How does soil work?

Soil is one of those things where every gardener deals with it extensively but most gardeners have no idea how it works or what it is made from. This can lead to disastrous gardening such as digging up a plant to put fresh soil around its roots or spending hundreds of pounds adding manure or soil a year when you could spend barely a fraction of that and achieve much greater effects.

Understanding how soil works and how to improve it involves looking at it from the point of a biologist, chemist and physicist (or a geographer, they are really under appreciated). Soil is massively variable and changes even in small areas. It is affected dramatically by both the underlying rock and the plants growing on top. Even humans or animals walking over it can have a massive impact.

Author Note: This is quite a science heavy page but it does focus on how soil can be improved practically. If you don't feel like listening to me babble on about clay, CEC, particle size and surface area then feel free to skip to the summery and soil types sections.

What Is Soil? - Components of Soil - Types of soil - Soil Metatypes - Soil Hand Test - Improveing Soil - Summery

- Advanced -

Water Retention - Cation Exchange Capacity (Fertility) - Why is clay so important?

What is soil?

At its most basic level, soil particles are minerals (mostly silica) bound to organic matter. For the purpose of gardening however, you should look at soil from the macro perspective. Soil contains soil particles (of differing sizes and continuance) and holes called pores. These pores can be further broken down into macropores, mesopores and micropores.

Both the bedrock and the established plants have a great impact on the type of soil you are likely to find, after all if you dig up a spade full of soil, not only will you have the mud but a great deal of microorganisms and plant matter as well!

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Components of soil:

Minerals:

As previously stated, soil is made from both organic matter and minerals. The minerals predominantly come from the base rock, in the case of Henleaze and a good portion of Bristol, this is mudstone and limestone which erode into soil that is alkaline and has a medium/large amount of silt and clay. This will be gone into later in greater depth.

Other places in the world have different base rocks that may cause an acidic sandy soil which would have vastly different properties and influence greatly which plants could be grown there.

Organic Matter:

Organic matter comes from decaying plants, animals or microorganisms. For an example consider a Galapagos island just formed from the see. It starts as nothing more than volcanic rock which is over time eroded and broken down. At that point you have little more than sand. Plant detritus such as seaweed are then blown or drift onto the rocks and they start to break down, therefore providing nutrients and establishing the basis for soil production.

This continues in even quite old soil, if you grow conifers for example, the fallen needles will decompose and acidify the soil. Organic matter is rich in nutrients and helps stabilise the soil. You should not ignore the effects of even large living matter, roots and such help increase drainage and even provide nutrients where they are worn away from pushing through the earth. Growing plants is one of the best ways to change the soil in your garden.

Pores:

The macropores are relatively large (above 70 nanometers) and they are generally too big to hold water, so when they drain they provide a source of oxygen, carbon dioxide and all the other vital elements in air.  This is really important for respiration and without these then the soil would become poisonous and plant roots would drown (more in the advanced section).

Mesopores are smaller, the type of soil has an effect on how big they can be and still provide their function but they are generally bigger than 35 nanometers. These are able to hold water but weakly enough that plants can access it. Without these then plants would be unable to obtain water from the soil and so most would be unable to grow. If these pores are saturated then that is normally the ideal condition for plant growth (barring odd plants like orchids)

Micropores are the smallest spaces in soil and a large proportion of these is detrimental to growing. These pores are so small that the soil partials are able to hold onto water so tightly that plants struggle to use it. In a compressed clay you can find soil largely comprising of these and fully saturated with water but it is unavailable to the plants as they are unable to access it (either the pores are too small for root hairs or the surface tension is too high and they do not have enough suction to utilise it)

If you now look at the entire picture, soil is extremely complicated! So far this is just the very basics, the size of partials in the soil and a number of other factors contribute to the type of soil and everything from drainage to what nutrients will be absorbed and what plants will grow there.

If you want to know more about the formation of soil then click here, some of these concepts may be better understood after looking at the Types of soil section.

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Types of soil

There are numerous types of soil, all of them have different benefits and downsides, the vast majority of soils are mineral soils but there are a few organic soils in the UK. The distinction is most soils have a very low organic matter content, the exact determination depends on how much clay is in the soil but if it is more than 20% organic matter, most people would consider it an organic soil. the ideal amount for mineral soils to grow anything more demanding than grass is 4-5%.

In addition there are two major bands of soil, topsoil and subsoil. The major difference is that topsoil has organic matter which often lends it a darker appearance. It is this that gardeners are really interested in. Subsoil is often much lighter and can look quite grey.

