By Bill Ward
Published in The Boerne Star on July 2, 2004
Some horticulturists, such as Pat McNeal, are questioning whether there should be much soil preparation when planting natives. McNeal, who spoke to the Boerne Chapter of NPSOT this spring, has found that many plants he uses for landscaping do better in what seems to be very poor soil, to which he adds nothing.
Soil preparation for native species might depend on how out-of-the-native-habitat the planting site is. Not all plants native to the Hill Country do well in all the different soil types in this region. Might some native plants do better in your landscape if the soil were altered?
There seem to be few scientific data on the relationship between soils and healthy native plants. The relationship of specific soil organisms to specific native plants is largely still a mystery. However, pioneering efforts to document the role soil organisms play in healthy plant growth are being made by Drs. Dick and Patricia Richardson and their graduate students at the University of Texas at Austin.
In her recent presentation to the Boerne Chapter, Dr. Patricia Richardson showed that soils are complex ecosystems richly populated by bacteria, fungi, protozoa, nematodes, arthropods, and earthworms, as well as plant roots. These organisms are matter in soils into nutrients necessary to transform organic which can easily be taken up by plants.
Soil organisms that nourish a tree are different from those which nourish a grass, which are different from those which nourish a wildflower. A given species of tree or shrub or flower probably has its own preferred assemblage of soil organisms that best provide nutrients to that plant.
Soil organisms feed plants, and plants feed the world. Our existence depends on plant life; therefore, our existence depends on the health of soil organisms that are necessary for plant growth.
It is my impression that there are few data on the effect of fertilizers on soil organisms. Does adding fertilizer change the types of soil organisms that exist at that site? How do soil organisms process compounds added to the soil?
One virtue of landscaping with’ natives is that they require little or no added fertilizer. This saves money and lessens the possibility of contaminating our water system with chemicals from fertilizers. Both organic and inorganic fertilizers can contribute to pollution of surface and subsurface waters.
My older daughter recently earned a PhD in Soil Science by studying the differences in phosphate in water running off of fields fertilized with cow manure compared to those fertilized with chicken manure. Apparently, the soil organisms process these manures differently and influence the amounts and kinds of contaminants in surface runoff.
In recent years there seems to be an increasing awareness of the importance of mycorrhizal fungi to healthy plant growth. Mycorrhizae are masses of fungal hyphae, some of which colonize the surface of plant roots and others of which grow within root cells. In exchange for carbon from the plant, mycorrhizal fungi help bring nutrients to the plant. Although research data are lacking, there seems to be the suspicion among some botanists that specific plants depend on specific mycorrhizae.
This might be a problem in restoring native vegetation to an area that has been populated by exotic plants. Mycorrhizae in soil where exotic plants flourished may not support healthy growth of native vegetation. It might take some time before the natives have developed their preferred mycorrhizae.
If mycorrhizae are indeed plant-specific, I wondered how tonics of mycorrhizal fungi could be marketed. After talking to McNeal and Richardson, I learned that the mycorrhizal potions for sale at nurseries are cocktails of many kinds of mycorrhizae. There is, therefore, a good chance that one of the mycorrhizae in the solution will be beneficial to a given plant.
Another method that may stimulate healthy growth of a native plant is to gather a bit of soil from around a naturally growing plant of the same species. Adding this natural soil when you plant the new native may inoculate your garden soil with just the organisms that make this plant happy.
