soil hardpan layer

What Is A Hardpan In Soil: identification and removal

Do you have an area in your garden or field in which plants never seem to grow well or thrive? Well, it could be any number of things, such as poor drainage or poor soil fertility, but if the soil is pretty good but has a lot of traffic on it – it might be a soil compaction problem. In some cases, a highly compacted area exists under the soil which is extremely hard to dig through with normal hand tools. This area is known as a hardpan and it will prevent roots and water from getting through which leads to a lot of problems when trying to grow anything.

What is a hardpan in soil?

Hardpan soil is soil that has a layer of densely compacted soil beneath the topsoil. The hardpan layer may be a few inches below the topsoil, or there may be several feet of topsoil before hitting the hardpan layer.

Hardpan soil is so densely compacted that neither water nor air can permeate it. It is rock-solid and may even be mistaken for a ledge or rock layer beneath the soil’s surface when you try to till an area for planting or digging in the soil.

The University of California Department of Agriculture and Natural Resources refers to hardpan as soil whose particles have cemented together to make sandstone. The hardpan layer ranges from a mere few inches to several feet in depth. Plant roots cannot grow in hardpan soil.

hardpan in soil infographic diagram
Hardpan in soil diagram


What are the causes of hardpan?

There are two causes of hardpan soil: soil composition and soil compaction. Hardpan soil caused by soil composition is called solonetzic soil, explains plant geneticist Asrinus Subha of Beriq. Solonetzic soil contains high amounts of clay and has a high pH. The high pH causes salts, like copper and iron salts, to crystalize and fill in the soil’s air pockets, preventing water from penetrating and sealing off air pockets that typically provide water and fresh oxygen to plant roots. This results in rock-hard soil.

Soil compaction from heavy loads can also cause hardpan soil to form. This may occur with farm equipment, landscaping equipment, or heavy traffic over the ground, mainly when the soil is wet. The pressure from above forces air and water from the pores in the soil and causes the particles to glue together to form a rock-hard layer of hardpan soil.

ploughing soil with a tractor
Repeat cultivations at the same depth increases the likelihood of a hardpan forming in soil.


Are there different types of hardpan?

Although the cause of hardpan soil can vary, hardpan soil refers to rock-hard soil that cannot be penetrated by water. Hardpan soil close to the surface may prevent plants from growing in the area because their roots cannot penetrate the hardpan, and the hardpan soil does not contain the water and nutrients plants need to grow.

Agroconection breaks hardpan soil down into five types.

  1. Caliche. This hardpan soil can be found in the Southwest and is the result of lime in the soil that binds the soil particles tightly together. Caliche hardpan looks light and chalky and generally has a high pH.
  2. Claypan. Claypan forms from a layer of soil with high clay content. It is sometimes found on top of a harder layer of hardpan soil. Claypan may soften when it is wet but is hard when it dries. Claypan prevents water from draining through the soil and makes it difficult for plant roots to get the water and nutrients they need.
  3. Fragipan. Fragipan is composed of a cemented layer of silt and sand. This type of hardpan is rock solid when it is dry but becomes brittle when it is wet.
  4. Ploughpan. Ploughpan is caused by moldboard ploughs that compress and compact the soil from the weight of the plough. It is typically a few inches thick and can be found on farmland.
  5. Traffic Pan. This hardpan can form from repeated foot traffic over some time. You may find traffic pan under footpaths, along walkways, and where repeated traffic occurs.

How do you know if you have hardpan soil?

Sometimes, hardpan soil can be seen on the surface if the topsoil has eroded over time, but it typically remains unseen and undetected until you try to prepare the ground for planting. There are several ways to identify hardpan soil. Here are some things to watch for.

  • Poor Drainage or Pooling Water: Hardpan soil prevents water from draining into the soil, which often causes pools of water to form on top of the topsoil after it rains.
  • Poor Plant Growth: Poor plant growth may be a sign of hardpan soil under your topsoil. If areas of your lawn remain patchy and lack good growth despite your efforts to water and seed the area, hardpan under the topsoil may be the cause.
  • Rock-hard Soil: If your rototiller struggles and bounces when you try to till the garden, you may think you have hit rocks, but hardpan soil could also be the culprit. Likewise, digging with a garden shovel or spade may be impossible in the area.
compaction from traffic on soil
Compaction from traffic on soil

How to break up a hardpan in soil

Breaking up hardpan soil can be challenging, but it is not impossible. Before attempting to break up hardpan, it is advisable to investigate and determine the hardness and thickness of the hardpan layer. Here’s how.

  1. Dig down to the layer of hardpan with a shovel. Remove enough soil from the area for you to access the hardpan for testing.
  2. Use a pick, crowbar, or other sharp tools to chip through the hardpan layer.
  3. Make a note of the depth of the hardpan to determine if breaking the hardpan up is feasible. Breaking up hardpan thicker than a few inches may be more work than is reasonable for the home gardener or homeowner that wants to plant a few flowers and veggies.
  4. Use a post hole digger or a chisel to break through the hardpan layer near trees or shrubs.
  5. Fill the hole with loose soil. This allows water to drain through the hardpan layer and may save your trees and shrubs.
  6. Hire a contractor with a backhoe, chisel plough or subsoiler to break up hardpan in a larger area.

What is the best tool to break up a hardpan?

Breaking up small hardpan areas can be done with an auger, pickaxe, or deep digging with a fork. A post-hole digger can be used to make holes through the hardpan layer to facilitate drainage in areas around trees and shrubs. A deep chisel plough or subsoiler on a tractor is probably the best option for larger areas. If you don’t have access to these machines you can hire a local contractor to do the work for you.


How to avoid a hardpan forming in soil

Hardpan forms when the particles in the soil are compacted and cemented together. You can do several things to prevent this from happening in your garden soil.

  1. Avoid working with wet soil. This means staying out of the garden when it is wet.
  2. Avoid heavy traffic or heavy equipment in the garden (especially when the soil is wet). Encourage family members to go around the garden instead of taking a shortcut through the garden every day. Excessive foot traffic along a path can cause the soil to compact.
  3. Plow or rototill the soil to a depth of 8 to 10 inches in the spring once the soil has dried. If soil sticks to the tines of the rototiller, it is too wet to till.
  4. Amend the soil with organic matter, such as compost or well-rotted manure.
  5. Add earthworms to the soil. Earthworms add nutrients with their casting and make holes through the soil, helping keep it well aerated.

Summary

Hardpan soil can be challenging to work with, but it can usually be managed. Unless your property has been exposed to excessive traffic or you have used a lot of heavy equipment in the area, your layer of hardpan soil is likely thin or limited to small spaces. Taking the time to break it up and amending the soil will typically improve it and reduce the risk of further compaction.