STEWARDSHIP OF THE MICROBIAL ECOSYSTEM OF AGRICULTURAL SOIL

Dr. Bridgitta Steyn Pr.Sci.Nat.

21 August 2018

“Soil is a basic treasure. Soils produce good yieldsand keep on doing so if they are well managed.The management of soil is among the oldest of thearts, but none is changing more rapidly than it. Weknow more about taking care of the soil than ourfathers and grandfathers did. There is much morethat we should know.”

Ezra Taft Benson

Soil fertility:• capacity of soils• to supply essential plant nutrients• in amounts and forms • readily available to plants

(or fertility)

Soil Biology and Soil Fertility

• Least well-understood component• Ancient practice dating back to 300BC• First inoculant ‘nitrogen’ – 1896• 15 years before the Haber-Bosch process

– Liquid ammonia– 450 °C– 200 atm

Soil Fertility

• Active– Immediately available

• Potential– Becomes available

• Chemical or microbial action on minerals and organic matter• Mineralization = Conversion of complex molecular compounds into

organic forms carried out by microorganisms

“A soil devoid of micro-organisms is a dead soil” Jacob Lipman

Some of the organisms associated with the roots are attracted and nourished by molecules secreted from the roots, and then take part in 2-way signaling and exchange of nutrients.

This situation clearly parallels the organisms associated with the mucus layer in the colon, where the mucus nourishes the microbiota, which in turn provides nutrients and metabolic signals (e.g. short chain fatty acids) to the host.

Kumar et al. 2016 Frontiers in Plant Science

https://www.noble.org/globalassets/images/news/ag-news-and-views/2013/10/pdf/soil-health.pdf

Building soil health can be accomplished by employing five principles:1.Armor the soil.2.Minimize soil disturbance.3.Increase plant diversity.4.Keep living roots in the ground all year.5.Integrate livestock grazing.

1. Armor the Soil• It holds the soil in place.• Reduces soil evaporation rates.• Helps maintain a more moderate range of soil

temperatures.• Reduces soil compaction by raindrops.• Provides a protective habitat for the surface dwellers of

the soil food web.

2. Minimize Soil Disturbance• Biological disturbance = overgrazing• Chemical disturbance = over application of nutrients and

pesticides • Physical disturbance = over time tillage

• Water erosion• Wind erosion• Water ponding• Easy crusting• Depletion of soil organic matter

3. Increase Plant Diversity

• The soil food web use to receive carbon exudates (food) from a

diversity of perennial plants, it now receives carbon exudates

from only one annual plant species at a time resulting in less

biodiversity in the soil food web.

• Diverse crop rotations provide more diversity, benefiting the soil

food web

4. Keep Living Roots in the Soil• Live plant root exudates = primary food source for the microbial

component of the soil food web.• Continuous growth of plants throughout the year = root

exudates feed the microbial component of the soil without interruption.

• Cropland systems = usually one crop per season with dormant periods before the next crop is planted. No live plant roots feeding the microbes exudates and they starve to death.

• Cover crops have the following benefits:• Carbon exudates = feed the microbes in the soil food web.• Building soil aggregates and pore spaces = improve infiltration.• Cover the soil = controlling wind and water erosion, soil temperature

and rainfall compaction.• Catch and release of inorganic nutrients = improving water quality.• Salinity management.• Provides food and habitat to pollinators.• Weed suppression.• Food, habitat and space for wildlife.• Integration of livestock to the system.• Adding crop diversity.• Adjusting the cover crop combination’s carbon/nitrogen ratio, to either

accelerate or slow decomposition.

5. Integrate Livestock Grazing• Returning livestock to the system will:

• Convert high carbon crop residue to low carbon organic material, balancing the C/N ratio with fall or winter grazing.

• Spring or summer grazing with short exposure periods followed by long recovery periods will allow plants to regrow and harvest additional sunlight and CO2, thereby feeding the microbes in the soil.

• Grazing of the crop residues in the fields recycles the majority of nutrients, minerals, vitamins and carbon back into the soil.

• Grazing reduces weed pressure.

‘Soil is essential for the maintenance of biodiversityabove and below ground. The wealth of biodiversitybelow ground is vast and underappreciated: millions ofmicroorganisms live and reproduce in a few grams oftopsoil, an ecosystem essential for life on earth…’

Australian Soils and Landscapes, An Illustrated Compendium

We are only focusing on the tip of the iceberg, which is the aboveground biodiversity, while the rest of the iceberg, which is the belowground biodiversity, is being neglected

Microorganisms = ‘cooks’

Rhizosphere = ‘kitchen’

SOM + fertilisers = ‘ingredients/groceries’

Microbial enzymes = ‘recipes’

Simple ionic compounds = ‘prepared dishes’