By Jeff Gillman, PhD – for GrowingAGreenerWorld.com
By now everyone knows that the best thing to add to a soil to make it work for their plants is organic matter. In your garden organic matter is usually added in the form of compost, though it can also come from organic mulches such as wood chips, straw, or leaves as they break down.
Organic matter helps your soil hold nutrients, water, and can even help air infiltrate the soil. In many ways organic matter is the perfect media to grow plants, which is why most potting soils that you can buy at the garden center are composed almost entirely of organic matter.
Knowing all this you may ask yourself why you don’t just fill your raised beds with nothing but organic matter and replace all of your garden soil with compost, after all, this stuff is fantastic and will work great all by itself! Right?
Actually no. Organic matter is great, but that doesn’t mean you can make a garden soil, and especially not a raised bed, with organic matter alone.
Organic material comes in many forms, but the most common one that you are likely to fill your raised bed with is compost. Other types of organic matter that may go into your beds might include decomposed wood chips or pine bark, decomposing leaves, or a host of other things – honestly there are too many possibilities to name them all.
With a very few exceptions (such as biochar), these materials are not done breaking down yet. It is true that organic materials has some excellent qualities, but over the course of months and years this organic material will continue to change, breaking down further into even smaller particles, and usually offering worse drainage as it breaks down.
To maintain good media structure while organic matter breaks down you need something besides organic matter, and that something is soil.
Deciding on a soil to add to your raised bed does not need to be difficult, usually the soil around your garden is just fine. A raised bed should consist of about 50 to 70 percent soil and about 30-50% organic matter if possible.
Raised beds are naturally well drained. If your raised bed is more than a foot or so tall then you should consider using a soil with more clay. If it is under a foot then more sand is appropriate.
In either situation the soil and organic matter will work together to provide nutrients, air and appropriate drainage.
There are mineral additives, such as granite dust, greensand, lime, and gypsum, that many gardeners have had success with over the years. These mineral dusts usually offer very specific minerals to the soil and may also offer better soil structure in very specific situations such as when you have a lot of sodium in your soils.
There is nothing wrong with trying these dusts, but you will have better success with them if you know what your soils are or aren’t offering to your plants, so it is a good idea to have a soil test done before considering them.
Granite dust and greensand offer potassium and some trace elements while gypsum offers calcium and sulfur. Lime offers calcium and magnesium while also increasing soil pH. Lime is only a good additive if your pH is too low and should only be added after having a soil test done.
Because of the high demand for calcium that certain vegetables, such as tomatoes, have, gypsum can be an excellent addition. I never grow tomatoes without a few eggshells or some gypsum added to the ground around their roots.
If you choose to add a mineral amendment then be sure to follow the directions on the bag closely. It is possible to add too much of a good thing.
With any plant the key to success is creating an environment that contains all of the things it needs to grow. Plants evolved to have their roots immersed in a mix of soil and organic matter, and that is what you should strive to offer them.