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Paydirt, Part I: Does Your Soil Save Water?

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 Well-aerated humus-filled container soil makes it easy for this tulip to emerge and flourish

It’s not a glamorous subject. But sometimes, you just have to get down to dirt.

The kind of soil you have is a big decider in how much water you use. Not to mention how well your plants do. Last June, I gave out ideas on how to make container soil ideal for conserving water. I still use those methods, and they still work.

But if you have a big garden, in the ground, a lot of those container methods just aren’t practical, unless you want to pay the big bucks to have your entire garden’s soil trucked in, pre-mixed. Or do it yourself, a task that puts you in superhero league.

Some container methods might work in a big garden: for instance, adding earthworms to your soil is an excellent idea in containers and out. By running it through their digestive tracts, earthworms turn soil into something more humusy, nutritious, and aerated. Humusy and aerated is ideal for conserving water; it means your soil structure is the perfect structure for holding water and delivering it to plants.

If you have time, you can actually accomplish a lot of this by earthworms alone. One way of making a garden bed is to lay cardboard, covered with straw or leaves or whatever organic matter you have a lot of, over the patch of ground you want to work next year. In our warm-winter climates, it’s best to do this in the fall; by spring, the ground is likely to be workable, unless it’s a patch of decomposed granite. Even earthworms can’t perform miracles.

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This high mountain soil is pretty much all decomposed granite. No earthworm action here, and laying on cardboard wouldn’t make it different

The cardboard forms a moisture barrier, the rotting straw or hay provides organic material, and all this is like hanging out a free lunch sign for the earthworms, who come from far and near to compost and digest.

How can you tell whether this method will work in your soil? The easiest, most ancient test, is to pick up a moist (not wet) handful, and squeeze.When you open your hand, the soil will do one of two things. It can break up, which means you’ve got sand or other types of decomposed rock. That means earthworms are not likely to gather around unless you give them more encouragement than a little cardboard and straw.

If the soil stays in a palm-shaped clod, that means there’s clay or silt in there, in which case, you go on to the second test.

The second test is just to take your thumb and tease out a ribbon from the soil in your palm. If the ribbon is shorter than an inch (2.5 cm) when it breaks, that means you have a lot of sand. If it makes a ribbon 1-2 inches (2.5-5cm) long before breaking, you’ve got soil with some clay and some organic matter. Longer ribbons mean clay combined with sand or silt (the very fine soil from waterways).

There are many more variations and shades to this test, but this is enough to let you know how much water your soil will use.

Basically, it works like this: sand has the biggest particles. Unless those particles are filled with clay (the dread cement-like sandy clay), the spaces between the particles form air chutes that water runs right through. That means plants barely get a taste of water before it runs on by, down into the secret crevices of the earth. Nutrients get washed down, too.

Silt’s particles are medium-sized. Silt is formed by  water-worn minerals, and acts a lot like sand in the soil – except that its particles are smaller and it has more available nutrients. In your garden, silt will hold water and nourish plants, but it will also form flat plates which resist water by forming a barrier.

Clay has the smallest particles, which means water seeps through slowly. Clay also has a negatively-charged ions that hold on to water and nutrients, giving your plants a wide range to absorb. The problem there is that plants need air as well as water, and the small clay particles don’t allow that. While some plants are adapted to grow in clay, a lot of garden plants get stunted in pure clay, and it’s very hard to work.

The ideal soil, which almost no one has, is a combination of all three, called loam. This is the soil that can feed your plants, and retain water without turning into a swamp. Everyone wants loam, and few people have it.

But wait – there is a solution. A savior. A panacea.

Organic matter.

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Tune in for our next post: Paydirt: Organic Matters

{ 11 comments… add one }

  • Sylvia (England) June 18, 2010, 2:01 am

    Great post Pomona, I look forward to the next one. I was thinking about this yesterday as I was weeding. The soil around the edge of the bed was rock hard and the roots of the weeds wouldn’t budge. But the rest of the bed the weeds came out and the soil was moist under the top half-inch. Yes you guessed where I put the compost last year! The trouble with my home-made compost is it has lots of weed seeds in – no matter how careful I am to keep the seeding plants out, the bins do not get hot enough to kill the seeds. My lesson from yesterday is that compost as mulch might add weeds to the bed but it makes weeding so much easier, as well as helping the soil structure and plants. I get lots of weeds blow in from the surrounding countryside so I am not going to worry about adding my home-made compost again.

    Thank you for a very timely post. Best wishes Sylvia (England)
    PS my soil is mainly silt and will one day be lovely loam!

  • Cyd June 18, 2010, 6:39 am

    Such a great idea Pomona! I have been using my compost pile a lot this year, the funny part is I have pumpkins sprouting in everything. I am definitely going to try the cardboard trick this fall.

  • Cheryl in Austin June 19, 2010, 3:16 am

    Nice information…it’s a reminder of stuff I learned in the Master Naturalist program. The best thing that has helped my compost is bunny poo. I have chickens as well and have been adding their waste to the compost, but when we got bunnies too…it really started cooking fast! We’ve got clay soil here in central Texas that we call Black Gumbo.

  • Pomona Belvedere June 19, 2010, 8:32 am

    Sylvia, I like your observation that the powers of compost overcome its weediness. I have never gardened in silt so I’m interested to hear what it’s like. (And roll on, your loam.)

    Cyd, could be worse! Pumpkins as escapees are pretty benign. Do your compost curcurbits start hybridizing into weird shapes?

    Cheryl, well, bunny poo! That’s the compost cooking secret! I’m going to have to start looking for sources. Our clay soil is red, with a lot of decomposed granite, and goes from slippery mush to cement depending on season.

  • Cyd June 20, 2010, 7:57 am

    Last year I had beautiful normal shaped pumkins, but their skins were so thick I almost had to use a dremmel to carve them. Who knows what I’ll get this year. I will take some pictures and we can see.

  • lostlandscape (James) June 20, 2010, 10:26 am

    You’ve given us some useful ways to figure out what kind of soil we’re working with. I’m always struck by how many different soils there are in my little suburban plot are. The house was cut into a slope, so some of the soil is probably fill dirt, while other parts are likely pieces of the earth’s crust that used to be sea-floor that hadn’t seen daylight until they were exposed 50 years ago.

  • Pomona Belvedere June 20, 2010, 2:18 pm

    I think suburban dirt must be the oddest mix, because as you point out, human hands (and machinery) have mixed it up, brought stuff in, and otherwise drastically changed it. I love the idea of sea-floor being part of the suburban landscape.

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