As per our “Plan of Attack” – This Series will investigate Soil via the plan laid out in Episode 1.
Episode 2 will outline the high level investigation into the Components of Soil and the impact of these on the Soil properties. Episode 2.3 will focus on the Resultant Soil Properties and will build on what we learnt about the different pieces of the “soil component pie” and what it means to us from the perspective of physical properties…..
Previously we identified the Components of the Soil as weathered minerals, organic matter, living organisms and pore spaces (typically filled with water or air). We examined each component from a high level view and considered the inter-relationships briefly.
Mineral Particles and Pore Spaces – See “Soil Series – High Level Components of Soil – (Episode 2.1) – Mineral Particles and Pore Spaces”
Organic Matter – See “Soil Series – High Level Components of Soil – (Episode 2.2) – Organic Matter”
But why do we care about soil components? So we can make an informed decision about how to make a difference to our soil. Okay, we agree on that. But…..
What difference do we want to make? Which direction should we be assisting our soil towards? If there could be a Champion of Soils that we all dream about, what would it be? And in what way could we monitor our path towards this Terra Olympia?
Permaculture Guru says…. it depends.
Oddly enough people start with different soils; they want to grow different plants; they want to integrate different animals, birds, insects, etc into the system; they want to put in different amounts of elbow grease; and they want to integrate the garden into their lifestyle to different degrees….. the ideal is to develop a system that can meet the owner’s needs and evolving abilities in an ongoing and regenerative manner.
Let’s then just attack the last question and the first answer….. if we are starting with different soils how do we monitor where we are and how we are traveling once we start to make changes…let’s pull this together in the form of assessing the Physical Soil Properties.
THE PHYSICAL PROPERTIES OF SOIL –
To Recap…We now know about size distribution of particles (clay, silt and sand); how important each and every mineral is (macro-nutrients vs micro-nutrients, essential vs beneficial, how available are they to the plant – the CEC); that pH has an impact; that pore spaces are critical to micro-climates allowing air and water to reside in the soil; and finally that energy is transferred in all directions (especially cyclically) as nutrients move around the food web.
Before attacking the list of physical properties, it is important to understand that there are 5 major factors which influence soil – the geological origin; the climate (rain and temperature impacts chiefly); the presence (and historical activity) of living organisms; topography (slopes and location on landscapes); and the duration of the exposure of the geological rock to these other three factors. The properties of the soil (chemical, biological and physical) differ depending on depth, due to the different combo of these factors. This change in properties is referred to as the Soil Profile (Elements of the Nature and Properties of Soils, Brady, N.C. and Weil, R.R., 2004).
Typically within productive systems we are looking at the top 300mm of soil, but with different systems and strategies there is benefits in the delivery (and holding) of nutrients and soil life much lower. One key benefit is expanding the potential Carbon holding capacity of the soil (called Soil Carbon Sequestration), simply by increasing the depth carbon (aka roots and other organics/life which follows) can be delivered. But we’ll get onto that later – so looking forward to that!
Having said that, don’t forget we can also raise the soil up too! Effectively increasing the height of the high carbon content soil food web above the poor soils (with compost, mulch etc), whilst improving the poor soil over time with nutrient dissemination.
In short-ish, when considering the soil properties, we need to think about the profile of those properties to understand the complete picture of our Soil Resuscitation requirements….do we just want to restart the heart or do we want the whole body functioning!
Get on with it…. I hear you saying!
The properties of soil are typically examined in the following order of descending importance:(Source)
- Colour and
With a bonus Chemical Property – for completeness….and because I curious as to how it fits!
Take a deep breath…. this is a tough/technical one…..
As mentioned previously, the relative proportions of particles – sand, silt and clay – within the soil are defined as the Soil Texture. As a general rule of thumb, as the particle size decreases the surface area increases as does what happens on that surface – i.e. lots of tiny gaps with edges rather than a small number of big gaps.
But what happens on those surfaces that is so important…… (Elements of the Nature and Properties of Soils, Brady, N.C. and Weil, R.R., 2004 – again I use a little bloggetic license on the info so it makes sense to me! Forgive me, Mr Brady and Mr Weil!)
