‘shall I compare thee to a summer’s day’
Agricultural sustainability can be loosely defined as the maintenance of productivity without loss in quality. Productivity which is similarly dependent on the maintenance of the soils fertility. The degree to which a system mines or replenishes the soils fertility is thus indicative of the sustainability of the system.
However fertility is not an exclusive or intrinsic property that can be measured, but a product of the interactions between the soil and the surrounding environment. An emergent property soil fertility, and by extension agricultural sustainability, can only be measured and understood in the context of the whole system. A system that begins by measuring soil properties.
The Soil Continuum
The problem with measuring soil properties is determining the extent of the property. Soils do not come in discrete homogeneous units but as heterogeneous plots that exist in a continuum. So whilst one piece of soil should relate to a neighbouring, it may not. The habitats that sit on top may be homogenous and have boundaries but the soils underneath may not.
Without clear boundaries it is necessary to divide this continuum into arbitrary units and then to group those units by similarity. The easiest and most useful property to determine in this context is textural class; the distribution of sand silt and clay.
Texture (% Sand, Silt, Clay)
Measured by hand [soil texture chart] or mechanical separation [particle size distribution] textural class can reveal the extent or lack of homogeneity. Indicative of changes in the soil horizon as well as the potential hydrological and chemical properties of the profile, soil texture is perhaps the single most important property to qualify. Soil maps, which are useful for reference, are at too low a resolution to serve as anything more than a guide.
Major Cations (PH, PBS)
The availability of the major cations (Ca, Mg, K and Na). affects plant nutrition. An approximation of the current availability of Calcium, the dominant (80% ) basic cation ,and any additions required can be made when pH is used in conjunction with textural class. PH is also an indicator of Aluminium toxicity. a major issue in highly weathered acidic soils. [Basic Nutrients], [understanding pH] [pH and Buffer Capacity]
Percentage Base Saturation (PBS)
A far better measure for determining the availability of the major cations is % Base Saturation (PBS). A measure of the relative distribution of the basic cations held by the clay mineral (Cation exchange capacity), PBS can identify the total pool and relative distribution of those cations. Imbalances can them be corrected to optimise availability. [Cation Exchange and PBS]
As the major cations ( Ca, Mg, K, and Na) are maintained by the clay mineralogy so the availability of Nitrogen, Phosphorus and Sulphur is maintained through the turnover of organic matter by the soil microbial biomass. [Soil Organic Carbon].
The dynamics of the soil microbial biomass can in theory be used to estimate the turnover of organic matter and the relative availability of N, P & S [Microbial Respiration].
Mineral Nutrition the 3-4-5 nutrient model
The rate and frequency of rainfall, the soils storage capacity and access to water sources (rivers, reservoirs, aquifers, etc) all affect the ability to grow crops and raise livestock. [Soil Hydrological Properties].
Knowing the quantity, frequency and fate of precipitation relative to crop needs can help to develop more efficient and sustainable use of available water resources.
Exchanges between the soil and atmosphere of Oxygen affect nutrient availability (redox), whilst agricultural practices (cultivation, manure and fertilizer inputs) contribute significantly to the evolution of the greenhouse gases Carbon Dioxide, Nitrous oxide (and Methane)* . DFM permits the modeling of these exchanges relative to cultural practices.
Global Flora in a Habitat Context
Conventional Linnaean taxonomy arranges the flora to reflect morphological and evolutionary relationships between taxon. With DFM the flora are arranged to reflect habitat relationships. These habitats are grouped by similarity into sets, which form bigger sets and these bigger again until all the habitats and associated flora are represented within three dominant biomes.
Data on the morphology, physiology and associations of the plants in a habitat can now be parsed and organized into a DFM. Flora are not bound to a single habitat and may be significant in more than one community.
Macro Fauna in an Economic Context
With the exception of soil fauna (i.e. annelids and nematodes), which are significant in organic matter turnover and thus a component of the microbial biomass, the fauna that can be found within an/or depend on the habitats are differentiated by their economic importance to Man.
The most economically significant being the domesticated. Categorized as livestock (i.e. ruminants) exotic livestock and companion (pets and working animals) the total number of domestic species numbers less than 30 [domesticated animals]. The remaining animals are all identified as Wild and are differentiated according to their importance to the habitat. Keystone, Managed and Enrichment species.
With details of the properties outlined above it should be possible to build an environmental inventory of any habitat; and from that to estimate the total pool and dynamics of resources under different management regimes.
The Local Environment in a Global Context
Taxonomic, yield and ecological data on the flora and fauna that is held in databases at institutions and research establishments could be parsed into the model to give local access to global data. Global data that could itself be enhanced by the local input.
GAIA IN THE GARDEN
PLANTS MINERAL NUTRITION
Free Cultural Works (CC BY-NC-SA) Malcolm McEwen (2016)