Modern soil testing systems reflect the level of background soil fertility, where the availability of plant nutrients are concerned, specifically phosphorous (P) and potash (K).
In other words, they do not provide an absolute figure regarding the availability of these nutrients throughout a crop’s entire growing season.
Prof. Mike McLaughlin, from University of Adelaide stated: “The higher the background soil fertility, the less the amount of fertiliser required to grow a crop.”
McLaughlin contributed to a discussion on soil health on the latest Tillage Edge podcast.
He confirmed the significance of soil health, adding: “There are a lot of soil health definitions out there; it really depends who you go to.
“But in essence the term reflects the overall physical, chemical and biological activity taking place within a soil. Organic matter is, obviously, central to this. It interacts with nutrient retention, aggregate stability and porosity.
“There’s no definitive test for soil health. Lots of people have got lots of ideas on the subject,” he added.
Soil fertility
A recent study in the United States, carried out by the Soil Health Institute over more than 100 locations, led to a definition of soil health centred on three key criteria: Organic carbon concentration; carbon mineralisation potential; and aggregate stability.
Carbon mineralisation is defined as the rate at which organic carbon can be converted to carbon dioxide (CO2) when dry soil is rewetted.
McLaughlin commented: “The figure relates the amount of carbon that is available to stimulate microbial activity. In turn, this reflects the potential for crop growth within a soil.
“Soils with good aggregate stability will have good water infiltration, aeration and water holding capacity.
“There are lots of other tests for soil health, including the measurement of bacteria elative to fungi.”
The professor explained that two of the three quality criteria identified by the Soil Health Institute can be directly replicated in Ireland – carbon concentration and aggregate stability.
“The issue of carbon mineralisation would have to be specifically assessed in an Irish context. But the principles associated with this criterion remain strong,” McLaughlin said.
He predicted that earthworm numbers within a soil are related to carbon concentration and aggregate stability.
“The US work did not look specifically at earthworm activity. However, they are important within a soil when it comes to determining its structure,” he added.
“However, it’s not possible to determine earthworm numbers within a soil sample that is assessed within a laboratory scenario; they will probably be dead by the time they get there.”