Understanding Soil Analysis Results
I Encourage growers to take soil samples to send of to a lab to be analyzed every year before the beginning of the growing season. The reason behind this is because it allows us to identify any nutrients that may be deficient, to know if we need to apply lime to adjust the pH or increase calcium in the soil, if adding gypsum should be considered to assist in salt drainage, if we require a foliar trace element spray, and to understand better the relationship between the fertilizer we put on and the soil, between those two and our crops. It doesn't take much to make a soil analyses that costs around $90 AUD (2022) worthwhile...for a canola crop we only need to produce 90kg of extra canola to pay off that soil analysis and I am confident we will overproduce that almost every time.
In this article I will walk through a sample report provided by APAL on a comprehensive soil analysis. The reason why I chose APAL is simply because I've used this laboratory very often and what you may learn from this article can be applied to most other laboratory results on soil samples. Please know that each lab can have its own processes and this may change the results slightly. It is not good practice to compare the results from two different labs that are on the same soil, so choose one lab that you prefer and use them to keep consistency.
Above is a PDF file of the soil results example provided by APAL. At the top of the report we will find all the details of where and when the soil sample was taken and submitted. Below the submission details we have a table with 6 columns, the first column with the test description, the second with the unit being used to measure what is being tested for, the third is the desired range of the measured nutrient, the fourth column is for measurement of the nutrient or ratio found, the fifth is centimoles/kg (we will get into that later), and last is a visual bar graph to easily identify deficiencies and potential toxicities.
Soil Texture
All soils can be classified as a type of texture made of three main components called sand, silt and clay. Sand being the most course particle, clay being the finest particle and silt being in between. once each particle size makeup is known the triangle below is used to determine the type of soil texture you have. It is important to know what the texture of your soil is as it dictates what your ECEC is (more on that later), your water infiltration rates and water holding capacity, and the capacity of holding fertilizer. You can determine your soil texture using your thumb and index finger but it is always good to have a lab confirm the texture for you.
ECEC
Effective cation exchange capacity (ECEC) measured in centimoles per kilogram of soil. This level found for ECEC is generally the sum of the cations found in the soil such as calcium, magnesium, potassium, sodium, aluminium, and hydrogen. This means the greater the ECEC the more fertilizer can be held with one kilogram of soil, the lower the ECEC the less fertilizer can be held per kilogram. ECEC is influenced by two factors which are the organic material in the soil and the texture of the soil. Organic matter in the soil is incredibly good at holding nutrients to itself, about 20 to 40 time better at holding nutrients than clay, this can be why there is so much emphasis on increasing your soils organic carbon content. Soil texture influences ECEC mainly from surface area but also the composition of the particles but we will mainly just talk about the surface area. The greater the surface area per kilogram of soil the greater the soils ability to hold on to nutrients and water because it can provide smaller spaces and 'pores' for water to hold onto. Sand for instance is 1000 times larger than a clay particle and so has a smaller surface area when compared to 1000 clay particles.
If you're struggling to understand the surface area think of a green apple, the total surface area of that green apple is just the green skin. If you were to cut the apple in half you still have the surface area of the green skin but now you also have the surface area of the white apple flesh that the cut has exposed while still having the same mass, this is essentially how a cluster of smaller objects will have a larger surface area of one larger object with the same mass as that cluster.
Organic Carbon (W&B)
Organic carbon can be a very good indicator of how well your soil can perform for you. A soil with high organic matter can hold a lot of water, fertilizer, provide ideal conditions for microbial communities, is the result of removing carbon from the atmosphere and can provide the slow release of nutrients into the soil for plant uptake. You can have too much Organic matter but I have never seen farmers to ever reach that level, if you're potting plants for you're own pleasure then I still recommend not filling your pots with straight compost.
The best ways to increase the organic matter in the soil is not by applying more to the ground but by changing your practices which can be as simple as not cultivating your soil when it is saturated with water or improving root growth for your plants and reducing the amount of ploughing the soil you do. Don't get me wrong, ploughing is an incredible useful tool but can be overused as well as underutilized.
pH 1:5 water and pH CaCl2
Potential Hydrogen (pH) is measured often in two ways. All labs and pH measurements are standardised with the measurements of 1 part soil to 5 parts water. This is because pH is measuring the concentration of hydrogen ions (what are responsible for acidity, what makes something sour ect.) so if we were to add less or more water to the same amount of soil then we would inevitably get different results making it impossible if a soil is acidic or not! This brings us to our next method, the CaCl2 which stands for Calcium chloride is a solution the lab will make by mixing CaCl2 with water and use that instead of using straight water. Calcium Chloride does not affect the pH directly but usually makes the results more acidic and this is because the calcium in the solution will compete with the hydrogen ions for negatively charged spots on clay particles and organic matter, this means we end up with not only of the pH of the soil solution but also of the hydrogen ions that will potentially be released were we to add lime to the soil. The pH results from a CaCl2 test can lead agronomists and growers to a better idea of how much lime they may need to apply as the two methods can vary widely in results.
Nitrate and ammonium
Nitrate and ammonium are the nitrogen forms that crops and plants will uptake, depending on soil texture, nitrate will often come up low in the soil because it is incredibly mobile through the soil and can easily leech out. Ammonium meanwhile is not readily mobile through the soil and can go through a microbial process to become later available to the plants as nitrogen. This section of the soil analysis is not too critical as nitrogen is very easy to apply and identifying nitrogen deficiencies is pretty straight forward.
