Monday, November 09, 2009

Building a large grow box... need some advise

Q. I'm building a large grow box 3' high 3' wide 15' long. I'm in St. George w mostly clay soil and have a couple of questions:

1. I'd first off like to promote better drainage, and I have a 3" soil auger. If I go down some 4 to 5 feet w 2 rows every 3 feet on center is it better to fill my holes w a gypsum and soil conditioner mix or a gravel and gypsum mix or would you use something else?

2. I'm hoping to get some County compost to mix w sawdust, sand, gypsum and St. George red clay to fill the mankiller in desguise. Last year I tried a 30 " high potato ring and filled it up w compost and potting soil twice. It setteled some 2 vertical feet both times over last years growing season. Does this sound like vermiculture and or anaerobic respiration to you? Next question does it ever stop settling or when can I hope for it to stabilize enough to plant?

A. If you have a real drainage problem, which would be rare in the desert country of St. George, I would fill your auger hole with course gravel. From what I've seen in that country, you would want to preserve every drop of water for the plants, rather than expediting the drainage. Let's talk about it. For example, simply making level, raised, ridged beds in the soil, as taught in the website at in the Gardening Techniques section, and having the planting area about 2” above the surrounding aisles, would solve any drainage problem in the low rainfall area in which you live.

Having a Grow-Box 3 feet deep is also unnecessary. Sometimes folks who can't bend over will make them higher than the recommended 8”, so as to more easily work in them, but 3' high is awfully high - even for that!

If you like working with a Grow-Box, rather than the soil, I recommend you consider building it 8" deep, and filling with a combination of peat-moss, sawdust, perlite, and sand, in equal amounts by volume. Any combination is fine, so long as the sand isn't more than about 30-35%.

I also recommend you build it 4' wide, rather than 3', if you have the space. This dimension will give you more productive use of your space, while allowing plants the light they need. Subsidence of this depth material is minimal, and you can plant immediately. And if it subsides an inch or two, you just add to it each year, and don't worry about it.

The reason I recommend 4' is that you can grow 4 rows of many vegetables in that width, whereas a 3'-wide box will only give you 2 rows. Look on the website at, in the Gardening Techniques section under Grow-Boxes for detailed instructions.

You speak of using gypsum as if it was going to be a major ingredient in your soil mix. If you were to build your Grow-Box 8" deep X 4' wide X 15' long, you should apply 2# of gypsum evenly over the surface of the soil in the Box before you fill with the planting mix. Then, after filling with the planting mix, you only apply another 2# evenly over the surface and work it into the soil mix. Do both applications once, and then, after each crop apply only the 2# to the soil mix and work it in. These instructions are all on the website for you to study.

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