How to test soil life and health


By Marleen Smith | 10 October 2017
soil
In healthy soil there is a balance between biological, chemical and physical processes. Agriculture disturbs this balance and it is up to the farmer to restore it.

Soil testing has rapidly advanced and now, instead of offering only chemical data as part of scripts, tests are increasingly being conducted in both laboratories and on farms at reasonable cost.

Old underwear and tea bags are two of the instruments American farmers are using to affordably measure biological life in soil. One of these farmers, Chris Teachout, is Iowa’s top conservation farmer and farms close to the town of Shenandoah. Teachout buries old, white cotton underwear in various locations on his land to compare biological activities from the sites.

He gets a clear picture of soil organisms’ activities in different areas by observing the colour and tempo of the underwear’s decay. He also weighs the underwear after removing it from the ground to measure results more accurately. He says cotton cloth that has been cut to the same shape and size of the underwear, will work just as well.

Teachout also employs another innovative and internationally recognised testing method – the so-called teabag index. He buries both black and rooibos teabags about 8cm deep for a specific amount of time, after which the bags are dug up, dried and weighed. This process can be augmented by also determining the carbon content of the tealeaves in a laboratory before and after burying and removal from the ground.

At the Menoken experimental farm in North Dakota, the policy is to keep soil tests as easy and affordable as possible. Recycled waste and commonplace consumer goods are used to test and compare aggregate stability in different soil samples, while chemical colour strips are used to determine the nitrate content, and thus carbon leaching levels.

MORE ACCURATE OPTIONS

Solvita: More sophisticated tests, like Solvita’s tests, to routinely measure biological soil activity, are also available. The test measures carbon respiration in soil samples as a key indicator of soil life. All soil organisms, whether bacteria, fungus, insects or other animals, have a metabolism, which means they utilise waste material and release carbon dioxide. Overhead soil respiration is commonly accepted as an indicator of how biologically active the soil is, which means it can be used to compare samples from various locations, as well as the improvement or decline of soil activity over time in the same location. Different types of Solvita tests are available. They vary from options that allow the farmer to do his or her own testing, to laboratory tests.

Haney: The Haney test was developed by Dr Rick Haney, agronomist at the American department of agriculture, after he found that soil testing methods which were affordable and easy to use for farmers, were hopelessly outdated. The test also measures microbial activity and was made available commercially after four years of testing. Haney’s test includes a Solvita test to determine the amount of carbon dioxide organisms released into a soil sample over 24 hours. It also measures the amount of organic carbon that can be extracted from the sample with water. This kind of carbon occurs roughly 80 times less than the total percentage of organic material in the sample and is the energy source that soil microbes feed on. It is also a good indicator for microbial activity in soil. Haney’s test also measures the amount of organic nitrogen that can be extracted from the sample with water. Inorganic nitrogen (NH₄-N en NO₃-N) is deducted from this. The result is related to the amount of organic nitrogen in the soil that can easily be broken down by microbes and released into the soil as inorganic nitrogen which can then be easily utilised by plants. Haney’s test results also give an indication of the carbon:nitrogen ratio in a soil sample, which is the measure of the tempo at which nitrogen and phosphorus mineralise in the soil and is made available for absorption by plants.

PLFA: The PLFA test analyses phospholipid fatty acids in the cellular membranes of soil organisms, which includes from bacteria to animals and plants. These fatty acids break down relatively rapidly as soon as an organism dies and the membrane begins to digest. A fairly accurate indication of which organism groups are present in the soil sample can be determined by analysing the fatty acids. These can include actinomycetes (a group of bacteria), arbuscular Mycorrhiza (a group of fungi that penetrate plant roots), Rhizobia (the bacteria that captures nitrogen from the atmosphere and imparts it to the soil) and protozoa (single cell organisms in the soil). Each of these groups plays an important role in the maintenance and improvement of soil fertility. A PLFA test provides a picture of the microbial community in a soil monster, indicates which functional groups are present in which concentration and how large the microbial community is in its entirety.

DNA analysis: DNA analysis has been widely used in academic laboratories and is used to determine the presence of specific, individual species in a soil sample’s microbial community. This kind of testing is, however, not affordable for farmers to use routinely. Dr Hendrik Smith from Grain SA in South Africa, says they are investigating possibilities on this front.

Enquiries: Willie Pretorius, email: willie@soilhealthsolutions.com; cell +27 83 458 9854