Soil structure can be described as the arrangement of soil particles (clay, silt, sand and humus) into aggregates, also called crumbs or clods, and the resulting air and water spaces in between those aggregates.
The structure of a soil may range from well-developed to poor and this describes the degree of infiltration rate of water and aeration of a soil.
These bonded larger particles, aggregates, have larger pore spaces and fissures between them than the individual soil particles, this allows for increased movement of water, air, plant roots and soil microorganisms.
Soil structure can be damaged by cultivation and by exposure to the force of raindrops.
Soil structure — aggregates of soil particles within a soil.
Advantages — increased water infiltration rate, greater soil aeration and increased organic life in soil.
Damaged by — cultivation, exposure to the force of raindrops.
Improved by — little or no cultivation, maintaining plant cover, adding organic mulches onto soil surface.
Climate change — release of carbon into atmosphere as soil structure deteriorates; or fixing of atmospheric carbon as soil structure improves with increased humus.
The tilth of a soil is another term often used in relation to soil structure, this results from tilling or cultivating the soil, and refers to a soil’s friability or crumbliness, whether a soil will run freely through your fingers.
Tilth also includes the degree of stickiness and plasticity when wet as a result of high clay content.
What is called a good, fine soil tilth provides an even, small aggregate size soil bed in which to sow seed or plant seedlings but the action of cultivation to achieve a fine tilth, damages the soil structure by mechanically breaking up those aggregates.
When I went to Urrbrae Agricultural High School in 1970 for my secondary education, in our Agricultural Science subject we were taught about tractor ploughing speeds, using a mould board plough, of a half to one mile an hour (metric was introduced in Australia in 1966 but imperial measurements remained in use for some time), these slow speeds were aimed at protecting the soil structure from physical impact. Then we were taught about the rotary hoe with the tractor power take off, PTO, having speeds of 540 to 1000 rpm, which reduced through the rotary hoe’s gearbox but still in the 250rpm range, which was completely contradictory to slow ploughing speeds but did achieve a fine tilth.
An example of this is the repeated use of the rotary hoe on market garden lands growing vegetable crops, with blades rotating at around 250rpm. This high-speed impact provides a fine tilth for planting but degrades the soil structure. Some market garden lands in the Adelaide Hills of South Australia can no longer support economic vegetable growing because of the rotary hoe. They have since been planted with wine grapes which are, of course, perennial and deep-rooted needing little or no cultivation and allowing the soil structure to slowly improve.
The removal of covering plant material can also damage the soil structure by exposing the soil to the force of raindrops. Try facing up to the rain in a heavy rainstorm to experience the power behind each droplet. Plant cover can be living vegetation or a layer of organic mulch.
Organic matter in the soil improves a soils structure by increasing the bonding between sol mineral particles. This occurs through the decomposition of dead plant material and animal manure by bacteria and fungi and in the process enabling the growth of biofilms and fungal hyphae which along with chemical bonds help soil particles to flocculate and form soil aggregates.
Decomposition of organic matter eventually results in the making of humus, a brown or black colloidal substance that cannot be broken down anymore and is a great benefit to the farmer and gardener as humus holds water and has a high cation exchange capacity meaning that it will bond with nutrients, preventing them being leached away and ready for absorption by plant roots.
The decomposition of organic matter is most often present very near the surface of the soil and this shouldn’t be cultivated into lower soil levels, let earth worms do the work of aerating and taking down soil nutrients. The soil surface biological activity should be helped by the addition of mulch or the planting to provide a cover and source of more organic matter.
The other lesser known product of organic matter decomposition are the small vegetative fibre particles that contain a lot of cellulose and as such are resistant to digestion by bacteria. These particles remain in the soil for a long time providing the service of water holding and preventing fine soil mineral particles forming impervious layers, as they mix with those fine particles keeping them apart, to allow drainage and aeration. This is of particular use on sandy and silty soils when organic matter is provided on the soil surface as a living or non-living organic mulch.
Soil structure which represents the degree of water and air movement through a soil, is affected by the amount or absence of tillage, the degree of vegetative cover and the amount of soil organic matter present.
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