- [email protected]
- (669) 696-3655
Evaluating soil quality is essential part of growing plants and the first step in the process of converting dirt into fertile healthy soil. Soil quality combines physical, chemical, and biological properties of soil and their interactions. You can assess some parameters by yourself but for most of them laboratory testing is required. This guide covers all the aspects of a biological soil test and explains some other tests to help you determine what tests are important and what are less relevant.
Soil testing can be a great tool for diagnosing plant problems and improving soil health. Choosing the right soil test or set of tests will help you find the root cause of the problem. As a result, you will be able to take the right steps at the right time and save a lot of money or even increase your profit.
There are multiple ways to test soil – for pH, nutrients, salts, heavy metals, microorganisms, compaction, texture – and the like. However, not all tests are accurate and some don’t reflect how plants grow in living soil and may lead to wrong conclusions and actions. Even though there are no generally accepted criteria to evaluate soil quality, it makes sense to assess these parameters:
Soil structure is the arrangement of mineral soil particles and organic matter bound together by physical, chemical and biological processes into aggregates. Plants require good soil structure to grow. Aggregates and pores between them allow roots, water and air to easily penetrate and move through soil. Stable soil structure also prevents erosion and increases water-holding capacity.
There are easy DIY methods to check compaction, root depths, aggregates stability and water infiltration that will give you a good idea about your soil structure. Check out this great practical tool – Soil Health Card that consists of 10 simple tests.
Healthy Soil Food Web creates well-structured soil, makes nutrients available to plants, protect plants from diseases and pests and sequester carbon! It is one of the key soil elements (1-minerals, 2-organic matter, 3-beneficial microorganisms) that is often missing in agricultural and residential lands.
Soil Food Web is comprised of macro- and microorganisms. We can not see microorganisms like bacteria, fungi, protozoa, nematodes and micro-arthropods with naked eyes. Therefore a laboratory soil biology analysis is necessary if your plants are suffering from pests, diseases, nutrient deficiencies, weed pressure and soil is compacted or doesn’t hold water. There are different methods to assess soil biology available: direct microscopy, fluorescent microscopy, DNA analysis, plate count, measuring CO2 respiration (biological activity), PLFA and some other methods. Each method has its own advantages and disadvantages, but some of them only suitable for specific purposes and will not show you the actual condition of the Soil Food Web in your soil. For example, plate count can be used to identify presence of specific pathogens in soil or compost, but inappropriate for total species.
For most farmers and gardeners a biological soil analysis via direct microscopy will be sufficient to evaluate soil life. This test provides all the basic information needed about the current population of microorganisms in your soil and soil health. Read in-depth explanation in the section below or visit our store to order biological assessment of your soil/compost. You can also do earthworm counts. If you want to go further, you can order tests for mycorrhizal root colonization and active microbial biomass (fluorescent microscopy). CO2-Burst test for biological activity loses to fluorescent and direct microscopy in accuracy because 1) it doesn’t differentiate between the groups of organisms in the sample generating CO2, 2) it doesn’t give the information about anaerobic organisms that can release methane, for example and 3) other chemical reactions in soil can also release CO2. In addition, the procedure itself can alter the biology in the sample. DNA analysis and evaluation of nematode population (e.g. Baermann Nematode extraction) are types of tests used by researchers and soil professionals.
Soil texture is determined by proportions of sand, silt and clay in the soil. Based on these proportions you can determine soil type using soil textural triangle. Without adequate biology soil texture strongly influences soil physical properties like porosity (water and air holding capacity), permeability (rate of water and air movement), carbon retention, and soil response to environmental conditions like drought or heavy rain. Once microorganisms start building soil structure, soil texture becomes less important. But it is always good to know the percentages of sand, silt and clay in your soil to choose the best management practices.
A number of tests can be used to determine soil structural class but keep in mind that they all have limitations.
– A simple jar test is an estimation of soil texture based on the speed at which differently sized soil particles settle out of suspension from a liquid (Stoke’s Law). In other words, larger particles (sands) will fall faster than smaller particles (silts, clays). Commercial labs offer hydrometer analysis based on Stoke’s Law which in combination with sieve analysis can give more accurate results. However, Stoke’s Law based methods give only approximate values for silt and clay. They cannot be used for saline or difficult to disperse soils and will give anomalous results with calcareous soils (the calcium carbonate should be removed before the test).
– Texture by feel test is a common field method to determine soil structure by squeezing a ball of soil into a ribbon and rubbing a small pinch of soil in palm with forefinger. You can find the full procedure in this USDA NRCS Texture by Feel Guide and watch a few YouTube videos on this topic. The downside of this method is that you must be experienced and familiar with the characteristics of the local soils to get reliable results. For instance, in some conditions clay aggregates are so strongly cemented together that they feel like fine sand or silt. More details by the link provided above.
