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What do I use to fill my garden beds?

Ray Scheel
9 years ago

Square foot and similar intensive gardening methods require an especially rich growing medium to support the plants, and raised beds impact water retention. One thing you don't want much of is plain garden or topsoil, especially the bagged stuff available in garden centers or dirt companies. There are different philosophies on how to fill the beds. Some people prefer (and some locations benefit from) "double-digging" the garden bed and working in amendments when a bed is initially built. This was the method Mel Bartholemew originally recommended, and it is still being published in his book.Similar but less effective is using a tiller to first till the soil, then till in amendments. Afterward, a thick layer of a soil-less fill mix is applied to the top, possibly putting a fine sprinkle of clean soil where starting fine seeds. However, many gardeners have experimented and found no difference in production between beds that were tilled or double-dug and the no-till methods where the soil-less mix is simply placed in the raised bed over a degradable liner of newspaper or cardboard to suppress the existing grass and weeds below. "Mel's Mix" is the most common fill substance and consists of a mixture of approximately equal parts of compost materials, durable "brown" organics for bulk & drainage, and a water retention substance. Mel's Mix, now being recommended by Mel Bartholomew via his website, is equal parts by volume of compost, peat moss, and coarse vermiculite. Mel recommends using compost from 5 different sources to ensure getting a wide variety of nutrients for the plants. Some people plant in almost pure compost, others use a mix of compost and leaves. Compost is talked about at length in the Soil,Compost, and Mulch forum. Essentially, it is a mixture of organically based materials that has been allowed to rot in a controlled manner. Finished compost does not stink, and a properly maintained compost pile also has almost no odor to speak of. Compost can be made of anything - chicken droppings and sawdust, various manures and straw, household scraps and leaves, or lawn clippings and chipped wood or small branches. A good variety is helpful, but use whatever is available and cheap. For the initial filling of the beds, price bulk compost from soil companies or see if there is a mushroom plant nearby. Because of its uncertain origins, municipal compost is not recommended for vegetable beds. For a cheap source of bulk compost, look for a nearby mushroom growing operation.The durable organics used for bulk are primarily shredded (or not) leaves or leaf mold, coir, or sprangham peat. The use of peat is currently debated as to whether it is renewable, but has advantages when making beds of acid lovers (though using leaves of certain trees can accomplish a similar result).Water retention substances are available in a variety of forms. Cheapest is vermiculite, a naturally occurring mineral deposit. Perlite (perlized mica) is a a mined product that is "popped" to make a water-retaining matrix. It retains the least water and has a tendency to rise to the top of the bed. Expanded shale is also mined then expanded, but has percolation properties not present in perlite and holds up longer than vermiculite. There are new polymers (man-made) that come as granules and form a gel that absorbs and slowly releases water. Some climates and situations will have more need of the water retention component than others. Vermiculite can also be bought from pool supply companies (sold as pool base), specialty packaging supply companies (it is used to ship glass bottles of chemicals - also check with places that receive chemicals using vermiculite as packing material - school labs come to mind), hydroponic stores, and some nurseries keep the bulk bags in storage so you have to ask for it. It can also be ordered by feed and grain stores or similar outfits, though you may have to ask persistently to get them to look it up and order it for you. The cheapest vermiculite you can find will do, you do not need the fine "horticultural" grade of vermiculite for filling beds, the coarser the better. Contrary to popular rumors, there is *not* a problem with the mines or asbestos in currently marketed products, though wetting it down before mixing to eliminate dust and prevent it from floating up in the initial watering is strongly recommended.These last two components are one-time additions to a bed; once a bed is built,adding compost to replace nutrients used by the plants is the only thing the bed needs.If you want more info on how to make compost or the pros and cons of various methods of amending your soil, the Soil,Compost and Mulch forum can teach you more than you've ever dreamed of knowing about compost and how to make it yourself.


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