Crop yield may be the best overall indicator of the condition of soil in a field. However, crop producers can check for signs of soil quality before harvest, says agronomist Denise McWilliams of the University of Minnesota extension service. She says the growing season is a good time to look at fields and evaluate soil according to 12 quality indicators.

"One indicator is the presence of earthworms," says McWilliams. "The best time to check for them is spring or fall, but you can also do it in the summer. Take a shovel full of earth from the top foot of soil and do a quick count. Ten earthworms per shovel-full are a good soil health indicator, according to the book 'Building Soils for Better Crops' from the Sustainable Agriculture Network. Birds following behind tillage in the spring looking for worms are also a good sign."

A second positive factor is organic matter, sometimes indicated by color. McWilliams says topsoil that is clearly defined and darker than the subsoil usually means excellent soil health. Organic residues on most of the soil surface are a third indicator, and the degree of subsurface compaction is a fourth.

The fifth quality indicator is soil tilth or mellowness, also called friability. If you can place wire flags into the ground easily down to the plow layer with only your fingers, soil tilth is good, says McWilliams.

If the soil crumbles well, is easily sliced and even spongy when you walk on it, you have a sixth indicator of quality. No gullies and no apparent soil runoff is a seventh factor. Eighth, the soil should hold water for long periods without signs of drought. Ninth, there should be no ponding or runoff from normal rainfall. Water should move steadily through the soil. A tenth indicator is crop color, which should be a healthy dark green throughout the season. Eleventh, the soil pH should fit the crop grown.

The twelfth soil quality indicator is nutrient holding capacity. Soil tests should show an upward trend in soil nutrients in relation to fertilizer applied and crop harvested. However, nutrient levels should not go into the "very high" category, says McWilliams.

"It's difficult for any piece of land to hit all 12 soil signs perfectly in one growing season," says McWilliams. "However, hitting as many as possible is a worthwhile goal."