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Garden Wise: Vegetable Gardening Starts with Good Soil

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Computer programmers are fond of the acronym GIGO. It stands for “Garbage in, garbage out.” The meaning is clear. If your inputs are poor, your outputs will be poor.

Gardeners would do well to adopt GIGO as a mantra. I’m often struck by how much time gardeners spend obsessing about their stunted fruit, deformed cucumbers, paltry tomato plants, and discolored spinach, while spending almost no time thinking about the health of their soil. A good vegetable garden starts with good soil.

Dr. Linda Schott, soil scientist with the University of Idaho Extension, usually corrects people when they confuse soil with dirt. As she puts it, “Dirt is dead. Soil is a living thing.”

Imagine this: In a typical teaspoon of healthy soil, you would find over a billion bacteria. In a square foot of soil, that microscopic biomass would be joined by a host of insects, earthworms, fungi and miscellaneous arthropods forming their own vibrant subterranean ecosystem. And that ecosystem is vital to plants.

Good soil has good structure, balanced pH, and adequate mineral resources to provide plants with essential nutrients. Good structure boils down to a balance between mineral content, air and water pockets, and organic matter.

Adding organic matter is the single most important step gardeners can take to improve soil health. Organic matter improves soil structure while providing essential nutrients. It increases water-holding capacity and soil aeration. Rather than enabling soil to hold more water at the expense of air, it increases the available supply of both.

It provides essential plant nutrients, aerates the soil, improves soil structure and tilth, and enlarges the community of soil organisms. Organic matter can help resolve a host of soil problems. Although a professional soil analysis will provide detailed information about the quality of your soil, adding organic matter will improve your soil, whatever its deficiencies.

Properly aged compost easily deserves the “best in class” award as a source of organic matter. It can be homegrown or commercial. Ideally, it reflects a healthy mix of sources, livestock manure, grass clippings, rotting leaves, pine needles, yard debris and kitchen scraps. Mixed compost is the best source for both macro and micro plant nutrients.

One perennial concern in using compost involves nitrogen (N). In the process of decomposing organic matter, considerable nitrogen is used up by soil organisms. Some nitrogen evaporates into the air where it is unavailable to plants. Consequently, aged compost is often relatively low in nitrogen, while providing healthy amounts of most other essential nutrients. Nitrogen, unfortunately, is the most important plant nutrient and is what botanists refer to as a limiting nutrient. This means that too little nitrogen will limit plant growth regardless of how much of other nutrients is available. While taking advantage of the virtues of compost, homeowners need to ensure that their soil is receiving adequate nitrogen.

How much compost should you add and how often? In garden areas, 5 – 8” of compost can be incorporated into the top foot of soil annually. Fall application is optimal though not critical. Some homeowners have had excellent results with several small top dressings of compost (1-2” per application) spread out over the growing season. Gardeners who mulch usually add a 2 to 6” layer in the spring or early summer.

Organic matter is the key to good soil which in turn is the key to successful vegetable gardening. Remember, soil organisms get their fuel from organic matter. As they eat and break it down, they discharge nutrients back into the soil in forms plant roots can use. The more organic matter you put in your soil, the more vibrant the community of soil organisms will be. Bacterial populations, for example, soar when new organic matter is incorporated into soil and drop rapidly when that matter has been processed.

A healthy community of soil organisms is essential for healthy soil. Along with breaking down organic matter into available plant nutrients, they aerate the soil and improve the soil’s water and air retention capacity while improving water drainage throughout the garden. As you add organic matter to your vegetable beds, you are helping the entire symbiotic ecosystem of living things within your garden. That is the foundation of healthy gardening.

There is little danger of adding too much organic matter to your soil. Optimal loam soil would have 4 to 6% organic matter. In southern Idaho, organic matter typically constitutes 1 to 2% of soil. Keep it coming!

Garden Wise is presented by the Magic Valley Master Gardener Association. We will try to answer questions of general interest submitted by the community. Email questions to


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