Soil microbes: small helpers, healthier garden soil

There is a lot happening beneath a garden that we cannot see. Bacteria, fungi and other tiny organisms help break down organic matter, cycle nutrients and influence the soil around plant roots. Looking after that living community is part of looking after your plants, whether you are growing a vegetable bed in Vancouver or caring for shrubs in Ridgefield.

The good news is that supporting soil life usually starts with sensible garden care, not a complicated routine or a bottle for every plant.

What do soil microbes actually do?

Microbes have different jobs. Some decompose fallen leaves and other organic material. Others help transform nutrients into forms plants can use. Certain fungi form partnerships with roots, while other soil organisms live independently. Not every microbe is beneficial, and not every beneficial microbe forms a root partnership.

Plant roots can absorb dissolved mineral nutrients directly. Microbes can help release nutrients from organic matter or make resources easier to reach; they do not replace the plant's need for suitable soil, water, light and nutrition.

Meet the root partners: mycorrhizal fungi

Mycorrhizae are partnerships between particular fungi and plant roots. The plant supplies sugars made through photosynthesis. Fine fungal threads explore the surrounding soil and can help the plant obtain water and nutrients, especially phosphorus.

• Many vegetables, grasses and fruit trees, including apples, associate with arbuscular mycorrhizal fungi, a common type of endomycorrhiza.
• Many conifers and trees such as oaks associate with ectomycorrhizal fungi.
• Rhododendrons, azaleas and blueberries commonly form ericoid mycorrhizae, a different partnership.

Some familiar vegetables, including cabbage and broccoli, generally do not form the usual mycorrhizal associations. A product meant for one plant group is not automatically useful for another.

Make your soil a good place to live

In our Southwest Washington gardens, pay attention to both soggy soil and dry summer beds. Most beneficial root-zone organisms need suitable moisture and oxygen. A wet spot or a compacted planting bed needs that problem addressed first.

• Keep foot traffic off planting beds, especially when the soil is wet, and avoid unnecessary tilling.
• Use an appropriate organic mulch and leave room around trunks and stems.
• Where the soil needs organic matter, consider a measured topdressing of compost rather than repeatedly digging through established roots.
• Base fertilizer choices on a soil test and plant needs. Adding extra phosphorus “just in case” can work against mycorrhizal activity.
• Check moisture below the surface and adjust irrigation to the season and planting, rather than keeping every bed constantly wet.

If you are building a new bed, choose the growing medium for that use. An in-ground bed, a raised bed and a container do not necessarily need the same mix.

Do you need to add microbes?

An established garden usually already contains a community of soil organisms. WSU Extension reports that adding packaged mycorrhizal inoculants to ordinary gardens and landscapes is generally unnecessary and often ineffective. That is different from some nursery situations involving sterilized growing media or severely altered soils.

If you are considering a microbial amendment, first identify what you want to improve. Check the actual organisms, intended plants, storage instructions and application directions on the label. A blend of bacteria and fungi is not automatically interchangeable with a specific mycorrhizal inoculant, and it is not a cure for poor drainage or the wrong soil pH.

A closer look at two products

At Shovel & Thumb, we carry Wind River Microbes products. The manufacturer positions BioSoil for trees, shrubs and woody plants, and SoilCare for lawns and gardens. Those intended uses are a starting point for reading the label, not a guarantee of results in every yard. Match any product to the plant and the situation, and follow its current directions.

See a root partnership in action

Watch “What Are Mycos?” by Down To Earth Fertilizer for a short animated introduction to mycorrhizal fungi. It is a manufacturer's explainer, not evidence that every garden needs an inoculant. In particular, rhododendrons and blueberries have their own ericoid partners, even though they are not a match for the common garden mycorrhizal products discussed.

For most home gardens, a good place to start is protecting the life already in the soil. Give roots room, avoid overwatering and unnecessary fertilizer, and let organic matter do its work over time.