Q: I’ve gotten my test results back from the soil lab, telling me to add some organic matter. What’s the best thing to add?

A: In the past, we’d just bop on down to the local garden center and load up a few bags of… something. Soil amendment, composted manure, planting mix, potting mix, top soil, compost… there are hundreds of products, and the names are pretty random.

So are the ingredients. Since there are no legal standards, these bags can contain whatever the manufacturer wants them to. There’s no labeling law, either. If there’s a label at all, often you’ll see something like, “Contains (peat, forest products compost, and/or compost), wetting agent, fertilizer.” You have no idea if this particular bag has peat or compost, much less what went into that compost. And what’s a forest product? Bark? Sawdust? Squirrels?

We don’t know if the contents are thoroughly composted, or if they’ll tie up nitrogen as your soil’s microbes finish the decomposition job. How much N, P, or K is in the bag? What’s the pH? Perhaps most significantly, how salty is it?

While we would hope that all these products are helping our gardens (or why else buy them), the truth is they could be ruining our soil for years to come.

The CSU Soil Lab ran an analysis on 65 commonly sold bagged soil amendments, and their preliminary results are alarming. Plus, when they compared two bags of the same product, the numbers varied widely. Sadly, there was no way to predict how good or bad the product was based on the manufacturer or label.

First of all, pH varied from 4.9 (extremely acidic) to 7.9 (the high end of what most plants tolerate). Adding an acidic amendment to an acid soil just compounds the problem. Here in Colorado, our soils often have a high pH. The last thing we need is an alkaline amendment!

The available NPK varied widely as well. Most had P (phosphorus) and K (potassium) levels far in excess of what is required in the garden. Adding an amendment with over 7000 ppm of potassium to your garden soil, when all that’s needed is 200 ppm, can injure plants. Even digging it into the top eight inches of soil doesn’t dilute it sufficiently, and over time these elements build to toxic levels.

Salt is an even bigger problem. While high rainfall levels leach salts in much of the U.S., our measly 15 inches isn’t enough to have that effect. Salt levels (measured in electrical conductivity) over 5 begin to be harmful to plants, preventing seeds from germinating and burning leaf edges. Plants die at higher salt levels. Yet many of the amendments contained salt levels of 10. Dried, composted manure (from feed lots and dairies) was even higher, with salt levels in the teens—and one had a high of 30! You wouldn’t take a salt shaker to your garden, yet adding these manures can amount to the same thing!

So what’s the best way to increase our soil’s organic matter?

The best compost is what you make yourself. Combining yard trimmings, grass clippings, fall leaves (shredded is best), and kitchen plant waste (leave out the salted leftovers) creates a low-salt, plant-based compost that is just what your soil needs.

(Adding “home grown” manure can be an option, depending on the source. Make sure it is thoroughly composted, and leach any excess salts before adding the finished compost to your garden.)

If you don’t have compost available, the next best option is buying bulk material from a recommended source. Ideally, the seller would be able to answer questions such as, “What ingredients were used to make this compost?” or “Have you had this compost analyzed? Can I see the report?” Repeatedly asking for this information may eventually spur these businesses to test their products and provide answers. (To be extra cautious, have your purchase tested before adding it to your garden, although time and lab fees may make this step problematic.)

Another option is to grow a cover crop. You thickly sow seeds for, say, an annual grass, legume (clover or alfalfa), or buckwheat. As the plants grow, their roots reach deep into your compacted and lifeless dirt. When the plants are killed by frost they act as mulch, protecting your soil over the winter. In early spring, till them back into the soil, adding organic matter. Let them decompose for at least 30 days before planting, to avoid having microbes and plants competing for nitrogen and oxygen. This is an excellent way to increase organic matter without adding salts or excess fertilizers.

The main drawback to a cover crop is that it takes up valuable space in your garden during the growing season. Depending on which crop you choose, it needs to be planted by September (earlier at higher elevations) in order to put on enough growth to benefit your soil. You may need to sacrifice a season to improving your soil, but the results will last for years.

Article and photos by Leslie Holzmann, Certified Colorado Gardener.

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