- Water adheres to these surfaces (surface tension, capillary action, adhesive forces, particulate charges etc all causing attraction) – i.e. the soil holds more water.
- Gases and dissolved chemicals within or passing through the pore spaces are “attracted to and absorbed by mineral particle surfaces” – i.e. the soil can retain more nutrients
- Weathering occurs on the surfaces releasing elements into the soil – the particles themselves deliver more minerals etc
- Soil life tends to colonise and thrive on the surfaces – i.e. more soil life.
The best description I have seen of how to chose the texture to aspire is from, yes you guessed it – Elements of the Nature and Properties of Soils, Brady, N.C. and Weil, R.R., 2004. It shows you why you don’t want to be at any one corner of our Soil Texture Triangle. Remember that Perth is typically sand, loamy sand, sandy loam…. towards the sandy corner of the triangle.
Generalised (again… it depends….there are exceptions) Influence of Soil Separates on Some Properties and Behaviour of Soils (Pg 98. Elements of the Nature and Properties of Soils, Brady, N.C. and Weil, R.R., 2004):So at the beach – so we can get a picture in our mind’s eye of pure sand and the very corner of sand – yep we have ripper drainage, yep great aeration, rapid decomposition but no organic matter (??guess the limited “soil” life about is ravenous, but perhaps more appropriate in the dunes!), yep hot sand even in spring, and yep susceptibility to wind erosion (or sand blasting!). Feeling pretty sorry for that scrub on the sand dunes now!
In Perth, as we move inland, this sand analogy is, sadly, still all to appropriate…. the soil life increases some what, the sand is grey instead of white and plants grow with less exposure to the wind and less salt spray, but we still suffer from rapid drainage (of water and nutrients applied!) and low organic matter. Let’s say as we move across suburbia, we add a little clay, some organic matter and some fertile inorganic mineral elements (fine rock minerals), we might just see ourselves leaving the bottom left hand corner and slowly gaining some clay properties and even a little of the silt properties from the above table. Now we’re approaching loam – a nice balance. But just adding these items does not mean you’ve fixed the problem. There a heaps of ways to help regenerate the soil and provide it with a means to maintain its place or even sustain its path towards loam…..Terra Olympia (in the very general sense!)
In “Soil Series – High Level Components of Soil – (Episode 2.1) – Mineral Particles and Pore Spaces” , I presented a couple of soil texture tests for your enjoyment…., so I wont repeat myself here. But feel free to head back and get your hands (or your jars) dirty.
The Structure of the soil is defined as the aggregation of the soil’s individual particles into larger groups – called aggregates or peds. The pattern of the aggregation effects the pattern of the pores and hence the movement of air and water (with potential nutrients) through the soil. Again its important to note that there can be many types of structural peds within different levels (AKA horizons) of the soil profile and that it’s easier to assess structure when the soil is relatively dry..
Structures are grouped in many ways. One example is:
- Spheroidal – Top or “A” Horizon – 1.1. Granular (very little agglomeration, rounded small lumps, porous – high permeability) Versus 1.2. Crumb (very little agglomeration, jagged edged small lumps, very porous – high permeability)
- Platelike – Usually very deep or “E” Horizon, but can be in any horizon e.g. exposed by erosion – (like a stack of plates, often due to parent material, caused by compaction and drying) – low permeability due to convoluted path through flat horizontal surfaces
- Blocklike – Usually lower or “B” Horizon, but sometimes in “A” – 3.1. Angular Blocky (pointy corners) Versus 3.2. Sub-angular Blocky (less pointy corners!) – both have moderate permeability, more common in humid regions.
- Prismlike – Usually lower or “B” Horizon – 4.1 Columnar (round tops) Versus 4.2 Prismatic (flat, angular tops) – typically arid or semiarid regions, both have moderate permeability
- Some websites also include a group which is delicately termed “Massive” which indicates one enormous clump with no discernible structure…. love descriptions that make sense! Obviously very low permeability
- Other websites also include the group of single grain indicating a lack of agglomeration in some soils.
