Digging In: Try your hand at green fertilizers

One of the best things gardeners can do for the environment is limit the use of fertilizers. Runoff from fertilizer can pollute lakes and streams. And synthetic fertilizers usually come from chemical reactions that require nonrenewable resources such as natural gas and coal.

Organic fertilizers can be a greener choice, because many of them come from waste materials or byproducts. For example, milorganite (which gets its name from Milwaukee organic nitrogen) is a treated sewage. Fish emulsion is made of fish byproducts. And corn gluten meal is a byproduct of the corn industry.

But all organic fertilizers are not created equal. And some, while seeming green, are unsustainable.

Mining for problems

Rock phosphate contains a high concentration of phosphorus, which makes it a popular organic fertilizer. (It's also used as a raw ingredient in synthetic phosphorus fertilizers.) Unfortunately, it's found only in a few places in the United States, most notably Florida, where it is strip-mined. And the reserves are limited. It's estimated that we'll run out of rock phosphorus in as few as 50 years.

So while it may be a natural product, rock phosphate is neither a renewable resource nor good for the ecology of the region where it's mined.

Other organic fertilizers that are mined include greensand (a source of potassium) and gypsum (which provides calcium and sulfur).

Aside from mined materials, there are organic fertilizers that are made from unsustainable ingredients. Perhaps the worst offender is bat guano. Guano is poo, but it is not a rapidly renewable resource or simply a waste material.

It must age in an arid location (usually a cave) for decades before it can be used. After it's aged, bat guano is harvested from caves, where it serves as an important source of nutrients for the many creatures living within those caves. Harvesting bat guano can badly disrupt the delicate cave ecosystem.

In addition, much bat guano is harvested outside the United States, so it takes fossil fuels to ship it from its point of origin to your garden.

Better choices

If you want to be a greener gardener, choose fertilizers that are organic, renewable and sustainable. Don't just buy anything that says "organic" on the label. Look at the list of ingredients.

Organic fertilizers that are renewable and come from close to home are even better. Compost and composted manure make great fertilizers, because they're a waste product that could have polluted our environment if they hadn't been recycled. And when purchased in bulk at local garden centers, they often are from local farms.

Jeff Gillman is an associate professor of horticulture at the University of Minnesota. He also is the author of three books, "How Trees Die," "The Truth About Garden Remedies" and "The Truth About Organic Gardening."

While organic and renewable, these fertilizers are based on byproducts that typically come from the ocean. Shipping those products to a land-locked state like Minnesota requires plenty of fossil fuels:

• Kelp

• Fish emulsion

• Alfalfa meal

• Cottonseed meal

• Corn-gluten meal

These fertilizers are organic, renewable and may be locally produced. If you can't find them in bulk (which is cheaper and more likely to be from a local source), and the source is not cited on the label, be sure to ask where they're from.

Also, because these fertilizers may contain a varying amount of nutrients, be sure to follow application instructions or ask a garden expert.

• Composted manure