April 9, 2012
Different plants prefer different soils. Understanding what soil is best for a group of plants in a given area can help in determining how to make adjustments. Three concepts that help in this understanding are soil succession, soil texture, and soil structure. Other related concepts are the effects of salt and lack of oxygen on plant roots.
Plants did not evolve by themselves. They co-evolved as part of a diverse ecology and established symbiotic relationships with a web of other organisms. Depending on the relative age of the soils along with climate, soils fall along a continuum from early soils that are little more than weathered bedrock to ancient woodland soils in old growth forests.
Early soils are generally lacking in organic matter, have a soil ecology that is bacterially dominant, and are more alkali (higher pH). By contrast, older woodland soils have a much higher percentage of organic matter, are fungally dominant, and are more acidic (lower pH).
Between these two extremes are soils that go from early weedy growth—mostly annuals with high seed production—through a series of trophic phases including the growth of perennial plants, the establishment of woody plants, and the accumulation and breakdown of organic matter into increasing amounts of humus. The very decomposition of the organic matter involves more and more fungi, both in number and species diversity, some of which establish symbiotic relationships with plant roots.
One of the most common soil problems seen is that plants that coevolved in later, older soils are planted in an earlier successional soil that has the wrong pH and not enough organic matter and related fungi. The result is nutrient deficiencies, especially iron, and stress. Stressed plants are more susceptible to pests and pathogenic diseases. This really isn’t a soil problem but rather a plant selection problem.
Another problem is that nutrient cycling, the gradual addition of organic matter and subsequent humus, is interrupted by the good intentioned gardener who rakes up and removes all the fallen leaves and branches over a prolonged period of time. If these materials are not replaced with/as compost and/or mulch or grass cycling, the plant’s health will soon decline and they will become stressed.
Soil is composed of different sized particles: sand, silt, and clay. If soils contain all three they are considered “loamy.” Just as plants evolved in different successional soils, plants have also evolved in different textured soils. Examples would be coastal scrub plants that prefer sandier soils or bay wetland plants that prefer clay soils. Regardless of soil textural class, these soil particles can be “glued” together into aggregates by humus and aerobic soil organisms. The more aggregation there is, the more soil pore space there is, and this increases air flow to the roots. This aggregation is referred to as pedogenesis or the building of soil structure.
Soil structure can be quickly destroyed by compaction and poor drainage. Avoiding compaction or taking measures to aerate compacted soils can help improve soil health. While some riparian plants do prefer saturated conditions, most garden plants do not and correcting drainage problems can dramatically improve soil aeration and related soil problems. Pushing soil succession forward through generous applications of compost and mulch can also quickly create soil structure and aeration.
Good irrigation management is a key component to healthy soils. Good soil structure will reduce run-off and increase root depth and water holding capacity. Leaks, poor irrigation uniformity, and/or poor irrigation programming can create anaerobic conditions and shallow roots. Deep, infrequent watering with no run-off is both an efficient use of a precious resource and a way to promote healthy soil.
Anaerobic conditions, or a lack of oxygen in the soil, can cause the loss of nutrients through volatilization, creating toxic gases such as ammonia and sodium hydroxide. These conditions also favor fungal yeasts to grow, which produce toxic alcohols that dissolve sensitive plant root tissues. Anaerobic bacteria can also start to grow and produce toxic substances and break down beneficial fungi. While some plants evolved in swamps and have adapted to these conditions, they will stress or kill most garden plants.
Synthetic fertilizers are salts, e.g., sodium nitrate, ammonium sulfate, potassium nitrate. Some of our bay soils are salty. Some water sources and even some fresh manures can be salty. Salt in a soil solution can make it harder for roots to absorb water and in extreme cases, can cause wilt and even death of plants through osmosis. Repeated applications of strong synthetic fertilizers, while good intentioned, can stress the plants they are intended to feed. Responsible use of organic fertilizers will avoid applying more salts to the soil and can also reduce pollution in the form of nitrates and phosphates in our waterways and aquifers. Mulch, humus, and soil organisms can all help buffer the effects of salty soils.
Reducing or eliminating the use of pesticides may also solve soil problems. All pesticide use creates collateral damage. The spreader-stickers, penetrants, and wetting agents found in pesticide formulations can affect soil organisms. “Weed and feed” products can stress nearby plants. Fungicides are especially harmful to the soil organisms responsible for the breakdown of organic matter, disease suppression, and pH regulation. Other insecticides can kill the microanthropods responsible for the breakdown of leaf litter.
Respecting the climate, soil texture, and soil successional level to select appropriate plants is the best way to avoid soil problems. If plants are desired that do not prefer the existing soil conditions, measures must be taken to adjust the soil. This is possible but can be expensive. Whenever possible, allow leaf litter to remain in place. Where this is not desired, compost top dressing and regular upkeep of a mulch layer can be employed to mimic this natural process. Feed the soil and let the plants eat what they want. Avoid excessive tilling but maintain good soil aeration by promoting a healthy soil ecology. Our soils are precious and we all need to do what we can to preserve them from compaction and erosion, as well as to promote improved soil structure. In this way we can all be good land stewards.