And a slight variation:
There is also a size descriptor applied – fine medium and coarse – plus a firmness index (also called grade) – strong, moderate and weak – to round off the technical structural descriptions. E.g. “weak, fine, subangular blocky structure”
For info on Perth soil – in great (and I mean brilliant and in depth!) detail head to this link…SOIL GROUPS OF WESTERN AUSTRALIA – A Simple Guide to the main Soils of WA – Ag Dept.
(Source) Mechanisms of soil aggregation:
- Soil microorganisms excretions – cementing agents, binding soil particles together.
- Fungi filaments (called hyphae) – extend through the soil, tying particles together.
- Roots excrete sugars – help bind minerals.
- Oxides – act as glue and join particles together (more common in weathered tropical soils).
- Natural attraction between soil particles due to electrostatic forces.
Soil Structure is also dependent on what the soil was originally developed from – eg erosion materials of exposed rocks deposited down wind or previously water sodden, but now dry river beds.
Then man steps in to add another factor to the mix. For example the soil structure may be damaged by cultivation – both through the use of heavy vehicles leading to compaction / shearing (causing a reduction in the amount of water entering the soil and being held in the root zone, restriction in root growth and the ability of seedling to reach the surface as well as a reduction in oxygen availability of oxygen all through the compaction of pore spaces) and tilling of the soil (which breaks up the formations and leads to exposure of the organic matter to the atmosphere (i.e. semi-infinite oxygen) leading to rapid decomposition / oxidation).
(Source) Soil Structure can be assessed with the following key goals in mind:
- INFILTRATION The ability of soil to accept rainfall, critical for moisture availability to plants.
- WATER RETENTION The ability of soil to hold the moisture (i.e. not run straight past roots).
- AERATION Plants need air to grow. If soils become highly compacted or waterlogged plants fail to thrive.
- FRIABILITY Refers to the ability of the soil to be broken into finer particles with little force… i.e. good seed/soil contact when planted, but allows the seed to germination, and the seedling stem and root to easily pass through the soil.
- SOIL STRENGTH – the dry dense surface crust that can form or other impediment which can restrict plant emergence and root growth
This reference also provides a good comparison but terrible photo of soil (or perhaps mud cake!) under a direct drilling wheat crop versus a conventional tilling wheat crop – this time from the NSW Environment and Heritage Dept.
There are two types of density to consider:
Dp = Particle density – mass per unit volume of soil solids only. This density is defined by the chemical composition and structure within the mineral. In soil the particles are typically quartz, feldspar, micas and colloidal silicates, giving the particle density a normal range of 2.6 to 2.75 Mg/m3 (i.e. Mega grams, 10^6).
Db = Bulk density – mass per unit volume of dry soil. This density accounts for the volume of the pore spaces as well as the soil solids.
Obviously the more the pore space volume, the lower the bulk density and therefore bulk density can be used as a gross reflection of the impact of texture and structure on the ability of water to infiltrate and to be held in the soil as discussed above.
For example, a high bulk density suggests either high sand content (good aeration) or soil compaction (poor aeration) both of which limit water retention within the root zone.
What are we aiming for:
I think we have done this one to death and will do more so in the future. To Recap in brief….. the pores are the gaps between mineral and organic matter and, in general, the more gaps the more air for decomposing, the faster the drainage of water and the more micro-ecosystems.
For completeness, pore spaces are classified as follows: (Source)
- Very fine pores: < 2 µm
- Fine pores: 2-20 µm
- Medium pores: 20-200 µm
- Coarse pores: 200 µm-0.2 mm
When pore space is less than 30 µm, the forces of attraction that hold water in place are greater than the gravitational force acting to drain the water. This is where our soil texture becomes critical….A medium-textured loam provides the ideal balance of pore sizes. Too big and the water and air movement is too rapid – free draining to the extreme – and too small leads to the soil becoming sodden and no air being available. Soil texture determines the pore space at the smallest scale, but at a larger scale, soil structure has a strong influence on soil, aeration, water infiltration and drainage. Please head back to those sections to see the discussion on the benefits, drawbacks and measurements…..
This is one we have not really touched on yet, but, instead of re-inventing the wheel I have found a ripper website of all you need to know about soils consistency….