Phosphorous
Olsen and Colwell are two different methods for testing the amount of phosphorous in the soil, in my experience Colwell has been more reliable than Olsen in Western Australia. The third one is PBI, which stands for Phosphorous Buffer Index, + Col P, meaning Colwell Phosphorous, this result can provide an idea of how available the phosphorous in the soil is, a higher result meaning less available. It is important to know how much phosphorous is in the soil because is it critical for plants to have access to it during the early seedling stages. Depending on what you're growing and on your program, applying 50kg/Ha of Phosphorous or 4 grams per plant is a good start (remember, that is 50kg of pure phosphorous so you'll need to work out how much you need to add using your own compost/fertiliser, ect). If you're just potting a plant, a premium potting mix should have everything the plant needs to get started. If you're planting native Australian plants they do not like nutrient rich environments so go with the cheaper potting mix options if you can or even just sand from your back garden.
Colwell Potassium
Potassium is very important to plant growth, fruit quality and tolerance to dry conditions. Some areas will apply as much potassium as nitrogen to their crops due to how much it can improve yield and quality. The results from this test are important to know if you have healthy amounts of potassium in your soil but this target range will change with your soil texture. For example, a sandier soil will only need so much potassium in its soil compared to a clay soil that requires a lot more. We will go further into depth on the next set of results.
KCl Sulfur (S)
Sulfur is often forgotten about when growers create fertiliser plans for their crops, every year your crops will remove a small amount of sulfur along with all the trace elements as well. After a few years, yields will start being reduced without any deficiency symptoms being visible. It doesn't require a lot of Sulphur to be applied every year to keep up maintenance levels. If you're an organic grower and you apply adequate amounts of compost then you wont have this issues, for those who are not organic growers, make sure you have a small amount Sulphate of ammonia or potassium sulphate into your ground to ensure nutrition doesn't suffer.
Exchangeable cations & Exch. cation %
My favourite part of the analysis and in my opinion where the next biggest return on investment is after fixing your pH and any deficiencies your soil may have. The exchangeable cations will provide the mg/kg of soil results but also the c.mol/kg (centimole/kg of soil) results which makes up the results for your soils Exch. cation % (exchangeable cation %). Exchangeable cations in the soil are best managed by comparing their quantities to each other and determining their percentage in the soil. The first comparison I look for is to ensure the potassium % is greater than the sodium, I then check if the calcium is greater than 60%, and lastly how much magnesium I want in the soil. Lets look at why for these three reasons:
Potassium > Sodium
Potassium and sodium are very similar in their properties making it hard for plants to distinguish between the two nutrients. Some plants that are tolerant to sodic soils have sodium ion pumps that can remove the sodium from within the plant. The issue for plants taking up large amounts of sodium comes when the weather starts to warm up which causes the cells to expand and rupture. Sodium will generally build up at the tips of leaves and so plants will appear to have been scorched on the edges of mature leaves. Therefore it is important the potassium is at greater concentrations to sodium to avoid this issue.
Calcium Percentage > 60%
Calcium is important for plant uptake and influencing the properties of the soil. Calcium has the ability to reduce the tightness in the soil and assist in water infiltration through the soil profile which can assist with reducing water run-off and removing sodium from the soil profile. The affects of calcium are most noticeable once the concentration reaches above 60%, through leaf analysis calcium uptake as well as magnesium are greatly improved. There is also building evidence that good calcium levels in the soil can help contribute towards healthier micro-organisms in the soil.
Magnesium %
Like calcium, magnesium can manipulate the soil properties as well but will work by tightening the soil, this means magnesium is best kept at 12% in heavier soils. If you're cropping in sandier soils, however, then it is okay to have magnesium go up to 18% without any major affects to the soil.
If you have already dealt with you pH then you should not have to be concerned about the exchangeable hydrogen and aluminium as they will only become more available when the pH becomes increasing lower. If you have exchangeable aluminium from a low pH, then this will be damaging your crops or plants.
Trace Elements
The trace element analysis results are the most straight forward, almost no matter the soil texture, if the results show you're low in iron then add iron, if not then don't, simple. There are incredibly small quantities of trace elements in the soil and are only required as such. From the APAL example soil analysis, nothing is required.
Salts
Salinity is measured by units called siemens which is a measurement of electrical conductivity (EC). The units used in the sample analysis is dS/m which is deci-Siemens per metre, the greater the quantity of salts in the soil the more conductive the soil solution will be. Don't forget, this EC measurement not only measures sodium but also all the other salts such as the fertiliser you apply such as calcium, nitrates, potassium, chlorides, magnesium, sulphates, ect. If you have a high EC reading, meaning that your soil is saline, means your plants or crop will struggle to uptake water. This is because of osmosis and how salts will attract moisture to itself.
With all this information you should now be able to test your soil all on your own! whether you're using Bunnings pH kits, pH and salinity probes, or sending your soil samples off to labs and interpreting the results. There are also leaf, sap, and soil microbial testing available to gain a better insight into how crops and soils are performing, you can check out APAL's website for more details and other soil, plant and disease testing laboratories available.
Happy Growing!