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Sandy soil, while providing excellent drainage, presents a challenge in terms of both irrigation and keeping nutrients in the soil. Vital plant nutrients, like nitrogen, phosphorus and potassium, which have a tendency to leach below the root zone of sandy soils when excessively irrigated, will remain present and available in the topsoil layer if more carbon is added. Adding carbon to the soil is as simple as adding more organic matter. I’d suggest adding successive top-dressings of mulch.
Ideally, a mulch layer will also soak up water like a sponge, keep the soil below it cool and prevent excess evaporation of water. Mulch also provides habitats for members of the soil life web, like microorganisms, invertebrates and the root-like organs of fungi called mycelia.
Adding a soil conditioner to your sandy soil before mulching will do wonders to improve upon the good work of the organic matter. There is no better ingredient to remediate sandy soil than raw sources of carbon, like biochar.
An ingredient in super-fertile Amazonian Terra preta soil, biochar is essentially charcoal. It’s plant matter—usually agricultural waste, like wood, plant stalks or brush—that is burned over a low temperature. Brown coal dust, called culm, can also be used in lieu of biochar.
These forms of carbon are largely unused by soil-dwelling organisms or soil processes, so they improve the soil structure by simply being there. This is unlike mulch, which is used up by soil-dwelling organisms and soil processes and will require you to reapply it.
Biochar or culm both have a high cation- and anion-exchange capacity, meaning they soak up and hold soil nutrition. Before blending either one into the soil, mix them with a nutrient-dense soil input, like manure or urine, so they’ll then soak up the nutrients in the input instead of the nutrients in the soil.
A variety of mulches can be used to remediate sandy soil. Each has benefits and drawbacks, and each breaks down over a different timeframe.
When applying mulch, take care to add compost or liquid fertilizers of some kind, as well. When soil organisms decompose high-carbon organic matter, they use nitrogen in the process, meaning that an application of mulch can temporarily tie up some of the soil’s nitrogen reserves.
Mixing layers of different mulches—called “lasagna gardening” or sheet mulching—can create a layer of organic matter that combines the qualities of several of these potential inputs and help sustainably balance the soil’s carbon and nitrogen reserves.
Here are some different mulch types you may consider using on your sandy soil.
Paper mulch is readily available in the form of newspaper, most of which is printed with soy-based ink these days. Lay several sheets of paper over your garden to effectively suppress the weed growth beneath. After a rain or watering, it’s relatively easy to make holes in the paper in order to plant the desired plants.
Paper mulch should be combined with some other mulch for top-dressing to prevent the sheets from blowing away in the wind when they dry out in between rains.
Seaweed or kelp is a garden “superfood,” providing broad-spectrum soil nutrition. If freshly collected, it should be rinsed so as not to add excess salt to the soil. It provides the basic NPK nutrients, as well as trace elements, like boron, chloride, copper, iron, manganese and molybdenum. Small-shelled organisms that are often tangled up in seaweed also provide soil with calcium.
Straw is a readily available and cheap mulching material that decomposes rapidly. The only downside to straw mulch is that is often contains weed seeds. But combine it with other mulches, like paper, and it will provide a nourishing and water-absorbent top-dressing for sandy soil.
Grass clippings provide a great boost of nitrogen to the soil; however, there are two caveats: They can sprout more grass, and if applied too quickly, they can form anaerobic mats that need to be aerated.
Another readily available material for mulch is leaf fall, which should ideally be decomposed for a season and then shredded before being applied to the garden. Leaves from trees like oak contain high levels of a woody compound called lignin, which means they are slower to decompose than other kinds of organic matter. They can cause the soil pH to lean towards the acidic side.
Due to the larger quantities of the aforementioned organic polymer lignin, wood chips are one of the longest-lasting mulches, sometimes lasting up to 10 years. A 2- to 4-inch layer of wood chips can suppress weeds and give your garden bed a “finished” appearance.
If sandy soil is the worst of your garden woes, don’t fret. With some time and attention and the careful application of amendments and mulches, you’ll be able to rehab your soil into a fertile, life-giving material where your crops will happily grow.