What is Soil Compaction, and Why Worry About It?
Soil compaction is the process by which you garden soil becomes more and more dense. Over the years your own garden soil may have compacted and you may not even realize it as there are multiple causes and soil compaction can happen both suddenly or over time.
Dense soil may be naturally occurring, but with plenty of plant variety and freeze-thaw cycles, the dirt should naturally aerate and become a little looser. When this doesn't happen, or when things like heavy machinery squish the soil unnaturally, new plants struggle to put down roots, literally, due to the force necessary to grow through compacted dirt.
Plants need air from the soil to thrive, so completely compacted soil poses a threat to the nutrients plants need. The process of losing access to air is called denitrification, or the removal of access to nitrogen for the roots. Certainly, there are contexts where compact dirt is helpful - drought-prone areas often rely on mildly compacted soil to retain enough moisture for plants to thrive - but overall, most gardeners are going to experience more trouble with soil compaction than if they keep lighter, well-aerated soil.
Ironically, if your soil becomes too compact, even by the act of too much rain, it loses its best drainage capacity. Water should trickle through soil, reaching your plants' roots easily but over an extended amount of time. Too much rain and soil compaction in combination can result in a few bad circumstances: water runs off at the surface, causing erosion, or water pools in the soil, promoting root diseases and still restricting access to air for the roots. Overall, you want your soil to get a good combination of moisture and air, so make sure your soil isn't getting too compact.
Ultimately, if you are growing fruits or vegetables, you'll see lower yields from plants experiencing soil compaction because they cannot range as deep as they want for nutrients. They'll conserve what they do get and keep their plants and fruits smaller, which isn't the goal.
Contributing Factors to Soil Compaction
Soil compaction arises for a variety of reasons. In larger-scale gardens, you may notice that planting the same crops in the same locations for years in a row will lead to soil compaction. This occurs because a single kind of root system will break up the soil in one particular way, while switching your crops around will result in a diverse variety of root systems, some of which break into deeper layers of the soil.
If you walk on your soil or run a plow or other machinery over it frequently, the soil can become tamped down in an undesirable way. For one thing, if frequent rain in the early part of the year flows through the soil, it can naturally reduce the air in the soil, compacting the soil's structure. Not growing cover crops, or leaving a raised bed with no plants in it for a season, can contribute to compaction as well, since soil without any roots in it is prone to losing its air.
Avoiding Dense Dirt Long-Term
Try not to walk on your flower beds, even if this means having to crane and reach to dig in them. It's a pain, but every time you press down on the dirt with your foot or a piece of equipment, you are pressing air out of that soil.
Water enough but don't overwater, and pay attention to the forecast so that you can avoid watering on days that will also have significant rainfall.
Plant cover crops or let plants die and remain in the beds over the winter; pulling them up in the spring will be part of a natural aeration process for your soil, and you might get some nice winter plants out of this process as well!
When choosing materials for raised bed soils, make sure you are offering enough items that will decompose and create air pockets in the soil, like compost, and a good combination of water-retaining and non-retaining substances. A popular mixture is equal parts compost, vermiculite, and peat moss which holds the water in, but the vermiculite keeps the water draining.
Be careful with tilling your soil. if you don't need to do it, just leave the soil and take advantage of the natural structure of the soil. Tilling when there have been heavy rains or drought can collapse what air pockets already do exist, so pick a good in-between time when the soil is fluffy and lightly moist if you decide to till.
Some people find that an aerator, a prong-covered rolling tool, does a good job of puncturing the surface of compacted soil, allowing both air and water to penetrate deeper and begin the work of rejuvenating the soil. This can be a good way to get started if you know you want to loosen soil long-term.
No matter how you do it, remember that avoiding compacted soil in the first place is much easier than "curing" soil compaction after it happens.