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. Key concepts such as the Soil Food Web will also be address.
THE COMPONENTS OF SOIL –
Soils are composed of weathered minerals, organic matter, living organisms and pore spaces. These pore spaces are typically filled with water or air. The ratio of the components by volume are generically indicated as: (Source)
Obviously the ratio will depend not only on where you are in the world, but also the historical treatment of your soil – by humans, by climate, by time. However it is as good a diagram as any from which to start dissecting soil components (and a good picture for your mind’s eye).
(As an aside for the less advanced of us – Soil Taxonomy (no, not Taxidermy – but that might fit well with the Soil Resuscitation (SR) philosophy!) is the classification of soil into groups of like properties. It seems to be acknowledged as a thankless job as, whilst classifications can be applied within three dimension and within a limited area, it is acknowledged that over a wider area and within the fourth dimension of time, the classification may be redundant depending on the rate of change within space and time. How very Doctor Who….Wibbly Wobbly Timey Wimey Stuff.)
Brace yourself – this is gunna get technical….. Bear with me though – the organic piece-of-pie is much more fun!
Mineral Particles –
Mineral particles present in soil are typically classified in different ways – typically by their physical properties and by their chemical properties (which, to a degree and from a plant accessibility stand point, will be dictated to by the physical properties).
The physical properties (size and number of each size) are the primary means of classifying different soils – (Source: Elements of the Nature and Properties of Soils, Brady, N.C. and Weil, R.R., 2004)
- Clay – <0.002mm
- Silt – 0.002 to 0.05mm
- Sand – 0.05mm to 2mm
- larger rock fragments – granules, pebbles, cobbles/stones, boulders, etc – >2mm
The proportion of each size range (in volume %) dictates the way we describe the overall soil in terms of the Soil Texture. The soils texture effects the suitability of the soil to meet the needs of a given use/user.
Whilst we will investigate the classification of soil further in a future blog, and with more relevance to Perth’s conditions, the following two diagrams are a great way to see how the combinations of the particulates are described and how you might go about determining what you have. Keep these in your back pocket, I’ll come back to them in the future.
In the mean time, have a look and a think. Where do you think Perth “soils” might fit? (Source)
Where does your soil fit? (Source)
Alternately you can use the “Jar Shake Test” to determine your soil. Information is taken from this (Source), but adapted for what we normally do with local soils…..
- Take a soil sample – consider what you want to know…. if you take only the top 50mm, then you will only learn about that; normally the growing zone will be about 300mm deep; however when planting a tree in the area, you might consider closer to 500mm. Having said that you also need to consider the size of the jar you are putting it in! It may also be that taken a few samples from different locations might be useful is areas have had various historical uses and hence a different treatment of the soil.
- If the soil is not dry, spread it out on newspaper out of wind until dry.
- Remove all rocks, stringy roots, etc.
- Crumble lumps and clods.
- Fill a tall, slender jar ~1/2 full of soil.
- Add water until the jar is short of full.
- Put on a tight fitting lid and shake hard for a minute. This shaking should break apart the soil aggregates and separates the soil into individual mineral particles.
- Check to see if it is well/evenly mixed (homogeneous). If not then shake rapidly for another few minutes or, if it looks like your mixing is not working at all, you might have a very hydrophobic sample. Therefore you’ll need to add a teaspoon-ish of dishwashing detergent to help the suspension of the particles.
- Set the jar where it will not be disturbed for 2-3 days (it might take that long if you have silty/clay soil. Alternately it may be a 2 hour experiment if you have silty, sandy soil or less if you are only blessed with sand!).
- Soil particles will settle out according to size.
- After 1 minute, mark on the jar the depth of the sand.
- After 2 hours, mark on the jar the depth of the silt.
- When the water clears mark on the jar the clay level. This typically takes 1 to 3 days, but some soils may take weeks and if this is the case, start dancing because you are on a winner.
- Measure the thickness of the sand, silt, and clay layers.
- Thickness of sand deposit ___________________________
- Thickness of silt deposit _____________________________
- Thickness of clay deposit ___________________________
- Thickness of total deposit ___________________________
15. Calculate the percentage of sand, silt, and clay.
- [clay thickness] / total thickness] = ___________________ percent clay
- [silt thickness] / total thickness] = ____________________ percent silt
- [sand thickness] / [total thickness] = _________________ percent sand
16. Then grab your handy soil texture triangle and look up your soil texture – chances are, if you’re in Perth and have not yet worked a little magic on your garden, you are in the bottom left corner of the triangle.
The attached link provides Perth-specific information for the different regions in a clever interactive arrangement. (07/10/2014 – unfortunately this site has now been closed, but I am currently in discussions with the provider to see if we can host it locally – I’ll update the link when we determine if we can resolve the issues. Thanks for your patience, but I’d hate to lose such an easy to use website due to compatibility issues!) Needless to say, the summary is the limey sands of the Quindalup soils versus the brown or yellow sands of the Cottesloe soils versus the sandy loams over a layer of clay in Guilford soils – so place us all in the bottom left corner of the texture triangle.
