During the soil texture test, a bolus of moistened soil is squeezed out to make a ribbon. The length of the ribbon indicates the texture, from sand to clay.
Soil colour
A skilled gardener or soil scientist uses colour to make observations about soil behaviour. The two most important things that colour can tell us is the organic-matter content and whether the soil is well drained or not. Organic matter is always dark brown or black, and it is darker when the soil is wet. Dry topsoil with high organic matter might be dark grey, but it will go almost black when it is wet. Topsoils are nearly always darker than subsoils.
Iron minerals often dictate the colour of natural subsoils. Indigenous people used these bright oxides and hydroxides for millennia as red and orange pigments for cave painting and body art. Today they are utilised in cosmetics (rouge and lipsticks), paints, cement colouring and medicines. This is because they are very stable, and a tiny bit goes a long way.
As well as playing a fascinating role in human cultural activities, the colours of iron oxides are interesting because they change depending on how wet the oxides are. This can tell us a lot about how well drained our subsoil is. Basically, the sequence is:
Grey/olive/greenish – permanently saturated, very poor drainage, completely anaerobic.
Light grey/yellowish – mostly saturated, poor drainage, moderately anaerobic.
Yellow/orange – intermittently saturated, reasonable drainage, periodically anaerobic.
Orangey red – mostly dry, good drainage, mostly oxidised.
Bright red – never saturated, extremely good drainage, always oxidised.
Desert soils are bright red in colour because they are almost never anaerobic (without oxygen), while mangrove mud or swamp soil is grey because it is permanently saturated. You may find that the top of your subsoil is bright red, but as you go down the profile and it becomes moister, the subsoil becomes more yellow or even mottled yellow and light grey. It’s all about the amount of air versus the amount of water. Consequently, it is worth digging a hole in your soil to check out the colours of the iron minerals in the various soil horizons, particularly the subsoil.
SOIL TEXTURE GUIDE
TEXTURE RIBBON LENGTH COMMENTS
Sand No ribbon can be made Gritty, coarse or fine sand; can’t be moulded
Loamy or clayey sand 0–1.5 cm Slight coherence, gritty
Sandy loam 1.5–2.5 cm Coherent but gritty
Loam 2.5–3.5 cm Smooth and coherent but not gritty; may be spongy if high in organic matter
Clay loam 3.5–5.0 cm Feels plastic and smooth; some fine sand may be present
Clay 5.0 cm or longer Like plasticine; some sand may be present
COPING WITH POOR TEXTURE
The fact that loams usually have the best balance of pore size for holding a good amount of water and air is not to say that sandy soils do not give good results provided that water can be kept up to the plants, or that clays cannot form the basis of an excellent garden. The art of turning a poor natural soil into one that is great for horticulture (a ‘hortic’ soil) essentially involves deepening the soil to increase rooting depth. A layer of heavy clay subsoil, for example, can be slowly turned into a highly porous, well-structured clay loam or cracking clay that allows water and oxygen – and subsequently roots – to enter. One of the classic strategies for achieving this is to add gypsum to the subsoil, which helps it to aggregate, thereby improving its structure. As a rule, clays are only useful when they are well structured, and the gardener with clay soil will need to keep up the organic matter and not over-cultivate the soil to assist in maintaining structure.
If the soil looks dry and is not cool to the touch, it is approaching Permanent Wilting Point.
Moisture content
It is important to be able to assess a soil’s moisture content in order to judge when it needs irrigating. Most people irrigate based on gut feelings or the dry appearance of the surface of the soil, which very often leads to either insufficient or excessive water and wastage of water. Most professional farmers use soil-moisture sensors and Bureau of Meteorology evaporation and rainfall data to accurately determine how much water to apply to their soil.
When measuring soil moisture, there are two extremes:
The upper end is called the Field Capacity, which is the amount of water the soil holds about 48 hours after it was completely saturated then allowed to drain.
