How Do You Increase Permeability Of Clay?
What is Clay Soil Permeability?
Permeability refers to the ability of water and air to move through soil pores. It measures how fast liquids and gases can flow through a material. High permeability allows for easier internal drainage and air exchange while low permeability restricts water and air movement.
Permeability is critically important for clay soils. Clay particles are very small and pack tightly together, leaving little space for air and water to penetrate. This results in low permeability and poor drainage. Excess moisture gets trapped, creating anaerobic conditions that prevent plant roots from absorbing oxygen.
Low permeability in clay causes major issues like:
- Waterlogging that harms plant growth
- Increased runoff and erosion
- Nutrient deficiencies from lack of proper soil aeration
- Reduced germination rates
- Greater susceptibility to compaction
Measuring Permeability of Clay Soils
There are several methods available for measuring the permeability of clay soils both in the field and in the lab. Common field methods include double-ring infiltrometers, single-ring infiltrometers, and borehole tests. In the lab, constant and falling head permeameters are often used.
Field methods like the double-ring infiltrometer involve driving two concentric metal rings into the soil and filling them with water. The rate at which the water infiltrates into the soil in the inner ring is used to estimate permeability. Single-ring tests work similarly with just one ring. Borehole tests involve drilling a hole in the soil, filling it with water, and measuring how fast the water drains away.
For lab tests, soil cores are collected in the field and brought back for testing. Constant head tests involve applying a constant water pressure to one end of the core while measuring flow rate through the core. Falling head tests allow the water pressure to drop as water infiltrates into the core. Lab tests allow more controlled conditions compared to field tests.
In general, field tests provide permeability estimates in a real-world setting, while lab tests allow more precise measurement under controlled conditions. The appropriate method depends on the goals of the test and resources available.
Factors Affecting Clay Permeability
There are several key factors that determine and influence the permeability of clay soils:
Clay Mineralogy
The specific types of clay minerals present in the soil play a major role in permeability. Clay minerals like montmorillonite have a swelling structure which can greatly reduce permeability when wet. Other clays like kaolinite do not swell as readily and have less of an impact on permeability.
Soil Structure
The overall structure and aggregation of clay particles affects water movement. Granular and blocky soil structures allow more permeability compared to platy or massive soil structures that restrict water flow.
Moisture Content
The moisture content of the soil directly impacts permeability. As a clay soil approaches saturation, the swelling of clay minerals and mobilization of fine particles can effectively clog pores, reducing permeability. Drier clay soils maintain more interconnected pores and have higher permeability.
Strategies to Improve Clay Permeability
There are several strategies that can help improve the permeability of clay soils:
Deep Tillage
Deep tillage involves plowing and breaking up soil to a depth of 8-10 inches or more. This helps create larger soil pores that allow water to infiltrate and drain more easily. Deep tillage should be done when the soil is dry to maximize fracture of the soil. It may need to be repeated periodically to maintain effects.
Controlled Traffic
Controlled traffic involves confining all machinery loads to permanent traffic lanes. This prevents excessive compaction in the rest of the field that can restrict water movement and root growth. The undisturbed soil maintains better structure with more large pores for drainage.
Adding Organic Matter
Incorporating organic matter like manure, compost, or cover crops helps bind soil particles into aggregates and opens pore spaces. The organic matter also feeds soil microbes that produce compounds to stabilize aggregates. This improves both stability and porosity of clay soils.
Using Cover Crops
Planting cover crops adds organic matter to the soil when the plants die back or are tilled in. Their deep root systems also help open up the soil profile. Leguminous covers like clover help add nitrogen. Covers should be allowed to grow as long as possible before tillage.
Deep Tillage
Deep tillage is one of the most effective ways to increase permeability in clay soils. It involves tilling and breaking up the soil to much greater depths than normal plowing. While regular plowing disturbs soil to 6-12 inches, deep tillage can go down 24 inches or more.
By disturbing soil to greater depths, deep tillage helps create larger passages and spaces for air, water, and roots to penetrate. It breaks up hardpans and compacted layers that restrict water drainage and root growth in clay soils. The increased depth and volume of loosened soil improves permeability throughout the soil profile.
Deep tillage requires the use of specialized implements like subsoilers, paratillers, rippers, or chisel plows. These tools have long, heavy shanks that can penetrate deep into the ground and fracture compacted clay layers. Using them at 12-24 inch depths and repeating every few years can steadily improve permeability. The deeper disruption of soil structure creates more continuous macro-pores for better internal drainage.
