Regenerative Agriculture: A soils-composition primer

Soil health, according to the USDA, is "... the continued capacity of soil to function as a vital living ecosystem that sustains plants, animals, and humans." Agriculture Victoria defines soil health as "... the condition of the soil in relation to its inherent (or potential) capability, to sustain biological productivity, maintain environmental quality, and promote plant and animal health."

Photo credit: USDA

Soil health is one of the key pillars of the Regenerative Organic Certification and a key objective of Regenerative Agriculture as a discipline. In order to fully understand the principles guiding the soil health requirement of Regenerative Agriculture, we need an understanding of soils. I present a soils primer in this post.

The essential functions of soils are (NRCS.USDA.gov):

• Regulating water -- soil helps control where rain, snowmelt, and irrigation water go. Water and dissolved solutes flow over the land or into and through the soil
• Sustaining plant and animal life -- the diversity and productivity of living things depend on soil
• Filtering and buffering potential pollutants -- the minerals and microbes in soil are responsible for filtering, buffering, degrading, immobilizing, and detoxifying organic and inorganic materials, including industrial and municipal by-products and atmospheric deposits
• Cycling nutrients -- carbon, nitrogen, phosphorous, and many other nutrients are stored, transferred, and cycled in the soil
• Physical stability and support -- provides a medium for plant roots.

The major soil components are illustrated in the chart below and then detailed in the text following.

Approximate composition of soil
(Source: https://www.ctahr.hawaii.edu/mauisoil/)

Soil Components
Soil is comprised of air, water, mineral particles (a mix of clay, silt, and sand), organic matter (decomposing plant material), and organisms (bacteria, algae, fungi, earthworms, insects, etc.).

Soil Minerals
The earth is made up of varying proportions of the 90 or so naturally occurring elements but, according to Alex Maltman (Vineyards, Rocks, & Soils), four of these -- oxygen at 48%, silicon at 28%, aluminum at 8%, and iron at 6% -- are responsible for 88% of its composition. In most geological materials, these elements combine to form minerals -- "a naturally occurring combination of specific elements that are arranged in a particular repeating three-dimensional structure or lattice" (opentextbc.ca, Minerals and Rocks).

In nature, minerals are found in rocks "and the vast majority of rocks are composed of at least a few different minerals." The picture below shows a piece of granite and its constituent minerals.

A close-up view of the rock granite and associated minerals
(Source: opentextbc.ca)

Jackson (Wine Science: Principles and Applications) stipulates that (p. 245) "... the mineral content of soil is primarily derived from the parental rock substrate." Soil minerals play a vital role in soil fertility in that (i) mineral surfaces serve as potential sites for nutrient storage and (ii) the weathering of primary minerals yields smaller particles that we call soil as well as releasing nutrients into the soil. The figures below show the weathering of rocks into minerals.

Source: geology.csupomona.edu

Soil Organic Matter (SOM)
What is SOM? "The organic fraction of the soil that includes plant, animal and microbial residues in various stages of decomposition, biomass of soil microorganisms and substances produced by plant roots and other soil organisms."

The soil components chart above shows organic matter comprising approximately 5% of the total soil but it plays an outsize role in the health of the soil and the crops raised therein. The distribution of SOM components are shown in the chart below.

Approximate distribution of organic matter in healthy soils
(Source: nrcs.usda.gov)

SOM makes up a small portion of the overall soil but has an outsize influence on soil functions (see Table 1 below).

Table 1: SOM influences on soil functions

Soil Function	SOM Influences
Water management	Helps soil create large pore spaces and channels that allow water to infiltrate and drain and small pore spaces that hold on to water Residues on the surface protects the soil surface from atmospheric elements
Soil structure	Plant exudates and microbial byproducts can be sticky substances that help soil particles held together to form and stabilize aggregates Physical benefits of increased aggregation     - Better aeration     - Better friability (crumbly): ideal rooting medium for plants     - Less crusting        - Crusting prevents water and air movement into the soil        - Can prevent seedlings from emerging        - Promotes water runoff Biological benefits of increased aggregation      - A home for soil microbes, worms, and insects     - Food storage — organic matter incorporated into aggregates and slow-released
Nutrient cycling and retention	As soil organisms break down and decompose soil organic matter will be consumed by soil organisms and released into the soil solution
Cation exchange capacity (CEC)	CEC measures the soils ability to temporarily hold on to many cations SOM provides between 20 and 80% of the CEC in mineral soils
Microbial diversity and resiliency	Organic matter is the main food source for many organisms in the soil Organic matter helps to create a mix of conditions and variety of homes to support the diversity that we rely on

Living Organisms
The soil food web is the community living all or part of their lives in the soil. The food web diagram below shows a series of conversions of energy and nutrients as one organism eats another.

Source: https://www.nrcs.usda.gov/wps/portal/nrcs

"As organisms decompose complex materials, or consume other organisms, nutrients are converted from one form to another, and are made available to plants and to other soil organisms. All plants -- grass, trees, shrubs, agricultural crops -- depend on the food web for their nutrition ... By-products from growing roots and plant residue feed soil organisms. In turn, soil organisms support plant health as they decompose organic matter, cycle nutrients, enhance soil structure, and control the population of soil organisms, including crop pests" (NRCS.USDA).

The fuel for the food web is the SOM discussed above.

Soil Water
Half of the overall soil content is pore space, a 50-50 mix of air and water.

Through the process of transpiration, water serves as a vehicle for moving material into, within, and out of the vine plant.  Water enters a vineyard through precipitation or irrigation and that water either runs off, flows to levels beyond which it can be accessed by the vine plant, or remains in the rooting zone where it is available for the plant's use. The plant uses water as an internal distribution vehicle (in addition to other functions) and facilitates this by expelling water through pores (stomata) in the leaves.  As water is transpired from the leaves, replacement water is drawn in at the roots.

Source: talktalk.co.uk

Water attracted to the vine root by transpiration moves undiluted nutrients to the root surface (bulk flow) but also carries dissolved nutrients into the roots as a part of its transit. Nitrogen is the nutrient most frequently acquired by the roots in this manner.  Nutrients are moved up from the roots to needed areas through the phloem by transpiration.

Soil Air
Half of the overall soil content is pore space, a 50-50 mix of air and water.  balance must be maintained as water can displace the air in the soil. Soil air differs from surface air in that it is heavily influenced by the soil's carbon content.

Soil Types
Please follow this link for an elaboration of soil types.


©Wine -- Mise en abyme