Talking Dirt-y

Last night was the third in the city’s xeriscape series (the second one I attended). The topic was soil, mulch, and composting, and the guest expert was my favorite soils professor. (That says something about majoring in horticulture: that I took enough classes about "dirt" to have a favorite professor.) He started off with some "wow" figures that I’d like to share with you.

If you take one acre of typical Mesilla Valley soil to a depth of 6" and with an organic matter (O.M.) content of 1%, the dry weight of the soil is 2,000,000 lbs. (The O.M. weight would be 20,000 lbs.)

Twenty-four hours after irrigating (to allow "free" water to drain), there will be 600,000 lbs. of water in that same acre of soil. Before irrigating, there can be as much as 300,000 lbs. of water in the soil, yet plants still will wilt. In other words, half of the water is bound to the soil such that plants can not use it.

A typical "uplands desert" soil (the sandier type, such as in the foothills) has no more than 0.5% O.M. A typical "valley" soil (a clay loam, in the river’s floodplain) has 0.75% - 1.5% O.M. In contrast, soils in the eastern Great Plains (such as Iowa or Minnesota) have 6% - 8% O.M.!

Burning summers, little water, difficult soils -- is it any wonder people tell me if I can grow plants here, I can grow them anywhere?