– Cation exchange capacity (CEC) is a useful indicator of soil structure because clays have higher CEC (more negatively charged sites on their surfaces that adsorb and hold positively charged ions) than sands. You can find the CEC in your standard/routine soil test report. Just keep in mind that aside from soil structure, CEC is also influenced by organic matter content and pH which is affected by soil biology.
Soil organic matter consists of plant, animal and microbial tissues at various stages of decomposition, living microorganisms and substances produced by plant roots and soil organisms. It is another key component of healthy soil. Organic matter in soil provides home and food for microbes (and eventually for plants), improves soil structure, increases nutrient retention, water holding capacity and provides a buffer against soil acidification. Aim for at least 3% organic matter in soil so that it starts performing all those functions. According to Dr. Elaine Ingham, 10% organic matter will be enough to support a thriving food web, but there is no upper limit to organic matter – it will continue to feed more and more microbes, assuming they were present in that soil in the first place.
Soil organic matter content can be roughly estimated by the color of the soil or in a lab. Soil organic matter (SOM) is difficult to measure directly, so labs usually measure and report soil organic carbon.You can multiply this total organic carbon value from your routine soil test by 1.72 to get an estimated percentage of organic matter in your soil. The conversion factor can differ for each soil, but 1.72 gives a reasonable estimate of SOM for most purposes. However, even lab methods are not completely accurate. Variation in the composition of organic matter causes discrepancies in the way that it is interpreted when different methods are used to measure it. Check out this research Comparing Four Methods of Measuring Soil Organic Matter in North Carolina Soils.
Soils can be contaminated with salts, heavy metals, petroleum products and pesticides like herbicides, fungicides and insecticides. These substances are toxic for plants, soil microbes and other living things including people. Soil salinization can occur naturally or due to applications of synthetic fertilizers, lime, gypsum, salty manure, salty irrigation water, especially in dry climate. Poor soil structure, leaving soil bare also increase soil salinity (poor drainage, high evaporation rate) as well as lack of soil microbes. Mining, manufacturing, land application of industrial/domestic sludge and the use of pesticides are main sources of heavy metal contamination of urban and agricultural soils. Heavy metals also occur naturally, but usually at non-toxic levels. Soil microorganisms can tolerate certain amount of salts and help immobilize those salts incorporating them into SOM and biomass. Fungi can clean up oil spills. Many bacteria, fungi and algae are also capable to completely degrade pesticides or break them down into smaller molecular compounds which have no or less toxicity. However, the process of degradation is influenced by many factors, such as the type of pesticide, the type of microorganism, temperature, humidity, acidity etc. Unfortunately, metals do not degrade like organic compounds. Mycoremediation is a promising tool for bioremediation of heavy metals, but it has its challenges and in general it is very difficult to eliminate metals from the environment.
With that being said, if the microscope soil biology tests of all areas showed that the Soil Food Web is flourishing there’s probably no need to test for contaminants. If not or you suspect that chemicals were applied on the property/it is situated next to the road where salts are applied, you might want to test your soil for contaminants. It is useful to test soil for soluble salts (Electrical Conductivity (EC)), pesticides and heavy metals to plan an extra application of biologically rich amendments if contaminants are found.
Extreme levels of soil pH (when soil is too acidic or too alkaline) create problems for plant growth and soil health such as decreased availability of plant nutrients, high concentrations of toxic compounds and leaching. For example, a small drop in pH can result in a significant increase in exchangeable and soluble aluminium, iron and manganese which are harmful for most plants and cause leaching of Ca, Mg, K and Na. Moreover, low pH values reduce the availability of most nutrients and prevent organic matter from decomposition (acids as preservatives). Soil pH is influenced by agricultural practices, soil biology and the kinds of parent materials from which the soil was formed. For instance, soils become highly acidic as a result of 1) oxidation of ammonium and sulfur fertilizers 2) production of strong acids by anaerobic bacteria thriving in compacted soil 3) leaching of basic ions (e.g. Ca, Mg) when water passes through the poor-structured soil. High soil pH are typical for tilled soils because they are dominated by aerobic bacteria that produce alkaline glues. Some soils are formed from basic rocks and have a naturally high pH but, again, it can be altered by biology. In soils with complete thriving Soil Food Web plants control the pH in the rhizosphere. A plant release specific exudates into soil to attract and grow certain species of aerobic bacteria and fungi that produce alkaline (bacteria) or slightly acidic (fungi) enzymes. Those enzymes change the pH in certain areas around the root system creating perfect conditions for that plant. Beneficial biology also retain nutrients in soil including basic ions thereby preventing the pH from dropping to toxic levels.
Considering the above, soil pH can vary greatly across a field and with the Soil Food Web in place may differ from millimeter to millimeter of soil which can’t be captured by pH tests. Anyway, these tests can show if soil pH at the particular spot is in normal range or out of whack. To measure the soil pH you can send a sample to a lab or use one of the in-field methods. In a study comparing 4 in-field methods of indicating soil pH, the hand-held pH meter produced results closest to the average from 82 laboratories.
