What makes good soil and what does “good” mean? Most of us would likely answer that good soil is full of readily available nutrients for our vegetables and flowers to uptake and robustly grow.
There is, of course, more to good soil than nutrients. Otherwise, why would we bother to grow plants in soil at all?
Soil, being an ecosystem, is teeming with life that supports plant growth, if conditions are right. The physical and chemical properties are what support soil biology, which in turn supports plant growth. Soil texture, structure, drainage, water and air are all important to consider whether you are growing vegetables or flowers.
Texture, which refers to the combination of different sized soil mineral particles, mainly depends on the amounts of sand, silt and clay present in your soil. Loam is the ideal combination of these soil particles.
The combination of particles leads to various properties of the soil. For example, sandier soils tend to hold less water and so have better drainage than clay, while clay tends to hold moisture and is more susceptible to compaction but can be more nutrient-rich.
Structure refers to the arrangement or aggregation of soil particles. A good soil structure is one where the structure is loose and porous, similar to a sponge with its pores and channels. This allows for roots to spread and easy flow of air and water. A good structure also supports healthy biological communities that include microorganisms, such as fungi and bacteria, and macro-organisms, such as insects and spiders.
Drainage is the rate that water moves through the soil. Too much or too little drainage is equally undesirable. Macro-pores promote good aeration and increase drainage, whereas micro-pores help retain water. Ideally, half of the soil’s pore space is occupied by water.
Lastly, air. It may come as a surprise that high quality soil has a substantial amount of air between soil aggregates. The best soil for plant growth contains about 50% solid material and 50% pore space. Imagine your garden bed and think that ideally 50% of that volume are the pores surrounding the soil aggregates that allow for air flow to and from the roots.
In many parts of Vermont, gardeners are challenged with clay-dominated soils. In those cases, adding organic matter (through compost or shredded leaves) may help improve soil texture and structure. Take care to avoid the “too much of a good thing” approach.
Since clay soils hold on to nutrients, overapplying compost may lead to excessive nutrients. Avoid this by monitoring your soils through regular soil testing (go.uvm.edu/soiltest).
We can also learn to limit practices that negatively impact soil properties such as routine tilling and compaction through repeated walking or driving. Ultimately, the more we learn about soil properties, the more we understand that “good” soil extends beyond fertility.
by Benjamin Block, UVM Extension Master Gardener Intern
Featured photo: The best soil for plant growth contains about 50% solid material and 50% pore space. Photo by Rain Photography/Pexels
