The Biosphere Blog

Knowing a soil’s texture gives us insights into its other properties. This was touched upon briefly here, but now that you’ve perhaps determined your own soil texture with a glass jar and some water, let’s go further into what the contributions of sand, silt and clay components are.

Sandy Soils
Soils high in sand, but low in silt and clay, are easy to cultivate, but are usually deficient in organic matter as they lack the negatively-charged clay particles that bind it. Water infiltration is generally greatest in sandy soils, though some sandy soils are water-repellent. Most do uptake water readily, though they drain easily as they are unable to hold this water. Sandy soils are less likely to compact under traffic (whether from people, animals or machinery) than other soil types. Higher silt and clay concentrations will improve water and nutrient retention.

Clay Soils
Soils with around 40-50% clay (so-called ‘heavy’ soils) are difficult to cultivate, but do hold organic matter, minerals and water well. Water infiltration is poorest in clay soils. Water is slow to enter and percolate even dry clay soils as a rule, owing to the nature of the clay particles. However, some clays crack so deeply when dry that heavy rainfall causes rapid flows and deep penetration of water until the clay minerals swell and close the cracks. Too much water in a clay soil can lead to a sticky consistency and even waterlogging. Much of the water in a waterlogged soil isn’t actually plant-available — plants just can’t ’soak it up’. Rather, too much water in soil displaces the oxygen plant roots need, leading to poor overall plant growth at best and root rot and plant death at worst. Clays are also susceptible to compaction to the point that plants may struggle to grow in them. Higher sand and silt concentrations in otherwise clayey soils makes them more ‘open’ and better draining.

Loam Soils
Loams have mixtures of sand, silt and clay that give them more intermediate properties to those of sands and clays. The best ones are easy to cuiltivate and hold good amounts of water and nutrients whilst still draining well. Loams in general hold the greatest amount of plant-available water, and silty loams hold the most of all. As a group these are regarded as the best soils for plants, with overall properties that encourage good root growth and nutrient retention. But as a group they also vary a lot in their proportions of sand, silt and clay, and some have less than desirable properties. Some loams set rock-hard, some develop surface crusts impervious to water, and some have very poor nutrient retention.

---

Knowing a soil’s texture can reveal a lot about its properties, as you’ve seen. But so far all we’ve done is extrapolate these properties from what we know about the properties of sand, silt and clay as distinct particles.

Soil is one entity to which the phrase “the whole is greater than the sum of its parts” could apply. The individual sand, silt and clay particles in soil group together to form aggregates or peds, and it’s the arrangements of these peds that give soil a characteristic more important than texture: its structure. Structure plays a major role in water infiltration and retention, nutrient retention, and how readily (or not) plant roots grow. A high clay soil with good structure could theoretically outperform a loam with terrible structure.

Next week we’ll begin a study of structure with first a look at soil horizons (the vertical layers that become apparent when you go deep enough). After that we’ll zoom in a little closer and describe the soil peds that make those horizons. And after that we’ll zoom in closer still and go over ‘exchangeable cations’ and ‘anions’. These are all well worth knowing, even if just for their own sake, as they will enhance your knowledge — and perhaps appreciation — of soil all the more!

Having read about Soil Properties, why not get out and assess (some of) your own? The following experiments are easy to do and will really help you understand your soil!

Soil pH

This is a fundamental test everyone should do. You’ll need to buy a pH kit but these are readily available from many garden centres on and offline, cost around $20 or less, and are very easy to use. Make sure you get one intended for soil though! (Some are designed for hydroponic systems, ponds, or pools.)

Soil Texture

Determine the proportions of sand, silt, clay and organic matter in your soil by feel! A soil with a high proportion of sand will feel gritty, a soil with a high proportion of silt will feel silky, and a soil with a high proportion of clay will feel sticky and can easily be rolled into a ribbon.

