Soil looks like dirt except that it is more interesting!!!
Soil is a really a pot full of weathered geological ingredients that have been stewed together through time with billions of tiny living creatures. We know that in one handful of soil there are millions of live organisms. So, when we are standing on the ground, we are really standing on the roof tops of a whole other world.
Living in the soil are plant roots, viruses, bacteria, fungi, algae, protozoa, mites, nematodes, worms, ants, maggots and other insects and insect larvae (grubs), and other animals. Together with climate, these organisms are responsible for the decay and cycling of both macro- and micro- nutrients, and their activities can affect the structure tilth, and productivity of the soil. Agricultural practices such as crop rotations and tillage affect the numbers, diversity, and functioning of the micro- and larger-organisms in the soil community, which in turn affects the establishment, growth, and nutrient content of the crops we grow. We have all heard our mothers and fathers say "you are what you eat". Indeed, the mineral nutrients that are in the food we eat have at some point come from the soil.
In this paper, I will introduce you to the activities of soil organisms (both micro and macro in size) in terms of how they affect the cycling and availability of nutrients to crops, disease cycles, weed management, and soil tilth and erosion potential. I will discuss the how these activities are influenced by soil management practices, and point to ways that we can manage and use soil biological activity to our advantage in agroecosystems.
Life in the Soil
Biological activities essentially control soil productivity. Soil invertebrates (such as mites, nematodes and collembola), and fungi and bacteria recycle all the carbon, nitrogen, and other mineral nutrients in plant and animal residues into forms that can be used by plants. Soil microbes (e.g. bacteria and fungi) regulate the destruction of toxic environmental pollutants like nitrous oxides and methane (greenhouse gases). Increasing the diversity of microorganisms in the soil will increase the ability of the soil to adapt and tolerate a variety of environmental conditions. The speed at which residues decay and nutrients are released from organic matter, and pollutants are detoxified, will be largely dependent on how we manage the soil.
In undisturbed soil, most of the nutrient cycling, roots, and biological activity are found in the top 20 to 30 cm, sometimes called the rooting zone, and known as the rhizosphere. More specifically the rhizosphere is the root and adjacent soil which is influenced by the root. It is characterised as a zone of intense microbial activity. The rhizosphere is bathed in energy rich carbon compounds, like sugars, amino acids and organic acids (products of photosynthesis) that leak from the roots. These compounds are called root exudates. Bacteria and fungi use root exudates and the dead sloughed cells from the root to grow and reproduce, but competition for a space on or near the root is stiff. In the battle for carbon, bacteria often produce antibiotics that remove the competition, and plant growth promoting substances which increase root growth. Scavenging and predator fungi, protozoa, nematodes and mites are also looking to increase their carbon and mineral nutrient intake. Mega fauna like earthworms feed in the nutrient rich matrix around the rhizosphere consuming large quantities of dead plant material, fungi, protozoa and bacteria. The castings left by earthworms are rich in available nitrogen and bind and stabilise smaller soil particles into larger aggregates. The sticky secretions and webs of fungal hyphae bind even smaller soil particles into aggregates further preventing erosion. Below ground community interactions are incredibly complex, invisible to the human eye, and yet can promote or inhibit plant growth and modify the soil habitat.
Every plant species leaks a unique signature of compounds from their roots. The quantity and quality of these compounds depends to a certain extent on the soil chemical and physical properties, but in all cases determines the microbial community of the rhizosphere. Symbionts like Rhizobium and disease causing pathogens may be particularly well tuned to the composition and quantity of root exudates. Vesicular-arbuscular mycorrhizal (VAM) fungi form a mutually beneficial or symbiotic relationship with 95 percent of all land plants, including cereals, pulses, forages, and some oilseeds. They appear to be essential to the establishment, growth and survival of many plant species. VAM fungi are known to increase drought and root disease resistance of the plant host. However, they are most well known for their ability in increase the uptake and content of less available mineral nutrients such as phosphorus (P), calcium (Ca), zinc (Zn), and copper (Cu). These fungi penetrate the cells of the root without harming the plant. From the root, the hyphae extend like a network of silk threads through the bulk soil where they absorb water and mineral nutrients, and glue and tie soil particles into more erosion-resistant aggregates. VAM fungi can be considered a highly effective transport system, like a pipeline, between the soil and the plant, moving water and nutrients to the plant in exchange for the carbon-rich products of plant photosynthesis.
Once plant roots are colonised by VAM fungi, their physiology and biochemistry change. Plants colonised by VAM fungi have higher rates of photosynthesis, better water use efficiency, and move more and different kinds of carbon compounds to the roots. Consequently, there is a different rhizosphere community associated with the roots of VAM-colonised plants; a rhizosphere with fewer pathogens, more nitrifiers, and others that we still don't know about. Nitrifying bacteria convert ammonia to nitrate, which is easier for the plant to absorb.
The rhizosphere is a partnership between the plant, soil and soil organisms. Plants provide the carbon and food source for soil organisms that bind the soil particles into aggregates and recycle soil nutrients, and soil provides the habitat, water, and mineral nutrients for both soil organisms and plants. Any factor or soil management technique that changes the amount and quality of carbon going into the soil as either residue or root exudates will effect change in the soil biological community. Change which ultimately has consequences for plant growth.
