How to turn Dirt into Soil
Hello, I recently expanded our garden with a 24 x 16 foot patch of land. However when we removed the grass, it exposed terrible dirt, the ground is hard and very clay like with little to no drainage. What should be done to turn this dirt into healthy growing soil for tomatoes? Should I sacrifice a growing season to plant a cover crop? Or should I simply till the ground and add a thick layer of compost? Thanks in advance!
Montgomery County Maryland
Healthy soil possesses a number of factors that include porosity (air spaces between the particles which help roots grow and breathe), permeability (how well water drains), and microbial life. Organic matter provides many benefits and improves soil quality in several ways, including "feeding" microbial life and promoting porosity by helping "glue" the fine soil particles that compact easily into larger conglomerates that allow for better airflow and water drainage through the soil. Microbes break soil organic matter's nutrients into forms plants can use, and will colonize plant roots symbiotically and help the plant absorb water and nutrients.
Your soil is likely adequately stocked with microbial life and mineral nutrients from the normal decomposed-rock portion of the soil, but it's possible your organic content is lower than desirable for gardening purposes. Housing construction will also have left the subsoil compacted and likely removed the topsoil entirely, which often has more organic matter than subsoil. It wouldn't hurt to get a soil test done this winter of the new bed so you know how best to prepare the soil come spring. This page provides some basic information and includes a link to regional testing labs: https://extension.umd.edu/hgic/topics/soil-testing
Advice concerning soil amendments and tilling of the soil is mixed and depends a bit on the plants being grown. Tilling can destroy soil structure, but if it's decimated to begin with, such an overhaul might be in order. Compared with long-term ornamental plantings, vegetable crops are producing a harvest and often entirely removed themselves each season, so nutrients are being used up at a faster rate, not completely recycled in-place, and may need to be supplemented. In addition, some heavily-compacted soils are not ideal for herbs or root vegetables that need a looser soil texture. Tomatoes, while not a root crop, still have a fairly deep root structure. You can either layer-on a topdressing of compost and let weathering (contraction cracks and fissures) and soil life (earthworms, beetles, etc) work it into the soil layers over time or you can mix it in manually with a tiller or pitchfork. The fork will alter the soil structure less than tilling if you wish to use minimal disturbance, though exposing large rocks with a fork may be of use if you intend to later add root crops. A soil test often gives results on current organic matter levels (and desired levels, in percentages) if it gives you the choice of denoting the sample is for vegetable gardening use.
Cover crops are a great way to minimize erosion and help improve soils (clay-busting deep-rooted radishes and nitrogen-fixing clovers are two examples). That said, except for periods of fallow land between crops or planting seasons, it isn't necessary to delay planting a season. Going forward, however - crop rotation is a good practice to avoid repeat occurrences of pest or disease outbreaks if they occur this year. Thus if another crop isn't going to be used in its place next year, a cover crop makes a good stand-in.