Clay provides structural integrity when dry, moisture retention, improved matrix-seed contact, and cohesion to your seed balls.
- You will need kaolinite, smectite, or bentonite. These are standards in the ceramics industry – high fire, low fire, and porcelain clay bodies are all fine.
- The clay can have some grog, which is just ground up sand-size ceramic material.
- I use recycled clay from studios and schools which has some plaster bits. Pick most of this out, but a little won’t hurt as long as it doesn’t affect the chemistry significantly.
- Avoid recycled clay that has glazes in it, especially for applications where seed balls will be near food-gardening sites. Glazes can have have toxic metals and salts.
- The color of the clay is not so critical. Some argue that red clay must be used. Red (oxidized) iron is immobile and cannot be taken up by plants. (Hmm- I wonder why so many recipes insist on red clay only without any substantiative support). Here’s a discussion and an experiment that I did that relate to iron in clay and seed balls.
- If you use a good compost, you don’t really need to worry too much about the nutrients in your clay.
IMPORTANT: Using local clay is frequently advised by other sources, however in doing so, you are likely to transport weeds with your seed balling endeavors. Please be thoughtful when using natural clay bodies, and don’t contaminate natural systems with weedy clay bodies.
Compost provides moisture retention capacity, structural integrity when moist, and nutrients for your seed balls. It can be from garden waste, lawn waste, composted mulch, or composted food stuffs. Bokashi compost must go through a secondary aerobic compost to be sufficiently broken down, however you can add bokashi tea to improve the microbial community in your seed balls. Vermicompost tea and bokashi tea are both good in moderation. Vermicompost, composted manure and guano, and post-ferment bokashi compost should be cut significantly with garden, leaf, or kitchen compost because they are SO strong.
Your compost should be well-aged and stable. It should be a balanced mix of sapric (smeary) and hemic (fibrous, but unrecognizably decomposed) materials, without much of any fibric (still recognizable) components. Well-aged compost is chemically stable and circum-neutral in pH. I try to use compost that is well over a year old.
Sieve the compost through 1/8″ – 1/4″ mesh for best results. You know, this is the most labor-intensive part of making seed balls, but it makes such a difference. But it IS labor intensive. Seed balls with finely sieved compost are so easy to roll and the finer compost helps hold the seed ball together and also provides better seed-matrix contact.
I try to use a 4:1 ratio by volume compost to moist clay. That’s A LOT of sifted compost. This mimics the A-horizon of the soil most realistically, where seeds evolved to germinate. Pump up the compost ratio, but use enough clay to hold the bombs together and provide good seed-matrix contact.
Use good seeds with the highest germination percent that you can find. Use local sources, when available. Make certain they are free of pests and that they aren’t mis-identified. Use the fewest seeds possible – usually only 1-2 per ball. See here for more detailed discussion of how many seeds in each seed bomb.
The size makes a difference. Smaller – Dime-sized (1-1.25cm) is optimal. Some seeds can go smaller. Few larger. A detailed explanation of my seed bomb size recommendations.
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