Controlled Traffic
One of the most effective ways to increase permeability in clay soils is to implement controlled traffic farming. This involves confining all machinery loads to permanent traffic lanes or tramlines.
Heavy machinery like tractors and harvesters running repeatedly over the same area causes significant soil compaction. Compaction reduces pore space in the soil, severely limiting permeability and drainage.
With controlled traffic, all field operations are confined to the least possible area of permanent traffic lanes. The undisturbed soil between traffic lanes is not compacted, allowing it to retain good soil structure and permeability.
Controlled traffic farming requires GPS guidance systems on machinery to keep them precisely within the designated permanent traffic lanes. The optimal design is to match the wheel spacing of all equipment to the traffic lanes.
Growers implementing controlled traffic typically see improved permeability within 1-2 seasons as the undisturbed soil regains its natural structure. In the long term, controlled traffic minimizes recompaction and enables stable improvements in clay soil permeability and health.
Adding Organic Matter
Adding organic matter to clay soils is one of the most effective ways to increase permeability. Organic matter creates spaces in the soil structure, allowing water to infiltrate more easily. It also improves moisture retention, nutrient availability, and microbiological activity in the soil.
There are several types of organic matter that can be added:
- Compost – Compost contains a diversity of organic compounds and beneficial microorganisms. It should be well-aged before application.
- Manure – Manures such as cow, horse, sheep, chicken, or rabbit contain valuable nutrients. They should be composted before applying.
- Cover crops – Cover crops can be grown and then tilled into the soil as green manure. Legumes like clover are especially useful.
- Crop residues – Leftover plant materials like straw or seed hulls can be incorporated into the clay soil after harvest.
Application rates will depend on the soil conditions, but typically 1-3 inches of organic matter per acre is recommended when trying to increase permeability of clay soils. The organic matter should be mixed into the top 6-10 inches of soil, either by tillage or natural processes like earthworm activity.
Applying organic matter to clay soils provides long-term improvements in permeability and soil health.
Using Cover Crops
One effective strategy for improving the permeability of clay soils is to plant deep-rooted cover crops. As the roots penetrate through the dense clay, they create channels and pathways for water, air, and future crop roots to travel through. Maximizing the biomass and depth of the cover crop root system is key to making substantial improvements in permeability.
Some of the best cover crop options for clay soils include cereal rye, annual ryegrass, red clover, sweet clover, and tillage radish. These species put down deep taproots that can extend 6 feet or more into the soil profile. Planting a mix of several cover crops creates diversity in the root structures formed.
Another important consideration is the timing of killing the cover crops. Allowing the covers to reach peak biomass before termination will enable the greatest improvement in permeability. Killing too early reduces the extent of root penetration and drainage channels created. Cover crops should be terminated through herbicide burn-down, mowing/rolling, or natural winterkill just prior to planting the cash crop they are intended to benefit.
Creating Natural Drainage
Two ways to create better drainage in clay soils without tilling are surface shaping and subsurface drainage tiles. Surface shaping involves contouring the land to facilitate the flow of water off fields. This can be done by creating gentle slopes, furrows, or crowns in the surface to direct water to ditches or drains. The goal is to shape the surface so gravity moves water downslope. This improves drainage and reduces ponding. Subsurface drainage tiles are perforated pipes buried 2-4 feet deep in trenches that channel water away from fields. The tiles are usually installed in a pattern to collect water from the soil profile. As water drains into the buried tiles, it moves through the perforations into the pipes and is carried away underground. Drainage tiles help lower the water table and provide an exit route for excess water in heavy clay soils.
Testing and Monitoring Permeability
Once you’ve implemented practices to improve permeability, it’s crucial to continue testing and monitoring your soil’s permeability over time. Regular testing allows you to track changes and determine if your improvement strategies are working as intended.
There are a few different methods for evaluating permeability on your property:
- Percolation tests measure the rate at which water infiltrates into the soil. These straightforward tests can be performed on-site.
- Infiltrometer testing quantifies infiltration rates. Different devices are available, from simple manual types to automated electronic infiltrometers.
- Soil core analysis in a lab provides precise permeability measurements based on undisturbed soil samples.
- Test pits allow visual assessment of subsurface soil structure.
Ideally, test in multiple locations to account for variability. Track and record results over time. Compared to initial baseline readings, follow-up testing will reveal if you’re making progress. If permeability remains poor, you may need to intensify improvement practices or try different methods.
Use the testing data to adjust your permeability enhancement strategies. For example, increasing the amount of organic matter additions or using deeper tillage in certain areas. Routine testing allows you to adapt and refine your practices to meet the specific needs of your clay soils.
