Last week focused on bulk density as a property of soil, and how lower values were associated with good soil structure and high organic matter content, while higher values were associated with soil compaction. We ended with the note that soils compact more when water is present. This post will cover the implications of soil compaction, and why it is something you want to avoid.

Soil pore space and bulk density are two sides of one coin. A high pore space means a soil is more ‘open’ and of a lower bulk density. Similarly, a low bulk density implies an ‘open’, airy soil.

From this we can see that the opposite also applies, in that a higher bulk density correlates to a lower pore space. And if a soil’s bulk density changes from a low value to a higher one, this is a good sign that it has become compacted. Compaction is usually from traffic, whether from foot, animals or heavy machinery.

Compaction, or compression, of a soil forces the individual soil particles together, which collapses the pores. Pore size, shape and number all reduce, and it is this change that has the flow-on effects on plant growth rather than bulk density per se.

One implication of compaction is lowered infiltration of water. If the pores in a soil reduce in size, shape and number, not only do they take up less water than before, but the soil also drains more slowly than before. In extreme cases, water not uptaken by the soil will pool on the surface, causing surface runoff and erosion.

This leads to reduced availability of water to plants. It follows that water not entering soil is also water not available to plants. Also related is that roots require more energy to access water in small pores. This is because of water’s surface tension and ability to cling to soil particles. The smaller the pore the tighter it holds water, and the more effort that is required by a root to extract that water.

Reduced air supply to roots is yet another consequence of compaction. Not only does water filling smaller and less numerous pores displace air in the soil, but smaller and fewer pores also increases drainage time. This can lead to waterlogging. Air cannot enter soils that are already filled with water.

Now, root cells do not photosynthesise, but respire like animal cells. Thus they need oxygen, and not the carbon dioxide above-ground plant cells use. Waterlogged soils become anaerobic (without oxygen). Without oxygen, both root growth and nutrient uptake will slow and eventually stop if such conditions persist. Prolonged waterlogging can result in root and plant death.

Soil strength increases under compaction. Much as we struggle to walk into a very strong wind, roots also struggle to push their way through compressed soil particles. If the wind is strong enough we will tire quickly, and probably give up. Roots too will give up if too much work is required. Pushing through requires a lot of energy and roots will always follow the easier routes through spacious soil pores if these exist. If a plant expends too much energy on root growth, there is less energy for leaves and flowers.

Plants in heavily compacted soil will tend to be stunted and pemanently exhausted, with a compromised root system attempting to support a compromised photosynthesising counterpart aboveground. And that compromised photosynthesising unit in turn will struggle to support a compromised root system belowground.

Plants in compacted soil tend to have a shallow and small root system, which makes them more susceptible to wilting. The roots are not able to penetrate widely or deeply enough to meet the full water and nutrient needs of the plant. A shallow and small root system may not be enough to anchor larger plants in the soil firmly enough, making them more vulnerable to strong winds.

And that concludes this whole section on soil structure! I do hope that you have a much greater understanding of what goes on beneath a soil’s surface, and how important a good structure is to overall plant health. You may even have recognised poorer areas in your immediate vicinity and have ideas on how to improve them?

I was always planning to move on to water movement in soil next, in a whole new section, but was recently asked to write about bore water especially. This a perfect opportunity to incorporate the two together and I’ll do my best!