By now, you gardeners have sunk your garden fork in the earth, gotten down on your dirty knees and, hopefully, run your fingers through some lovely, healthy soil. Unfortunately, many of you are frustrated with soil that is too hard, or too sandy, or too something else. How do you go from frustration to joy?
Healthy soil is alive, with abundant air, water, and organic matter. The foundation of soil, the mineral part, is inert. It consists of tiny clay particles, slightly bigger silt, sand grains, pebbles and rocks. The proportions of these different particle sizes vary tremendously and determine what is known as the soil texture, such as a sandy loam, silt loam, or coarse sand. This mineral fraction of soil makes up about 45 percent of the volume of a typical soil. Organic matter makes up about 5 percent. The other half of the volume is air and water. In a healthy soil a complex web of life is busy, from tiny microbes to ants, earthworms, and moles.
If you have to stomp on your garden fork to get it into the soil, it probably has a much higher percentage of minerals, and much less organic matter, air, and water. Soil can be hard for different reasons. We talk about “hardpan,” but that usually refers to a very compact layer down below the plow zone. Those compact layers can be caused by ancient compaction by glaciers or modern compaction by equipment and tillage. There is little room for air and water, and roots have difficulty penetrating. If a compact layer occurs within a foot or so of the surface, and isn’t too thick, it can be broken up by deep tillage with a chisel plow, or by the penetrating roots of certain plants.
Real clay soil is naturally very hard when it is dry because the flat shape of the particles allows them to stick together with very little pore space. Clay is not common in Caledonia County because it only accumulates in standing water such as a lake or ocean. The Champlain Valley has huge areas of clay soils because it was at the bottom of the Champlain Sea for thousands of years. In the Connecticut River Basin, clay is only found in areas that glacial lakes once covered, notably glacial Lake Hitchcock, which stretched from Connecticut north to St Johnsbury. Because the lake was long and narrow and confined to the river valley, most of the clay deposited on the lake bottom has washed away in the millennia since the lake drained, or covered up by more recent flood deposits. Today, clay deposits can be found along some benches on the sides of the valley. Clay becomes compacted easily, especially if it is worked when wet. Farmers in the Champlain Valley have customarily plowed in the fall so that the hard clods would be broken up by freezing and thawing over the winter.
Nearly all gardens around here are in loam soil, which contains a mixture of particle sizes. In which case, any compacted soil is probably due to human causes. Vehicle compaction is very common. Try to dig into a part of a field where farm equipment typically concentrates and you will find compacted soil. Those huge tires you see on modern manure tank spreaders are to avoid compaction by the enormous weight of several thousand gallons of liquid manure - the wider the tire, the fewer pounds per square inch.
Compaction at the base of the tillage zone is also very common. Any tillage implement puts pressure on the soil directly under it. Moldboard plows are particularly bad, and many fields suffer from a compacted layer about eight inches down, caused by decades of plowing. Tilling when the soil is too wet aggravates the problem.
Dealing with compaction is not easy. A thin compacted layer such as that caused by plowing can be broken up with deep tillage with chisel plows, but there are drawbacks. Any type of tillage causes a decrease in organic matter, as it is broken down faster due to increased oxygen. A longer term solution that is becoming more common is the use of tap rooted plants to penetrate the compacted layer. Grown as a cover crop, plants such as forage radish and rapeseed send their taproots in search of nutrients and water. Forage radish is particularly well suited to penetrating compacted soil. Once the root is through the compacted layer, the channel it has made serves as a conduit for air, water, and organisms. As organic matter increases, so does biological activity.
What about sandy soil? Once again, sand is usually found on terraces along the edge of river valleys, deposited there by streams flowing into ancient lakes. Sand can also be found in floodplains along the river, deposited in meadows every time the river floods. If the meadow is in a steeper part of the valley where the flood water rises but continues flowing, the sand left behind will be coarse; the finer particles will continue down the river. If the meadow is in a very flat part of the valley where flood water sits for a long period, those fine sand and silt particles have time to settle out and the new soil that is deposited will be very fine and very productive.
If your sand is coarse, it does not hold water well and consequently your garden suffers in a drought. The coarseness of the particles means large pore spaces which allow water to drain through easily. Coarse sand is also inherently infertile. Soil nutrients are held on the surface of soil particles and the total surface area of coarse sand is less than that of finer textured soil.
The best way to improve a sandy soil is to incorporate lots of organic matter in the form of compost, manure, and cover crops. In one respect, organic matter (also known as humus as it decomposes) takes the place of the missing finer mineral particles. Humus has enormous capacity to retain nutrients and then exchange them with growing plant roots. Humus also ties sand particles together, allowing the formation of soil aggregates, clumps of soil that hold together well and in turn form larger clods. It is the aggregation of soil particles that results in the crumbly texture that we associate with healthy soil. A third major benefit of organic matter in sandy soil is its ability to absorb and retain water until called for by plant roots.
It feels right to work organic matter into the soil with your fork or rototiller. Remember, though, that tillage hastens the decomposition of organic matter. If you are dealing with a sandy or clay soil that badly needs organic matter, go ahead and work in lots of compost and/or manure, but be ready to plant a cover crop as soon as possible. The object is to maintain a high level of humus in the soil by continuously replacing decomposing humus with new organic matter, i.e. roots growing through the soil which eventually die and decompose in their turn. As this transformation takes place, the organisms that feed on organic matter will also increase and begin to do your tillage for you. Ants, earthworms , and other organisms are continually mixing the soil. Let them do their work.
Healthy soil is a joy to work with. May your clods be fruitful.