SOIL DENSITY
OBSERVATION DENSITY SUITABLE FOR
Soil can be scooped up easily by hand Very low, fine state of tilth All forms of annual horticulture, root and other vegetables, floristry and seedlings
A hand trowel can be pushed in easily Low Most forms of annual horticulture, fruit and nut trees
A hand trowel can be pushed in with some effort Low to moderate Long-lived annuals and perennials, fruit and nut trees
A spade can be pushed in with some assistance from the foot Moderate Perennial crops, fruit and nut trees
A spade can only be pushed in by jumping on it Moderate to high Fruit and nut trees, shade trees, lawns
A fork can be pushed in with some assistance from the foot High Shade trees, windbreaks, rough grasses
A fork cannot be pushed in except with extreme effort Extreme Rough grasses, weedy forbs, highly tolerant trees
If there is no combining of soil particles into peds, then the soil exists as single grains and is said to be without structure, or ‘apedal’. Loose sand is the perfect example of this, as it has very good aeration but an extremely poor water-holding capacity.
In heavy clay with very small particle sizes, the only way air can get into the soil is if the clay is structured, allowing cracks and larger pore spaces. The more cracks and pores the clay has, the better the aeration will be. Clays with no structure are among the worst for erosion and the restriction of plant roots, while well-structured clays, such as the black earths of the Darling Downs in Queensland, are among the very best growing soils in the world. Well-structured clays combine the good aeration of sand with the water-holding capacity of clay, providing the perfect mix of air, water and solid support.
An unstructured growing medium such as mangrove mud has no air at all. Mangroves can live in it because they put up little snorkel-like roots (called pneumatophores) to allow air to enter the rest of their root systems. Other plants that grow in unstructured waterlogged soils, such as rice, have an aerated structure along their roots, like foam rubber, that allows air to get to their roots. It’s not that these plants tolerate low oxygen levels – they just have ingenious ways of getting oxygen to their roots other than through soil pores.
Vegetable crops derive their nutrients largely from the topsoil, where water, oxygen and mineral levels are optimal.
Stability and dispersion
A well-structured soil is useful, as it admits air and water into the profile, but how stable is the structure? Will it collapse easily into sludge when you start working and wetting it to create your urban farm? This is a common problem in new housing estates, where natural soil has been disturbed and poorly structured clay is brought to the surface.
The collapse of soil structure upon wetting is known as ‘slaking’. When unstable soil is cultivated and then receives rainfall or irrigation, it turns into mud that won’t conduct air and water. Even worse is ‘dispersion’, where fine particles of clay become suspended in water and won’t settle out. A muddy brown river or dam indicates dispersive clay. This is one of the worst soil scenarios to encounter, because once the soil disperses it will set hard upon drying and be difficult to transform back into a well-structured soil.
Testing a soil for the stability of its structure is not difficult, particularly with loams, clay loams and clays. Take some air-dried peds that are 5–10 millimetres in diameter, and place them carefully in a jar of clean water. Observe their behaviour in the first minute or two.
Classes 1–6 peds slake – they fall apart into a little pyramid of mud.
Class 7 peds are stable – they swell but don’t fall apart much.
Class 8 peds are highly stable – there is no observable change except in colour.
Slaking is undesirable, as it indicates that a soil is prone to collapse when wetted, but most soils will slake at this stage. Classes 7 and 8 are the most desirable form of peds, and they are usually associated with high organic matter, which binds soil particles together.
Now leave the peds in the water for half an hour. What changes have occurred?
Class 1 – the water surrounding the ped is completely muddy. This indicates highly dispersive soil.
Class 2 – there is a muddy ‘halo’ around the ped. This indicates an unstable dispersive soil.
Soil structure can be seen by grabbing a handful of soil and shaking it a little – it should fall apart into aggregates.
SOIL PORES
Water is held more firmly by the soil in fine pores and is released more easily from coarser pores. The finer the pores, the higher the water will rise up in the soil from a water table. This is easily demonstrated by purchasing two sponges from the supermarket, one with coarse pores and the other with fine pores. Wet them up with some coloured water, and then place them on their side to drain. The finer sponge retains more coloured water – showing a higher water table or ‘capillary fringe’ – than the coarser sponge. Remember also that the coarser the pores, the more air will enter once the water has drained. This is called ‘air-filled porosity’.
In the middle is dispersive clay. As seen on the left, organic matter coagulates clay particles; on the right, gypsum does the same. The most fertile soils combine organic matter and calcium to create structure.
CLAY, CLAY – GO AWAY!
If your urban farm has a clay soil, don’t despair! It is often said that clay soils are difficult to work with, but in reality all the world’s most fertile agricultural soils are well-structured (well-aggregated) clays or clay loams that combine the magnificent water-holding capacity of clay with the perfect drainage of sand. The secret is well-behaved clay. A badly behaved dispersive clay soil sets like concrete, while a well-behaved one – which is full of organic matter and plenty of calcium – breaks up or crumbles into aggregates. This means the soil allows in air but also retains moisture. If you have a badly behaved clay soil, assess its structure and then work out a plan to improve it.