This has very little organic matter and is far inferior to grow on. In gardening, the main reason people encounter subsoil is when builders take away topsoil to bury stone and rubble before replacing it with subsoil. This happens fairly often as topsoil is quite valuable and can be sold on. If this happens then adding organic matter will go a long way to bringing it back to full capability. Other than this there is very little to interest gardeners as other than tree roots, very little plant activity happens in that zone.

Soils are classically composed of three sizes of particles, each with their own properties. If you test soil for structure you work out the percentage of each and this tells you what you have and its base properties. There are various methods for this and they range from crude hand tests to quite sophisticated chemical tests.

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Mineral Particles: (skip to soil meta-types)

A/N: You may want to look at certain terms to fully understand the importance of particle size, see Cation exchange capacity and water retention)

Sand: This is the largest soil particle (anything larger than 0.5mm and smaller than 2mm), it has a very low nutrient/water retention meaning it is prone to nutrient leaching and is nearly impossible to compact. It has very large pore sizes and virtually no micropores. Due to its size, it dose not mix well with soil and therefore it tends to have a minimal effect when added to soil (unless adding monolithic amounts) despite popular opinion that adding sand will massively increase drainage.

Its benefits are it has high aeration, high drainage and it can be worked soon after rain. It is also very easy to adjust the pH and very stable physically. As it warms up quickly it has an earlier growing period. The downsides are it is prone to leaching of nutrients, and struggles to retain water.

In a relatively sandy soil, it should feel gritty if moistened and worked in your hands.

 

Silt: This is the intermediate soil particle, it is smaller than 0.5mm and larger than 0.2 nanometers although the easiest way to distinguish it from clay is that clay is far more easily deformed. Silt is a good particle for soil as it has good nutrient retention and high water retention due to a high number of mesopores in a silty soil. It can in many ways be seen as a rather unremarkable substance. It is often found where shallow seas or rivers were once prevalent.

Silty soils are generally good for gardening with a long period of workability so are able to withstand digging even when wet without much soil damage. They have a good retention of nutrients and water meaning plants are less prone to drought and deficiency whilst not being prone to water logging.

High silt soils have no major downsides but are also not remarkable for any specific use and relatively poor structure. Due to this, some people disregard silt as a soil particle believing it to be fairly similar to loam.

When you work moist silty soils in your hand, they should feel very smooth and almost soapy.

 

Clay: Clay particles are the smallest and because of that they have the greatest impact when introduced to the soil. Due to their small size but high surface area and Cation exchange capacity, these soils are extremely fertile and are extremely good for growing highly demand plants. They however have the unfortunate problem of compressing easily when wet and once compressed are very hard and slow to return to their former state. This leads to worked clay soils having a very high proportion of micropores and potentially becoming 'glays' where they become pale and have very little organic matter. Glays are largely anaerobic and very hostile for plants to grow in.

Whilst clay is fantastically useful in the soil, for storing nutrients and resisting drought, a high clay soil is also prone to structural damage which may require a great deal of effort to fix. In addition, pure clay is a very suitable material for lining ponds or beds to improve water retention and even create reservoirs to prevent drought. Clay is self-working as it expands and contracts greatly when adsorbing water so needs little maintenance

When working with a moistened high clay soil then by putting pressure on it with a finger and wiping it, you should see a smear or sheen appear.

Clay is a very complex particle with many permutations, to see more click here

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Soil Meta-types

Soil texture is in most cases, a combination of the three particles, the percentages of which determine the qualities of the soil, if a soil is balanced then it is called a Loam, which is why topsoil is sometimes sold as shredded loam.

                                                      This is a soil analysis map, by taking the percentage of clay, silt and sand you can determine what your soil is.

Knowing what soil you have is useful as you can predict how it will act, but in most cases a simple hand test will do for the texture and a basic pH test is all you need.

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Soil hand test

This test is to be done with slightly damp soil, over time working it in your hands, it may dry out and you should add very small amounts of water. If the soil is overly gritty then it can be sieved if you have a suitable mesh. Grit will distort the results of this test.

