The Effect of Soil Compaction on Tree Health
Phoebe Gordon, UCCE Madera and Merced and Mae Culumber, UCCE Fresno and Kings
Soil compaction can be a severe issue in agricultural soils causing slowed water infiltration, ponding, runoff, and soil erosion. It can negatively affect plant growth and root development, which can lead to poor water and nutrient uptake and decreased yields. These issues can be difficult to diagnose, as symptoms can be assigned to other issues or not recognized at all.
Orchard traffic is concentrated between tree-rows, however compaction issues can impact the entire orchard depending on the historical land use and the method used to prepare the ground prior to planting. Tree roots expand rapidly during establishment and a full-grown tree requires a large volume of soil for their roots. One study estimated that a peach tree on a plum rootstock utilized 2.5 cubic yards of soil in the first year and expanded rapidly to use 130 cubic yards by the fourth year of growth (Vercambre et al. 2003). Anything that impedes root growth can impede tree growth and yield. The combined effect of compromised roots and reduced movement of water through the soil profile can produce either droughty or waterlogged conditions. If water is applied at a rate faster than it can move through compacted soils, it will pond on the surface, which can trigger stress responses in trees, including stomatal closure and reduced photosynthesis. While it may seem paradoxical since stomatal closure is also triggered during water deficits, it is simply how trees will respond to many stressors. Conversely, because infiltration is reduced, water cannot move as easily down into the root zone, which can result in water deficits, tree stress, and stomatal closure.
To understand why compaction is an issue, it is helpful to understand the components of the soil environment. Soils are comprised of solids (mineral soil and organic matter), liquids (soil water with dissolved nutrients and salts) and vapor (soil air). The liquid and vapor fraction of the soil can only be found in soil pores. The factors that influence porosity are complicated, but primarily have to do with soil texture and good soil structure.
Figure 1: McFarland loam with a loose, friable structure. Source: Blake McCullough-Sanden
Soil texture is determined by different proportions of sand, slit, and clay that comprise the solid fraction of the soil. Soil texture determines the rate water drains through soil after rain or irrigation. A variety of soils occur in the San Joaquin Valley, ranging from coarse sand to heavy clay. Loams (Figure 1), for example, tend to have an equal proportion of sand, silt, and clay, and if well structured, are some of the best soils for agricultural production.
In a well-structured soil, soil particles are not packed in tightly against one another; there are spaces between each particle, called soil pores. Soils that have larger particles have larger pores; this is why sandy soils have rapid water infiltration rates and lower water holding capacity in comparison to heavier textured clay soils. Soils with good soil structure have a wide variety of pore sizes. In general, large pores permit the rapid movement of water and air into and through soil. They don’t retain much, if any water, however. Very tiny pores, also known as micropores, typically occur within soil aggregates and sometimes between soil particles. These hold water too tightly to be plant accessible due to capillary action. Moderately sized pores hold water loosely enough for plant roots to access and typically will hold air as well. In all but the smallest soil pores you can find both water and air unless the soil is saturated from irrigation or heavy rain. Water clings to the soil particles, and air fills the center of a pore.
Soil aggregation also significantly affects soil porosity. Aggregation begins when individual soil particles are bound to one another through calcium and soil organic matter, particularly organic matter excreted from fungi and bacteria. Aggregation can increase soil porosity, creating larger pores in soils that may naturally have small pores, like finer textured soils. Pores can also be created by spaces left by dead and decayed roots as well as soil fauna like earthworms and larger vertebrates. Most of the pores in soils are connected to other pores, and these interconnected spaces are how water and air move through soil. You may have heard the term ‘bulk density’ applied to soils: this is a measure of soil weight for a given soil volume. You can hire a lab to do this for you, or do it yourself with the right equipment, a scale, and your kitchen oven.
Soils range in bulk density based on texture (Table 1) but for a given soil texture, the lower the bulk density, the higher the porosity and/or organic matter. Not all soils compact to the same degree, nor are the restrictive bulk densities the same. Coarser textured soils generally compact less than clay soils. Loams, such as in Image 1 are especially vulnerable to compaction because of the wide distribution in particle sizes; clay particles can move into the larger spaces between sand particles, increasing bulk density. Clay soils become more restrictive to root growth at lower bulk densities than coarser textured soils (Table 1).
