Garden Soil.

It’s a good time of year to learn some things about the soil you use for growing ornamental plants or food crops.  Vashon’s native soils are notoriously poor, but there are ways to improve them.

What’s so poor generally about Vashon’s soils?  People often refer to the dreaded Vashon Till, a hardpan that lies, in some locales, at or near the ground’s surface and proves difficult to cultivate.  But even where this layer isn’t at the surface, the Island’s soils are less than ideal.

That’s because there’s generally low topsoil accumulation in a bioregion like ours.  Topsoil is, as the name suggests, the layer of soil that sits near the surface of the ground.  It’s the product of plants and animals dying on that surface and being broken down by soil organisms in a process known as decomposition.  Decomposed matter mixes with existing soil to create a rich blend of minerals and scarce elements available for plants to take up via their roots.  It’s also a medium rich with the above organisms, which interact with plants in myriad beneficial ways.

To produce such an ideal growing medium naturally, there needs to be lots of organic matter contributed to the soil over time.  And here’s one reason why Vashon soils are poor.  Conifers, our predominant groundcover historically, shed relatively little organic material.  Conifers also block the growth of shrubs and grasses, huge contributors of organic matter to the soil.  As a result, in most places, the Island’s topsoil is (the technical terms says it all) thin.

But topsoil’s composition is only one part of its fertility.  There’s also soil structure to consider.  Plants don’t thrive in ground compacted by animals or equipment.  Nor do they do well in soil that’s too loose—often from too much sand being present.  Ideally, topsoil has good tilth:  a goldilocks texture of not-too-firm but also not-too-loose.  Good tilth features a variety of soil particles (clay, sand, silt) and lots of organics mixed in with inorganics.  It has textural irregularities that foster soil life and allow air penetration.

Then’s there’s also soil layering.  Healthy soil retains soil horizons.  If you mix the horizons by, say, over-tilling, you disturb the soil biome (organic soil community) that works the soil and makes food available to plants’ roots.  Generally, layers should descend from dark (signaling organics are present) to light, in distinct horizons.

Finally, there’s soil moisture.  Plants can’t thrive without water, vital to their growth process and uptake of nutrients but also essential to their physical structure.  If you have good tilth, you usually have good soil moisture, provided there’s a periodic source of water from rain or irrigation.  Too much water drowns roots, though, so having soil that drains well, as tilthy soil does, is important too.

For healthy garden soil, you should work to achieve these features in planting areas.  Anything else?  Mulch on top!  Next time.