Compost Doesn’t Cut It

With very different gardening experiences in different areas of the country, I learned that fertile soils need much more than compost to be truly healthy…

I’ve been a gardening freak ever since I was in junior high, right at that oh so tender age.  I dreamed of getting armloads of roses from boys, but that never happened, not even remotely close, so I started my own rose garden with the help and enduring patience of my mother.  My gardening hobby took a break during college and a move to Pittsburgh, but once I was in my first house, I started gardening with earnest and was pretty successful at it.  I thought I had the greenest of all thumbs.  The flowers were beautiful, shrubs grew a foot every year, veggies were very tasty, and on and on.

In Pittsburgh I knew very little about soil.  All I could tell was I had very heavy, dark clay.  I had heard of the virtues of compost, so I turned in shovels full of that with sand to lighten up the clay.  Miracles!  The garden sprouted forth and took on almost tropical-like growth in Pittsburgh’s short growing season.  Easy success!

A few years later, I moved to my husband’s farm in Southern Maryland.  With so much room outside, I started buying plants like crazy.  This time it didn’t work out so well.  At first inspection, the soil was very sandy, so I thought it just needed organic matter.  I added wheelbarrows full of compost and planted.  Next year, everything looked run down and weedy.  Perennials were stunted and tomato plants would barely produce.  I thought to myself, ‘What in the world is going on?  Why doesn’t organic matter cut it in Maryland?’

That’s when I started researching soil and learned that soil fertility isn’t just organic matter.  A highly fertile soil has a healthy amount of organic matter PLUS all the minerals required for life in abundant amounts.  Our Maryland soil needs everything – organic matter, calcium, phosphorus, all the micronutrients, etc.

It’s important to realize that organic matter is humus, mostly carbon.  Compost is not going to have calcium unless the ingredients that went into making the compost are high in calcium.  Makes sense, right?   Then why isn’t this common gardening knowledge?  I have many organic gardening books, and none of them talk about soil minerals.  No mention of calcium, but organic matter is touted as the end all, be all.  Organic matter is indeed where it’s at, but only if you already have heavy soils that are loaded with minerals.

I’m not dissing organic matter here, no way.  Our farm’s organic matter is so low (under 2%) that its water holding capacity is greatly reduced.  Organic matter can also hold minerals in the soil and make them available to plants, so our soil’s ability to do that is diminished as well.  We’re aiming for the ideal soil as described by William Albrecht:  50% air and water, 5% organic matter, and 45% minerals and hydrogen.  Out of the 45%, calcium should be around 65%, magnesium 15%, micronutrients including copper and zinc at 10%, and 10% hydrogen.   Each ingredient is needed in different amounts, but they’re all absolutely necessary for fertile soil, healthy plants, and healthy animals.

If you’re interested in learning more about minerals and compost, including why compost gets more attention, I highly recommend this very enlightening conversation:


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