What is soil?
Soil is a key part of gardening but most people know very little about it, in reality soil is much like a complex organism, make up of interlocking parts that changes in response to its environment, it even has a microbiome like us and other animals that changes over time and keeps it in good condition. Soil can become ill by drying out
or an imbalance in its components, it can even drown by flooding, being deprived of oxygen or become depleted and starve due to a lack of nutrients.
Soil is the medium that plants grow in, an established tree can have roots 1.5 to 3 times the size of the canopy and a surface area vastly more than that. It holds the water and nutrients plants need to grow, like a battery and it provides support to stop them falling. Soil is arguably behind light, the most important thing in the world to plants.
That said, what makes up soil?
Minerals:
Soil is normally about 45% minerals, these change depending on what the soil developed on and consist of Sand, Silt and Clay. It is how the soil stores the vast majority of its nutrients and water but also it’s structure and how it responds to working.
Bristol is built on Limestone and Mudstone, this erodes largely into Clay and Silt.
Organic Matter:
Organic matter is what differentiates pure soil and rock. It consists of two main types, Humus (stable) and decaying (unstable). Both of these are mostly formed from dead plant matter that decays down.
The humus is very long lasting, it has decayed down as far as it can go and acts as the glue that holds soil together, it is responsible for holding a lot of the water and nutrients that don’t attach to minerals well.
The unstable organic matter is not fully rotted down and keeps decaying, this means it releases nutrients constantly as it breaks down further, charging the soil with nutrients essential for plant life. It’s why you don’t need to constantly feed soil to let plants grow. It’s also why plants that are cropped need more feeding, nutrients are being taken away rather than breaking down in the soil.
In mineral soils, organic matter is fairly low, between 2-29%. These soils are what most people would expect to have. Mineral soils are extremely durable and less prone to damage than organic soils.
In organic soils, there is 30% or more organic material, these are generally found in peatland and very wet areas. They are extremely fertile and tend to hold nutrients and water well. They are generally more productive in agriculture but also tend to deplete and become damaged when they dry out too much.
Bristol is a Mineral soil, so very durable.
Pores:
Just like in skin, the soil has pores, these make up to 50% of the soil volume in some cases. Pores are important because that’s how Water and Air move through the soil. If there is not enough pore space the soil becomes compacted, if there is too much, the soil struggles to retain water.
Macropores are large pores that plant roots can access, they are how the plant gets water from the soil and it allows the soil to drain, these are more common on sandy soils and less common on clay soils. Plants establish quicker when the soil is high in macropores but the soil drys out faster as well.
Micropores are pores that are too small for water to escape via gravity or roots to penetrate, they are how the soil retains water and why clay soils hold more water than sandy soils. However an over abundance of micropores means the soil is prone to being waterlogged and hard for roots to establish in.
Bristol is predominantly clay based so tends to have more micropores than macropores. This means it is slower to dry out but harder for plants to establish and harder to work.
Micro-organisms:
The soil also consists of micro-organisms, just like our gut bacteria, they are responsible for breaking down of organic matter, releasing the nutrients, fixing nitrogen from the air for plants to use, even some symbiotic fungus that attaches to plant roots and help them infiltrate micropores for nutrients and water.
The exact contents of the soil changes constantly and depends on what is available to them. In a healthy soil then they use aerobic respiration and this provides a lot of benefits to the plants, they break down organic matter well and help plants grow. In waterlogged soil then instead anaerobic respiration causes poisons and toxins to build up which can badly damage already stressed plants. This is why consistently wet soil can smell sour.
Most healthy soil will contain an abundance of micro-organisums and need little help in that regard, to improve a depleted soil, add organic matter, either manure or simply planting in there will help.
What is Bristol soil?
The quickest easy way to test your soil type is here: http://www.landis.org.uk/soilscapes/, looking at Henleaze for example, we have a clay loam.
Clay based soils have:
High fertiltiy: As they have more micropores than macropores, clay is good at retaining nutrients.
Poor drainage: As above, they retain water, this can cause waterlogging but also stops the soil drying out too fast.
High Stability: Clay soils are very dense and structural, when a plant has rooted it is well supported.
Slow rooting: As clay is very dense, rooting may take longer than a lighter soil but will form a better root system because of it.
Poor workability: Clay soil is easily damaged by working or walking on while wet, this collapses macropores into already dominant micropores and should be avoided.
Improvements:
To improve a clay dominant soil, add organic matter as this helps open up the soil, increasing rooting speed, drainage and structure. This can be done by adding well rotted manure (stable organic matter) or by planting in it. Do not add compost as in best case, it will achieve nothing, at worst it will degrade in the soil causing the soil to shrink.
COMPOST CHEAT SHEET
Type | Use for |
Multipurpose | seeds, pots (3yr or less), annuals in pots, annual fruit and veg, DO NOT ADD NUTRIENTS, DO NOT EXCESS AMOUNTS OF SOIL. |
50L Ericaceous | Ericaceous plants in pots |
John Innes Seeds | Seeds |
John Innes No1. | Cuttings, seedlings |
John Innes No2. | Small shrubs. Shrub and trees for winter planting, most fruit and veg, Pots 3 yrs or more DO NOT add nutrients |
John Innes No3. | Mature trees or shrubs, Pots 3yrs or more. |
John Innes Ericaceous | Ericaceous plants in the ground DO NOT add nutrients |
Topsoil | Raising soil level, providing soft layer for seeds, mixing with manure compost for a raised bed. |
Manure compost | Improving moisture retention, improving nutrient retention. breaking up clay, improving soil quality. DO NOT direct plant DO NOT add nutrients, mix well with soil before use. |
Bulb fibre | Pots with no drainage for bulbs |