Local gardening stores offer a multitude of soil mix choices this time of year. This overview of the attributes of soil and how to amend commercially produced potting soils will increase the success of your container and raised bed plantings. This article is posted on the Ask a Master Gardener Blog and provides a comprehensive explanation and strategies for improving commercially produced soil.

All soil primarily consists of sand, silt and clay mineral particles. Sand is visible to the naked eye. Clay is the size of bacteria. In a natural setting, soils have pores made by plant roots and insects, essential for drainage and allowing air to enter the soil for roots to breathe. The size of the pores is also essential for water movement and the soil’s ability to retain water.

In contrast to well-balanced mineral soil, commercial potting soil typically has very few of these properties and has several problems. Most bagged potting soil consists of ground wood and bark from recycled forestry products. The potential hazard with some commercially bagged soils is the possible presence of weeds such as horsetail (Equisetum) and the potential presence of herbicides from sprayed landscaping or hay that persists throughout the composted state.

Amendments

Commercially produced soils, often recycled forest products, are sold in large bark chunks as mulch or finer-grained material as potting soil. These products have inherent problems, such as nitrogen deficiency, which can be solved with amendments.

Fertilizer with some phosphorous and potassium is one way to address nitrogen deficiency. Some fertilizers also contain micronutrients, which can be beneficial. Use a significantly diluted solution (1 tablespoon per gallon) of soluble fertilizer and frequently monitor your plants to see how they are doing.

Since the RFP in purchased soil bags is very porous, much of the fertilizer will drain through the pot in one application. Adding well-composed organic matter or humus materials will help provide effective nutrient retention. Humus is very good at holding water. Adding well-composted materials to a soil mixture will also add beneficial bacteria and fungi essential for many processes. Another benefit is that humus has a net negative charge that can hold onto cations such as potassium, calcium and magnesium, giving the soil cation exchange capacity essential to plant growth. For a deeper dive into soils, including amendments that will address cation exchange capacity, the soil’s capacity to hold water and the addition of slow-release fertilizers, go to the link for the full article: skagitmg.org/potting-soils/.

While forestry products recycled as commercial potting soil have drawbacks, amending these insufficiencies is an easy process. It is much better to use RFP products than to burn, bury or dump them into the overflowing waste stream. Armed with awareness of the limitations of modern “potting soil” and knowing some remedies, gardeners can buy these products feeling confident with scientific knowledge.

Bruce Lindsay became a Skagit County WSU Extension Master Gardener in 2015. He has an MS degree in geology and an MS degree in soils and has mapped soils in Nevada, Arizona, California and ­Washington.