In a summary of such a great resource (which is completely inadequate), soil consistency is described as the way soil sticks to itself and the surfaces it comes into contact with (cohesion and adhesion). It is classified in the following way:
1. Wet Soil Consistency (e.g. immediately after rain)
(a) Stickiness – squeeze wet soil lump between thumb and finger then open – non-sticky, slightly sticky, sticky and very sticky – i.e. the capacity of soil to stick to other objects.
(b) Plasticity – roll a ball of between your palms to make a sausage – non-plastic, slightly plastic, plastic, very plastic – i.e. the degree to which a soil can be molded or reworked causing permanent deformation without rupturing – how well can you make a sausage with it?
2. Moist Soil Consistency (eg 24hours after rain)
(a) Friability (also known as Rupture Resistance) – take a small ball of the soil and squeeze it between your thumb and finger or in your fist if more effort is needed to crush it – loose, very friable, friable, firm, very firm, extremely firm
3. Dry Soil Consistency (eg air or oven dried soil)
(a) Hardness (also known Rupture Resistance – yes this is true, but on a different scale…) – same as Friability only with dry soil so might not be able to make a ball, labelled – loose, soft, slightly hard, hard, very hard and extremely hard
Alternately the soils can be assessed in terms of the Atterberg Limits: (Source) “The Atterberg Limits (ASTM Test D-4318) define the ranges in moisture content that a soil will behave as a solid, plastic and liquid. The Liquid Limit (LL) of a soil is defined as the moisture content above which the soil behaves as a liquid, and the Plastic Limit (PL) is the moisture content above which the soil behaves plastically (we can make a sausage!). The numerical difference between the Liquid Limit and Plastic Limit is termed the Plasticity Index (PI).
It is important to note that the plasticity is typically seen in finer particle soils, especially clay, and is typically assessed in this detail for construction rather than gardening purposes – eg road bases, dam construction. But when it comes to gardening…. Us plebs can get a rough measure of soil plasticity by the good old ball test or sausage/worm test. (Source, and experience)
The Ball Test – A ball formed with the wet soil and toss it repeatedly from hand to hand. Low-plasticity soils such as silt, fall apart when tossed; Non-plasticity soils, such as sand, cannot be formed into balls; and clay soils, being highly plastic, hold together well.
The Sausage or Worm Test – Using the palms of both hands to roll a sausage with the wet soil. More plastic soils can form longer sausages. A sandy soil, which has no plasticity at all, cannot be rolled. Silt or low-plasticity soils can yield a sausage 3/4 to 1 1/4 inch long. A plastic clay soil can be rolled into a sausage 6 inches long. A sausage from a clay soil can be compressed and made into a new worm many times.
Whilst some clay is good as discussed in other sections, high clay content is not. Adding water to clay may turn it from a solid into a fluid state. A low plasticity soil like sand is subject to erosion by prolonged rains, however saturated clay on a steep slope can suddenly turn the soil into a liquid, resulting in a landslide (or “slip” if you come from NZ!). Check out the texture section to see where we stand any where close to the coast you’ll be several sausages short of the bbq, but in the hills you’ll be cookin’ with gas.
Soil temperature is depended on energy produced, energy absorbed and energy lost. There are various factors that affect soil temperature: (Source)
- Solar radiation – The Sun delivers ~2.0 cal/cm2 min -1 and the impact of this on the soil temperature depends on angle of the Sun relative to the soil.
- Moisture content – A soil with higher moisture content is cooler than dry soil. Due to –
- Evaporation: Whenever water evaporates in the soil, it absorbs heat energy, cooling the soil.
- Rainfall: Rainfall cools down the soil due to the high altitude water source being cooler.
- Condensation – Whenever water vapour condenses in the soil, it releases energy, heating the soil, however this indicates that the soil is cooler than the air containing the vapour and hence is unlikely to be in extreme heat conditions.
- Vegetation – Bare soil quickly absorbs heat readily and becomes very hot during the summer. Vegetation acts as a insulating agent, regulating the seasonal extremes.
- Colour of the soil – Black colored soils absorbs more heat than light coloured soils.
