Compaction Effects on Corn

Two stunted plants (arrows) growing in sprayer wheel track (dashed lines).

The picture is from a strip-tilled field with most corn at the 7-leaf to 8-leaf stage. The farmer sprayed preemerge at a slight angle from the planted rows. The sprayer had 60- foot booms and the tires left a 16 or 18 inch foot print. The soil was wet from excess rainfall and the sprayer left tracks which are still visible. Typical soil texture for the field is silt loam. Every place the tracks crossed the row, there were one or two stunted corn plants or none at all due to root restrictions from compacted soil, as shown in the picture.

I wondered what might be the potential impact, so pulled up my calculator app. A sprayer with a 60-foot spray width would travel 726 feet to cover one acre. The two tires would have a combined width of about 32 to 36 inches, or about 3 feet. Thus, the wheels left tracks over 4.5 percent to 5 percent of the field and essentially destroyed the yield potential of plants in the wheel track. Corn yields were 220 bu/ac in 2014, so - all things being the same - the 2015 yield would be expected at 209 bushels. This is a potential yield penalty of 11 bushels, just for the compaction caused by one pass across the field.

We talk about "preventing" compaction, which may be unrealistic. The application window for most herbicides is very narrow to start with. The wet conditions this spring narrowed the window further. In this case, it wasn't a question of doing it right, but of doing (or not doing) at all. It may be important to remember these kinds of small-scale problems when reviewing yields after our next harvest and start planning for the 2016 crop. 

Bulking up may be good for lifting, but not for growing crops. Bulk density is a measurement used to identify soil compaction and based on the density of water. For example, the text book bulk density for unrestricted root growth in a silt loam soil is 1.40 g/cc (grams per cubic centimeter) or less. Water weighs 62.4 lb/cu-ft (pounds per cubic foot). A soil bulk density of 1.40 g/cc is equivalent to a density of 87 lb/cu-ft. 

Root growth begins to be affected when a silt loam has a bulk density of 1.55 to 1.60 g/cc (97 to 100 lb/cu-ft). Root growth is restricted at bulk densities of 1.65 to 1.75 g/cc (103 to 109 lb/cu-ft) or greater.

Suppose the wheels in the left a one-inch track in the original cubic foot of soil, compacting it to a depth of 11 inches rather than the original 12 inches. An inch of compression doesn't seem like much, but this would change the bulk density from 1.40 g/cc to 1.53 g/cc, close to the point of affecting root growth. 

Wet soil compacts worse than dry soil, so assume 2 inches of compaction under the wheel lugs. The resulting bulk density would be 1.68 g/cc in the block of soil under the lug. This would be like a tiny wall extending downward into the soil, severely restricting the seedling root system growth, with the pictured plants as a final result. 

- Fred Vocasek, Senior Laboratory Agronomist