This soil test provides information on levels and diversity of microorganisms from different trophic levels. Basically it is a snapshot of the current state of the Soil Food Web in a soil. Monitoring soil biology is important because it drives essential ecological processes like nutrient mineralization, aggregate formation, water purification or carbon storage. Read more about the benefits and interpretation of the results of the Soil Biology Assessment.
This type of Soil Biology Assessment is performed using a light microscope according to the technique developed by Dr. Elaine Ingham and other soil microbiologists. One gram of soil/compost is mixed with 4 gram of water (or more if necessary) in certain manner to release microorganisms from water-stable aggregates into the solution. A drop of the solution is placed on a microscope slide, spread evenly and covered with a coverslip. 100X and 400X magnifications are mainly used to identify bacteria, fungi, protozoa, nematodes, micro-arthropods and evaluate aggregation.
When the slide is ready, a quantitative or qualitative approach can be used to assess the levels of Soil Food Web organisms in the sample.
The quantitative approach is useful in a lot of situations: to get a baseline of soil health, test the quality of compost, correct the soil F:B ratio, apply the right amount of compost, tea/extract or monitor the effectiveness of management practices. It implies counting and measuring parameters of organisms in a sample and calculating their biomass and numbers. Results are expressed in estimated numbers per gram of soil for nematodes and protozoa, and in micro-grams per gram of soil for bacteria, fungi and oomycetes:
Fungal biomass and bacterial biomass are used to calculate fungal to bacterial biomass ratio. The values for each functional group and F:B ratio are then compared with optimal ranges for a desired plant based on the stage of succession (e.g weeds, brassicas, grasses, shrubs, trees). For compost results are compared with minimum requirements for bio-complete amendments.
Sometimes the amount of some organisms varies greatly from field of view to field of view, from reading area to reading area. In this case the Standard Deviation of the Mean will be high which indicates low precision of the data. If the drop distributed evenly on the slide, it usually means that there’s not enough organisms of this type in the sample. We always do several assessments to ensure the accuracy of the data and make a note in the report in the comment section. Please see the example of our Full Soil Biology Assessment Report for more details.
*Fungal strands can vary in wet and dry weight significantly. Young fungal hyphae may contain 80% water while the oldest fungi can contain only 5-10% water but may have higher dry weight because of denser cell walls. The value of 1.5 g/cm3 for mostly cytoplasm-filled or cell wall layered fungal hyphae was chosen by the Soil Food Web School based on the research paper of Bakken and Olsen (Bakken, L.R. and R.A. Olsen. 1983.).
**A bacterium has different weight when it is cytoplasm-filled (fully active), partly cytoplasm filled (going dormant) or fully dormant (dry weight). Biomass of bacteria at these different stages cannot be easily differentiated and measured. The factor of 0.9 picograms per bacterium was chosen by the Soil Food Web School as a value in the mid – to – cytoplasm-filled range given by different researchers (Bakken, L.R. and R.A. Olsen. 1983; Lee, S., and J.A. Fuhrman. 1987).
The qualitative approach can be used when rough approximation of biomass or numbers of microorganisms is acceptable. For example, if we only need to know some general information about the sample like what groups of organisms are missing or imbalanced, if the sample is fungal or bacterial dominated, if aerobic or anaerobic organisms prevail, presence of pathogens etc. When we need to assess large amount of samples quickly or save money, this type of assessment might also be the right choice.
For the qualitative assessment the same observation strategy is used as for the quantitative method but without counting or measuring parameters of microorganisms and preparing higher dilutions. Based on the main dilution of a sample and density of the organisms the amount of bacteria, fungi, oomycetes, protozoa and nematodes can be roughly estimated (no/low/high/enough). For the details checkout the sample of our Quick Soil Biology Assessment Report. Sometimes, depending on the experience of a lab-tech, bacterial biomass can be estimated in μg/g without further dilution.
We offer both quantitative and qualitative assessments. For more information and pricing check out our services. In addition to levels of microorganisms each assessment provides information on the diversity of speces.
A diversity of bacteria, fungi, protozoa and nematodes in a sample can be evaluated based on the diversity of morphological characteristics within each functional group. For instance, if we are able to find bacteria of 12-16 different sizes and shapes, that means the sample has a great diversity of bacterial species. Or if we see a lot of bacterial feeding nematodes, and they differ from each other in mouth shapes, lips, internal parts, that means there is a good diversity of bacterial feeding nematodes in the sample. For most purposes this level of evaluation is sufficient for monitoring soil biodiversity.
Changes in oxygen levels, moisture, temperature or other conditions may result in significant changes in soil biology. Therefore samples for soil biology analysis should be collected, packaged and shipped differently from samples for standard soil test. To get representative data the samples should be collected, stored and shipped with minimum disturbance and as fast as possible. Please refer to our Sampling and Shipping instructions for more information.
Copyright © 2024 Veriterra Lab. All rights reserved