What is soil structure?
Soil particles are bound together by organic matter, plant roots, bacteria and fungi, to form larger clumps.
Humus and the exudates of soil organisms are ‘gummy’. They form the strongest and longest-lasting bonds to hold soil particles together to form aggregates. The shape and size of these aggregates determines the ‘structure’ of soil.
If there are continuous pores between the aggregates soil fauna can move around easily and water and oxygen can shift in and out of soil faster.
The stability of the aggregates is important too. As they have to stay together through winter and summer, during hard rain and under traffic.
How to test your Soil Structure?
- Place a handful of soil crumbs in a tumbler
- Carefully pour enough water into the glass to cover the soil
- A good, stable soil will hold together even when the crumbs are wet.
- Poor structured soil will fall apart
Sand
- Feels gritty between your fingers.
- Wet sand can be rolled into a pencil-shape, but will not retain the shape.
- Water disappears into sandy soil fast, and plants wilt quickly. Must water plants often.
Loam
- Feels smooth and silky between your fingers.
- Moist loam can be rolled into a pencil-shape and will retain the shape.
- Water disappears into silty-loam soil quickly. Holds water better than sand.
Clay
- Feels very smooth between your fingers.
- Moist clay can be rolled into a pencil-shape, will retain the shape. With high-clay content the cylinder can be carefully rolled into a ring without breaking.
- Water holding capacity is high - can leave longer between watering.
I encourage you to use your spade as a diagnostic tool. As gardeners, getting to know our soil is fundamental. Most plants when plucked from soil do not survive long. We are all aware that the mineral and organic elements of soil provide plants with essential nutrients, yet this is only a fraction of what plants need from soil.
Plants function with distinct tops and bottoms, soil gives roots anchorage. The soil provides the water and oxygen roots need to respire. Soil acts as insulation against temperature extremes. For our plants to prosper, we need to insure all these needs are met.
Soils consist of mineral particles and organic matter, with the pores between filled with either water or air. The variety of soils reflects the distinct parent material, vegetation and climate in which they were formed. Soil texture We categorise soils according to their texture. Soil texture is based on the particle size of the mineral elements.
Moisten some of your soil and rub it in the palm of your hand. A sandy soil has larger particles and will feel gritty. If you attempt to roll it into a sausage it will crumble and have no plasticity. A clay soil has very fine particles, will feel sticky or slippery and will form a sausage when rolled.
Between the extremes of sand and clay lie loams that possess a balance of clay and sand particle and silts composed of fine, silky particles.
Sandy or light soils are easily dug (lifting potatoes is a joy) and they are free draining. Conversely, they retain little water, are leached of nutrients and easily eroded by wind or rain. Clays have the advantage of retaining water for long periods, likewise storing high amounts of nutrients that are available to plants. However, as any gardener with a bad back will tell you, they are hard to dig, prone to water logging in winter and rock hard in summer.
The texture of our soil is difficult to alter without large inputs. It is possible to improve a sand by adding ¼ volume clay to the top soil but to improve a clay to loam would require adding more than 60% sand.
The aspect of the soil we can best tackle is the structure. Soil structure is the way the individual mineral and organic particles of the soil group together.
Humus is key to the formation of soil structure. Humus is that part of soil organic matter that has decomposed as far as possible and become stable; it is the ultimate carbon sink. It is a dark sticky substance that gives good topsoil its rich colour and earthy smell. It acts as glue grouping soil particles into crumb. The colloidal chemistry of humus means it makes nutrients available to plant roots and improves water retention.
As well as humus, mycorrhizal fungi are the living elements of the soil essential in forming pores between particles that allow the roots to receive water and oxygen; here the earth worm is the hero.
A good garden soil can be almost half pore space. From the plant's perspective, a well structured soil has a crumb that is stable and doesn’t breakdown in rain. It is loose enough to allow roots to grow and water, nutrients and
oxygen to penetrate; if we squeeze a hand full of well structured soil it should noticeably reduce in volume.
It has fine aggregates, good tilth rather than large clods. A poorly structured soil collapses in rain and forms crusts, it is grouped in large clumps or sheets, it is hard to dig. It is inclined to water logging in winter and erodes easily. Some of the poorest structured soils are the silts often bought in as top soil, they appear rich and fluffy when first applied but quickly compact to form a hardpan.
Be very wary of buying in top soil. Unless carefully formulated it is often best, especially in environmental terms, to work with the existing soil and improve it or select plants adapted to it.
While good garden soil is perfect for growing plants in the ground, it is usually inadequate for growing plants in containers. When plants are grown in pots both their drainage and aeration are compromised by the confines of the container. To achieve good aeration and drainage most growing media are soil-less and based on sands or other inorganic elements of fairly coarse particle size combined with organic matter, pine bark or peat, to increase water and nutrient holding capacity.
The high organic content of potting mixes also helps to keep them light. The qualities of good drainage, aeration and high organic matter also have a downside. Growing medias drain so well they dry out easily and leach nutrients, and the high organic matter encourages fungi that produce waxy substances that repel water, making the mix hydrophobic.
To compensate, various water holding gels and wetting agents can be used. As home gardeners we need to be aware that our pot plants will dry out far more quickly than plants in the ground. The extreme end of soil-less gardening is hydroponics, where all nutrients are provided in solution and the media becomes an inert free draining substance like Rockwool. From an organic perspective it is very hard to replicate the full suite of nutrients provided by a living soil.
This article won't give you all the answers about soil structure but hopefully it does is encourage you to ask questions about your own soil and to ask those questions with a spade in your hand. Go out and have a dig around your garden, have a feel of your earth.