Dispersion of clay is undesirable, as it indicates a soil that is highly prone to erosion. Such dispersive clay soil will usually be low in organic matter and light in colour (commonly either white, yellow or yellow–brown). Dispersive soils will respond to gypsum.
If the water around the peds in the jar is not muddy after half an hour, move on to the next step in the testing process. Shake the jar hard, then set it down and let it settle for 10 minutes.
Classes 3–5 – a significant proportion of the soil remains dispersed, leaving the water muddy. This means the soil will be dispersive in running or high-energy water. These kinds of soils are likely to seal up during periods of rainfall or irrigation.
Class 6 – the water clarifies completely. The soil may slake, but it is not naturally dispersive even if provoked.
Dispersion is caused by too much sodium and not enough calcium in the soil. Sodium causes clay particles to repel each other when suspended in water.
Garden soils should always be Class 6 or higher. A little slaking is acceptable, but they should never disperse, even if vigorously shaken in water. These soils are always dark in colour, with plenty of organic matter ‘gluing’ the soil particles together. They should never be sodic (in other words, full of sodium), and will always contain sufficient calcium to prevent dispersion.
Basically, dispersion is caused by too much sodium and not enough calcium in the soil. Sodium causes clay particles to repel each other when suspended in water, whereas calcium causes them to coagulate together and drop out of suspension. This is why gypsum (calcium sulphate) – affectionately known as the ‘clay breaker’ – is used to improve dispersive soils. It displaces the sodium and causes the clay particles to coagulate. Highly organic soils are very unlikely to respond to gypsum, because the organic matter prevents the clay from dispersing.
Test for gypsum responsiveness
If your soil shows any dispersion (Classes 1–5), then you should establish if it would respond to gypsum. Take two jars, and place a tablespoon of soil in each one. Drop a sma
ll pinch of gypsum (or plaster of Paris) in one jar, and fill both jars with water. Place a lid on both jars, and then shake them vigorously for about 5 minutes or until the soil is thoroughly dispersed. Set them aside for an hour.
If, after an hour, the soil in the gypsum-treated jar has completely coagulated and the water is clear, but the untreated jar is still muddy, then the soil is gypsum responsive. If the untreated jar is only slightly muddy, use around 100 grams of gypsum for every square metre of soil in your garden.
If the untreated jar is so muddy that you can’t see through it at all, then you will likely need up to 500 grams of gypsum for every square metre of soil. If the untreated jar is muddy but you can see through it, then you will likely need between 100 and 500 grams per square metre. Gypsum is best dug in, but it is not entirely useless if it is left on the surface to incorporate slowly.
Structure in Artificial Soils
Even when making artificial soil mixes for rooftop gardens and containers, we need to consider the basic principles of physical soil fertility. Therefore, we have to design the mix so that it has highly granular components, which – despite having no structure – are very well aerated. Ingredients such as pine bark, sphagnum moss, perlite and horticultural ashes (with a large particle size) all hold water well within the particles, but allow water to drain out and air to enter between the particles. In this respect, we are mimicking a strongly structured soil with its good balance of fine and coarse pore spaces.
The aggregate stability test. On the left the soil is slaking (falling apart), in the middle it is stable and on the right it shows dispersion.
Encouraging good structure
In natural soils, good structure develops if you have the following situations:
wetting and drying cycles that cause shrinking and swelling, thereby creating cracks and structure
high organic-matter levels that also cause shrinking and swelling, and bind soil particles together to form peds
the right balance of clay minerals that shrink and swell (if you don’t, then you can amend your clay to improve its behaviour)
the correct level of calcium (see the test for gypsum responsiveness)
restricted traffic and compaction.
Most important of all is a regular application of organic matter that stimulates biological activity as well as the soil organisms (such as worms and ants) that create pore space in the soil. This will greatly reduce your need to cultivate your soil (a practice that tends to destroy structure).
The structure of soils can be ruined or diminished by bad practices. The following situations do not encourage good structure and porosity:
compaction by stock, traffic and pedestrians
waterlogging that prevents the much-desired shrink–swell cycle that leads to cracking and structure
low organic-matter content that also reduces shrinking and swelling, which in turn decreases biological activity
constant or repeated vigorous ploughing (excessive rotary hoeing is a common culprit here).
Ploughing or digging to fix the density and ‘fluff up’ a soil creates a temporary increase in pore space. However, if it is repeated too often – especially when there is insufficient organic matter, so there is a tendency for the soil to disperse – then the soil structure collapses like a failed soufflé.
Soil texture
The mineral fraction of a soil consists of three different particle types differentiated by size: sand, silt and clay. Soil texture is a way to quantify what sort of soil particles you have and in what balance they are found.