The method outlined below is routinely done ‘in the field’ when there is no access to more accurate equipment. It can take some practice to perfect, so don’t be afraid to do this several times to be sure of your result:

1. Take a handful of soil and remove any stones, sticks, leaves or bark. Don’t worry about any really tiny pieces of organic matter you may see, you just want to remove the ‘ obvious’ pieces that are easy to see and pick out. 
2. Knead this sample between your fingers and thumb. You want a moisture content such that the soil just fails to stick to your fingers. If too dry, wet the soil slightly with a small amount of water and keep kneading. (An eye-dropper or squirt bottle is handy.) Keep kneading until you reach both this moisture content and you can no longer detect a change in consistency. Be sure to break down all lumps of soil so as not to confuse these for sand grains
3. Can you see any visible sand? Or did you hear and feel any when working your sample?
4. Squeeze the sample hard to see if a ball or cast forms. Does this shape hold or does it fall apart?
5. Squeeze this shape between your thumb and forefinger with a sliding motion. Does a self-supporting ribbon form, and if so, what is its length?

Here’s how to interpret your results:

a. Doesn’t stick together and can’t be formed into a ball or cast. Single sand grains stick to fingers — sand
b. Forms a cast that just bears handling and which falls apart easily. Forms a very short 6 mm ribbon that breaks easily — loamy sand
c. Forms a fragile cast that is sticky when wet, with many sand grains sticking together. Forms a ribbon 5-15 mm long— clayey sand
d. Forms a cast that just bears handling, and in which individual sand grains can be seen and felt. Forms a ribbon 15-25 mm long — sandy loam
e. As for sandy loams, but individual sand grains can’t be seen, though can be heard and felt. Also forms a ribbon 15-25 mm long — fine sandy loam
f. Holds a cast that feels spongy. No obvious sandiness or silkiness, but may feel greasy if a lot of organic matter is present. Forms a ribbon about 25 mm long  — loam
g. Holds a cast but will crumble. Feels very smooth and silky. Forms a ribbon 25 mm long — silty loam
h. Forms a very firm cast in which sand grains can be felt. Forms a ribbon 25-40 mm long — sandy clay loam
i. Forms a very firm cast with a spongy feel and which is pliable when squeezed between thumb and forefinger and smooth to manipulate. Forms a ribbon 40-50 mm long — clay loam
j. Forms a pliable cast in which sand grains can still be seen, felt or heard. Forms a ribbon 50-75 mm long — sandy clay
k. Forms a very pliable cast with slight resistance to shearing between thumb and forefinger. Forms a ribbon 50-75 mm long — light clay
l. Forms a smooth, pliable cast which handles like plasticine. Can be formed into rods that don’t break, but with some resistance to ribbon-forming. Forms a ribbon at least 75 mm long — medium clay
m. Forms a smooth, pliable cast which handles like stiff plasticine. Can be formed into rods that don’t break and without resistance to ribbon-forming. Forms a ribbon at least 75 mm long — heavy clay

This is a very quick and handy test to know that can be performed on the spot and with access to water. Sometimes, though, it can produce misleading results, as soils that feel alike can differ quite a bit from each other in their proportions of sand, silt and clay. If you wish to know the actual proportions of these components you can have a particle size analysis done at a laboratory that determines soil particle sizes with calibrated sieves and a mechanical shaker, or you can do a good-enough one yourself at home with a glass jar and some water!

Here’s how:

Soil Composition

Determine the proportions of sand, silt, clay and organic matter in your soil visually, and compare with what you felt above!

