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Underground Give and Take

One of the most widespread wrong ideas in horticulture is that roots simply serve to anchor plants in the soil and suck up nutrients. This perspective leads to many mistakes, not the least of which is the overuse of “plant food”. I won’t go into any detail about the process of photosynthesis, but the gathering of solar energy by plant leaves is crucial for healthy soil.

As a plant’s roots grow and push out into surrounding soil, they exude a thin layer of mucus which acts as a lubricant. This gel is rich in nutrients and provides nourishment for microbial life underground. Mycorrhizal fungi spores which may have laid dormant for decades are brought to life by a signal from the root exudates, as are many types of beneficial bacteria that convert nitrogen, phosphorus, and other elements into forms that the roots can uptake.

Soils without growing plants tend to be relatively lifeless and compacted. But when a seed germinates and a new root extends into that soil, there is a powerful reaction from the living organisms, something like children hearing the sound of an ice cream truck coming down their street. Here comes food!

Fueled by the photosynthates, fungi attach to the roots and send their own root-threads (hyphae) out into the soil to forage for the nutrients their host plants require. Soon the surrounding soil is loosened by the fungi hyphae and great populations of bacteria develop, supported by the roots and fungi.

The fluffed-up soil allows more oxygen to penetrate deeper, which further promotes the microbial life that is now busy seeking out and also creating plant nutrients. For example, a major source of N is the excretions and dead bodies of soil microbes - it is to a plant’s advantage to support the living organisms in soil. The further out the plant roots extend, the more soil becomes alive with helpful “associates” that are all benefitting from the aboveground plant leaves absorbing solar energy. Of course, in return the plant is receiving valuable nutrients and improved soil conditions for its roots.

For the grower, this all argues for avoiding bare soil and also for using companion plants as much as possible, such as legumes or wildflowers between the rows in orchards or vineyards. The more leaves that are gathering and sending energy to underground organisms, the better. This is why “living mulches” increase yields so much in USDA trials, as compared to inert mulches. In reading the reports of these trials, the scientists seem rather puzzled as to why the yields are higher when tomatoes are planted into vetch fields, but the answer is quite simple and predictable to a soil biologist: More solar energy has been captured and “sent downstairs” to helpful soil critters. The tomato plants then benefit from the increased life in the soil that has been supported by the vetch.

All of the above can be disrupted by fertilizing with fast-acting synthetic fertilizers. It’s hard to improve on a system that has taken many millions of years to build. We simply need to learn how to better work within that elegant system instead of trying to impose our mistaken ideas about feeding plant roots.

Cheers, my friends,

Don Chapman
President, BioOrganics, Inc.

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