Dirt matters. A lot.
In comments about my post about the Harris County problem, Luke pointed out that I’d not really looked at the soil profile under Houston and its neighbors and upstream watershed. That’s in part because I wanted to compress a lot of data into a small post, and because I’m just not familiar with all the details of the soils in that part of the state. Sand or clay, loam, caleche, all respond differently based on the physics and chemistry of the soil and its component parts. I’m not a soils expert, and I’m not a construction engineer. So this is a sort of Dirt and Water 101: The quick and dirty version.
Soil comes in a number of flavors depending on how much organic matter it has, how much sand versus other components, and if it has a lot of clay. There are few “perfect soils” unless you are planting vegetables in the black loam of Iowa, in which case you may very well have a field of the world’s most perfect soil. But not perfect for other things. The age of the soil, how much it has weathered, what grew or grows in it, the climate in the past and currently, if there are critters that stir it up, all those determine what might grow, what will grow, and how much water the soil can absorb. You can buy thick books about soils, guides to the soils in your area, and engineering texts on soils for construction. What I’m going to focus on is “how fluffy is it?” Or how much water will the soil hold when the heavens open up and you start looking around to see if the animals are walking along two by two.
Sand is the most porous. Water flows through sand, be it fine or coarse. There are large spaced between the grains, relatively speaking, with lots of room for air or water. It drains wonderfully, but doesn’t hold together too well. There’s a reason Jesus told the parable about the man who built his house upon the sand. The rains came, the wind blew, and the house washed away. Water, enough of it, will mobilize the sand. Depending on just how fine the sand is, and how much fine silt is mixed with it, you can get quicksand, from “quick” meaning alive, or moving. Even beach sand, or dune sand, moves with wind and water. But it drains quite well, so sandy soils do not hold moisture as tightly as other kinds of ground. Water comes through and then goes away.
Silt is a very fine, powdery soil. Loess is one form, the powdered rock left behind by glaciers and glacial rivers. It does not drain as quickly as sand, but it still drains well. Silt feels smooth in your fingers, but if you get some wet and roll it, it doesn’t act like modeling clay. It holds together better than sand, however.Plants grow well in silt, unless it gets too compacted. There is a goodly amount of airspace between the bits of silt, so it can hold a good amount of water, but when wet it compacts. Silt also refers to the suspended sediment in rivers, or what makes the Canadian River red, the Yellow River yellow, and the Amazon brown. It floats along with the water if it is fine enough, or slowly settles out, creating river deltas. it is not so great for rapid drainage, but it is better than clay. The “perfect soil” for many crops is loam, the even blend of sand, silt, and clay. The clay holds water, the sand has large pores and drains, and the silt keeps it fluffy and has nutrients that are easy for plants to absorb.
And then there’s clay. Clay is wonderful, frustrating stuff. When you look at clay under a scanning electron microscope, it is little plates. Some kinds of clay are more coarse than others, and some are infamous because of how they swell when exposed to moisture (montmorillionite is one. Used in clumping cat litter). Once they swell, there is less space for water in the pores between the little plates, and it cam become impermeable, or just very, very, very slow to drain. The bottom of playa lakes are Randall Clay, which absorbs water quickly when cracked, but once it wets, the cracks close and it becomes almost impermeable. The water will soak in eventually, but is likely to evaporate even faster. If you pick up a handful of clay soil and get it wet, you can roll it into snakes or balls and it sticks together. The following website shows how to do the ball test. http://www.finegardening.com/improving-clay-soils
The ideal soil for building is one that drains well, but not too fast, that will not settle and compact too much, and that holds enough water to encourage plants but not so much that it causes stability problems for buildings or can’t absorb rainfall. In reality, you work with what you have, in some cases trucking in more suitable dirt. The composition of your sub-soils and how well they drain, soil compaction, the amount of organic material in the soil, the slope of your site, and a lot of other things also determine drainage ease or difficulty.
A low-lying area, with heavy clay soil, at the end of the watershed drainage area, with high groundwater, is one of the worst-case scenarios as far as “How do I keep this place dry in very wet weather?”