The Rhizosphere

The rhizosphere is the area in the soil around the plant roots. It consists of soil, insects, microbes, and roots. Some soil fauna, such as spiders and their relatives, the mites, are not true insects and are called arachnids. They have eight legs. Spiders do not eat plants; they eat insects that hurt the garden. Other creatures, such as slugs and snails, are also found in the soil and feed on plants. Microbes are organisms that are too small to be viewed without a microscope and include bacteria, viruses, fungi, protists, algae, and nematodes.

Some soil microbes are opportunistic pathogens. They do not affect healthy plants, but if a plant is weakened by environmental conditions, the microbe will enter the plant and cause infection. Beneficial soil microbes make nutrients available to plants and in exchange get carbohydrates from the plants. Beneficial rhizosphere microbes are also responsible for the generation of compost from animal and plant matter. Earthworms are also helpful in the composting process and in the recycling of nutrients (Figure 5.1). Worms should not be introduced into ecosystems where they are not naturally found, however, because they can cause extensive damage to the organic layer in forests and reduce the habitat for native creatures.

Figure 5.1 Earthworms are an important component to composting. In vermicomposting, or worm composting, red earthworms are used to consume and digest organic matter in order to produce castings, an odor-free soil additive.

Composting of green plants directly in the soil is known as green manuring. Green manures are usually supplied from young weeds that have been mechanically removed with a hoe and turned back into the soil to decompose. Some growers plant a cold-tolerant cereal or legume crop that grows over the winter and then turn it into the soil in the spring. The soil microbes break down the plant and recycle the nutrients into the soil.

The nutrient that is most often deficient in the soil is nitrogen. Soil microbes that belong to the genus Rhizobium form nodules on the roots of legumes and convert nitrogen from the atmosphere into a form that becomes soluble in the soil and available for plant uptake. Free-living nitrogen-fixing bacteria such as Azospirillum and Cyanobacteria are also found naturally in the soil.

Mycorrhizal fungi form long hyphal filaments that stretch into the soil in search of nutrients; they also form associations with plant roots. The fungi bring nutrients such as phosphorous to the plant in exchange for carbohydrates and may also protect the plant from fungal pathogens. Additionally, the hypha helps to bind the soil particles together and give the soil good tilth. Many plants form symbiotic associations with mycorrhizal fungi. Plants that are normally found in association with mycorrhizal fungi will not thrive if they do not form the association. It may take years to establish the associations, which are very important in forested and natural ecosystems. Wildflowers often die if you try to dig them up in the wild and plant them in your garden, because their symbiotic mychorrhizal fungi do not survive the trip.

Some soil bacteria, such as Streptomyces, produce antibiotics that kill other microbes. Streptomyces also produce a substance called geosmin that is responsible for the earthy smell of garden soil and ripened compost. Bacteria from the genera Bacillus and Pseudomonas are used to kill fungal pathogens. They are added to seeds or sterilized potting media and are labeled and treated as fungicides. These organisms are naturally present in fertile soil and in properly aged compost, so there should be no need to purchase them if you add high-quality compost to your garden.

Fungal and viral diseases can be spread by water droplets that bounce off the soil and land on leaves. The use of plastic mulch for plants that are susceptible to viral diseases has given good results. The mulch creates a barrier between the soil and the leaves. Many other types of mulch, such as cedar or pine bark (which smells like the trees), are available. They are yellow or reddish in color. Cocoa mulch smells like chocolate and has a dark brown color. Gravel is also sometimes used, especially with drought-tolerant plants. Straw and hay are very popular and can be easily picked up and moved when needed, although you have to be careful that the hay does not contain weed seeds.

Mulch should not be placed too close to the stem, as overly moist conditions may lead to fungal infections. Mulch is used to lessen temperature fluctuations and to conserve water in the soil and prevent erosion. Temperature affects plants at all stages of growth. The temperature of the surface of the soil, where plant growth occurs, may fluctuate widely over daily and annual (season- to-season) intervals.

Crop rotations also help because disease organisms can survive the winter in the soil. Many disease-causing microbes and insects are specific about which plants they will infect. If no host plants are available for a few years, the pathogen population will die back from lack of a host. Crops that grow in the same place year after year can cause an increase in the insect pest population in the soil. This is why growers who practice monocropping often use large quantities of insecticides. For example, conventional commercial strawberry growers used to routinely fumigate the soil with methyl bromide to kill disease microbes as well as weeds. This substance has been recently banned by the EPA, however, because it is toxic to humans and depletes ozone. It also kills the beneficial organisms in the soil.

Figure 5.2 A weed called giant foxtail grows between rows of corn on the right-hand side of this photograph. The corn on the left has been treated with an herbicide that prevents foxtail growth. Weeds compete with crops for nutrients and water, and they spread diseases.

Weeds are plants that grow in a place where you do not want them to be, such as in a vegetable garden or in the middle of a lawn. Many weeds are escaped garden plants from European settlers that are no longer in fashion or foreign grasses that hitched a ride in grain shipments. Weeds compete with garden plants for nutrients and water and may cause overcrowding and lead to conditions that spread disease (Figure 5.2).

Weeds can be pulled by hand or scraped off the surface with a hoe. Hoes have a sharp edge that is used to remove the weed just below the surface of the soil. Any weeded plants that are not edible can either be turned back into the soil as green manure or put into a compost heap. Plants release compounds from their roots into the soil. Some of these are weak acids that help to release nutrients from the soil. Plants also can release allelochemicals that affect other plants. Some weeds, such as tamarisk and knapweed, produce chemicals that prevent the growth of other plants.

Herbicides are chemicals used to kill weeds and are widely applied by both commercial growers and home gardeners, especially on lawns. Some target the broad leaf dicot plants but allow the monocot grasses to continue to grow, whereas others may target the monocot grasses. Misuse of commonly used herbicides can result in contamination of water supplies and may be toxic to humans. They are not used in organic horticulture, as mechanical methods of weed removal are effective and the weeds are a source of compost material. Weeds can be prevented by the use of smother crops, which are planted in between rows or in other places where weeds might grow.