A growing interest in soil health and conservation agriculture, as well as criticism of unsustainable farming practices, have increased the need for soil testing techniques that provide a more holistic assessment of soil health. According to a recent paper by Prof Alan Franzluebbers from the Department of Soil Science at North Carolina State University in the US, soil management worldwide is under threat because of practices such as high-intensity agriculture.
This approach to farming largely ignores soil’s biological function and relies solely on chemical inputs. This has led to soil depletion reaching critical levels in many countries.
In his research, Franzluebbers found that traditional soil testing services overlooked the importance of biological soil health. Soil analyses have always focused on total soil organic matter and chemical indicators of inorganic nutrients such as nitrogen, potassium and phosphorus.
Dr Hendrik Smith, soil scientist at NviroTek Laboratories in Hartbeespoort, North West, supports this view. “Soil health has to be determined on a chemical, physical and biological level, but the latter is often ignored in soil testing,” he says.
Although the Solvita test developed in the US to measure the biological health of soil is commercially available in South Africa, he told Farmer’s Weekly that this technique was still underutilised in the local agricultural sector.
“Soil is the farmer’s most important asset and the engine driving productivity. There is tremendous value in using this soil testing technique.”
What does the test measure?
According to Dr William Brinton, inventor of the Solvita test and founder of Woods End Laboratories in the US, the Solvita test measures the rate of carbon dioxide (CO2) respiration and the presence of organically bound nitrogen in soil. These are both key indicators of soil fertility.
The Solvita kit replaces traditional CO2 tests, can be completed within 24 hours, and is more cost-effective for South African farmers at a cost of about R200 per test. This makes it possible for farmers to manage their soil with greater precision.
To obtain a representative soil sample, Smith recommends that farmers start in one corner of a field and traverse it in the shape of a ‘w’. This should ensure that all the various soil types present are tested.
The dried, weighed sample is moistened with water, which triggers a flush of CO2. The intensity of the burst is measured with a digital colour reader. Measuring the rate of carbon exchange via soil respiration can determine, with greater accuracy, the potential release of nutrients such as nitrogen and phosphorus, which are components of plant residues, microbes and humus.
Information supplied by Solvita indicates that declining rates of CO2 respiration in soil are associated with intensive tilling, soil compaction and excessive fertilisation. As soil quality is depleted, the rate of CO2 respiration also declines.
If farmers can measure CO2 respiration, they will have a more accurate picture of the condition of their soil and can then implement measures to counter soil depletion. For example, if the respiration reading is low, it could be improved with practices such as cover cropping, crop rotation, reduced tillage, and ploughing plant residues back into the soil.
Smith says that the test will also indicate whether farmers are over fertilising by applying too much nitrogen. This occurs often because farmers are unaware of the amount of organic nitrogen already available in the soil. “What many farmers don’t realise is that this forms part of the pool of nitrogen available for crop growth. In some maize production areas, there are up to 800kg nitrogen already present in the soil, so why do farmers apply more?”
According to Franzluebbers, improved nitrogen management is essential. “Sufficient nitrogen should be supplied to crops that need it, while excess nitrogen must be avoided when crops [have an adequate supply] to avoid environmental deterioration,” he says.
Irresponsible soil management
According to Brinton, “astounding” yield gains in the agricultural sector worldwide have been positive for the industry in terms of profitability, but these have come at a price. There are considerable indications that yields are stagnating in many countries in Europe and Asia. He attributes yield declines to factors such as climate change stress, pest and disease build- up, soil erosion, as well as the depletion of soil fertility.
Smith believes that farmers, particularly those with intensive or specialised operations, should take a greater interest in the CO2 and nitrogen levels present in soil.
“There’s room for improvement in South Africa’s soil. If farmers don’t measure [their soil] regularly, there is no way of knowing whether soil management practices are effective or should be adjusted,” he says.
Visit www.solvita.com for further information.