When Good Soil Is Bad
The horticulture of native plants sometimes gets tangled up in traditional horticulture or agriculture, especially as it relates to the understanding of soils and soil fertility. The gardening dogma has been to extol the wonders of rich, fertile soil, the use of compost and commercial fertilizers that promise faster, bigger, and better. Although rich soil may be beneficial for your vegetable garden, agricultural crops, and some perennials, this is not the case for growing most native plant species.
There are three types of customers that commonly visit our nursery. One type lament their soil is too dry and sandy and plan to haul the soil away and bring in topsoil. The second type have clay soils and complain it is so compacted they cannot do a thing with it and plan to haul it away and bring in topsoil. The third boast that they have been composting for years and have created very rich soils. Unlike the story of the three bears - none of these customers have it "just right".
Soil is a precious resource. As an environmentally oriented native plant gardener, hauling soil away and replacing it with "top soil" is an option that should almost never be considered. The one possible exception is if the soil is contaminated with heavy metals or other pollutants and even in many of these situations removing the soil is ill advised. What is commercial topsoil anyway? At one time it commonly was Michigan peat – muck soil mined from drained wetlands. This is still available, but is becoming increasingly more expensive and difficult to find. Remember that mining this material cannot be done sustainabily and results in the destruction of wetlands. Today topsoil is commonly made up of sand with a lot of organic matter added to it to create a dark and rich (manufactured) soil. Regardless of the source, adding topsoil is generally not necessary and can lead to unanticipated problems such as the introduction of weeds through seeds, rhizomes or other propagules. In addition, laying one soil type upon another without mixing the layers can result in drainage problems. Finally, the extra fertility will not be an advantage, but a hindrance.
Soil is much more than the sum of its parts. It is made up of inorganic mineral parts (sand, silt and clay), decaying organic matter plus an abundance of living things including fungi, bacteria, insects, roots, etc. If you think of the soil as a recycling factory – the mineral component would be the factory building, the organic matter (dead stuff) would be the product being recycled, and the microorganisms and other animals the workers. Under natural conditions the rate of accumulation of organic matter in the soil is generally in equilibrium with its rate of breakdown. This is important since the soil organic matter provides space for air exchange and water retention. In addition, organic matter ties up nutrients in a form unavailable to plants and releases the nutrients only when the organic matter is decomposed. Under natural conditions soils sometimes do accumulate organic matter, but this takes hundreds if not thousands of years. If the organic matter is being recycled faster than it is being replenished the accelerated breakdown will result in a decrease in water holding capacity and soils become more prone to compaction. In addition, there will be a short-term spike in nutrient release. Plants will respond by growing vigorously. Intuitively this may not seem like a bad thing but the plants may grow faster than desired, attaining an unanticipated size and require a lot of pruning. Having the right microbial workers working at the right speed, maintaining this equilibrium is critically important.
Tilling the soil also changes the balance of accumulation to breakdown of organic matter. Tilling loosens the soil and provides aeration. This is not necessarily a bad thing, but it does alter the community of soil microorganisms, which accelerates the degradation of the organic matter. In addition, tilling brings weed seeds that lay dormant in the soil (often called the seed bank) to the soil surface where they will germinate.
There are certainly instances where tilling and adding organic matter to the soil is beneficial. When organic matter is needed in your garden to provide water holding capacity on sandy soils, or for soil aeration on clay soils you should use organic amendments that are high in carbon and low in nitrogen. These are products like rice hulls, wood chips or straw. This will provide the most sustained, although temporary, improvement in soil water holding capacity, soil aeration, and drainage. This is accomplished by slowing the decomposition of organic matter. Adding amendments rich in nitrogen to the soil stimulates microbial growth - in effect you have added a third shift at the recycling factory and given all the workers strong coffee. Adding high carbon, low nitrogen organic matter will slow decomposition, and support a different complement of microorganisms such as mycorrhizal fungi, Rhizobium and Frankia spp. bacteria. - all soil symbiotes. Symbiotic microorganisms coexist with plants in a manner that benefits both plant and microbe. Mycorrhizal fungi, for example, enhance uptake of soil nutrients and water and are found associated with most terrestrial plants. In exchange the plant provides photosynthates (mostly sugar and starches) to the fungi. Rhizobium spp. and Frankia spp. are two types of soil microbes that fix atmospheric nitrogen for certain species on the most infertile sites. These organisms work for free and deliver the nitrogen directly to the plant. Interestingly the presence of these types of soil organisms is inversely correlated to the amount of soil nitrogen. Native species have co-evolved with these soil microorganisms to tolerate soils of moderate to low fertility. While native plants, like most other plants, respond to greater soil fertility, weedy species in general can take better advantage of increased nutrients. Low soil fertility will generally give native species a competitive advantage over the high feeding, non-native weeds. Many beloved native plants such as lupine, bush clovers, harebell among others are only able to survive when plant competition is minimized by low soil fertility.
Soil testing and pH
I know I am in the minority, but I find traditional soil testing for native plantings completely unhelpful. These tests provide valuable information for agriculture and some types of horticulture but not for natives. A great deal is made of soil pH and indeed it is an important soil parameter. The fact is there is very little you can do long term to alter soil pH. Turning a couple of shovelfuls of soil over will help you determine if the soil leans to the sandy or clay side of the continuum. The degree and depth of soil compaction will also be obvious. Don't forget to also look at the surrounding vegetation, which will reveal useful information about the soils and which natives are best suited to the site.
My advice is don't obsess over soil fertility - after all you are growing native plants not field corn. Embrace the soil you have and work with it by choosing plants that will grow under those conditions. There is a time and a place for soil amendments but it is important to be mindful that all amendments are temporary. They should be used as an interim step toward establishing vegetation that is not reliant upon regular soil amendments. The good news is that all you need to do is get some natives established and time will take care of your soil problems. This may require adding some low nitrogen / high carbon organic matter initially to get things going. This will prove to be the simplest, lowest cost and more environmentally sensitive solution.