  • 1) Gather enough soil to make an inch round ball from just below the surface (so it is not just baked dry from the sun)
  • 2) Dampen it with water until it is slightly sticky
  • 3) Dose it form a cohesive ball? If No go to Sand
  • 4) Dose the ball fall apart under gentle pressure? If Yes go to Loamy Sand
  • 5) Dose it form a short stubby cylinder (about 5cm by 1.5cm)? If No go to Loamy Sand
  • 6) Roll it back to a ball, Dose it roll into a thread? (about 13cm by 0.5cm) if No go to 7, If Yes go to 10
  • 7) Does it feel predominantly sandy? If Yes go to Sandy Loam (should be mildly tacky)
  • 8) Dose it feel particularly smooth or silky/soapy? If Yes go to 9 if No go to Loam
  • 9) Dose it feel very silky? If Yes go to Silt, If No go to Silty Loam
  • 10) Will the thread form a U shape? If No go to 12
  • 11) Will Is the U cracked? If Yes go to 12, if No go to 14
  • 12) Dose it feel particularly soapy? If Yes go to Silty Clay Loam, if No go to 13
  • 13) Dose it feel strongly sandy? If Yes go to Sandy Clay Loam, if No go to Loam
  • 14) Will the U form a ring shape (using 8-9cm length)? If No go to 12
  • 15) Is the ring cracked? If Yes go to 16, if No go to 17
  • 16) Does it feel soapy? If Yes go to Silty Clay loam, if No go to Clay Loam
  • 17) The soil is predominantly clay. Dose the soil feel sandy? If Yes go to Sandy Clay
  • 18) Dose the soil feel soapy or silky? If yes go to Silty Clay, If No go to Clay

This is a table of the different textures, if the feel is wrong then try double checking the hand test.

Soil Type Feel Uses
Sand Very sandy feel and barely sticky Can be worked well when wet, avoid thirsty or hungry plants
Loamy Sand Very sandy feel but more adhesion Can be worked well when wet, low retention of water and nutrients
Sandy Loam Slightly sticky, should stick to fingers when worked Easy to work, better retention of water and nutrients than loamy sand
Silty Loam Slight adhesion and reasonably smooth texture Easy to work, structure is not easily damaged. Decent water and nutrient retention
Silt Very smooth and easy to work, slightly sticky. Very easy to work and has smooth texture. good nutrient retention and decent water retention
Loam Readily workable but slightly sticky Near base soil, good retention of both water and nutrients.
Sandy Clay Loam Sandy texture that may smear when put under pressure. Semi sticky Will compress less and drain slightly more than normal clay or loam, better workability for less nutrient/water retention
Silty Clay Loam Slightly soapy texture that smears when put under pressure. Sticky. Medium workability, holds nutrients and water well but should not be worked heavily when wet.
Clay Loam Smears under pressure, less sticky than pure clay. Sticky and only moderate workability Treat as clay, high nutrient and water retention but should not be worked when wet.
Sandy Clay Slightly gritty texture, smears. Hard to work. Treat as clay with slightly better drainage, should not be worked when wet.
Silty Clay Slightly soapy texture, smears. Hard to work. Treat as clay with slightly better structural resistance, should not be worked when wet
Clay Near pure clay, hard to work and smears easily Fantastic nutrient and water retention but poor workability and should never be worked when wet
 
In most cases soil types are some form of loam, although if you look at the corners of the soil percentile graph then it is easy to
see if you have a significant amount of clay in the soil, it alters the soil's properties and has more of an impact than other types.
This is why it is never as useful to add sand to soil as people tend to think, but adding small amounts of clay can give a sandy or depleted peat soil a lot of structure!
 
pH
pH is the measurement of acidity in the soil, an acidic soil is commonly called ericacious and needed for the growth of calciphages (azalea, pieris and heather for example). It is important because the pH determines what nutrients are available to the plants, and how easily they are absorbed. This is why many ericacious plants suffer from iron deficiency.
 
The soil pH is of course determined from its parent rock, in Bristol it is mudstone and limestone, therefore giving a generally alkaline soil. 
 
Organic Soil
 
The term organic soil, means having at least 20% of its mass as organic carbon, these soils tend to be extremely rich and fertile, with a dark colour that helps trap heat, In the UK, some areas even have an additional cropping period over mineral soils! This sounds like a fantastic soil to grow in, however the only environments that are conductive to this is where organic matter is slow to decay, such as marshland or peat zones. This is fine for casual growing but the very act of cultivation destroys these environments and makes them rare and difficult to adapt for use.
 
Improving and altering soil
 
When looking at how to alter soil, you should always bare in mind that it takes time and soil will try to revert to its original condition. Despite numerous customers asking how to improve their soil, there is no single solution and you should think what you want out of it, for example a dry thin soil would benefit from adding clay or loam whilst a heavy soil would benefit from lime and organic matter. In addition it is a widespread misconception that fresh soil should be added to old soil, in many cases this is not only pointless, it is quite destructive.
 