1. Collect about two cups of soil and remove any large sticks, leaves, stones or bark
2. Place one cup of this “cleaned” soil into a straight-sided 500 mL glass jar with a tight-fitting lid
3. Add water to a level of about 2cm from the top of the jar
4. Place the lid tightly on the jar and shake vigorously for 3-5 mins
5. Place on a table or even higher at eye level if possible — you want to be able to observe and measure the jar later without disturbing the contents, and having the jar at a height beforehand makes this easier and more comfortable. Allow the contents to settle undisturbed for a couple of days or even a couple of weeks. The more clayey your soil the longer it will take these lighter particles to settle. The water layer may still be discoloured after this time owing to any dissolved organic matter present

Assuming your sample has all four components, small rocks (if present) and sand particles are the largest and heaviest, and will settle on the bottom. The next layer will be silt, followed by clay. If your soil is mostly clay you’ll likely only see clumps of clay on the bottom.

Above these solid layers will be the water. Any discolouration is due to rotted organic material that has dissolved in this layer. Any non rotted organic material will be floating on top of this water layer. Here’s my garden soil:

(I stuck a ruler onto the jar above with a loop of sticky tape for this photo, and lined the ruler up to be flush with the very bottom of the sand layer, not the very bottom of the jar which was about 2.5 mm thick glass. The ruler appears to be lower than this owing to camera position and parallax.)

This is where choosing a jar with straight sides becomes important! Ignore the water level and measure the total height of the combined soil fractions, as well as the height of each layer. Divide the height of each layer into the total soil height and multiply by 100 to express as a percentage.

For me, the soil was 39 mm high. Sand particles formed a 27 mm layer. Silt formed a 9 mm layer, and clay formed a 3 mm layer. Thus this soil is 69% sand, 23% silt and 8% clay. (Check that your numbers add up to 100 so as to pick up any calculation errors.)

(If you’re struggling to see the silt layer in the above photo, it isn’t you! It’s definitely visible when viewed at eye level in direct sun.) I had to keep picking up the jar to confirm with my own eyes it really truly was there too!)

Reading these results from the Soil Texture Triangle below tells me I have a sandy loam. To read your results, find the sand % value along the bottom of the triangle and follow the diagonal that slopes up to the left. For silt, find the % value along the right of the triangle and follow the diagonal that slopes down to the left. For clay, find the % value on the left of the triangle and follow the line that goes straight across to the right. As each fraction adds up to 100%, the three lines will converge at a single point, and the section that point is in corresponds to your soil type.

Now that you know a bit more about your soil up close and personal (or, perhaps, mine!), we will examine soil structures in more detail over the next few weeks. Soil structure has a big influence on plant growth, and is well worth understanding.

This week’s soil experiments conclude next week with a wrap-up discussion as to what your soil results mean.

Soil properties of most interest to gardeners and horticulturalists are colour, texture, mineral content, structure, and chemistry. Each of these could be lengthy posts in their own right (and maybe they will be, later!), but the following overviews should get you up to speed on the concepts.

Colour

Soil colour mostly comes down to two things: the type and amount of iron oxides present, and the amount of organic matter present.

Iron oxides are compounds of iron (Fe) and oxygen (O) — common rust (Fe₂O₃) is the best-known example. Iron oxides form in soils in the presence of air and water and their colours range from yellows, through to oranges, browns and reds. Most soils contain mixtures of different types, the proportions of which determine the overall colour.

Soil colour can provide clues as to drainage and aeration, particularly of the subsoil. A uniform brown or reddish-brown colour extending into the subsoil signifies a well drained and aerated soil — the colour indicates iron oxides, which indicates the presence of air and water oxidising iron at those depths.

Yellow shades indicate moderate waterlogging, and extremely waterlogged soils consist of the ‘gley’ (not a typo!) colours. ‘Gley’ colours (dark greys, green-greys, green-blues and blacks) indicate the presence of iron hydroxy-carbonates (’green rusts’) which form in very wet and air-poor environments.

Organic matter darkens soils, and topsoils, where organic matter accumulates, are usually darker than subsoils. Organic matter or certain minerals like ilmenite can be behind the colour of black soils. Soils made of lighter greys and whites are a sign of no organic matter and significant leaching over extended periods.