Table 1: Ideal and restrictive bulk densities of several soil textures. Adapted from USDA NRCS publication “Soil Bulk Density / Moisture / Aeration”
| Soil texture | Ideal bulk density | Restrictive bulk densities |
|---|---|---|
| Sandy loams, loams | <1.4 | >1.8 |
| Silts, silt loams | <1.4 | >1.75 |
| Sandy clays, clay loams | <1.1 | >1.58 |
Soil compaction reduces soil function by reducing pore size and pore distribution (Figure 2), which increases bulk density, and could reduce the volume of water that is available for plant uptake. It also interrupts the continuity of soil pores, which again, is necessary for the movement of water and air through soils. Compaction happens when a high level of downward force pushes soil particles closer to each other and into nearby physical voids (the pores). In orchard systems the severity of compaction is largely influenced by soil type, equipment traffic, and repeated tillage. Water is an important lubricant: at field capacity water films allow soil particles to slide against one another more easily. Dry soils compact less easily than wet ones, and because of this we recommended to stay out of orchards when the soils are near field capacity. The result of this process is soils with smaller pores and less pore continuity, which can reduce water infiltration and soil water holding capacity.
The amount of force imposed by vehicles and heavy machinery is determined by the weight of the equipment and the surface area that weight is distributed over. Lighter vehicles generate less force and heavier vehicles generate more. The smaller the surface area that force is spread over (wheel surface area, for instance), the more force is transmitted to the ground. In other words, more of the soil has a force generated on it, but the intensity is less since the weight is distributed over a wider area. Compacting forces are most intense close to the point where the force originates (the soil surface directly under the wheel, for example) and decrease in intensity the farther away they are from the wheel. The stronger the force, the deeper the soil that is affected. A study comparing the effects of two tractors commonly used in Spanish almond orchards found a heavy tractor generated 5.5 ton-force, and a light one that generated 1.65 (the units reported here have been converted from metric). However, as the number of passes increased from 1 to 8, both tractors increased the bulk density of the soil, but the lighter tractor did more damage to the surface soil. This may be because the tires were much narrower in the light tractor than the heavier tractor. The contact surface area of the front tires of the light tractor was approximately 1/5 of the heavier tractor and the rear tires were ¼ the surface area, but the force that the light tractor did not proportionally decrease to the same degree – it was about 1/3 smaller. The heavier tractor increased bulk density more than the lighter tractor approximately one foot below the soil surface. One pass of the light tractor reduced topsoil porosity by 2.8%, and the heavy tractor reduced it by 3%.
Compaction is not discussed in orchards perhaps as much as it should be, but compaction is well recognized in row crops because of plough pans. Tillage loosens up top foot (or so) of soil but does not loosen soil beneath the tillage implements. Continual field traffic will continue to compact the soil that the tillage implements cannot reach, and eventually the plough pan becomes too dense for roots to penetrate. In orchards, tillage is much less common, but there is plenty of traffic that can compact the soil.
Roots grow and expand by exploiting already existing soil pores, but also by physically pushing soil particles out of the way. Compaction both reduces the rate at which water can move through the soil as well as impedes the plant’s ability to explore the soil, which could reduce uptake for nutrients with limited mobility. While orchards are routinely fertilized in California and nutrient availability is high, nutrient uptake may still be hindered due to reduced root function. Oxygen levels in compacted soils can be reduced, and roots are not as efficient at generating energy. Nutrient uptake is an energy intensive process, and thus nutrient uptake can be less in compacted soils even if adequately available.
Compaction has been thoroughly studied in agronomic systems, but to the authors’ knowledge, less so in orchard systems. Ferree and Schmid (2001) examined the effects of compaction on several different apple rootstocks planted in pots filled with a sandy silt loam and found that greater soil compaction (1.5 g cm-3) reduced growth and uptake of potassium and boron, but increased uptake of magnesium and manganese. Very mild compaction (1.2 g cm-3) increased growth, perhaps because of greater root soil contact. While there were differences in rootstock response to compaction, vigor did not affect the apple trees’ ability to tolerate compaction. This study was done in pots, however, with uniformly compacted soil, and does not reflect real orchard conditions which have zones with greater and lesser compaction. Vercambre et al. (2003) found that roots were more twisted and branched less in compacted zones in plum orchard root systems. Compaction that is generated by traffic between tree rows can extend into tree rooting zones (van Dijck and van Asch, 2001) and the entire between-row area can be compacted. This is because orchard rows are often much wider than the width of most equipment, and traffic does not necessarily stay in the same ruts and is distributed over more soil surface, compacting more soil. Despite the compaction, van Dijck and van Asch did not find a reduction in infiltration rate of the top or subsoils even though pore size was reduced.