The lower end is called the Permanent Wilting Point, which is the moisture content below which the plant cannot get any more water and wilts beyond the point of recovery.
The best farmers know the texture and water-holding capacity of their soil, so they allow their soil to dry to about halfway between Field Capacity and Permanent Wilting Point before irrigating. They apply just enough water to bring it up to Field Capacity again, so they do not use more water than the soil can hold.
In practice, of course, the average urban farmer is not going to get out the calculator and consult the Bureau of Meteorology evaporation records every time they irrigate. However, like professional farmers, they need to get to know the particular moisture-holding characteristics of their soil and how much water they need to apply to wet the soil down to the bottom of the root zone.
There are all sorts of fancy electronic devices that measure soil moisture, but – with a little training – the human senses are very good at judging moisture content. The number one mistake people make is judging moisture by the condition of the soil surface, so ensure you dig down about 100 millimetres, pick up a handful of soil and observe it in good light; also touch the newly exposed soil with the back of your hand. Use the Assessing Soil Moisture table to help you work out the moisture content of your soil.
The next issue to deal with is how much water to apply. If your soil is about halfway between Field Capacity and Permanent Wilting Point, then the approximate amount of water to apply to the top 300 millimetres of the topsoil (the root zone) is shown in the Water Application table.
Note that the amounts of water added are in millimetres of depth, and they are quite significant. This emphasises the fact that it is more efficient and better for plants if you water them deeply and infrequently rather than lightly and often, as the latter situation will allow water to evaporate from the surface and encourage shallow root growth. Deep watering to bring the whole root zone up to Field Capacity encourages deeper rooting and allows the surface to dry out between waterings. This dry surface layer acts as mulch and prevents surface evaporation, reducing the net amount of water needed.
There are cases where farmers want to cause their crop some moisture stress by allowing it to approach – if not actually reach – the Permanent Wilting Point. Called ‘regulated deficit irrigation’, it is common when growing wine grapes as it concentrates the juice and leads to the production of superior-quality wines. It is also used for stone fruits, such as peaches and figs, where excess water is disastrous for the ripening fruits. The fruits will ‘blow up’ or split if too much rain or irrigation occurs at this time. The urban farmer may like to experiment with encouraging ripening and concentrating the nutrients and flavours in common vegetables by keeping them a little on the dry side before harvest.
FOLLOW YOUR NOSE
Smell is a very reliable guide to the balance of air and water in your soil. In the presence of oxygen, soil smells earthy. In its absence, it smells like old socks or the water at the bottom of a vase of long-dead flowers. If soil has a particularly nasty aroma, then it has probably ‘gone bad’ and something needs to be done about it. The classic smell of waterlogged swampy soil is ‘rotten-egg gas’, or hydrogen sulphide – sulphur in its reduced form. While generating hydrogen sulphide in the science lab has long amused schoolchildren, in an urban soil it is no laughing matter – as your crops will quickly object to such toxic growing conditions.
Make sure you dig down to at least 100 millimetres below the surface to assess soil moisture; ignore the surface.
Once crop plants
start to wilt, there is a good chance that you have lost significant yield.
ASSESSING SOIL MOISTURE
OBSERVATION MOISTURE CONTENT CONSEQUENCE
Dark, glossy, shining wet, sodden; feels cold to the back of the hand Above Field Capacity Exclusion of air, waterlogging and root death by asphyxiation
Dark, matt, obviously moist; feels cool to the back of the hand About Field Capacity Usually about right, adequate balance of air and water content
Dark, matt; feels relatively cool to the back of the hand Around or below Field Capacity Still good, don’t need to water
Dark, matt; feels just cool to the back of the hand About halfway between Field Capacity and Permanent Wilting Point Might need to water soon
Lighter in colour, matt; neutral temperature to the back of the hand Approaching Permanent Wilting Point Need to water, as yield is starting to seriously decline
Light, matt; feels warm to the back of the hand About Permanent Wilting Point Water now, as you have suffered yield decline
Very light, matt; bone-dry powder that feels warm to the back of the hand Below Permanent Wilting Point Only the toughest plants are alive, only succulents give better than zero yield
CHEMICAL FERTILITY
As we saw in The Needs of Plants, plants require 12 elements (or nutrients) from the soil to grow and thrive – this is the chemical fertility of the soil. We could argue that there are actually 14 essential elements, because there are two others whose necessity remains debatable: sodium and chlorine, which together make sodium chloride (common salt). These elements are present in all plants, but only a few plants seem to respond positively when they are applied to soil. Small amounts of common salt are certainly necessary for animals, but plant deficiencies of common salt are effectively unknown. Generally speaking, there will be enough common salt in any soil for you not to have to worry about applying it.
