Tuesday, February 23, 2010

Don’t tread on me

Whenever soil structure is discussed, compaction is addressed. You might wonder, “What is soil compaction and why does it matter?” Soil compaction is something you want to minimize to keep your soil structure and ecosystem in top condition.

At its most basic, soil compaction is structural alteration of the soil as a result of outside pressure.  If you have ever attempted breaking through hardpan while digging in the garden, you are well acquainted with severely compacted soil. Just so, it is also difficult for soil dwelling organisms, such as worms and even microbes, to tunnel and forage through compacted soil. They cannot get the air, space and nutrients that they need to survive. Talk about your huddled masses yearning to breathe free-give them some space!

Soil structure determines the ability of soil to hold and conduct the water, nutrients, and air necessary for plant root activity. A typical cultivated loam soil is approximately 50% solid material, 30% water and 20% air. Water and air move through pores in the soil between the solid particles. Soil compaction reduces total pore space, interfering with drainage. Resulting low soil oxygen levels are the primary factor limiting plant growth in landscape soils.

So, we see that soil compaction is detrimental because increased soil density limits root growth, drainage and a healthy soil ecosystem. Compaction in home gardens typically occurs during construction and later as a result of foot traffic, especially when the soil is wet. I garden in the Pacific NW, for goodness sake. The soil is always wet. Except when it is really dry. What can be done to alleviate this situation?

If you are planning a garden, you can cultivate compost into the top 8-10” of soil. Depending on the type of soil the compost should make up 25-50% of the top 8-10” when you are done. Clay soil will require the heavier application of compost. If your garden is already planted, consider a 3-4” layer of compost mulch every year to gradually create the ideal home for earthworms and beneficial micro-organisms as you improve the structure and drainage.

Restrict traffic through the planted area. I was taught that moist soil can reach 75% maximum compaction the first timeBeckypictures2 011 it is stepped on, and 90% by the fourth time it is stepped on. Mom was right; you shouldn’t play in the mud-or plant in it, either. Raised beds are one solution. Paths may be needed to divide the bed  into manageable sections to eliminate compaction in the growing area. Ideally, you should be able to reach all parts of the bed from either outside of it or from a pathway through it.  Good luck with that. I’m just sayin’.

Once you have begun to create good soil structure, avoid excessive cultivation. Soil cultivation is any activity that redistributes the soil. Farmers cultivate using a plow or disc to break up the soil's surface. In the landscape, this is usually done by hand with a shovel or fork. Even so, use moderation. There is some punditry that cultivation disrupts the soil’s natural structure. Some experts eschew mechanical cultivation, stating that it can pulverize the soil's structure and destroy earthworm populations, ultimately creating more compaction rather than less. For your entertainment, here is a forum where the subject is debated-sometimes heatedly! Apparently, there is no agreement between actual gardeners on cultivation, or any other topic, generally!

Actually, all gardeners can agree on certain basics. Good soil is crucial to healthy plants. Good soil is determined by its structure and by the health of its ecosystem. Organic amendments, such as compost, contribute to both. And that's all I have to say about that-for now, anyway!

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