Soil is a complex mixture of organic matter, minerals and countless organisms both alive and dead. To understand whether or not soil is healthy, all of its interconnected properties — physical, chemical and biological — must be evaluated.
Through their interconnectivity, these three properties play a crucial role in soil health and affect key parameters such as aeration, soil organic matter and pH. Coming full circle, those same soil health indicators play a vital role in microbial activities, such as carbon and nutrient cycling.
Why it matters: A good soil assessment looks beyond chemical and physical attributes and also considers biological health. The diversity of soil microorganisms is great and their needs should be considered in your soil management plan.
A good soil assessment, therefore, looks beyond chemical and physical attributes to further evaluate that third component — biological health. Yamily Zavala, Soil Health Lab manager at Chinook Applied Research Association (CARA), provided an overview of soil health assessments at the last November’s Farm Forum virtual conference, with particular attention to soil microbial communities and their functions.
As manager of CARA’s Soil Health Lab, Zavala’s focus is on the evaluation of physical and biological soil properties. Regular and thorough assessments allow growers to monitor changes in soil properties as they work to better soil management decisions. There is a great diversity of soil microorganisms. Their needs should be considered when starting a new soil management plan, said Zavala.
Regardless of the lab you choose to carry out your soil assessment, make sure the most important soil health indicators are evaluated, recommends Zavala.
Soil aggregates stability
Samples are first evaluated for soil aggregate stability. This is done by placing soil in a sieve and running water through it.
“When the soil is stable, aggregates will stick together and remain in the sieve,” said Zavala.
The higher the number, the better the aggregate stability.
Soil microbial activity
Lab technicians also assess samples for bacteria, fungi, protozoa and nematodes. In fact, soil microbial respiration is one of the most important biological soil health indicators. Using a test developed by Cornell University, soil microbial activity can be measured by releasing the carbon dioxide trapped in the soil into a chamber. Larger respiration values indicate higher activity by soil microbial communities, said Zavala.
Having a strong soil microbial community will improve physical and chemical constraints. When soil is managed appropriately, active carbon is an indicator of the small portion of soil organic matter readily available as a food and energy source for soil microbial communities. Having food for the biology in the soil helps maintain a healthy soil food web, Zavala said. It’s also a good indicator of soil health responses to changes in crop and soil management.
Most farmers understand the chemical aspect of their soil health assessments. However, the biological aspect is just as important, she said. The fungal-to-bacterial ratio, for instance, is an important indicator of soil health.
In the lab, soil samples are diluted into special solutions and a portion is processed to assess biophysical indicators. Bacteria are then counted and measured.
“Each creature in the soil has a function,” said Zavala.
Ciliates, for example, are indicators of physical soil health constraints and play an important role in the soil food web. Protozoa are single-celled microbes that eat bacteria, other protozoa, soluble organic matter and sometimes fungi — and when they do, they release nitrogen that can be used by the plants and other members of the soil food web.
Nematodes are also a good indicator of soil health, said Zavala. Where there are too many nematodes, particularly of just one species, soil health is likely poor. Too few nematodes is also an indication of poor soil health.
Nematodes play an interesting role in the soil. They feed on other soil microbes — some of which are bad for the health of the crop — including bacteria, fungi and predators.
“Nematodes, in general, are considered the bad guys,” said Zavala. “But less than 10 per cent of them do damage to your crop.”
The rest of them are “good guys,” she said, because they improve nutrient cycling.
Fungal-feeding nematodes attack fungal communities in the soil, including the ones that are not good for the crop. If fungal communities are not present, warned Zavala, they will feed on the roots of crops. When microbial communities are in balance, however, they act as a natural biological control in the soil. Applying too many chemicals to the soil kills natural predators, she added.
A soil test that includes a complete assessment of the soil’s physical, chemical and biological attributes is critical, Zavala emphasized.
“It is important to benchmark your soil to monitor changes in soil biology and correlate them with any improvements on the specific constraints you have in the soil,” she said.
Too often, soil tests evaluate micro and macronutrients present in the soil, as well as organic matter, pH, soluble salts and cation exchange capacity, but forget to assess the microbial communities present.
“Healthy soils have lots of microbial interactions,” said Zavala. “Soil without biology is dead soil.”