All 12 elements supplied from the soil are taken up by the plant’s root system in a simple (ionic) form after they have dissolved in water. This is the same regardless of whether the elements are derived from an organic source (such as compost and manure) or an inorganic (mineral) source (such as lime and gypsum). The elements are present in the soil water (soil solution) as salts – but we don’t just mean common salt (although sodium chloride is one of the salts found in soils), as there are many others (such as magnesium sulphate and sulphate of potash). A salt is a substance that dissolves in water by separating into a positively charged part (a cation) and a negatively charged part (an anion). For example, sodium chloride or common salt (NaCl) separates into Na+ + Cl-, where sodium (Na) is the cation and chloride (Cl) is the anion.
It follows then that plants in a dry soil won’t have enough water not only for photosynthesis, but also for nutrient uptake – they actually starve from lack of water! Hence the expression we hear used sometimes: ‘Water is the best fertiliser.’ A crucial point about the delivery of nutrients to plant roots is that not all salts (nutrients) dissolve as readily as each other in water. Thus, delivery of some nutrients is much easier than others. Being highly soluble, nitrogen and potassium are much easier to deliver to plants in liquid form while they are in active growth. But calcium has low to moderate solubility, and it is therefore very important to ensure there is adequate calcium present in the soil before you start a crop. The Plant Elements and Their Solubility table shows the dissolvability of the essential plant elements.
If we remember that the number of cations always has to be balanced with the same number of anions, you can start to mix and match nutrients to make up all sorts of fertiliser salts that can be used to improve the chemical fertility of your soil. There are several common combinations:
All Prunus species (such as peaches and plums) are prone to fruit splitting if excessive water is applied close to ripening time.
WATER APPLICATION
SOIL TEXTURE WATER TO APPLY (MM)
Sand 23
Fine sand 30
Sandy loam 27
Fine sandy loam 33
Loam 26
Organic loam 35
Clay loam 27
A SPECIAL CASE
In general, all the compost and organic forms of nutrients that we apply to our plants must break down to their ultimate ionic forms for the plant to take them up. However, there is an exception to this rule. Some simple organic ‘chelating’ compounds, such as sugars and amino acids, are small enough to pass through the walls of the root cells. Insoluble elements that have been ‘chelated’ to improve their availability to plant roots can be purchased to correct specific deficiencies. You may have seen chelated iron in garden centres, as it’s a useful way of overcoming iron deficiency in plants growing on alkaline soils. There are also chelated forms of other trace elements that are commercially available, such as zinc and copper, as well as blends that combine various trace elements into a single fertiliser.
calcium sulphate (gypsum) – CaSO4
ammonium sulphate – (NH4)2SO4
iron sulphate – FeSO4
magnesium sulphate (Epsom salts) – MgSO4
sulphate of potash – K2SO4
calcium carbonate (lime) – CaCO3.
BIOLOGICAL FERTILITY
Healthy soils contain huge numbers and varieties of macroorganisms and microorganisms, and research has shown that the larger the number and diversity of organisms, the more fertile the soil usually is. Biological fertility is the ability of a soil to support an array of beneficial organisms.