It’s important to know the texture of a soil, because middle-of-the-road textures such as loam and clay loam tend to be the easiest soils to manage. This does depend on the structure, of course, as only well-structured soils have the necessary range of pore sizes for good aeration and drainage as well as good water-holding capacity. Consequently, soil texture combined with structure is a measure of the mix of pore sizes and the proportion of water and air that is held.
Texture is linked to a soil’s ability to hold water (for example, sands hold little water and clays hold a lot). However, when it comes to growing plants, it gets a bit more complicated, because some soil textures won’t release their water to plants readily even though there is a lot of water in the soil. Clay, for example, holds up to 60 per cent water by volume, but it won’t release all its water to any plant growing in it because it has such fine pore spaces (capillaries). As plants start to extract water, the difficulty of obtaining more water increases until it gets to the point where the plant cannot get any more. This is called the ‘Permanent Wilting Point’ for a pretty obvious reason – the plant wilts and won’t recover. Think of squeezing out a wet sponge. The first gentle squeeze releases most of the water, a firm squeeze discharges some more, and by twisting it tightly between two hands you can get the last drops out – but you will never get it completely dry by squeezing. There is water there, but you – like the plant in clay soil – simply haven’t got the strength to get it out.
In natural soils, the texture of the subsoil will be ‘heavier’ (in other words, more clay) and less well structured than that of the topsoil, because fine clay particles easily leach out of the topsoil. This is true for all but the deepest coastal sands, where there is no clay. So, if you have a natural soil with crumbly clay loam in the topsoil, you might expect to have a fairly heavy, plastic and poorly drained subsoil.
To see the textures of your soil layers, dig down to a depth of around 600 millimetres (if your soil is deep enough). This is not easy; a post-hole digger will help, otherwise dig a 600-millimetre square pit to get to the 600 millimetres in depth. This is all the soil depth most crop plants will need, fruit trees included. Note any changes in colour and texture, or if rocks appear.
As you dig down into natural soils, you will usually see less of the dark colours and more of the ochre colours of iron oxide emerging. The structure will be less obvious, and there will be none of the fine crumbs we usually see in our topsoil. This lower part of the soil profile may even form into coarse blocks or lumps of clay. This is not good, and may need correction. Remember, texture will nearly always get ‘heavier’ with depth in natural soils.
In urban soils, you might not find the normal profile of A, B and C horizons. There might be layers of clay alternating with layers of sand and loam. This is evidence of human impact, and it often occurs on building sites. Layering in this way is not good for water movement in the soil profile. If the layering is severe enough, consider homogenising the top 300 millimetres of soil by deep digging.
The new clay soil at the London Olympics site is only five years old, but it is already showing topsoil humus development. The topsoil is now stable, and the subsoil has good structure.
Colour is a guide to a number of important soil properties. On the left, the yellow hue of this subsoil indicates poor aeration at depth. In the middle, this bright reddish subsoil has good drainage. On the right, this topsoil has a darker grey colour due to its superb organic-matter content.
Tests for soil texture
The simplest and, with a little experience, the most accurate way to assess soil texture is the hand-texture method. Obtain about a cupful of the soil layer being tested, remove any obvious stones, sticks and foreign matter, and then proceed as follows:
Take a small handful of your soil, and wet it slowly. Add just enough water so that the soil does not stick to your fingers but is moist enough to glisten to the eye. If it becomes sloppy and wet, add more soil until you get the mixture right.
Work the soil with the palm and fingers of one hand to form a ball of soil, or bolus, that is evenly mixed with no lumps remaining. This may take a few minutes of working. Hold the bolus up to your ear while working it – can you hear the grinding of sand?
Try pulling out a ribbon of soil between your forefinger and thumb. How long can you make it?
Use the information in the Soil Texture Guide table on the opposite page to assess your
soil type by its texture.
If you can hear particles rubbing together as you work the bolus, and if it is also quite hard to make a ribbon of any length, it’s sand. If it feels smooth and silky, and you can easily make ribbons over 5 centimetres in length, it’s in the clay-texture category. Most people can tell sand from clay, but it’s the in-between textures that are a bit harder. The basic texture divisions are sands, loams, clay loams and clays, but we can have degrees of these as well, such as ‘loamy sand’ (sand with a bit of loam in it) and ‘sandy clay’ (clay with a bit of sand in it). Sands are said to be ‘light’, while clays are ‘heavy’ in texture.
Another common method you can use to determine your soil type is known as the jar test. Take a handful of soil, place it into a jar of water, replace the lid and then vigorously shake the jar until the soil has completely broken down. This may take some time with heavy clay.
The large particles of sand will settle first, followed by silt and finally clay. You will then be able to see the bands of the different soil particles, thus allowing you to estimate their relative proportions in your soil. This method offers a crude breakdown of your soil; however, we feel that the best way of identifying and understanding your soil is the hand-texture method.
Grow Your Own Page 8