Dealing with Compacted Soil

Rocks_Hwy50-BlueMesaReservoir-CO_LAH_8224-001My soil is rock hard! That’s a common complaint along Colorado’s Front Range. Our soils tend to extremes—we find that we’re either dealing with sand and decomposed gravel (the remains of glacial moraines), or clay. Then, to make things worse, soil becomes compacted over time. Roots can’t force their way through compacted soils, plus there’s no place for air or water. How do we turn compacted dirt into soil that nurtures life?

You can determine your soil structure and humus content with this easy test. You need a large glass jar, water, and dirt. Place dirt in your jar until it’s about three-quarters full. Then add water to almost the top. Shake with great enthusiasm until the contents are evenly distributed. Then place the jar where it will remain undisturbed for a day or two.

soil jar test-001Now look at your jar. The largest particles (gravel, sand) are at the bottom. Medium-sized particles (silt) are layered on top of that, and the finest, clay particles—which stay suspended the longest—finally settle at the top. Any organic matter is likely on top of everything else. Using a ruler, you can measure the percentages of each particle size. Then compare your soil to the chart pictured here.

soil-triangle-usda-smAnother test is a percolation test. Dig a large hole (as for a small tree or #5 plant container) and fill it with water. How long does it take that water to drain out? If it’s gone in less than 12 hours, you have “well-drained” soil. More than that, and you’re dealing with clay. (If it’s still holding water after 24 hours, consider creating a koi pond instead!)

soil_breakdownIdeal soil is 45% dirt (as shown in the chart above), 25% air, 25% water, and 5% organic matter. Most Colorado soils have little or no organic matter, and if they’re compacted, there’s no room for air or water, either. Compacted soil is a major cause of “failure to thrive” in our landscapes. What can we do about it?

Next week: some suggestions for dealing with compacted soil.

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