Disclaimer: The views and opinions expressed in this article are those of the author and do not necessarily reflect the views of the Arboricultural Association.

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Too much soil in your soil

Author:  Mark Roberts
Last Updated:  21/11/2018

As I write this, I’m flying back from the UK after attending the Arboricultural Association’s annual conference; Soil and Trees. It was a fantastic conference, well worth the effort and it was truly an honour to be able to speak at it.

Soil compaction was a recurring theme, and so it should have been. The ideal soil is a mix of 45% soil particles, 5% organic matter and 50% air and water – based on that the bulk of an ideal soil, isn’t actually soil at all. So what happens if you have too much soil in your soil?

When a soil has been squashed (or mercilessly pummelled), the space for air and/or water is reduced, the soil is said to be compacted – effectively, you end up with too much soil in your soil.

Compacted soils are far too common in urban areas and one of the main reason for surface flooding and sediment loss in agricultural land. As far as agriculture goes, we are literally washing our soil out to sea and polluting the oceans while we do it (we are flushing our country away). In urban areas, there are 101 reasons that a soil becomes compacted, some natural and some… not. But why is having too much soil in your soil a bad thing?

Starting with the most obvious; it’s just hard – too hard for roots to grow. At the conference, there was some marvellous time-lapse photography of roots growing through soils at various degrees of compaction. As you would expect, as the soil became more compacted, the roots ability to grow through it reduced and kept reducing until they just couldn’t grow at all. No surprises there, but the footage nicely dispelled the myth that roots can grow through pipes and into foundations – they can’t. But compacted soils do more than just slow or stop root growth.

In a compacted soil the amount and size of micro-pores are greatly reduced. Macro-pores are the spaces around soil particles where a film water exists – roots are alive and need to drink. OK, saying roots breath and drink is a gross oversimplification – but I have been flying for a very, very long time. For soil nutrients to get into a plant, they need to be suspended in water (the soil solution). So with less water, nutrient availability becomes limited. But compacted soils do more than just reduce the roots ability to take up nutrients.

The nutrients absorbed through the roots are moved up and through the plant via the xylem (one half of the vascular tissue). Nutrients are needed wherever metabolic processes take place; almost everywhere. With limited access to water the plant’s ability to move nutrients to where they are needed is also limited. But compacted soils do more than just reduce the plant’s ability to move nutrients.

Photosynthesis requires a stack load of water (6:1, water to carbohydrate), with limited access to water the plant’s ability to photosynthesise is also limited. But compacted soils do more than just reduce the plant’s chemical ability to photosynthesise.

For a leaf to achieve its maximum photosynthetic area it must be rigid. Leaf rigidity is maintained through hydraulic pressure (turgor pressure), with limited access to water the plant’s ability to achieve the maximum photosynthetic surface is also limited.

But compacted soils do more than just reduce the plant’s ability to create and maintain turgor pressure. The carbohydrates that are made through the process of photosynthesis are moved down and around the plant through the phloem (the other half of the vascular tissue). Carbohydrates are needed where cell division takes place, where growth happens. With limited access to water, the plant’s ability to move carbohydrates and grow are also limited. But compacted soils do more than just reduce the plant’s ability to grow.

In a healthy soil, a non-compacted soil, there should be a mycorrhizal connection between plants and fungus in the soil (mycorrhizal fungi). The fungi have the ability to extract, move and/or convert nutrients in a soil that the plant may not be able to access. The plant ‘gives’ the fungi carbohydrates and the fungi ‘gives’ the plant access to nutrients. Without the mycorrhizal fungi, the plant may not be able to get all the nutrients that it needs, without the carbohydrates, the fungi may not be able to get all that it needs. But compacted soils do more than just limit mycorrhizal fungi and nutrient access.

Mycorrhizal fungi can also form a connection between plants. By being connected, if something is happening to one plant, it is also happening to the other connected plants (albeit to a greatly lesser extent). When the first plant responds to the attack (whatever that attack may be) those responses can be signalled through mycorrhizal connections to the other plants and those plants, in turn, can also prepare for a potential attack. Mycorrhizal fungi can create an early warning system – being forewarned is being forearmed. But compacted soils do more than just limit the effectiveness of the plants early warning system.

Worms. In grazed, fertile pastures the weight of earthworms can easily exceed the combined weight of the animals grazing on it – in the ideal soil, there can be a lot of worms! Worms mix and drag organic matter into the soil, they provide pathways for water and space for air – they create habitat where other soil organisms live which in turn creates habitat for other soil organisms. Without the worms, organic matter on the soil is not readily mixed into the topsoil and further broken down into the subsoil. In a compacted soil there are few worms. But compacted soils do more than just limit the number of worms and the mixing of organic matter.

Organic matter also binds soil particles into aggregates and improves the water holding capacity and… its back to the start with issues when water is limited and nutrients are restricted and the types of things that live in the soil are affected.

At the conference, there was a nice comment about 80 to 90% of all tree problems being directly attributable to the soil. I’m not a numbers person – but it would seem that we can reduce tree problems by simply reducing the amount of soil in our soil.

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