Managing Garden Soils - The Dirt on Dirt

Gary Westlake


Most of us don't think much about our garden soil, perhaps because it's beneath us. The truth is that it is the most important aspect of gardening to manage. The soil allows water, air, and nutrients to get to the roots, and provides space for your plants to grow. Without good soil your garden just won't amount to much.

Anyone can have decent garden soil with a little work and even the beautiful loam soil that most gardeners wish for can deteriorate if neglected. Gardeners with challenging soil types may have to work a bit more but they too can have productive soils.

Sandy soils feel gritty when you rub them in your hands. They are difficult because they do not hold moisture or nutrients well. If your soil feels smooth, it probably has a lot of silt and if your soil feels sticky when wet, it probably has a high percentage of clay. Clay does not allow moisture to pass easily and has very small air spaces. When gardeners try to amend clay soils by adding sand, it often results in disaster. It would take tons of sand to make a difference even in a small lot and if only a small amount of sand is added to clay, it can form concrete making the situation worse.

Fortunately, all soils can be improved by the addition of organic matter but you need to understand what organic matter does for the soil in order to do it well. Good garden soil is not just made up of small particles of sand, silt and clay. It is alive with bacteria, fungi, and a host of other creatures. These inhabitants are busy breaking down dead plant material and this process causes the soil particles to clump together in small aggregates. Sandy soil with this granular structure, full of organic matter, will hold moisture better and a clay soil with this living structure will drain better. With the addition of organic matter, all soils will resist compaction and erosion better, they are easier to work and warm up sooner in the spring. So the first thing to remember is that the soil improvement is caused by the biological process rather than the organic matter itself.

Some of the largest of the inhabitants of the soil are earthworms. Earthworms can eat their own weight in dirt each day, and collectively they can move around and improve 20 tons in every acre each year. They do, however, have a dark side because they are not native to North America. In our forests, they break down the dead plants too fast, making it hard for our native plants to survive, so be careful not to dump garden soil in the forest.

One of the most common tiny creatures in soils are mites. A square foot of soil can contain 100 different species of mites, all specialized in feeding on a different aspect of the breakdown process. Fungi are even more common. We normally think of fungi as mushrooms but that is only the fruiting body. Most of the fungus is made up of long thin filaments in the soil that feed on living and dead plants. In fact, some called micorrhyza are essential because they extend the root system of plants and provide them with nutrients directly through this close partnership. Also in huge numbers are bacteria. The characteristic smell that you get when you dig into the ground in the spring is caused by a group of bacteria called Actinomycetes.

There are two kinds of gardeners - ones that water too much and ones that water too little. I confess to being the kind that waters too little. The downside to this is that our plants grow slowly and the roots tend to stay where the water is. Nutrients have trouble getting to the roots because there is no water to carry them. The gardeners that water too much tend to set their plants up for drought. Also too much watering can fill the air spaces in the soil and suffocate your plants.

Garden soil contains huge numbers of weed seeds. Most weeds are successful because they produce many seeds and these seeds are very long-lived in your garden. For example, the common Lamb's Quarters produce 70,000 seeds for each plant. When you factor in to this the fact that each seed can survive in the soil for up to 40 years, the numbers of seeds-in-waiting can be staggering. Most weed seeds germinate in response to clues that they have been brought to the surface, such as increased light. So one of the challenges a gardener faces is how to keep these seeds from germinating. Two of the best ways are to avoid deep tillage as much as feasible and to cover the surface with weed-free compost each year. Another way, which is often used in the lawn, is to apply Corn Meal Gluten that kills the weed plant as it germinates by affecting its first tiny roots.

There are some types of organic matter that are not good to add directly to your garden. If the organic matter contains too much carbon in relation to the nitrogen, it will rob the soil of its nitrogen as it breaks down giving your plants a hard time. Sawdust, for example which can contain 700 times more carbon than nitrogen can bring growth to a standstill especially if it is dug in. It needs to be composted first. For the same reason, although bark mulch can do many things for you including improving moisture retention, it should never be mixed into the top layer of soil.