You might have heard it said that the ideal soil is a “loam” which (on the triangle) is a blend of sand, clay and silt. All three have excellent properties when present in a mix, however the extreme corners of the triangle result in “soils” unable to meet the needs of growing plants. Too much of anything is a bad thing…. a rule for life?
But what does this mean? Where should we aim to get to in our urban or rural edible garden? What additions can we make to our “soil” that have the potential for sustained benefits into the future with limited ongoing input? Big questions, but we’ll figure out the answers together.
The other key classification, Chemical Properties, within the Mineral Particles piece of pie, is based on the presence of specific minerals. Whilst the size and size distribution is the key to water and nutrient holding, but without any nutrients you have nothing to support life. So, onto the nutrients….
The nutrients can be split into Macro- and Micro-nutrients. Macro nutrients are required by plants in large amounts, whilst the remainder are required in trace amounts.
Macronutrients: (Life Sciences Vol I, Ahuja, M, 2006) – We’ll look in more detail at the most interesting of these at a later date. The following is quoted from the book as it provides the best summary (and most copied across the internet without reference!) that I have been able to locate.
- Nitrogen (N) is a major component of proteins, hormones, chlorophyll, vitamins and enzymes essential for plant life. Nitrogen metabolism is a major factor in stem and leaf growth (vegetative growth). Too much can delay flowering and fruiting. Deficiencies can reduce yields, cause yellowing of the leaves and stunt growth.
- Phosphorus (P) is necessary for seed germination, photosynthesis, protein formation and almost all aspects of growth and metabolism in plants. It is essential for flower and fruit formation. Low pH (<4) results in phosphate being chemically locked up in organic soils. Deficiency symptoms are purple stems and leaves; maturity and growth are retarded. Yields of fruit and flowers are poor. Premature drop of fruits and flowers may often occur. Phosphorus must be applied close to the plant’s roots in order for the plant to utilise it.
- Potassium (K) is necessary for formation of sugars, starches, carbohydrates, protein synthesis and cell division in roots and other parts of the plant. It helps to adjust water balance, improves stem rigidity and cold hardiness enhances flavour and colour on fruit and vegetable crops, increases the oil content of fruits and is important for leafy crops. Deficiencies result in low yields, mottled, spotted or curled leaves, scorched or burned look to leaves.
- Sulphur (S) is a structural component of amino acids, proteins, vitamins and enzymes and is essential to produce chlorophyll. It imparts flavour to many vegetables. Deficiencies show as light green leaves. Sulphur is readily lost by leaching from soils and should be applied with a nutrient formula.
- Magnesium (Mg) is a critical structural component of the chlorophyll molecule and is necessary for functioning of plant enzymes to produce carbohydrates, sugars and fats. It is used for fruit and nut formation and essential for germination of seeds. Deficient plants appear chlorotic, show yellowing between veins of older leaves; leaves may droop.
- Calcium (Ca) activates enzymes, is a structural component of cell walls, influences water movement in cells and is necessary for cell growth and division. Some plants must have calcium to take up nitrogen and other minerals. Calcium is easily leached. Deficiency causes stunting of new growth in stems, flowers and roots. Symptoms range from distorted new growth to black spots on leaves and fruit. Yellow leaf margins may also appear.
Micronutrients (trace elements): (Life Sciences Vol I, Ahuja, M, 2006).
The micronutrients (also called trace elements in common speak!) are defined as either essential or beneficial. Essential is defined as a nutrient where a plant is unable to complete its lifecycle in the absence of the mineral, the function of the element is not replaceable by another mineral element and the element must be directly involved in the plant’s metabolism. Beneficial mineral elements are those that can compensate for the toxic effects of other elements or can replace a mineral nutrient in function not directly related to plant metabolism. Omission of the beneficial elements for a plant will result is the suboptimal performance of the plant.
Having said that a beneficial element for one plant may be essential for another, so the definition should not be taken as the recipe for all plants.
The following is quoted from the referenced book (Life Sciences Vol I, Ahuja, M, 2006) but with italic-ed comments from me to include information from elsewhere in the book:
- Iron (Fe) is necessary for many enzyme functions and as a catalyst for the synthesis of chlorophyll. It is essential for the young growing parts of plants. Deficiencies are pale leaf colour of young leaves followed by yellowing of leaves and large veins. Iron is lost by leaching and is held in the lower portions of the soil structure. High pH (alkaline) conditions render iron unavailable to plants. Essential
- Manganese (Mn) is involved in enzyme activity for photosynthesis, respiration, and nitrogen metabolism. Deficiency in young leaves may show a network of green veins on a light green background similar to an iron deficiency. In the advanced stages, the light green parts become white, and leaves are shed. Brownish, black, or greyish spots may appear next to the veins. In neutral or alkaline soils plants often show deficiency symptoms. In highly acid soils (low pH), manganese may be available to the extent that it results in toxicity. Essential
- Boron (B) is necessary for cell wall formation, membrane integrity, calcium uptake and may aid in the translocation of sugars. Boron affects at least 16 functions in plants, including flowering, pollen germination, fruiting, cell division, water relationships and the movement of hormones. Boron must be available throughout the life of the plant. It is not translocated and is easily leached from soils. Deficiencies kill terminal buds leaving a rosette effect on the plant. Leaves are thick, curled and brittle. Fruits, tubers and roots are discoloured, cracked and flecked with brown spots. –Sounds pretty Essential to me!
