Gardening for Geeks. Christy Wilhelmi

      Home soil-testing kits test levels of nitrogen, phosphorus, and potassium as well as pH.

      The second type of soil test yields more elaborate results. A complete test involves sending a soil sample out to a laboratory. Soil technicians provide you not only with N, P, K, and pH ratings but also with ratings for trace elements, salinity, and heavy metals.

      Why is this important? Well, let’s say you live in a city, near a busy road. Your test might show high levels of zinc in the soil. Where did that zinc come from? Zinc disperses from braking systems on vehicles, from roads, from airplanes overhead, and from galvanized metal gardening tools (like watering cans or buckets). Excessive zinc prevents the uptake of nutrients in plants. That’s right—it actually blocks the plant’s ability to extract N, P, and K from the soil. A soil test might also show that the pH of your excessively zinc-laden soil is acidic. This would tell you that, if you were to raise your soil’s pH, you would bind up the zinc, making more nutrients available to your plants.

      It’s always a good idea to get a complete soil test to rule out the presence of lead (residues from turn-of-the-century oil drilling, leaded gasoline, and house paints), arsenic (used previously for years near railroads as a weed killer), mercury, cadmium, or aluminum. Most university departments of agriculture offer inexpensive soil tests (just search the Internet for “university soil test” to find one near you), or you can send a sample to a soil lab, such as Wallace Laboratories (wlabs.com) in El Segundo, California. To find out the active microbiological composition of your soil (bacteria, fungi, nematodes, and more), look up “biological soil testing” or send a sample to a lab such as Earthfort (earthfort.com).

      Make Amends

      Once you know the nutrient levels of your soil, you can amend accordingly. Soil amendments—also called inputs—can increase available nutrients, but they also can alter soil texture or improve drainage. Let’s revisit the soil textures: clay, silt, and sand. What can you do to improve these conditions?

      Surprisingly, the organic solution to hard-packed clay soil, compacted silt, and loose, anemic sandy soil is one thing: compost. In clay and silty soil, compost serves to create space between particles and allow more airflow, which then helps the soil drain better. In sandy soil, compost works as a sponge to retain moisture and provides structure to hold nutrients. Compost brings these extreme conditions closer to the perfect texture for growing vegetables and fruits; this ideal is called loam. Loamy soil holds nutrients but drains well. It supports the easy proliferation of root systems and is nearly effortless to dig. Loam is the goal that every gardener hopes to attain with soil.

      So how much compost should you apply to your soil? The general rule of thumb is to add compost in inches. Add a 1/2- to 1-inch (1.25- to 2.5-cm) layer at the beginning of each season before planting and, if needed, again during mid-season to boost production. You can either top-dress, meaning spread it out on the surface and leave it, or you can work it into the top few inches (7 or 8 cm) of soil. Soil food web aficionados prefer to apply compost on the surface, without disturbing the delicate strands of fungal hyphae and microbial life hard at work in the soil. The microbes will utilize the compost as it filters through the soil during regular watering and as larger insects and earthworms till it into the soil for you. All of this activity makes nutrients available to plants with less work from you.

      Gardeners with very sandy soil may choose to ignore the 1/2- to 1-inch (1.25- to 2.5-cm) rule of thumb and add compost with reckless abandon. It’s OK. Add compost, then add more compost, and when you think you’ve added enough, add more. Your soil will be just about right at that point.

      In addition to benefiting soil texture and structure, compost adds nitrogen and inoculates your soil with those stagehands we talked about earlier. Compost does much more than feed the soil; it brings it to life with fungi, bacteria, microscopic insects, and earthworms. It supercharges your soil with the microbiology needed to help plants thrive. You can buy bagged compost from nurseries, but why not make it yourself? It’s a great way to recycle nutrients in your garden and cut down on waste that goes to the landfill. Food waste happens to be one of the top contributors to climate change, by the way. So if you aren’t composting, now is a great time to start! Best of all, making your own compost means you know exactly what’s in it, and you don’t have to drive anywhere to get it.

      Start a Compost Bin

      A compost bin can be any structure that holds garden biomass (use this term instead of waste, because you’re not wasting anything). A compost bin can be a cylinder of hardware cloth, an old trash can with the bottom removed and holes punched in the sides, or an official store-bought compost bin. You don’t even need a bin, per se, to store your compost. You can make a pile in your backyard and let it cook. The important thing is to start using your own garden biomass to give back to your garden. Here are a few guidelines for creating a viable composting system.