Verdure: Nitrogen won’t always fix it

Can the adverse effects of soil compaction be overcome by applying high levels of nitrogen?


Soil compaction is a hidden yet possibly serious factor in poor turfgrass performance. Vehicle and foot traffic consolidate underlying soil particles, creating declines in root and shoot growth. If rooting is affected, nutrient and water uptake may be reduced.

In 1983, while at Kansas State University, Bob Carrow, Ph.D., and his graduate student, M.J. Sills, set out to determine how soil compaction affects the ability of Pennfine perennial ryegrass (Lolium perenne L.) to use nitrogen and water.

In the greenhouse, they created two different levels of soil compaction. The sandy loam soil (60% sand, 30% silt and 10% clay) was either not compacted or subjected to compaction via a dropped weight. This resulted in soil (0 to 3 inches; 0 to 7.6 centimeters) having a bulk density of either 1.27 or 1.34 grams/cubic centimeter, with the higher bulk density for the compacted soil.

Next, nitrogen was added at either a “normal” rate (1 pound/1,000 square feet; 0.5 kilogram/100 square meters) or twice that rate, with each rate applied via two different sources: ammonium nitrate or isobutylidene diurea (IBDU). All of these nitrogen-compaction treatments were monitored for water use, using tensiometers installed in each core at 2 and 10 inches (5 and 25 centimeters). Tensiometers were read every day, and when the 5-centimeter tensiometer reached -0.70 bar, water was applied until that reading returned to a bar value of zero (saturation).

For reference, at -0.33 bar (field capacity), the volumetric water content at that depth was 17.3% in the uncompacted soil and 18.9% in the compacted.

All the nitrogen fertilizers were applied once, and the experiment was conducted for 72 days. Collected data included clipping yield and nitrogen content, clipping carbohydrate content, visual turf quality, and end-of-study root weight. Soil bulk density, moisture retention and aeration porosity were also determined at the end of the study.

Compaction had the greatest effect on turfgrass visual quality, regardless of the source or rate of nitrogen. Turfgrass quality declined as compaction increased, and higher rates of nitrogen did not improve that quality. Clipping yield also declined as compaction occurred, and, again, neither nitrogen rate nor nitrogen source alleviated the adverse effects of that compaction.

However, total nonstructural carbohydrates (TNC, a measure of plant hardiness and recuperation) were unaffected by compaction and increased slightly at the higher nitrogen rate when that nitrogen was IBDU.

Finally, root weight was decreased by an average of 30% when compaction occurred, and this was exacerbated when high rates of nitrogen were used, for a 45% decrease in root weight. Roots were shorter, too, with fewer at the depth of 6 to 10 inches (15 to 25 centimeters).

And water use? Compaction reduced evapotranspiration (ET) from 20 to 15 inches (51.5 to 37.1 centimeters) (water used per pot during the study). That lower ET was a result of the perennial ryegrass in the compacted soil having slower shoot growth and reduced rooting, and the soil having limited aeration because of shifts in pore-size distribution (other data collection showed that overall porosity declined from 25.1% without compaction to 20.3% with compaction).

However, water-use efficiency (milligrams dry shoot tissue per milliliter of water used) was not affected by compaction. This was because even though ET declined under compaction, so did shoot growth. Therefore, the water-soluble nitrogen carrier was more important here, making most efficient use of applied water, even under compacted conditions. Basically, nitrogen-fertilized plants were more efficient at using the water, regardless of compaction.

Overall, compaction had the greatest impact on the growth and quality of the perennial ryegrass. While application of nitrogen did improve water-use efficiency, it could not overcome the other negative effects of compaction on the perennial ryegrass. These effects included decreased rooting and clipping yield, and poorer quality.

The conclusion was: If compaction is your issue, best practices are to alleviate that compaction by cultivation and/or traffic control.

Source: Sills, M.J., and R.N. Carrow. 1983. Turfgrass growth, nitrogen use, and water use under soil compaction and nitrogen fertilization. Agronomy Journal 75:488-492.

Editor’s note: Read all of Beth Guertal’s recent Verdure columns.

Beth Guertal is the Rowe Professor of Soil Fertility in the Department of Crop, Soil and Environmental Sciences at Auburn University in Auburn, Ala., and past president of the Crop Science Society of America. She is a 22-year member of GCSAA.