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Testing for 5 Key Compost Characteristics

by Organic Gardening staff

We tested 30 brands and found that it’s a mixed bag.

We asked Will Brinton, Ph.D., and the staff members of the Woods End Research Laboratory to test bags of compost from all regions of the United States. Products included composts made from cow, chicken, horse, and sheep manure; used mushroom “soil” and food wastes. The lab measured five key compost characteristics, and as you’ll see, many of the brands flunked several of the tests. (We didn’t include any products that contained sewage sludge, often labeled as “biosolids,” because we believe most sludge-based composts should not be used in home gardens due to probable contamination with toxic wastes and heavy metals.)

What We Tested: Organic Matter Content

Why it matters

Nitrogen is the nutrient that demands the most attention because it’s the most likely to be in short supply in your garden and because it’s also the nutrient most likely to cause pollution problems if it’s overapplied. The nitrogen content should be a key factor in determining appropriate application rates of compost.

What the tests showed

16 out of our 30 composts were too old or had been diluted with soil, resulting in an organic-matter content less than 30%. Although using them wouldn’t harm your soil, they were definitely not good buys. Only a third of our samples fell within the preferred range of 30 to 60% organic matter. Four products contained levels over 60%, indicating that they were probably not yet fully composted.

What We Tested: Content

May vary from 0.5 to 2% or more; should be indicated on the label if above 1%.

Why it matters

If the organic-matter level is over 60%, then the compost isn’t yet mature, and it could temporarily inhibit plant growth when mixed into the soil (although it could still be used safely as a surface mulch). If the level of organic matter is too low, then the compost simply won’t improve the soil as well as a better-quality product would. “Organic matter is the essence of compost,” Dr. Brinton explains. “It’s the energy source that feeds soil microorganisms and regulates the steady release of plant nutrients. It also creates the ‘glue’ that improves soil texture, and it increases the soil’s ability to hold moisture.” Should be 30 to 60%.

What the tests showed

Only one-third of the bagged composts listed the nitrogen content on their labels. But Dr. Brinton’s lab tests revealed that the producers were not using nitrogen content to set their recommended application rates. The rates provided on the bags were often too high. No labels made a distinction between annual rates and higher one-time rates for new beds.

What We Tested: pH

Should be in the neutral range, between 6.5 and 7.5.

Why it matters

pH is a measure of acidity or alkalinity, represented by a number on a scale in which 1 is very acidic, 7 is neutral, and 14 is extremely alkaline. For optimum plant growth, you want to maintain a nearly neutral soil pH in your garden. Regular applications of good-quality compost help maintain neutral soil pH, but you should avoid using overly acidic composts on soils that are already naturally acidic, and avoid high-pH products on already alkaline soils. (If you don’t know your soil’s pH, have it tested.)

What the tests showed

About half of our samples fell within the 6.5-7.5 pH range, nine were too high (as alkaline as 8.3), and four were too low (as acidic as 4.5). If you think a difference in pH of just one point or so probably doesn’t matter much, think again: the pH scale is logarithmic, which means that for each one-point change, the alkalinity (or acidity) increases or decreases by 10 times.

What We Tested: Carbonate

Levels Should be indicated when high.

Why it matters

Using an overly acidic compost won’t usually do any long-term damage to your soil, but using one that’s too alkaline might. High-pH composts often contain carbonates, usually in the form of lime (calcium carbonate.) If you have naturally alkaline soil (most common in drier regions) or if your soil is acidic and you already apply lime to reduce the acidity, Dr. Brinton warns that you should avoid using a high-pH compost. “Once a soil contains too much carbonate, other nutrients, such as phosphorus and zinc, will become unavailable,” he says. “And there is no easy way to bring the soil back into balance.” What the tests showed:
Dr. Brinton found that 30% of the composts had high carbonate levels, making them poor choices for use on alkaline or recently limed soils. And not a single compost producer had included information about carbonate levels on its label.

What We Tested: Salinity

Should not exceed 5 mhos/centimeter. (An mho is a unit used to measure salt conductivity.)

Why it matters

As organic matter decomposes, minerals are slowly converted to salts that dissolve in water and become available for plant roots to absorb. If compost production is not managed properly, or if a large amount of chicken manure is used, salts can sometimes accumulate to a level high enough to injure plants-especially seedlings. Low salinity is particularly important in dry regions, where soils are already naturally high in salts because there isn’t enough rainfall to leach the salts down into the subsoil. You should also choose a low-salt compost for heavy applications before direct seeding and for container mixes.

What the tests showed

25% of our samples exceeded the 5 mho/cm standard. And none of the compost products we evaluated listed the salinity level on the label.

What Our Inspections Revealed

Here’s what we found when we examined our bagged samples:

One out of every four of the 30 brands we inspected contained compost that was so sticky and clumpy that it would have been impossible to spread in the garden. (I made a beautiful “clay” pot from one brand, while another dried into serviceable rock-hard bricks!) Several others were obviously too woody and not fully composted.
The color of all the brands was similar while the composts were moist, but when we dried them out for a few days, three of the brands were too light in color to be good-quality composts.
All of the the sticky composts-one out of every four-were too wet. Most others were appropriately moist. Only one product had a very low moisture content, and it was labeled as “Chicken Manure Fertilizer-dry, composted, will not burn.” We suspected a problem because of the product’s strong odor, and, sure enough, Dr. Brinton’s tests confirmed that the chicken manure was not fully composted.
We found several brands that smelled sour or reeked of ammonia, indicating poor-quality immature products. Only one bag had that desirable earthy, woodsy smell when we first opened it, but several more developed the earthy odor after they had been exposed to the air for a few days.

Reprinted with permission from Organic Gardening Magazine