Soil Compaction in Arizona

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Soil Compaction: The Invisible Bandit Below Our Feet

R. Andrew Backhaus, PhD

Whenever I go to my son’s soccer game at the local middle school, I’m always disappointed by the lack of shade for all the soccer moms and dads on the sidelines. Although there are trees scattered along the perimeter of the field, none have the growth you’d expect after 20 years in the ground. Why? Because of soil compaction. Soil compaction is one of the leading problems of trees growing in suburban settings. Arborists estimate that 40 percent of all commercial and residential properties suffer from soil compaction near trees. Compacted soils prevent trees from growing to their full potential, reducing their height and spread and making them susceptible to disease, insects and wind damage. Despite its widespread occurrence, this problem is usually overlooked, making it the invisible bandit that robs us of beautiful trees and durable turf.

Sir Clement Attlee, prime minister of England in the 1950’s once remarked about Winston Churchill that "nothing grows under a heavy roller." So it is for trees and turf. Soils become compacted by the simple application of pressure from foot traffic, vehicles and even rain drops. The greater this pressure, the greater the soil compaction. As soil particles are squeezed together their pore space shrinks (see figure above), leading to compacted soil. The lack of pore space prevents air movement to roots that, in turn, cripples nutrient uptake and creates conditions for

anaerobic microbes to build up. Roots growing in compacted soil must exert more energy, robbing above-ground growth. Compaction diminishes water infiltration and drainage capabilities creating damp soils that foster anaerobic disease organisms, soil erosion and further compaction

The vulnerability of soils to compaction varies with soil texture (% of sand, silt, and clay), moisture content, and the amount of pressure applied. One can measure compaction by using a penetrometer, a probe-like tool that measures resistance when it is inserted into the ground or by measuring the change in a soil’s bulk density. Compacted soils weigh more than non-compacted soils. It is possible to improve compacted soils by increasing their overall % of pore space. This is done either by tilling or aeration treatments or by adding soil amendments. However, it is generally easier to avoid compaction in the first place. This is accomplished by reducing wheel or foot traffic wherever possible, and by enforcing this especially when soils are wet.

Soil compaction affects: Water availability, N and K uptake, plant growth and yield, delayed planting and operations due to colder and wetter soils

PRESSURE IS THE CAUSE OF COMPACTION

Pressure is defined as pounds per square inch or psi. A 200-pound golfer standing on one foot wearing a shoe with 20 sq. in. of sole generates about (200 pounds/20 sq. in.) = 10 psi. That same golfer, when walking, creates 25 psi on his heel and 16.6 psi on the ball of his foot. His 17-pound pull cart with two, 2”-wide tires generates 2.1 psi. An electric golf cart (empty, 950 pounds) with four 8”-wide tires produces 3.7 psi, while that same cart with one person and gear (1,200 pounds) places 4.7 psi of pressure on the soil. A maintenance pickup truck (3,000 pounds) = 25 psi (four tires) (see chart below), whereas a 20 ton tractor with 2, 8x2 foot tracks generates 8.7 psi. Compare that to a women weighing 125 pounds standing in stiletto high heels. Those heels can generate up to 8000 psi! Now, that’s pressure, weight divided by area.

Although the psi is higher for a stiletto heel than a bulldozer, the overall compaction damage to a soil is naturally far greater for the vehicle, because it covers a greater area. This was revealed in a fascinating study by David Wienecke, who compared the compaction damage to a golf course by golfers who walked vs. rode in golf carts. (see chart above). The results are clear. Carts do far more damage than walking. The same can be shown for tractors on athletic fields

Deflating tires to the lowest safe PSI will help reduce compaction.

Use flotation tires to reduce compaction.

That is not to say that foot traffic causes little damage; it does, especially on athletic fields or foot paths where repeated walking on ground makes it difficult for grass or other plants to get established. The effect is even more pronounced with farm animals. The hoofs of a 1200-pound horse generates about 100 psi. At a walk, trotting or cantering horses can increase the psi even more.


Severely compacted soil on soccer field created by construction trucks used during installation of night lights	Compaction on foot path going into the woods. If left alone such compaction will last for hundreds of years.

Soil compaction is a continuous, ongoing problem that requires constant attention especially in high traffic areas and soils that were previously subjected to heavy construction impacts. In the next article I will describe a several ways that compaction can be treated in both turf and tree areas.