Texture

Soil is made up of differently-sized particles, summarised here:

Particle	Diameter (mm)
Clay	less than 0.002
Silt	0.002 - 0.02
Fine sand	0.02 - 0.2
Coarse sand	0.2 - 2
Gravel	greater than 2

The proportions of each of these five in soil determines a soil’s texture, or ‘feel’. The word ‘feel’ comes from the technique of working a moistened sample between fingers and thumb. Soils with a lot of clay will become sticky and easily from ribbons when rolled, while soils with a lot of sand will feel gritty and barely form a workable ball. Many other soils will fit somewhere within this range.

A soil laboratory would of course analyse a soil sample more accurately, but as a field technique this is a great way to get, pardon me!, a feel for your soil. We’ll actually go over how to do this technique, as well as some other simple soil experiments you can easily do at home in an upcoming post.

The following Soil Texture Tirangle diagram shows how soils are classified based on their proportions of clay, sand and silt.

A soil’s composition and texture reveals much about its properties and characteristics, though this is not definitive, as the mineral content and organic matter present (or not) also has an influence. General statements can still be made however.

(Go here for details on how to assess your own soil texture and composition!)

For example, soils high in sand but low in clay and silt tend to drain easily. This can be to your advantage if watering with, say, bore water of moderate salinity, as the salts are likely to leach away rather than build up to toxic levels. This can also work against you as sands are not able to hold organic matter.

Too much clay, with little sand and silt, can result in heavy, sticky, poor-draining and waterlogged soils when wet, and hard, cracked surfaces when dry. Clay soils tend to be rich in organic matter though, as they are particularly good at holding nutrients.

Silt’s properties are partway between those of sand and clay. Its drainage ability is less than sand’s but higher than clay’s. On the other hand, it can hold onto more nutrients than sand, but not as much as clay can.

An ideal soil, from a plant-growing point of view, is one of a roughly balanced mix of sand, silt and clay — the loams.

Mineral Content

Sand minerals in soil have small surface areas relative to their volume, low water retention ability, and low chemical activity. These properties account for their inability to hold nutrients in the soil. Silt minerals also have small surface areas relative to their volume and low chemical ability. Soils high in silt can compact under heavy traffic, which impacts water and air movement.

Clay minerals are the ones that most influence soil properties. These minerals exist as very small rod and plate-like crystals with very large surface areas relative to their volume. This and the fact they are also very chemically active is why they hold onto water and nutrients so well.

Structure

Soil structure refers to how the solid parts (peds) and the spaces (pores) are arranged.

A good soil structure is one with fine soil particles (crumbs) distributed somewhat evenly amongst spaces through which air, water and plant roots can travel smoothly. A high organic matter content, roots growing through the soil, and earthworm activity all contribute to the creation of channels and pores through which air and water can percolate. If soil is left undisturbed these form a positive feedback loop and both structure and organic matter increase still further over time. Better flow of air and water increases root growth and attracts earthworms and other biological activity from arthropods, insects and microorganisms. These have a combined effect of breaking larger soil peds into smaller ones, creating still more organic matter, and forming still more pores and channels which further increases air and water movement, attracts further root growth and biological activity, and so on.

A poor structure is characterised by soil peds compacted into clods with poor air and water percolation. High traffic can exacerbate this compaction and impede air, water and root movements. Excessive digging of soil will destroy any channels formed by earthworms and possibly lead to clod formations, especially in more clayey soils. Both of these can be overcome, and the structure improved, by incorporating high amounts of good quality compost and/or manure into the soil and leaving it to resettle and reform.

Chemistry

Soil chemistry has a major effect on plant growth. It is the means by which minerals and organic matter are broken down in the soil and made accessible to plants. This chemical activity can be non-biological, biological, or a combination of both.

Much of this activity occurs because of the negatively-charged surfaces of clay minerals and humus particles. (Humus is the dark, spongy and jelly-like end result of organic matter decompostion.) The negatively-charged surfaces attract and hold positively-charged ions of calcium, potassium and magnesium, which may then be uptaken directly by plant roots or dissolve into the soil water for uptake that way. These negatively-charged surfaces also attract many microorganisms that break down organic matter.

