Is Your Soil in Good Physical Shape?

Soil health is broken down in to three components - physical, biological and chemical. Each component is important, but the physical component is one that can have a significant impact on yield. Soil physical aspects break down into two main areas:

  • soil structure, including aggregate stability, and
  • available water capacity.

Aggregate Stability

Aggregate stability is a measure of how well soil aggregates or clumps of soil stay together when they are subjected to a test with water or simulated rainfall. Soil aggregates with good stability will largely stay intact when exposed to the test. Soils with poor aggregate stability will breakdown into individual soil particles. (Figure 1). So what are the implications of aggregate stability in the field? A soil with poor aggregate stability is prone to water erosion. When raindrops hit the soil the individual particles are easily broken away and washed down the slope and potentially off the field. The same is true for wind erosion. Soil particles are more easily broken off and blown with the wind. Poor aggregate stability increases the incidence of crusting. The rain hits the soil surface breaking the aggregates apart and spreading them across the soil surface filling in the soil pores. When the soil dries, it is a solid layer that slows water movement into the soil and makes it difficult for the crop to emerge through. (Figure 2).

Soil structure refers to the way in which soil particles cling together to form aggregates. When individual soil particles are aggregated, they form larger, relatively stable primary structures. Structure also encompasses how soil particles are bound together and the spaces or pores in between. Structure has a significant effect on soil water properties and the ability of a soil to resist erosion. Good soil structure increases porosity, aeration, drainage and permits easier root penetration, all of which are important on soils with limited internal drainage, such as clays. Conversely, poor soil structure in the topsoil produces hard, massive clods, which makes a poor seedbed for germinating crops. Poor structure in the subsoil results in dense, compact properties which limit root and moisture penetration.

Clay particles tend to cling tightly together and resist separation more than sand particles. As organic matter decomposes to humus, a variety of compounds are released which "glue" soil particles together. Soil structure is formed through the actions of:

  • drying and wetting or shrinking and swelling, this creates cracks and planes of weakness for roots
  • freezing and thawing
  • roots through the removal of water, root exudates or organic materials that bind soil particles and the formation of root channels
  • soil animals like worms and beetles, moving soils, mixing and releasing organic materials
  • microorganisms breaking down plant and animal residues
  • tillage - reduces the size of aggregates, an artificial way of creating structure in some soils, generally not long lived.

Tillage

Tillage breaks down soil structure, so minimum tillage or no tillage is the best. Increasing organic matter in the soil through the use of manures, other organic materials such as compost, a diverse crop rotation and cover crops will improve soil structure.

Compaction

Soil compaction is a part of the soil profile that has poor structure. The compacted layer lacks good structure, making it difficult for roots to grow through, (Figure 3) and for air and water to move through. Taking measures to minimize compaction, such as keeping axel loads below 5 tons per axel, reducing traffic in the field, making sure the soil is fit and using radial tires and reduced tire pressure, will help prevent the problem.

Water Holding Capacity

Plant growth depends on the water stored in the soil. Available water capacity is a measure of the soil water that is available to the plant. Water is stored in the soil in the pores and in organic matter. Adding organic matter, such as manures and composts, is a good way to improve the water holding capacity of sandy soils.

Soil that is in good physical shape will have good soil structure. A well-structured soil will allow water to move through the soil, air to enter the soil, will not crust and favours good root growth. (Figure 4). Minimizing tillage and compaction, using a multi-crop rotation including cover crops and adding organic materials such as manure and composts, will greatly improve the structure of the soil.

similar sized aggregates in petri dishes

Figure 1: Two similar sized aggregates were placed in these petri dishes of water for 5 minutes. The soil on the left has very good aggregate stability and the one on the right is poor.

Photo of a seedling emerging from dry crusted soil.

Figure 2: Soil crusting due to poor aggregate stability can impede emergence and significantly reduce crop stands.

Photo of a clump of soil showing roots

Figure 3: The roots went down until they hit the compacted layger and then went horizontal. The resulting shallow root system will suffer moisture stress during dry periods.

Photo of a hand holding well-structured soil.

Figure 4: A well-structured soil allows for easy rooting and water and air movement through the soil.


For more information:
Toll Free: 1-877-424-1300
E-mail: ag.info.omafra@ontario.ca