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From Our Landscape Architect:
"Soils 101 Continued - Organic Matter"
 by Janielle Guzinski
So if the texture of our soil (clay, sand, etc.) is a fixture, what can we change? The answer is the organic content of our soil.
Organic matter is the portion of soil that is not tiny bits of weathered rock that we call clay or sand. Soil is a mixture of these physical/mineral particles and the bits of organic matter. In the forest this organic matter comes from decomposing leaves and twigs, fallen berries, and waste from woodland creatures. In our landscapes, organic matter usually comes from application of compost, manure, other products of that nature, or from mulch that had decomposed over time.
Organic content affects how our soil behaves in many ways. Organic matter contains natural versions of the chemicals we add in with fertilizers or soil activators. However, the compounds in organic matter are bound together and don't wash away the same way chemical additives can. Also, they won't burn your plants like chemicals since they are bound together and less accessible. The organic matter has two separate effects on water retention in soil. Sandy soils lose water due to superb drainage, and organic matter provides more surfaces for water to hold on to instead of draining between sand grains. In clay soil, it has the opposite effect by allowing water to drain as it breaks the clay aggregates into smaller chunks. It also encourages a healthy microorganism community in your soil, which improves plant performance. In the landscaping department, we amend the soil in every flower bed, garden, or planting area we create with organic matter.
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A cross-section of a good soil showing the rich, dark brown organic matter at the top of the profile.
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What is the best way to add organic matter to your landscape? You should mix it into your existing soil. If you are planting a new tree, dig the hole bigger than you need and mix the surrounding soil with the compost and fill in with that mixture to the size you need for the tree. If you create a harsh line between surrounding soil and the compost, it can actually act like a barrier. You can end up with a pot-bound plant as the roots grow only in the amended soil and don't extend into the rest of your soil. And if your soil, like many in Galveston, is not that great, you can keep adding compost, manure, earthworm castings, or any number of products year after year and build up a great top layer of soil full of organic matter.
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Mix new organic matter into the soil,
in this example, with a pitchfork.
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For more information of soil science basics, see http://www.soils.org for the Soil Science Society of America. They also have a blog with lots of useful information, http://soilsmatter.wordpress.com/.
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