We saw earlier how pH (essentially the chemistry of hydrogen ions in solution) affects the availability of nutrients:

Lowering pH by applying large amounts of acidifying fertilisers such as ammonium sulfate or ammonium nitrate can lead to a molybdenum deficiency or a manganese toxicity. Raising pH by applying large amounts of lime may lead to iron and/or manganese deficiencies.

Nitrogen wouldn’t even be available to plants were it not for the action of soil microbes (and to a much lesser extent, lightning strikes) ‘fixing’ atmospheric nitrogen into nitrogenous compounds plants can uptake.

Chemical reactions can also act in deleterious ways. Excessive amounts of potassium can inhibit magnesium uptake, leading to a magnesium deficiency. The reverse also occurs, in that excessive amounts of magnesium can lead to potassium deficiencies. Excessive use of phosphatic fertilisers can cause zinc deficiencies even in zinc-rich soils. And soils with high levels of manganese can prevent Rhizobium bacteria from accessing the cobalt they need to fix nitrogen for their leguminous hosts.

 

A Definition of Soil

Soil makes up the pedosphere layer on Earth and is easy enough to recognise. Unless you’ve studied it though, it’s actually quite hard to articulate what it is you’re recognising. So what exactly is ‘soil’?

Soils differ markedly in colour and texture, but all contain the same five components of:

• inorganic* (mineral) particles, derived mainly from weathering of rocks
• organic* materials derived from dead and decaying plants, animals and animal products
• water, also known as the ’soil solution’ in which plant nutrients are dissolved
• air, which fills the spaces between soil particles not filled by soil solution
• living organisms, such as microorganisms, insects, earthworms, and even small burrowing animals

---

* Please note that to a chemist, ‘organic’ means ‘contains carbon’ and ‘inorganic’ means ‘doesn’t contain carbon’. The presence of carbon in soil implies an origin from living and once-living organisms. Even carbon-rich coals and charcoals originated from once living organisms.

---

How Soils Form

Soil formation occurs over thousands of years of ‘weathering’. Weathering is the breaking up of rocks and sediments by physical, chemical and biological activities.

Physical weathering is the break up of rocks without physical change. Rocks fragment into smaller and smaller particles until they eventually become the mineral component of soil. Wind, temperature, water and pressure are behind this type of weathering. Wind is abrasive and has a sand-blasting effect. Temperature fluctuations over days, seasons and years cause rock to expand and contract until it cracks. Water continually freezing and thawing in holes or cracks will also break rocks with time. Plant roots in cracks can exert pressure and push rocks apart as they grow and thicken.

Chemical weathering is the break down of rock through chemical changes. Water and oxygen are the major forces here. One example is water acting on granite. The water reacts with feldspar crystals in the granite to create clay minerals. The clay minerals weaken the rock and increase the likelihood of it fragmenting. Another example is oxygen reacting with iron to create iron oxides. These oxides are more fragile than iron and compromise the rock’s structure, leading to it eventually breaking away.

Biological weathering is a form of chemical weathering, in that some plants and microorganisms release acids that break down rocks and mineral compounds. Lichens (which are not single organisms but rather fungi and algae or fungi and cyanobacteria living together in beneficial relationships) produce a weak acid capable of dissolving rock.

---

The extent to which each of these occurs is a function of the climate, environment, organisms and rock type specific to a region, and the reason soils vary so much globally. Rocks rich in quartz, such as sandstone, will form sandy soils. Rocks poor in quartz, such as shale or basalt, will not. Rocks in hot, dry deserts will break down differently to rocks on a wet, temperate coast and form different soils as a result.

At the end of the day we have the soil we have, and must work with it and its characteristics accordingly. The next post will look at the properties of soils: their colour, texture, mineral content, structure, and chemistry.