Soil: the literal and biological foundation upon which all gardens are built. Healthy soil provides nutrients, structure and moisture-holding capacity that contribute to plant growth. Ideally, it also contains a healthy microbiome including a wide variety of microscopic insects, mites, bacteria and fungi, which serve to break down organic matter into the elemental forms that plants can use.
Many gardeners, well aware of the importance of good soil, seek various amendments to that end. Let’s take a look at some of the top ways that home gardeners can improve soils where plants are not thriving. Keep in mind that in some cases, a baseline measurement from a professional test (or soil pH test from your local Master Gardeners) will be necessary to determine how much amendment is needed. Each of the following soil amending techniques is complex enough to warrant its own discussion, but the following will serve as an introduction.
The quantity and quality of organic matter in the soil influences many different functions. Organic matter, which comes in three types, improves water retention and soil structure, provides nutrients and helps the soil aggregate into “peds” that allow for stability and pore space.
Organic matter includes plant residues, microbes, detritus and more microbes. As these plant materials are broken down by the microbes, the material eventually becomes humus, a more stable form of organic matter. Sources of organic matter for the soil include compost, animal manure, crop residues and cover crops. In your yard, these may also include leaves, grass clippings and kitchen compost.
It would be difficult to add too much organic matter to your garden soil, but it is possible to add wrong ratios of brown and green organic matter, causing a temporary depletion of nitrogen that will be detrimental to your plants. For great detail on adding organic matter to your soil, check out OSU’s guide.
Different types of plants use nutrients in different ways. Varying soils have different mineral and nutrient compositions. Some nutrients drain out of soils very quickly, while others remain in the soil much longer. Agricultural techniques to address this, used for centuries, include field rotation and allowing some plots to remain fallow.
Depending upon goals and yard or bed size, home gardeners may also be able to apply some of these techniques. In turf, phosphorus is much less important to healthy grass growth and color than potassium, so many turf fertilizers will contain less phosphorus than other nutrients. For more about lawn fertilizing, see OSU’s guide. This document will also provide some insights into fertilizing in other situations.
Till or no till?
Like many controversial either/or debates, the answer to the “till or no till question” is in the details. We know that generally, tilling can compact soil and move weed seed closer to the soil surface, resulting in a flush of weeds. Tilling can also promote soil erosion. But there are circumstances where the benefits of tilling can outweigh the drawbacks. These are going to vary from one crop or garden system to the next and are influenced by climate, soil type and the pests present in the system. For an example of a study on the effects of tillage in a western Oregon system, see OSU's guide.
Some crops, like blueberries, azaleas, rhododendrons, and hollies, enjoy a more acid soil. It’s fairly commonly understood that pH can be raised with lime and lowered with sulfur, but lots of gardeners are quite surprised by the amount of these amendments needed to make a real change in the soil or by the amount of time, often more than a year, before a change is noticeable. To move the pH “needle” from 7.5 to 6.5, for instance, might take anywhere from 10 to 50 pounds of elemental sulfur per 1,000 square feet of garden space! For more info, read OSU's guide on acidifying soil in landscapes and gardens.
Sometimes, the effort or expense of modifying a soil to fit our needs is just too much.
This is often the case in places with heavy clay soils that do not drain. (Often, sand is added to the clay in hopes that the sand will loosen up the clay, but, in fact, sand plus clay equals brick.) Adding enough organic matter — and incorporating it — is not practical for all soils. In other cases, the soil may be very thin, with rock just underneath. These challenges make raised beds worthy of consideration. For a closer look at raised bed gardening in Oregon, try this guide from OSU.