A compacted “plow pan” layer often forms about 8 to 10 inches below the surface in both mowed and tilled orchards. This is a widespread problem in orchards regardless of soil type. Growers are often unaware of its presence and impact. In some cases, no-till orchards haven’t been ripped for decades.
Just ask Mark Schmidt, who grows almonds and walnuts in Waterford, CA, located in the northern San Joaquin Valley. He has one block of 20-plus-year-old walnuts that has sandy top soil, but just about 8 inches down a hard pan, or plow pan, has formed. “Just doing normal work on the ground over the years compacted it down,” he says.
Last spring, Schmidt deep-ripped the orchard, and the results were dramatic. “In one year, the growth was just unbelievable, from 1 foot a year, to 3 to 4 feet this past year,” he says. “It was all because of the compaction; the hard pan was cutting off the roots. The roots were just getting by.”
Schmidt, who’s also known in the ag community as a beekeeper, certainly learned a lesson he won’t forget. “This year I ripped that ground both ways; last year I ripped it just one way,” he says. “I did a real massive break-up job this time.”
Let Roots “Breathe”
A greater understanding of soils and the effect of compaction on tree growth and yield will help in developing approaches to minimize the problem and increase productivity. Soils are complex mixtures of mineral particles that vary in both size (sand, silt, clay) and arrangement. Soil texture refers to the proportion of sand, silt, and clay in a soil. Soil structure means how the particles are arranged; it largely determines the amount of pore space, the spaces between the particles, in the soil.
Pores are an important part of healthy soils for several reasons. They affect water intake and movement through the profile, the amount of water soil can hold for use by the tree, and drainage below the root zone. Soil water content increases above compacted layers and may create conditions that encourage root and crown diseases. Compacted soils are more subject to runoff and erosion.
We have seen a number of orchards where the ground was wet in the top few inches but water never reached the roots.
Compaction also affects air exchange between the soil and the atmosphere. Did you know that roots need to “breathe?” Roots and the beneficial microorganisms that live in the soil need to take in oxygen and release carbon dioxide.
Microorganisms fix nitrogen from the air and help break down organic matter which releases nutrients to the tree. These are important points to keep in mind because cultural practices have a direct impact on soil quality and sustaining tree health.
As Schmidt learned, soil becomes compacted primarily because of the pressure applied by the weight of orchard equipment. It compresses the soil particles together, causing the soil to be denser with less pore space. It is more difficult for roots to push through dense soil and so root growth can be reduced. The best strategy to minimize compaction is to prevent it from occurring in the first place. The risk for compaction is greatest when soils are wet. Driving on wet soils is the single biggest cause of compaction.
A dry soil is much more resistant to compaction than a moist or wet soil. Sandy soils are particularly susceptible to compaction as the weight of equipment can cause the sand grains to interlock. Soils with high organic matter are also less susceptible to compaction. Test for compaction by probing the soil with a pointed metal rod, bucket auger, soil tube, or shovel. It is more easily done on a somewhat moist soil.
Apply even pressure and push the probe into the soil. Compaction is the resistance found about 8 to 10 inches below the soil. Alleviate compacted layers by ripping or chiseling when the soil is dry. A shank depth capability below the compacted area is necessary. Curved shanks require less draft than straight shanks to loosen the same amount of soil. Space the shanks no further apart than the depth of ripping and cover the width of the middle to within a few feet of the trunks to avoid injury to larger roots. A single shank down the middle is of limited value. Do not worry about injuring smaller roots; root pruning actually stimulates root growth in much the same way that heading cuts in the canopy promote new shoot development.
For nine ways to minimize compaction, go to the next page.