Common Mistakes (Please don't do these)
Problem Action Result
Old Soil Add new soil At best, improve soil texture, at worst destroy soil structure, damage plants and dry out soil
Hungry soil Add Manure This will improve soil structure but not feed much at all and potentially (with raw manure) could damage any plants.
Heavy Soil, add drainage Dig Sand in At best, absolutely nothing (unless you use massive amounts). At worst, damage soil structure, dry out surface soil
 
To refresh or improve soil, please consider what is missing and what you want your soil to be!
 
Problem Method of alteration Result
Low nutrients Add fertiliser (Click Here) Extra feed tailored to the plants increases growth
Heavy Soil Dig in organic matter
 (manure or green manure)
Flocculates clay particles and improves drainage, workability and rooting speed
Sandy Soil Add organic Matter or Topsoil/loam based compost Binds to sandy matter and improves retention of water and nutrients. encourages micro-organisms.
Poor Drainage (lawn) Spike lawn and brush sand in the holes This creates tunnels of sand for water to drain down, just making holes or digging in sand separately dose nothing
Poor Drainage (Landscaping) Use Underground drains, pipe work can channel excess water away Soil has less water and less likely to waterlog. (Needs a run-off)
Light dusty soil (often caused by using too much multi-purpose) Add organic matter but especially add loam or topsoil to provide structure. Only common in raised beds Improves retention of water and nutrients whilst providing a structure to root into
Change the pH (to acidic) Add sulphur over time to pots, or line a hole and fill with ericacious compost. (It is very slow and difficult to adjust pH in clays) Soil pH will be reduced, meaning it is better suited for growing ericacious plants, this is a slow process.
Change the pH (to alkaline) Apply fine lime compounds to the soil, check what you use as different materials have different neutralising values and a material that is fine is much quicker to work Soil pH will be increased meaning it is better suited to growing lime loving plants, this is a slow process.

 

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Summery

  1. To improve soil nutrients: Add Fertiliser.
  2. To improve soil texture: Add organic matter.
  3. To improve clay soil: Add Lime, Gypsum or Calcified Seaweed depending on the soil acidity.
  4. To avoid compaction: Do not work or walk on clay when wet.
  5. Adding sand or grit is nearly useless for increasing drainage.
  6. Look above for methods of improving soil.
  7. To reduce damage to soil that is often walked on, try mulching with coarse bark to retain soil structure.

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What is soil? (advanced)

Water retention and available water - Cation/anion exchange capacity - Soil texture and clay

How does water retention and available water work?

Water is a polar molecule, this means it has a high adhesive ability, for a liquid. This enhances the natural surface tension of liquids and helps it cling to other materials, such as skin or soil.

Water is held by the soil by surface tension, small or charged particles hold water to them stronger than large or neutral particles. In addition pore size is an important factor, in small pores (created by small particles) there is less room for water and therefore it is held stronger and harder to access. This is countered by gravity attempting to drain water from the soil.

Therefore if there is a lot of small particles (leading to small pores) then a lot of water is retained due to the high surface tension, if there are large particles such as sand, then there is a lot of water lost because gravity is stronger than surface tension. This is the root of manipulating field capacity. By flocculating clay, you increase the size of pores or by compressing or adding clay you decrease the size of pores.

Macropores: These are large gaps and water is not retained. Normally full of air.

Mesopores: These are medium sized pores and water is normally retained when the soil is at 'Field Capacity' This is where the vast bulk of available water is.

Micropores: These are able to retain water however they hold it so strongly that plants are unable to use it and therefore it is not 'available water'

Field Capacity is how much water is held in the soil after gravity has drained away what it can. A simplified example is a sponge used for washing dishes. After it has been filled with water, water runs out, drawn by  gravity. After it has stopped dripping however it still retains moisture and that is field capacity. Now if you squeeze the sponge, you will still find there is water that is very difficult to remove and that is analogous to the water held in micropores.

Permanent Wilting Point: A plant root is capable of exerting only so much force on the soil, if it cannot withdraw water then it has reached the permanent wilting point and new water must be made available or the plant will die.

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Cation/Anion Exchange Capacity (CEC)

Context: Cations are positively charged atoms, many nutrients are cations as they are positively charged soluble salts.

When you feed plants, you add nutrients to the soil. Why do the nutrients stay in the soil and not simply wash out when it rains and how can this be harnessed?

Cation exchange capacity is, for the purposes of most gardeners, the measurement of the ability of soil to store nutrients, measured in the amount of cations the soil can hold per dry weight. Organic matter, especially Humus (stable decayed organic matter) has the highest CEC commonly found in soil, up to three times that of montmorillite.

