Look closely at the corn row just above the ruler. Nearly every plant is 6.5 inches apart. If that's your desired spacing, it's a perfect stand.

Now look at the second row. You can tell one pair of plants is closer than 6.5 inches. It's still a reasonable stand.

Bob Nielsen, Purdue University Extension corn specialist, spent many years demonstrating the importance of even spacing. He found yield losses up to 5% based on uneven spacing alone.

Good spacing- The row just above the ruler is spaced nearly perfectly. However, there is a pair of plants closer than 6.5 inches together in the row at the top of the picture.

Nielsen refers to 'standard deviation.' In layman's terms it's a measure of how far off the stand is from being perfect. He says usually the best you can hope for is a standard deviation of about 2.0 when measuring spacing around established plants. That's what he compares other stands to when determining if there is a potential yield loss.

You can measure spacing in 1/1000th of an acre- that's 17 feet, 5 inches in 30-inch rows. When Nielsen's main purpose is documenting plant spacing uniformity, he typically measures plant-to-plant spacing in 25 linear feet of several rows at several spots in the field. Record inches between plants. If you have access to any computer spreadsheet program that can calculate standard deviation, determine how well your planter performed.

Potential yield losses get larger at standard deviations above 3.0. Spacing is more erratic.

Erratic stands result from many factors, often starting with worn parts or wrong settings. Nielsen was an early proponent of getting planter units checked and replacing worn or broken parts. He used data from his spacing trials to make his point.

How far apart should plants be? That depends upon row width and desired population. According to the Purdue University Corn & Soybean Pocket Guide, in 30- inch rows at 32,000 seeds per acre, that's 6.5 inches.

That's the spacing of the plants above the ruler. This picture was taken in the Crop Watch '15 field. The farmer was aiming for 32,000 seeds per acre.