- Zinc (Zn) is a component of enzymes or a functional cofactor of a large number of enzymes, including auxins (plant growth hormones). It is essential to carbohydrate metabolism, protein synthesis and intermodal elongation (stem growth). Deficient plants have mottled leaves with irregular chlorotic areas. Zinc deficiency leads to iron deficiency, causing similar symptoms. Deficiency occurs on eroded soils and is least available at a pH range of 5.5 – 7.0. Lowering the pH can render zinc more available to the point of toxicity. Essential
- Copper (Cu) is concentrated in roots of plants and plays a part in nitrogen metabolism. It is a component of several enzymes and may be part of the enzyme systems that use carbohydrates and proteins. Deficiencies cause die back of the shoot tips, and terminal leaves develop brown spots. Copper is bound tightly in organic matter and may be deficient in highly organic soils. It is not readily lost from soil, but may often be unavailable. Too much copper can cause toxicity. Essential
- Molybdenum (Mo) is a structural component of the enzyme that reduces nitrates to ammonia. Without it, the synthesis of proteins is blocked and plant growth ceases. Root nodule (nitrogen fixing) bacteria also require it (Exciting topic for the future). Seeds may not form completely, and nitrogen deficiency may occur if plants are lacking molybdenum. Deficiency signs are pale green leaves with rolled or cupped margins. Essential
- Chlorine (Cl) is involved in osmosis (movement of water or solutes in cells), the ionic balance necessary for plants to take up mineral elements and in photosynthesis. Deficiency symptoms include wilting, stubby roots, chlorosis (yellowing) and bronzing. Odours in some plants may be decreased. Chloride, the ionic form of chlorine used by plants, is usually found in soluble forms and is lost by leaching. Essential
- Nickel (Ni) is required for the enzyme urease to break down urea to liberate the nitrogen into a useable form for plants. Nickel is required for iron absorption. Seeds need nickel in order to germinate. If nickel is deficient plants may fail to produce viable seeds. Essential
- Sodium (Na) is involved in osmotic (water movement) and ionic balance in plants. Essential
- Cobalt (Co) is required for nitrogen fixation in legumes and in root nodules of non-legumes. The demand for cobalt is much higher for nitrogen fixation than for ammonium nutrition. Deficient levels could result in nitrogen deficiency symptoms. Beneficial
- Silicon (Si) is a component of cell walls. Plants with supplies of soluble silicon produce stronger, tougher cell walls making them a mechanical barrier to piercing and sucking insects. This significantly enhances plant heat and drought tolerance. Silicon may be deposited by the plants at the site of infection by fungus to combat the penetration of the cell walls by the attacking fungus. Improved leaf erectness, stem strength and prevention (or depression) of iron and manganese toxicity have all been noted as effects from silicon. Beneficial
Tip from the Guru is that Selenium should also be considered as an important element and is deficient in WA soils…. luckily you can get this by using kelp additives. I’ll research this more and get back to you, but so far it looks pretty important for cattle when it comes to cellular membranes!
You will see from the above discussion that pH is a really important factor in soils with respect to the availability of the elements. So many factors influence pH, but the most conducive conditions for plant utilisation of the minerals available is a pH of 6.5 to 7. The topic of pH is a whole long-winded blog of its own, so again, let’s just keep this in mind until we get the chance to run through it in detail. What do we aim for, and what we can do to help the system achieve (and maintain) that goal in the long term?
Another important factor in understanding the availability of nutrients in the soil is called the Cation Exchange Capacity (CEC). This reflects the extend to which the soil can store the mobile mineral elements (primarily N, P, K).
Pore Spaces – (i.e. the holes between mineral particles and organic materials)
Notably omitted from the above elements are Carbon (C), hydrogen (H) and Oxygen (O). These are supplied by the air (both in the soil and atmosphere), and the water (typically from the pore spaces of the soil). We’ll go into the interaction between air, water and the plants in more detail in a future blog.
Organic Matter – Still to come….. “Soil Series – High Level Components of Soil – (Episode 2.2) – Organic Matter”
THE PROPERTIES OF SOIL – Still to come….. “Soil Series – High Level Components of Soil – (Episode 2.3) – Resultant Soil Properties“
Information Added Post-Publishing –
Just to follow up on a query asked regarding organic matter behaviour in the Jar Shake Test –
Although you should remove the large pieces of organic matter to avoid them interfering with your settling, the remaining large organics will typically float to the top of the water. The finer “wettable” organics remaining within the soil will typically settle between the silt and clay layer. (Some good photos can be found here) It is however not recommended that you use this test to determine if you have “adequate” organic material present. Organic material will be transient due to the nature of the soil food web and hence I will refer you onto the Organic Matter section of the Components of Soil Series. The more organic material the better as a general rule.