Sunday, February 22, 2009

How to Make Free Soil Blocks. Part 2 of 4

In our next free soil block technique, we will be revisiting the ancient Aztecs of Mexico City. For a primer on this technique, known as the Chinampas soil block, read up on the history of the first soil blocks ever made at We may not have shallow lake channels to scoop nutrient rich mud and make blocks, but with a garden hose, a bucket, a hoe, some scrap wood, straw and soil, we'll come pretty close. Here's the Outline: (Makes one wheel barrow full of mix, about 100 2" blocks, 40 3" blocks, 25 4" blocks.)

1. Test Soil. 2. Make Forms. 3. Make Mix. 4. Fill Forms. 5. Remove Forms and Cut Blocks. 6. Seed.

Your first step is to determine if your soil is suited for the Chinampas soil block technique. You will need to do a home-made soil composition test. You will want to sample soil that is from your garden and has been weeded recently. You'll also want to take soil from perhaps a stream bank or some areas that are prone to flooding or look black with a humus build up. If you find yourself looking at the soil in your pasture, you might just want to make our turf blocks described last week. Now, take a clear glass jar with a tight fitting lid and fill it slightly over half way with your proposed soil. Take the first few inches of soil minus any heavy mulch or leaves and sticks, we're after the broken down humus and soil particles. Next, fill your jar with water almost to the top. Add two teaspoons of salt. This will help settle the clay. Close the lid tightly and begin shaking vigorously! Shake until all the soil is completely suspended and it looks like chocolate milk. Let it sit over night and check in the morning. You should notice that the soil sample has been seperated into layers of different colors and different particle sizes. Here's the break down: The bottom layer is Sand, the next layer up is Silt, the next up is Clay, and finally the top layer is Humus or Organic Matter(note how it appears to be floating on the top). You can determine rough percentages of content by taking a permenant marker and scoring some lines from the top to the bottom, about 10 lines evenly spaced will give you a percent in ten percent increments. So, what do you got? How much Sand, Silt, and Clay do you notice. My soil here in the foothills of the Willamette Valley are: 30% Sand, 35% Silt, 30% Clay and 5 % Humus. "What am I looking for anyway?", you might be saying. You're looking for at least 30% Clay. Clay will bind the blocks together where you have no other source of peat moss or "channel mud" which is the binding material source for other blocks. Clay is a fantastic soil medium when used with organic matter and lots of water, perfect for soil blocks! If you have too much clay or all clay, that's fine, too. Clay is composed of mineral rich powdered rock dust with a natural binding element. Sounds good for soil blocks! You may be thinking how can you turn sticky clay into rich potting blocks? This takes us to the next step. Provided you have some clay, you will want to get some straw from somewhere. It should come free. At last resort, you can buy some, but look around for some spoiled bales that nobody's using. If it's wet, start chopping it up with a machete or shovel and make it smaller. If it's a dry, start walking on it and break it up with your feet making the stalks smaller and smaller. You could sift this stuff with 1/2" hardware cloth. The wet stuff, keep chopping by hand to make little pieces. Make a big pile, like the size of a 2'x 2'x 2' pile. This is about half a bale of dry, crushed straw, much less of a wet, chopped bale. Our next ingredient is compost. Sift with 1/2" hardware cloth and bring that in a 5 gallon bucket. Now, go out and pick a spot where you will be mixing. Bring in a half a wheel barrow of clay/soil. Wet it down with a garden hose, and mix it up with a hoe. The point is to thoroughly wet the clay, but not soupy, stiff but moist, but no excess water. You will need some hard packed ground to mix the straw in or lay out an old piece of plywood. Do you have an old concrete pad somewhere? Dump out the clay and add the compost slowly, like adding flour to the creamed butter. Now mix in the straw. Chop it in with the hoe, turn it, flip it, get the straw mixed in thoroughly, through and through the clay. If it's too hard, add more water. This will make it easier to mix, but it will be heavier, so go easy. Your over all objective is to make a wet but stiff paste like a muffin mix, or like a cake batter that can be moved easily and spread in a pan.

Now, we need the pan, or the block form. There will be two ways that your block mix can be used now: 1.) in individual cells created with wood scraps or 2.) one large form that can be filled to it's capacity. The first way is to build grid like patterns of thin stocks of wood like 1/2" plywood and make your forms into 2"x 2"x 2" squares, 3"x 3"x 3" squares, or 4"x 4"x 4" squares. It depends on what you are growing. Check out the sizechart at You can make these grids any size you want. The idea is to make a form, fill it, remove it and fill again. These grids are like a honey comb. Make sure the form is wetted down or evenly soaked in water overnight in a tub with pond-like water that is sticky and thick with algae. This "pond water" will coat the wood form with algae that will help it lift off the blocks. Set your forms and blocks on wetted down plywood or a large sheet of plastic. Fill them with a shovel and level off the tops. After you have filled them, you should be able to lift the form up and the blocks stay put. If they do not leave the form, your mix should be wetted down again and tried again. You should have nice shaped wet squares of Chinampas soil blocks.

For the other method you will make only one form with a thickness of 2", 3", or 4". It can be made out of any stock and built as large as you want, provided you have made enough mix to fill it. You can put your form over a sheet of plywood, a large sheet of plastic, or a concrete pad, just make sure you spray everything down with lots of water to prevent sticking. After filling the form full of muck, smooth out the top with a trowel or board, sort of like screeting concrete. Now, lift off the form and what is left is a big block of mud soil. Take an old knife, or machete, or a piece of hardwood board sharpened into a shim-like cutting tool, known as the coa, or digging stick of the ancient aztecs(see entry photo) and cut and slice your brick up into the same size as the thickness of your board. You can score out the pattern first, and then cut through completly. Either way works well. I like the one form method because it's a lot less work and cutting those bricks or blocks are so much fun. On the other hand, a well made honey comb form made with wet soil spaces the blocks out just right for the air pruining technique. No need to move them any further or cut the roots when transplanting. Finally, it is time to seed. Poke a hole in your newly formed blocks with your finger, a stick or dibble, plant your seed, sprinke a little compost or sifted soil over to cover and wet them down. You could cover with black plastic to keep the heat in if it's chilly. Keep them moist, as the clay will have tendency to dry out. Transplant out when the white roots have poked out of the block, or when the canopy of plants are shading each other out of sunlight. Cover the block completely in the soil so the clay will not be in contact with the air. Mulch over for more moisture retention. Drip irrigation is best in Chinampas blocks. Top dress with manure, or fertilize with compost tea.

You now know how to make the famous Chinampas soil block. A very low tech, high performance soil block that costs nothing with large yeilds and enhances garden fertility with clay, compost and straw. Let me know how it goes or if anyone in the states has done authentic Chinampas type channel muck gardening. Lettuce get growing!

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