Starting from the Ground Up...Soils 101

Av Singh, Ph.D

Farmers and researchers alike, this is the time of year that we’re both itching to get working the land. But as we prepare for another cropping season, maybe we should step back and take a closer look at what we planting our next crop in – our soil.

It all begins with the soil food web. There exists an incredible diversity of organisms at this level – ranging from one celled bacteria, algae, fungi, and protozoa, to the more complex nematodes and micro-arthropods, to the visible earthworms, insects, and plants. An acre of living topsoil contains approximately 900 pounds of earthworms, 2400 pounds of fungi, 1500 pounds of bacteria, 133 pounds of protozoa, and 890 pounds of arthropods and algae. These organisms are an integral part of agroecosystem processes.

• Nutrient cycling – when organisms consume food, they of course, will excrete wastes. The most important waste for crop growth is ammonium (NH4+). Organisms, including plant roots, quickly take up ammonium and other readily utilized nutrients. Certain soil bacteria fix nitrogen from the atmosphere, making it available to plants, reducing our reliance on external sources of N.
  
  
• Nutrient retention – in addition to mineralizing nitrogen or releasing nitrogen to plants, the soil food web can immobilize or retain nitrogen when plants are not rapidly growing. For example, N in the form of soil organic matter is less mobile and less likely to be lost from the rooting zone (and enter groundwater) than inorganic nitrate (NO3-) and NH4+.
  
  
• Improved soil structure - bacterial activity, organic matter, and the chemical properties of clay particles are responsible for creating microaggregates from individual soil particles. Earthworms and arthropods consume these aggregates and generate larger fecal pellets. These fecal pellets become part of the soil structure. Fungal hyphae and root hairs bind together to help stabilize larger aggregates. Improved aggregate stability, along with the burrows of earthworms and arthropods, increases porosity, water infiltration, and water holding capacity, thereby reducing runoff.
  
  
• Disease suppression – a complex soil food web contains numerous organisms that can compete with disease-causing organisms. These competitors may prevent soil pathogens from establishing on plant surfaces, feed on
  pathogens, or generate metabolites that are toxic to or inhibit pathogens.
  
  
• Degradation of pollutants - Soil organisms decompose organic compounds, including manure, plant residue, and pesticides preventing them from entering water and becoming pollutants.

Having thousands of different species and strains of organisms living in our soil, its hard to single out a few and do justice to the incredible web of life that exists, but below are some of the more important ones that many of you are familiar with:

• Nitrogen-fixing bacteria (Rhizobium spp.) form symbiotic associations with the roots of legumes like alfalfa and soybeans. The bacteria infect the root forming nodules in which the plant supplies simple carbon compounds to the bacteria, and in exchange the bacteria convert N (N2) from air into a form the plant host can use. (Remember – often different legumes require different species of Rhizobium inoculant).
  
  
• Actinomycetes are a large group of bacteria that grow as hyphae like fungi. They decompose a wide array of substrates, but are especially important in degrading hard-to decompose materials such as cellulose. Actinomycetes are responsible for that sweet earthy scent that you can smell after freshly tilling the field. They are also important as antibiotics both within the soil and for humans.
  
  
• Vesicular-arbuscular mycorrhizal (VAM) fungi colonize plant roots. In exchange for carbon from the plant, mycorhizal fungi help solubolize phosphorus and bring soil nutrients (P, N, micronutrients and water) to the plant. Agricultural practices affect the formation of VAM fungi. The number of mycorrhizal fungi will decline in fallowed fields or in those planted to crops that don’t form mycorrizal associations (e.g., broccoli, canola, spinach, sugar beets). Also, frequent tillage and broad-spectrum fungicides will reduce VAM numbers.
  
  
• Earthworms, probably need no introduction, they are major decomposers of dead and decomposing organic matter. Earthworms enhance soil quality by increasing the surface area of organic matter thus stimulating microbial decomposition. They improve soil stability, porosity, and moisture-holding capacity by burrowing and aggregating soil. Earthworm burrows enhance water infiltration and soil aeration. Fields that are “tilled” by earthworm tunneling can absorb water at a rate 4 to 10 times that of fields lacking tunnels. The castings left by earthworms are essentially nutrient-lined channels excellent for root growth. Lastly, benefits receiving recent attention are soil turnover and burying of organic matter. In no-till or reduced tillage systems, surface residue builds up and triggers growth in earthworm populations. Earthworms pull more and more residue in their burrows, helping to mix organic matter into the soil, and reducing disease by removing the overwintering substrate for many pathogens.

So, how do we create or maintain this healthy soil? Several factors affect the level of organic matter in a soil. First and foremost would be the amount of organic matter added to the soil, but other factors include moisture, temperature, tillage, N levels, cropping, and fertilization. High rainfall and temperature promote rapid plant growth, but these conditions are also favourable to rapid organic matter decomposition. So, similarly in contrast low rainfall or low temperatures slow both plant growth and organic matter decomposition.

Tillage can be beneficial or harmful to a biologically active soil, depending on what type of tillage and it’s timing. Remember the moldboard plow? Well excessive use of the moldboard plow brought organic matter levels less than 1% (biologically dead soil), because it would bury crop residues and topsoil to a depth of 14 inches where the oxygen-limiting environment would hinder decomposition. Shallow tillage systems incorporate residues near the surface (where the oxygen is) and speeds up decomposition.

Excessive N applications stimulate increased microbial activity, which in turn speeds up organic matter decomposition. Typically soils have a carbon to nitrogen (C:N) ratio of 12:1, however extra N will narrow this ratio and disturb the balance of microbial populations. Bacteria populations explode when inorganic sources of N are used in excess, and even though there is a dramatic increase in organic matter decomposition, there may not be enough C in the system to keep populations healthy. Thereafter, applied N is not recycled to plants it becomes subject to leaching. It is for this reason that green manures and composted animal manures work so well – because they maintain a proper C:N ratio.

So, is it worth making changes to your farming practices to save your soil? Well, let’s look at it this way. Say your % organic matter has decreased over time and you want to increase it. Well the first step is to ensure that you additions must be higher than your removals. But you must remember that generally 60 to 70% of the carbon contained in organic residues is lost as carbon dioxide, and 5 to 10% is assimilated into the organisms that decompose the organic residues, leaving only 20 to 35% to become new organic matter. It takes ten years for this organic matter to become the stuff we all know and want (humus). So, if you added a ton of residue, you would get 400 to 700 pounds of new organic matter. One % organic matter weighs 10 tons per acre…so as you can see building organic matter is a slow process!

So, it is more feasible to stabilize and maintain the existing organic matter in the soil than try to rebuild it…and your first step is taking care of the web of life that exists in your soil.


Av Singh, Ph.D, is the Extension Coordinator at the Organic Agriculture Centre of Canada and may be reached for comment or questions at 902-893-6275 or via email asingh@nsac.ca


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