Maintaining a good soil structure is far cheaper than rectifying a damaged one. A good soil structure will reward a farmer, or a gardener, many times in increased production,ease of tillage and lower irrigation costs.
Soil structure refers to the grouping of soil particles into aggregates. These aggregates are commonly called peds. In topsoils peds are commonly spheroidal or polyhedral, and in subsoils they are commonly blocky or prismatic. Soils with no structure are described as being single grained when the particles are held together loosely and massive when they are compacted together.
Soils having well developed peds of moderate or high strength are relatively resilient to management impacts such as cultivation or stock treading. Soils having poorly developed peds are more prone to damage under inappropriate management.
Soil structure is described initially by the shape, grade and size of peds.
The common shapes of Peds are shown in this diagram.
The grade of Pedality describes the distinctiveness and degree of development of peds (single grain or massive for Apedal soils and weak, moderate or strong for Pedal soils).
Equally important to soil structure are the spaces between the peds or the holes in the soil. these are variously formed by worms, termites, plant roots and soil shrinkage during dry periods. The presence of Macropores or large air spaces is very important for transmission of water and to allow root growth. Soils with plenty of macropores are usually described as being well-aerated and permeable.
Good soil structure is important for -
- Soil aeration
- Soil temperature.
- Soil water movement.
- Soil biological activity.
- Soil strength and stability.
- Plant root movement in the soil.
The following example is a very powerful demonstration of the importance of maintaining good structure in your soil.
The example above shows a small hobby farm in the Manawatu region of the North Island of New Zealand. The farmer had purchased the property as a lifestyle choice but was determined to make the small farm profitable by growing maize crops to sell to his dairy farmer neighbours.
The photo on the left shows what the soil looked like under pasture before the farmer started growing an annual crop of maize every year for eleven years.
The topsoil had a crumbly very well aerated structure with its dark chocolate colour indicating lots of organic matter in the topsoil layer. The subsoil layer had a light grey colour and numerous orange mottles indicating the profile was subject to seasonal wetness from a rising water table. This was a superb alluvial soil on a low terrace near a large river and certainly very suitable for regular cultivation. The well drained nature of the soil meant that at the start, there was no problem getting the farmer’s tractor onto the paddock for seasonal tractor work.
The farmer wouldn’t have probably noticed these changes in his soil, even if he had been looking. Maize is a high nutrient demanding crop and growing continuous crops of maize on the area quickly altered the nature of the soil in the paddock and its structure.
Within four years of cropping, this is what happened to the soil profile. Much of the organic matter had disappeared from the topsoil layer which also had lost much of its structural character and was already changing to look more like the subsoil.
What he should have observed was a steady decrease in maize yields as all of the nutrients in the soil were depleted by the crop. What he did notice, and passed on to the Research Scientist Graham Shepherd (Landcare Research NZ) who took these photos, was that his tractor started getting stuck more often when he was working the paddock up for the next crop.
However, the farmer persisted in trying to grow more annual crops of Maize and after eleven years of continuous cropping, this is what his soil profile in his paddock looked like.
He had managed, through neglect of his soil structure, to convert an impressive topsoil into something that is almost indistinguishable from the sub-soil. He was also complaining of getting his tractor stuck in the paddock regularly and of very poor yields from the crop (quite understandable really!!).
Could the paddock be restored? This is highly unlikely because it requires a long period of time for recovery and a mountain-load of money.
When I show this example to groups of farmers throughout Australia and New Zealand, there is usually lots of collective gasps and righteous head shaking during the retelling of the story, but when I ask how many of them regularly monitor their own soil for soil structural deterioration, not a single hand goes up.
Detecting soil structural deterioration can be quite simple if you have a spade to dig a hole and a camera to take a photo of the soil profile. If you go back to nearly the same spot every year or so at the same time of year and dig another hole and take a further photo, in a few years you will build up a photo sequence of the dynamic soil structural change in your soil. Its a good fun exercise to do on a warm, sunny weekend and, I suggest you take your family with you when you do it. If you explain what you are doing (and why) to them, you will find that they will take ownership and greater interest in the process and in understanding your soils.
This next demonstration is another practical example of the benefits of a good understanding of your soil structure and properties. It is always important, when cropping your soil, to maintain it in a healthy condition by using good soil management practices. So, how do you tell if a paddock is in optimal condition to plough or cultivate?
Primary cultivation of a paddock should be done when the soils are not too wet; otherwise wetness prevents soil fracturing, causes smearing of soil particles into less moisture penetrable clods and creates thin plough pans that reduce infiltration of water.
To assess the suitability of your soil for cultivation, roll a small piece of your soil in your hand to form a thin worm, reducing the diameter by exerting pressure with your fingers in about 15 to 20 complete forward-and-back rolling movements.
Conditions are most suitable for cultivation if the soil cracks before the worm is made or if you cannot form a worm at all, for instance in sandy soils. The soil is too wet to cultivate if you can make a worm without cracking.
Some Australian and many New Zealand soils are known as “Sunday Soils”- too wet to cultivate on Saturday and too dry on Monday. Soil clods that are too dry, won’t break down with further cultivation to give any sort of fine seedbed, required for successful planting of many crops. If cultivated too wet many clods will simply deform and pancake under wheel traffic. This is because the adhesive forces holding the clod together are greater than the energy being applied to break the clods up.
Southland (NZ) farmers have a significant problem with soil vulnerable to structural collapse, particularly when there is a combination of prolonged heavy rains and vulnerable soils.
The photo above shows two highly productive Southland Soils. One is prone to soil structural collapse and the other is less so. Can you tell which one is? The problem was neither could most Southland farmers.
Because of this issue, as part of the Topoclimate South Soil Mapping Project, we devised a simple INDEX from the mapping data that we were gathering that gave a quick guide to farmers of the relative vulnerability of their soil to structural collapse. This Index was known as the Structural Vulnerability Index or SVI. The scale was shown as a range from 0 to 1; if your soil had a SVI tending towards zero, then it was less prone to structural collapse, if it tended towards 1 it was very prone to this problem.
This tool was a very simple but effective mechanism, based on some quality science and objective measurements, that farmers could use to improve their understanding of their soils.
The SVI’s for the two soils are now shown in the picture above.
For more information refer to the article Soil Structural Vulnerability Index or SVI.