Populations of soil organisms will vary dramatically from soil to soil, and even from day to day within the same soil, depending on how much food (organic matter) for growth and reproduction there is, the moisture content and the temperature of the soil. These organisms benefit from the same physical fertility factors as plants – they need a balance of nutrients, as well as access to air and water in the soil – and they offer many benefits in return. For example, soil organisms exude a range of soluble organic compounds that are known to stimulate root growth and hence nutrient uptake.
Some people claim that they can tell the ‘health’ of a soil by analysing the proportions of various microbes (for instance, fungi versus bacteria), but in our opinion there is very little evidence that this is of any real use. In fact, soil microbiology provides the most fertile ground for all kinds of unproven (and unprovable) claims, which has led to a raft of ‘cures’ being peddled to the unwary. While we are great advocates for creating your own healthy living soil by using homemade compost and worm-farm castings to bring all the beneficial organisms you need to your urban farm, we have seen many commercial products aimed at aiding soil biology that appear to be a complete waste of money.
Commercial potions abound that are said to contain microbes that will ‘unlock’ soil fertility. ‘Probiotics’ that list benefits such as ‘increases root mass’ and ‘activates and restores your soil’ are increasingly being stocked in garden centres. When asked to produce any objective evidence of their claims, a deathly silence often follows. Indeed, it would seem somewhat illogical to assert that adding a little bottle of microbial solution to a soil that already contains trillions upon trillions of the same organisms would change anything.
PLANT ELEMENTS AND THEIR SOLUBILITY
ESSENTIAL ELEMENT CATION ANION SOLUBILITY
Nitrogen (N) Ammonium: NH₄+ Nitrate: NO₃- Very high
Potassium (K) K+ Very high
Calcium (Ca) Ca²+ Low to moderate
Magnesium (Mg) Mg²+ Moderate to high
Iron (Fe) Fe³+ Moderate to very low
Manganese (Mn) Mn²+ Moderate to very low
Zinc (Zn) Zn²+ Low
Copper (Cu) Cu²+ Low
Phosphorus (P) PO₄3- Low to very low
Sulphur (S) SO₄2- Moderate to high
Boron (B) BO₄2- High
Molybdenum (Mo) MoO₄3- Low
Carbonate CO₃2- Very low
Organic matter of all kinds is the fuel for a soil's microbial life, and it helps to
increase biological fertility.
SALTS AND PH
A salt is a substance that dissolves in water, but did you know that water is actually a salt? It dissolves to a small degree in itself:
H2O H+ + OH-
This is the basis of pH, or acid/alkali chemistry. H+ cations cause acidity, while OH- anions cause alkalinity. Note that water has an equal number of acid (H+) and alkali (OH-) ions. This makes water pH neutral: neither acid nor alkaline.
Now let’s add hydrogen chloride (HCl) to the water to make the salt hydrochloric acid (which is commonly used to lower the pH of swimming-pool water):
HCL H+ + CL-
There is now an excess of the acid H+ ions, and the water into which the hydrogen chloride is dissolved is said to be acidic.
If we mix sodium hydroxide, otherwise known as caustic soda (NaOH), with water:
NaOH Na+ + OH-
There is now an excess of the alkali OH- ions, and the water into which the sodium hydroxide is dissolved is said to be alkaline.
CATION EXCHANGE CAPACITY (CEC)
Regardless of the fertiliser you are using, it must first dissolve into an ionic form before plant roots can take up its nutrients. But what stops the nutrients from simply being washed away by rainfall? Soil has a very elegant system for storing nutrients.
Humus (fully decomposed compost) and clay particles in the soil act like a sponge to soak up and store both water and nutrients. The way they do this is that both carry an overall negative electrical charge, which attracts positively charged ions (cations). The nutrients are thus held in the soil so that they do not leach out. The cation exchange capacity (CEC) of a clay or organic particle refers to the negative electrical charge that it carries, which allows it to attract cations. Plant roots have to ‘exchange’ one cation for another to get the nutrients they need from the soil particles, and they do this by exuding a hydrogen ion (H+).
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