Sunny days in March offer the perfect opportunities to prepare your soil for planting. What do our soils need that they don’t already have? – oxygen-air and biology-carbon. Increasing the air and carbon in our soils support soil life which in turn supports plant life. Our tight clay soils that tend to exclude oxygen and are deficient in organic matter can use some help.
If you are starting a new planting bed or reviving a neglected one, adding compost is key to your plants’ success.
Our basic recipe is to spread 2” of local compost on the surface and dig or fork it into the top 6”-8”. We especially like to use a garden fork, which creates holes that allow the compost to fall down into the root zone, leaving more of the soil’s microbial and fungal networks intact. This will add air, carbon and soil organisms, while also sequestering carbon and reducing release of carbon dioxide in the atmosphere. If you’re planting individual trees and shrubs, dig each hole 2-3 times the diameter of the pot and the same depth. Mix 30% compost with the existing soil (20% is okay for native shrubs) – 50% is way too much. By the way, clay is not ‘crap’; it contains lots of minerals and it holds water; it just needs some help.
If your soil is very dense clay, hard for water and trowels to penetrate, you can add 1”-2” of expanded shale on the surface and dig it in with your 2” of compost, or 10%-20% in with the compost for a shrub or tree. Expanded shale is a local product, shale fired to 900 degrees, creating microscopic porosity that holds both air and water, and provides excellent breeding grounds for beneficial microbes. It helps build soil structure (porosity and tilth) and, unlike compost, it is permanent and won’t need renewal. Expanded shale is valuable for roses and fruit trees that don’t like “wet feet”, and it can prevent fungal problems.
It’s OK to add some organic fertilizer, but use half the recommended amount. You can always add more later and it’s better to let the biology digest the nutrients.
Water deeply and seldom (once a week is good, increasing to twice a week in July). Too much water drives out the oxygen. Plant roots need air and healthy soil life needs air.