As mentioned before, Clay is polar and electrostatic (normally negatively charged), as is humus and this means it is able to attract cations and hold them in the soil, thus there is very little wastage and fertilisers are far more efficient. The soil can only hold as many cations as it has negative charge (so the overall charge becomes 0 or equilibrium. The same is true in positively charged soil, it draws negatively charged nutrients and so holds on to different nutrients! (these are very rare and effectively not present in the UK)

pH

The soil pH (potential of hydrogen ions) does matter. as the soil becomes more acidic then there are more available hydrogen ions (+) which will attach to the soil and therefore lock up soil charge. They are not plant nutrients so basically act as blocking material that stops the soil holding anything useful. This is why acidic soils are generally lower in nutrients than more natural or base soils, and why adding lime to a slightly acidic soil can have such useful benefits. It also explains why it is very easy to change the pH of a sandy soil (H+ ions are effectively taking up more space , 1 in 5 ions is 20% CEC used) but hard to change the pH of clay soils (1 in 30 ions is only 3% CEC used)

Organic Matter

Organic matter is a fantastic way to increase the CEC of the soil, it can hold vast amounts of nutrients and at the same time flocculates its surroundings, providing a better environment for plants to grow in. The downside of this is it is not stable.

Most organic matter degrades over time as it is broken down by bacteria and fungi, in a sense this is good as it releases nutrients but this also makes it hard to utilise in hot places and is why many people will apply manure every year to their garden. Humas is the exception to this as it is already broken down to its most stable form and therefore acts as a permanent reservoir of nutrient storage.

Is there a better option?

Possibly.

The problem with organic matter is that it degrades, therefore we need to find a substance that is not used by anything and therefore is not broken down. Charcoal and coal dust are the obvious materials. Charcoal is slow to break down and holds nutrients well whilst coal dust can persist in the soil almost indefinitely and act as a store for plants, thus enriching soil. However if you have been paying attention then you should realise the entire point of a high CEC soil is to store nutrients and these should be charged with a nutrient source first or they will simply such it out of the surrounding ground. Mixing with raw manure or a liquid feed for 2-3 weeks should provide the nutrients you want before you apply it.

The problem with this approach is that charcoal and such are acidic and therefore reduce the CEC of the surrounding soil, whilst reducing the effectiveness of certain garden chemicals. This has not been fully studied but is an extremely interesting possibility. Wood based materials will linger longer in the soil but they will degrade over time whilst burnt bone and such has been proven to last longer.

Speculation: Adding calcium, magnesium and potassium may reduce acidity and therefore increase available CEC as these are base cations.

For those interested then look up Terra Peta

Looking at the table below (data taken from a wide range of sources), impure garden clays can have upwards of 30 times the nutrient holding and potential fertility of sand!

Average CEC of materials at pH 7
 
Sand 1-5
Sandy loam 5-10
Loam 15-18
Silty loam 15-26
Clay loam 15-30
Clay (garden clay) 30+
Clay (Montmerilliate) 65-100
Vermiculite 70-140
Humus (Pure) 100-300
Depleted Peat 3-5
Biochar 25-130

 

Nutrient Cations
Used in plant pH Reduceing
Ammonium (nitrogen compound, releases Hydrogen ions) Aluminium
Calcium Hydrogen
Magnesium  
Potassium  
Sodium  

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Soil Texture and Clay

Why does clay distort soil texture and drainage so much?

The reason clay is so important in the soil again comes down to size. Take for example 1cm3 of clay and 1cm3 of sand. Clay can be over 1000 times smaller than sand so its hard to get an accurate visual representation but an inaccurate one is almost as good.

If you fill up a tin or bowl with a layer of malteasers (or marbles but malteasers are edible), or some other small object and take note of the empty space you can see roughly the amount of air pores you could find in the soil and you can probably count the number of 'particles'.

Now in another bowl put in a layer of salt or sugar. and try the same. Even with a magnifying glass you will struggle to see reasonable gaps between the grains. Clay has an almost inconceivably larger surface area and a far smaller 'pore space'. In reality most clay compounds are flakes and therefore lie on top of each other and compound this.

Whilst this is quite abstract and it deals with only pure sand and clay, consider how much more surface area clay must have (surface area to volume increases almost exponentially the smaller the object) and therefore how much more of an effect it has on the soil.

Ideally soil should have a 50% volume of air. In a clay soil this is unlikely and in a sandy soil this is generally reached, however the big problem occurs when sand is mixed with clay as not only will it not increase drainage (as sand has a tiny surface area) but the clay will simply fill in the pores between sand particles and create a soil that has the worst aspects of both!

(Important: Don't forget to eat the malteasers (but don't eat the marbles))

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