Managing the Soil as a Habitat
Soil management is defined by Nyle Brady (1984) as the sum of all tillage operations, cropping practices, fertilizer, soil amendments, and other treatments applied to the soil for the production of plants. Once again, putting emphasis on the interconnectedness between all farming practices and the soil.
Management practices that affect the placement and incorporation of residues can make it harder or easier for the soil organisms responsible for recycling nutrients. Tillage directly affects soil porosity and the placement of residues. Porosity determines the amount of air and water the soil can hold. Placement of residues affects the soil surface temperature, rate of evaporation and water content, and nutrient loading and rate of decay. In other words, tillage collapses the pores and tunnels that were constructed by soil animals, changing the water holding, gas, and nutrient exchange capacities of the soil. Conservation tillage and particularly no tillage reduce soil disturbance, increase organic matter content, improve soil structure, buffer soil temperatures, and allow soil to catch and hold more melt water. These soils are more biologically active and biologically diverse, have higher nutrient loading capacities and release nutrients more continuously.
Earthworm numbers increase dramatically with no tillage. The burrowing activities of earthworms increase soil aeration, water infiltration, nitrogen availability to plants, and the microbial activity in the soil. Earthworm burrows can be stable for years, acting to increase the extent and density of plant roots as well as stabilising soil aggregates to improve soil structure and limit erosion. It has been suggested by a number of researchers that earthworms are major contributors to the breakdown of organic matter and N cycling in zero- tillage systems. Earthworms appear to prefer plant material that has been colonised by fungi and bacteria, which can lead to the reduced incidence of fungal diseases in crops. Indeed, earthworms are probably most important in no tillage and other conservation tillage systems, not only because these systems encourage earthworm populations but, because without mechanical mixing and loosening, earthworm casts and burrows are left intact to encourage better root development. In long-term dryland tillage experiments at the Lethbridge Research Centre, we have found as many as 300 earthworms per square meter under no tillage compared with none under conventional tillage (Clapperton et al., 1997b). In this same field experiment there was a significantly lower incidence of common root rot under no tillage compared with conventional tillage, demonstrating the long-term benefit of maintaining the soil habitat. In my research group, we are now beginning to explore the use of earthworms as biological indicators of more sustainable land management practices.
Conservation tillage practices also create a more undisturbed rhizosphere environment. Colonisation by VAM fungi is reduced under conventional tillage compared with no tillage. The links between the VAM fungi, plants and the soil are cut during tillage operations reducing colonisation by VAM fungi which can have consequences for plant nutrient uptake and growth. A study in the United States recommended significantly lower rates of P fertiliser for no tillage compared with conventional tillage wheat production because of the increased benefit of VAM fungi under no tillage (Jackson et al, 1994). Studies in my laboratory (Clapperton et al., 1997a) have shown that reducing the colonisation by VAM fungi can in turn reduce the mineral nutrient content of wheat.
There can be disadvantages to conservation tillage in terms of N immobilisation, weeds management, and having too much residue affect seed germination. However, these negative consequences can be reduced and prevented by diversifying crop rotations to include pulses, forages, grasses, oil seeds, and fall-seeded crops.
The benefits of diversified crop rotations married together with conservation tillage can dramatically increase soil productivity while reducing off-farm costs. Low residue crops like peas, mustard, or canola can be rotated with higher residue cereals to reduce the trash loading. Rotating cereals and oilseeds with peas, forages, or underseeding cereals with annual legumes increases the amount of N available to plants in the cropping systems. The residual benefits of N from these crops can be persistent for a number of years depending on which legume was used and how the soil was managed. Tillage can accelerate the rate at which the N is released from residues resulting in large short-term plant-available supplies of N (Mohr et al., 1998). More research is needed on sychronising the N release from residues with the N uptake by the subsequent crop, allowing for more efficient use of N by the crop. Legumes do well in biologically active soils, and also act to build biologically active soils. Legumes form partnerships with two symbionts; N-fixing bacteria (Rhizobium species), and VAM fungi to provide the increase P required for high rates of N-fixation. Earthworms have a preference of legume and oil seed crops, and under no-tillage will increase dramatically over one season (depending on the weather) and their burrowing activities can increase the amount of carbon and nitrogen deeper in the rooting zone. Increasing the amount of available nutrients at depth allows for the continued availability of key nutrients throughout the growing season. Residues from some crops inhibit the growth of other plants either directly, or indirectly from the by-products produced from the microbial decay of the residues. Fall rye, mustard, George Black medic, and sweet clover will inhibit the growth of some weeds in no tillage as well as conventional tillage.
The Land Resource Sciences Section at the Lethbridge Research Centre has undertaken a new long-term field experiment, that has been largely funded by farmers, to study the effects of new flexible dryland cropping systems with integrated livestock grazing under conservation tillage on yields, pest and disease populations, soil quality and economics. This study emphasises a systems approach to agriculture, where the effects of soil management are considered in light of the benefits of improving the soil biological, chemical and physical properties for crop productivity, economic viability, and environmental sustainability.
It is generally understood that the soil microbial community benefits soil productivity, yet we know so little about the organisms that live in the soil and the functioning of the soil ecosystem. Continued research aimed at understanding the interactions between soil management practices and the soil biological, chemical and physical properties of soil will be the key to sustaining soil, and our future generations.
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