- Tillage – The cultivated soil has greater temperature amplitude as compared to the uncultivated soil.
- Soil texture – Soil textures affect the thermal conductivity of soil. Thermal conductivity decreases with reduction in particle size due to larger pore volumes.
- Organic matter content – Organic matter reduces the heat capacity and thermal conductivity of soil, increases its water holding capacity and has a dark color, which increases its heat absorption tendency. The decomposition of the organic matter also releases heat.
- Slope of land – Solar radiation that reaches the land surface at an angle delivers less energy than the same amount of solar radiation reaching the surface of the land at right angles. The greater the angle the less heat is delivered to the soil.
Soil temperature regulates seed germination, plant and root growth, and the availability of nutrients through composting. The perfect temp depends on the plant, but typically 18 to 24oC is ideal, but with diurnal fluctuations, the minimum night time temp and the maximum day time temps are perhaps more important to consider. Mulching can help to reduce the fluctuations in surface temperature. Fluctuations in soil temperature are much lower with increasing soil depth, becoming insignificant at about 500mm. (Source)
Cornell University has a great site which goes through each common vege and describes many aspects including the temperature requirements for germination etc…. well worth a look. Colorado State University also has a good summary. Unfortunately all temps are in Fahrenheit.
Info from the locals….
The above list of factors affecting soil temperature is a great start if you want thought triggers for adjusting your soil conditions to minimise the extremes (normally just in Summer here in Perth). Other things you can consider are shade clothes, clever plant selection of limit exposed soil and working on that soil texture through soil building measures (which we will get to one day!).
Soil color, while easily discerned, does not affect the behavior and use of soil, however it can indicate the composition of the soil and give clues to the soil’s evolution. The horizons within the Soil’s Profile are often marked by distinct colour variations (Source)
“The process which determine the soil’s colour include the weathering of geologic material, the chemistry of oxidation-reduction actions upon the various minerals of soil, especially iron and manganese, and the biochemistry of the decomposition of organic matter. Other aspects of Earth science such as climate, physical geography, and geology all influence the rates and conditions under which these chemical reactions occur.” (Source)
Soil gurus use the Munsell System of Color Notation to describe the soil based on its hue (a specific color), value (lightness and darkness), and chroma (color intensity). Here is the link, feel free to go wild …. my favourtite is 10YR 2/1. Sounds good, doesn’t it? Or a simplified scale is presented in this link, which in turn references a great little Aussie website, which has documented much of what we are learning here…
For our purposes, the following might satisfy our interest: (Source)
- Dark brown or black colour in soil can indicate a high organic matter content.
- Wet soil will appear darker than dry soil. However the presence of water also affects soil color by affecting the oxidation rate. In well drained (and therefore oxygen rich soils) red and brown colours caused by oxidation are typical. In wet (low oxygen) soils where the soil usually appears grey.
- Aerobic conditions produce gradual colour changes, while anaerobic result in “rapid colour flow with complex, mottled patterns and points of colour concentration.” (Source)
- The presence of specific minerals can also affect soil color.
- The following gives you a good summary of the main contenders (Just to note the exception…..Manganese and nitrogen causes a black color, like sulphur.)
- Bright colours (normally accompanied by strong reds) indicate that the soil is well drained (is not typically under prolonged saturation). Dull colours (normally yellows, and greys, often found together in mottled horizons) indicate the opposite. (Source)
Soil resistivity is a measure of a soil’s ability to retard the conduction of an electric current and is subject to great variation, due to moisture, temperature and chemical (salt) content. (Source)
With increase in the moisture (or increased electrolyte) content, the soil resistance decreases and conductivity increases until about 22% of moisture content, after which there will be very little change in the soil resistivity. (Source)
Above the freezing point of the water, temperature does not impact the soil resistivity significantly but below it, the soil resistivity rises significantly.(Source)
Again, this measure gives a little indication of moisture content, the presence of minerals and the texture of the soil system. The following chart is an example of this interpretation:
9. BONUS CHEMICAL PROPERTY…. PH – just because it seems to fit here too!
Soil pH refers to the acidity or alkalinity (a logarithmic scale of free hydrogen ions (H+) in the soil matrix – note that as a logarithmic scale, small change in pH indicates a significant change in the chemical environment and hence the biological reaction to the change – chemical addition tends to cause more significant short term fluctuations in pH whilst the application of organic matter migrates pH gradually and is regulated by the bacteria who thrive within a limited pH range… more on this shortly). The pH scale ranges from 0 to 14 with pH neutral at 7; values > 7 being basic or alkaline and values < 7 being acidic.
