If you send us a soil sample, we will perform a biological analysis following Dr. Elaine Ingham’s Soil Food Web methodology. The analysis includes counts of bacteria, actinobacteria, fungi, oomycetes, ciliates, and flagellates. But, before you decide to spend the money for a biological analysis, there are few other less expensive tests that are easy to perform and can provide useful insights into your soil health. Here are some basic tests that you can perform:
Soil Texture Test
Find a glass or clear plastic straight sided jar with a tight fitting lid. Quart size canning jars or empty peanut butter jars are ideal. Fill the jar 1/3 full with the soil sample after removing all pebbles and large debris from the sample. Fill the rest of the jar with water leaving a 1 inch air gap at the top. Screw the cap on tightly and shake the contents vigorously. After the contents are well mixed, set the jar on a level surface. After 1 minute, the sand will be settled out. Mark this level with a marker on the side of the jar. After one hour, place another mark representing the silt layer. Finally, place a mark at the top of the clay layer when the water is relatively clear, which may take 1 to 3 days. Measure the thickness of the 3 layers. The height of each layer divided by the total height of all 3 layers represents the relative amounts of sand silt and clay in the sample. The amount of organic matter is not included in this analysis and will mostly be floating in the top part of the water. Finally, use the calculated percentages of sand, silt, and clay to determine the soil texture using the following chart.
Image from: https://i.imgur.com/dhrWN1J.png
Soils in the loam region are a great start for healthy soil, but most soils do not fall in this region. For example, most soils in Houston, TX are in the clay region. Clay soils are difficult to work with because they do not drain well, easily become anaerobic when wet, and turn rock solid when dry. The easiest and most effective way to improve any soil not in the loam region is to add organic matter such as compost. Soils can always use more organic matter. For example, I grow most of my vegetables in 100% compost.
Soil Compaction
Soil compaction is one of the issues that plague many Houston soils due to the high clay content. Compaction can be caused by foot or vehicle traffic on top of the soil which presses the air out over time leading to anaerobic conditions. Penetrometers are used to measure the amount of compaction in soils and work by measuring the amount of force needed to press a spear a certain distance into the soil.
But, instead of using an expensive penetrometer, I like to use a clothes hanger that I’ve straightened into a thin spear. I press the spear into the ground as far as possible before it bends. Then I measure the depth of insertion. In my raised beds that have no compaction, I can easily insert the clothes hanger 16 inches into the soil. This is my benchmark for all other measurements. I can only insert the close hanger an inch or two in the grass pathway between my back door and my raised beds. This tells me that my pathway is extremely compacted.
Macro and Micro Nutrient Soil Test
In the United States, soil samples can be sent to the agricultural extension office of local universities for analysis. Here in Houston, I send my soil samples to the Texas A&M Agrilife Extension. They are capable of testing pH, NO2-N, P, K, Ca, Mg, Na, Conductivity, Zn, Fe, Cu, Mn, B, organic matter, and texture (sand, silt, clay). These tests are great for characterizing soil and pointing out deficiencies in macro and micro nutrients. Here is a link to the Texas A&M Agrilife Extension: http://soiltesting.tamu.edu/
Biological Soil Testing
The above tests are great for learning about the physical characteristics of soil, but provide little insight into the biological processes taking place. Is the soil alive or dead? For example, many commercially available potting soils are sterilized and are therefore “dead” biologically. They may contain all the micro and macro nutrients plants need, but the biology is nearly non-existent. The good news with these potting soils is that nature eventually takes over. Bacteria are the first to arrive, and the other forms of life arrive soon after creating better soil over time.
The results from a soil food web biological soil test can be used to gain insight into the succession of the soil. For example, poor quality soil with little organic matter will be bacterially dominated and will have little to no fungal matter. This fungal to bacterial ratio (F:B ratio) can be used to determine the succession of soil as follows:
F:B ratio = 0.05 Bare soil. No vegetation, compacts easily, plants show signs of low nutrient availability.
F:B ratio = 0.1 Weeds. Soil supports weed growth, problems with compaction, plants have limited root depth
F:B ratio = 0.3 Early successional. Soil has some structure, brassicas (kale, broccoli, mustards) grow well.
F:B ratio = 0.5 Vegetables and grasses. Grass, root crops, lettuce, greens begin to grow well. The soil is starting to be more compact resistant. Soil food web is becoming more diverse.
F:B ratio = 1 Mid-successional. Grasses, vegetables, annual crops, and flowers grow well. The soil is resistant to compaction. Biological diversity is high with an active soil food web.
F:B ratio = 5 and above Shrubs, bushes, vines, trees, forest. As the soil matures, the fungal matter increases. The nitrogen will be mostly in the form of ammonium with very little nitrate.
Send us a soil sample
- The soil or compost sample should be placed in a quart size or larger ziplock bag.
- Write your name, date, location, and type of sample on the outside of the bag.
- If preparing a soil sample, place 3 core samples in the bag. Ideally, the core samples should be 1 inch in diameter and 3 inches deep after scraping off the top layer of leaves and other organic debris. If sampling the soil of a plant or tree, the 3 samples should be taken half way between the trunk and the drip line.
- If the sample is of compost, take 5 or more tablespoon size samples throughout the compost pile and place them all in the ziplock bag.
- The ziplock bag should be 1/3 full or less. Do not press out the air from the bag. The air will allow the microorganisms to survive the trip to the lab.
- Ship the sample to us as quickly as possible, and let us know when the sample will arrive so that we can complete the analysis as quickly as possible.