil involves assessing its ability to hold and transmit water, air, and nutrients. Here are some methods commonly used to evaluate soil porosity:
1. Visual Inspection
Conduct a visual examination of the soil to observe its structure and texture. Well-drained soils with good porosity typically have a loose, crumbly texture with visible spaces or pores between soil particles.
2. Soil Texture
Soil texture influences porosity. Sandy soils have larger particles and tend to be more porous, while clayey soils have smaller particles and tend to be less porous. Assessing the soil texture can provide an initial indication of its porosity.
3. Percolation Test
A percolation test measures the rate at which water moves through the soil. Dig a hole or pit in the soil and fill it with water. Observe the time it takes for the water to percolate or drain away. Faster percolation indicates higher porosity.
4. Water Infiltration Test
This test assesses how quickly water penetrates the soil surface. Create a small depression or ring on the soil surface and pour a known amount of water into it. Measure the time it takes for the water to infiltrate the soil. High porosity allows for quicker water infiltration.
5. Soil Core Sampling
Use a soil auger or sampling tube to extract a soil core from the ground. Analyze the core for its structure and pore spaces. Well-aerated soils with visible and interconnected pores are indicative of good porosity.
6. Soil Porosity Calculation
Porosity can be calculated by measuring the volume of voids or pore spaces in the soil. This requires collecting a known volume of soil and determining the volume of water it can hold. The water volume ratio to the total soil volume provides the porosity value.
It’s important to note that soil porosity can vary within a single location due to soil compaction, organic matter, and water content. Therefore, it is recommended to use multiple methods and observations to comprehensively assess soil porosity.
How To Improve Soil Porosity?
To improve soil porosity, which refers to the air and water-holding capacity of the soil, you can take several steps:
1. Organic matter
Add organic matter such as compost, well-rotted manure, or leaf litter to the soil. Organic matter improves soil structure, increases pore spaces, and enhances water infiltration and drainage. Spread a layer of organic matter over the soil surface and incorporate it into the top few inches of soil.
2. Avoid compaction
Avoid heavy machinery or excessive foot traffic on the soil. Compacted soil has fewer pore spaces, reducing porosity. If compaction has already occurred, use aeration tools like a garden fork or aerator to loosen the soil.
Apply a layer of mulch, such as wood chips or straw, to the soil surface. Mulch helps retain moisture, moderates soil temperature, and prevents erosion. It also promotes the activity of soil organisms, which contribute to soil porosity.
4. Cover cropping
Planting cover crops, such as legumes or grasses, during periods when the soil would otherwise be bare can improve soil porosity. Cover crops protect the soil from erosion, add organic matter when incorporated, and their root systems enhance soil structure.
5. Avoid overwatering
Overwatering can lead to compacted soil and reduced porosity. Water your plants appropriately, ensuring that the soil is moist but not waterlogged. Proper watering practices encourage deeper root growth and improve soil structure.
6. Avoid excessive tillage
Excessive tilling can break down soil structure and reduce porosity. Minimize unnecessary tilling and consider alternative practices such as no-till or reduced-till gardening. These practices preserve soil structure and increase organic matter content.
7. Soil amendments
Depending on your soil’s specific characteristics, you can consider incorporating soil amendments to improve porosity. Adding sand or gypsum can help loosen the soil structure for clay soils. For sandy soils, adding organic matter or compost can increase water-holding capacity.
8. Crop rotation
Implementing crop rotation practices can improve soil porosity. Different crops have varying root structures and depths, which can contribute to soil aeration and break up compacted layers.
Remember, improving soil porosity is a gradual process. It may take time and consistent efforts to see significant results. Regularly monitor the soil’s condition and make adjustments accordingly.
What Happens When There Is Low or High Soil Porosity?
The porosity of soil plays a crucial role in determining its fertility, drainage capacity, and overall health. Let’s explore what happens when there is too little or too much soil porosity:
1. Too little soil porosity (low porosity):
When the soil has low porosity, there are fewer open spaces or pores for air and water to move through. This condition can lead to several issues:
Insufficient porosity restricts water movement, causing water to accumulate on the soil surface. This can result in waterlogging, root suffocation, and promoting anaerobic (low-oxygen) conditions, which are detrimental to plant growth.
Insufficient air movement within the soil limits the oxygen supply to plant roots. This can lead to the development of anaerobic microorganisms that produce harmful byproducts, negatively affecting root health and overall plant growth.
Inadequate porosity hampers the movement of water carrying essential nutrients through the soil profile. This can result in poor nutrient availability for plant roots, leading to deficiencies and reduced crop productivity.
2. Too much soil porosity (high porosity):
There are excessive open spaces or pores when the soil has high porosity. While some degree of porosity is essential, excessive porosity can cause problems:
Soils with high porosity drain water rapidly, leading to excessive leaching of nutrients. This can result in nutrient depletion from the root zone, reducing plant growth and causing nutrient imbalances.
High porosity soils have reduced water-holding capacity, making them more prone to drying out during low rainfall or irrigation periods. This can lead to drought stress for plants and increased irrigation requirements.
Increased erosion risk
Excessive porosity can make the soil more susceptible to erosion by wind or water. The lack of fine particles and cohesive forces within the soil structure can cause soil particles to detach and be carried away, leading to soil erosion and reduced soil fertility.
Finding the optimal balance of soil porosity is crucial for maintaining healthy soil conditions. This balance depends on soil type, organic matter content, compaction, and management practices. Soil amendments like organic matter additions, cover cropping, and appropriate tillage techniques can help improve soil porosity and create a more favorable environment for plant growth.