Hydrogen ions are made available to the soil matrix by the dissociation of water, by the activity of plant roots, and by many chemical weathering reactions.
Typically the nutrition and hence growth of plants improves as the pH approaches an optimum level for the plant. Optimum soil fertility is typically pH neutral (6.0-7.2). This is due to the process both within the organic mater and in the mineral “pieces of soil pie”.
MINERAL PARTICLES –
Soil fertility is directly influenced by pH through the solubility of many nutrients.
At a pH lower than 5.5, many nutrients become very soluble and are readily leached from the soil profile. I.e. can lead to deficiencies of phosphorus, calcium, magnesium and molybdenum. Also low pH soils / acid soils can also have excess minerals like aluminum and manganese which may reach potentially toxic levels for some plants.
At high pH, nutrients become insoluble and plants cannot readily extract them. The following schematic presents a useful indication of the impact not only of pH on the nutrient availability to the plants but the difference in that availability dependent on the dominant soil structure.
ORGANIC MATTER –
Bacterial populations are highly dependent on pH and tend to suffer away from 7 and be significantly depleted out to the range of 5-9. Fungi are happiest more at a pH of 5, but can exist within the range of 2-7. Whilst all of this is a generalisation across a huge number of species, it can be seen broadly in the effect on minerals, nitrogen and decomposition occurring within the soil. Organic matter mineralization is accelerated as neutral pH is approached due to better microbial activity linked to happy bacteria. Nitrification and nitrogen fixation are also inhibited by low pH.
It is also interesting to note that some diseases thrive when the soil is alkaline or acidic.
pH is easily measured with home test kits which can normally be bought from any plant stockist and allows us to pick a direction we want to travel. Of course pH may vary in different areas of the garden as previous activity on a particular site may have skewed the pH from its “native” value. “Typically, Western Australian soils have a pH range between 4 and 8.5. In the metropolitan area, soils are more alkaline near the limestone-based coastal sands. Soils further inland and in most agricultural areas are naturally acidic.” (Source 3)
Having said that similar to the consideration of change through the soil profile, the pH of the soil may also change with the proximity to the root systems. The plants release chemicals into the soil for various reasons – sometimes this is to modify the micro climate to attract mutually beneficial organisms which may directly or indirectly alter the local pH. Therefore whilst testing and modification of soil towards the neutral may be beneficial, it may also be unnecessary. Perhaps consider modification if your pH appears extreme, but it is more recommended that pH be used as an key tool when investigating why a particular plant is failing to thrive.
If you do have a problem or are concerned about one approaching…. one thing outside the soil that you might investigate is the type of water you are using – tap, bore, grey etc -, the time of exposure and the amount as these factors both contribute ions to your system as well as mobilising those already present (as a positive) or washing them away. (Teaming with Nutients – The Organic Gardener’s Guide to Optimising Plant Nutrition, Lowenfels, J., 2013)
As an aside this chemical release (whether it be pH altering, pest deterring or/and any other role) should be something considered when planting different plants as companions or replacements. The soil suited to one plant (or modified by that plant) may or may not suit the peer or “superseder”.
The Cation Exchange Capacity (CEC) fits in here too, but is such an interesting topic…. and I have overshot on this discussion already…… that I will devote a topic to this in the future…
To cut a long story (sorry, but congrats for making it this far!) short, most of the physical properties (plus the bonus pH) determine the aeration of the soil; the ability of water to infiltrate and to be held in the soil; and the nutrient availability within the root zone of the plants we wish to nurture.
Next time….. NEMATODES – it takes more than one nematode to make a phylum!
Following that …. SOIL CARBON (will try not to drown us all in this, but this will be a “Polly Pipe” not a warren I need to go down!)