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An Introduction to Soil Biology - Part 1: The Soil Food Web
“Preparations help the biology do their work” spoken by Rudolph Steiner in relation to the Biodynamic preparations; which are central to the long-term building of soil fertility on Biodynamic farms. The Biodynamic preparations as such, are not adding vast quantities of beneficial microorganisms into the soil; but instead, they stimulate and wake up the dormant soil, facilitating its natural process of becoming a dynamic living entity.
Since Roman times it has been well known that only by feeding the soil, can the plant be suitably fed as nature intended. However, with the advent of the industrial revolution, this notion seems to have been forgotten in both farming and scientific communities. Ever since its inception, organic and Biodynamic agriculture has emphasised the importance of soil health and fertility as the foundation of sustainable and wholesome farming.
This article aims to shed light on the life in our soils and highlight how organic and Biodynamic methods contribute to enhancing soil life.
Beneath the Surface…
With day to day running of farms, it’s often easy to focus too heavily on the above ground management of farming; much to the neglect of the below ground. Beneath the surface, soil is teaming with an incredible diversity of micro-organisms which make up the complex underground ecosystem called the soil food web (Figure 1).
These organisms range in all shapes and sizes from the smallest virus, bacteria, algae, fungi and protozoa to the more complex nematodes and micro-arthropods, through to the visible earthworms, insects, small vertebrates and plants.
As these organisms grow, eat and move their way through the soil, they perform a vast array of functions. Beneficial microbes decompose organic materials including manure and plant residues; fix atmospheric nitrogen and solubilise soil minerals into plant available form; store and recycle soil nutrients; enhance soil aggregation and porosity; build soil humus and hence increase nutrient and moisture retention; prey on crop pests and even be consumed themselves by higher level predators from the intertwined soil food web.
Who’s Who in the Soil Food Web
The soil food web is fuelled by the primary producers, namely: the plants, algae, lichens, moss and certain groups of bacteria that have the ability to fix carbon from the atmosphere.
Other soil organisms then obtain their energy by consuming those primary producers and their waste products. As organisms decompose organic materials or consume other organisms, nutrients are converted from one form to another and some are made available to plants and other soil organisms. All crops – grass, vegetables and orchards all depend on these interactions of the food web for their nutrient supply.
The four key players in the soil food web of particular interest to agricultural soils are bacteria, fungi, protozoa and nematodes. Bacteria are single celled organisms and reside in the soil in vast numbers – a teaspoon of soil generally contains between 100 million and 1 billion bacteria.
Most bacteria are decomposers of simple carbon compounds but they also hold nutrients in the root zone, improve soil structure and filter and degrade pollutants. Fungi are multi-celled organisms that grow as long threads or strands called hyphae. Fungal hyphae can span in length from a few cells to many yards.
Saprophytic (carbon degrading) and Mycorrhizal (symbiotic) fungi perform important services related to soil-water dynamics; they physically bind soil particles into aggregates thereby improving soil structure.
They also decompose complex carbon compounds, retain nutrients in their fungal biomass and compete with plant pathogens. Protozoa are single celled animals that feed primarily on bacteria, but also eat other protozoa, organic matter and sometimes fungi. When protozoa eat bacteria, excess nitrogen is released into the soil in plant available form.
Nematodes are non-segmented tiny worms and many growers are familiar with the nematodes that cause crop losses and plant disease, when in fact, there are incredible varieties of beneficial nematodes. These beneficial nematodes consume bacteria, fungi or even other nematodes and in doing so (similarly to protozoa) release nutrients in plant available form.
It is this process of predators consuming lower hierarchical organisms and recycling nutrients in which highly productive natural ecosystems can maintain their fertility in the long-term without the application of fertiliser year after year.
Fuelling the Food Web
Plants do not have a digestive system of their own and are entirely reliant on the soil and soil organisms to pre-digest their food, making it available for subsequent uptake.
This reliance is evident by the fact that the plant secretes 30% of its total photosynthesised energy (sugars and carbohydrates) out of its root system to feed the living microbial workforce that crowd around the root system awaiting their daily feed. Why on earth would a plant waste 30% of its reserves releasing them out into the environment instead of using it for its own growth?
Of course, this is not a waste at all, but an extremely valuable future investment for the plant. In exchange for these sugars, soil microbes supply the plant with the all important essential minerals that the plant requires for all growth processes and to further photosynthesise more sugars and hence more microbe food.
It is this symbiotic mutual exchange between plant and microbe in which timely applications of horn silica (BD 501) and other plant nutrients play such a valuable role and can improve this process. Since 501 enhances the quality and quantity of light that the plant absorbs; its use not only directly improves plant growth, but it also indirectly stimulates microbial growth via this symbiotic interaction.
Crop Type and Ecological Succession
Ecological Succession refers to a predictable and orderly change in the composition or structure of an ecological community. In nature, we find a range of plant ecosystems such as pioneer species or early colonizers, grasslands, shrublands and soft and hardwood forest ecosystems. Over time, there is a slow and gradual change through successional ecosystems as they increase their complexity eventually moving from annual pioneer lands to perennial forest ecosystems (Figure 2).
With these visible changes in the plant communities above the ground, also comes a correlated change underground in the soil micro-organism structure. Different plants (including agricultural crops) have evolved in a range of varied environments along this succession and as a result, require a specific balance of soil life for its optimum production.
Each type of agro-ecosystem (be it grassland, vegetables or orchard) have a characteristic microbial balance in which they will thrive. In particular, the ratio of fungi to bacteria is a key characteristic of the type of system.
Grassland soils usually have bacteria dominated food webs, meaning most of the biomass will be in the form of bacteria. Productive vegetable soils tend to have ratios of fungal to bacterial biomass nearer to 1:1, while forest and orchard soils tend to have fungal dominated food webs.
The presence of predators in the soil food web reflects their food source, ie. protozoa or bacterial feeding nematodes are abundant where bacteria are plentiful. Management practices can change food webs; for example tillage practices, fertiliser and chemical applications, can have a negative influence on soil microbial communities while application of biodynamic compost, liquid compost extracts and other organic amendments can have a positive influence on the soil microbial communities.
Soil Fertility Test at Laverstoke Park
The Soil Foodweb laboratory service at Laverstoke Park in Hampshire has been testing soil samples from UK and Europe for almost three years. Our analysis has identified major deficiencies and imbalances in microbial communities amongst agricultural soils. In particular, beneficial fungi are a common shortage as they appear to be much more sensitive to excess (i) cultivation, (ii) fertilisation and (iii) chemical application. The lab service follows peer-reviewed scientific methods to measure the microbial biomass and diversity in soil.
The Soil Foodweb labs have an evolving database currently consisting of biological results of over 100,000 samples from all over the world. The laboratory accesses this database to compare the client’s results to soils where the plant species are growing in native ecosystems. This information can be used by growers to determine the need for any organic amendment programs required to correct microbial imbalance in their soils in order to improve growing conditions for their particular crop.
Growers are well aware of the importance of soil analysis for monitoring chemistry and the nutritional balance of their soils; however, monitoring the soil biology is much less commonplace even though, the biological activity of the soil is equally important to soil health (Figure 3).
Understanding the many roles that microbes perform in the soil highlights the importance of testing and monitoring how this living component of the soil is faring under current management practices. Once the microbial balance has been determined, the necessary management practices or best course of action can be set in place toward improving the biological and overall fertility of the soil.
Part 2 of this article will be published in the next edition of Organic Matters and will focus on practical strategies to improve soil biological health with particular emphasis on the use of compost and liquid compost extracts or compost teas.
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