What can be done to alleviate compaction? Tillage is a classic way to do it (Figure 3), however repeated tillage damages soil structure and it will only temporarily loosen the surface of the soil and tilled soils are vulnerable to re-compaction. Tilling the soil may regenerate orchard growth. Medeiros et al (2013) found that ploughing the soil to a depth of 3 feet alleviated compaction to up to 2.5 feet below the soil surface and increased fruit yield and size in a 14 year old citrus orchard (Medeiros et al., 2013). These options should be evaluated carefully in established orchards, as they could severely damage roots and leave some plants, such as walnut, vulnerable to crown gall infection. However, deep ripping can sometimes be the only option for alleviating deeper soil compaction. Time will also reduce soil compaction; Van Dijck and van Asch found that a vineyard abandoned for five years had much less soil compaction than an actively managed vineyard. This is a relatively short time in recorded recovery rates of compacted soils; some soils have been reported to remain compacted for decades (Kozlowski, 1999). Allowing soils to remain fallow for years is not applicable to most planting situations, however if you are planting an orchard into land that has not been managed for several years, compaction may not be an issue. This should, however, be confirmed with backhoe pits.
Figure 3: photo of a recently ripped field. Notice the large clumps of cemented soil. Source: Allan Fulton
Planted cover crops like mustards and forage radishes have been found to be very effective at penetrating compacted soils, as well as providing forage for bees during critical times. Cover crops can alleviate compaction via their roots acting as resistance to compacting forces on the soil, and their roots, once dead and broken down, can create channels for new root growth. However, these pores are vulnerable to compaction forces. Oliveira and Mervin (2001) found that frequent mowing of sod treatments during the growing season resulted in similar soil bulk density measures to herbicide treatments in orchards. This study also found that hardwood bark mulch was the most effective at reducing compaction, however this is impractical in systems that do not use off-ground harvesting. In contrast to this, a similar study conducted in California almond orchards by Terry Pritchard et al. (published 1989) found that growing brome or allowing resident winter vegetation to grow in between tree rows in both a young and mature orchard resulted in less compacted soils compared to complete control or chemical mowing. In the mature orchard, multiple years of brome continually reduced soil compaction. Surface compaction increased in the chemical mowing and residual herbicide treatments in that same orchard. The brome did not increase overall water use in the orchard; while early season water use was higher, when the grass residues were allowed to remain on the soil surface after mowing, it acted as a mulch and decreased evaporation, which countered the early season effects.
Soil compaction could affect the success of replanted orchards. Many older orchards are at a wider spacing than what is commonly planted today. Without proper soil mixing and preparation, you may be placing tree rows in compacted soil, which will negatively affect their growth. Do your homework and conduct extensive soil analyses with backhoe pits in several areas to identify potential compaction issues at the orchard preplant stage. If the soil is compacted, rip and plough, or cross-rip the site to loosen the soil and promote root growth and development (Figure 3). Most importantly, take care to not re-compact it before planting the trees.
Preliminary research with whole orchard recycling (WOR), an alternative to burning or removing orchard biomass that grinds and incorporates orchards into the soil on-site (Holtz et al. 2016), have found increased infiltration rates in replanted orchards compared to conventionally planted orchards. The chips of woody biomass have been shown to enhance soil moisture retention, buffer temperature changes, increase soil organic carbon and the abundance and diversity of microbial populations, characteristics associated with improved soil structure.
What’s the bottom line? Why should you care about compaction in orchards, especially since traffic is relegated to row middles, where roots aren’t located? Compaction indicates a deterioration of soil structure, which negatively affects soil function. It decreases infiltration rate, can reduce plant available water either through soil storage or through reducing water movement to roots, and can reduce root growth and nutrient uptake. While most compaction occurs in row middles, these row middles are important for the infiltration of winter rainfall, especially in high rainfall areas. Roots may grow in row middles in areas where there is enough annual rainfall to fill the soil profile, especially if the soil can hold significant amounts of water. Additionally, while this would need to be tested, it is reasonable to expect that compacted soils could reduce soil recharge rates. Flood irrigation can also provide an opportunity to increase soil moisture levels in the alleyways.
The best way to deal with compaction is to prevent it when possible. In established orchards, minimize vehicle traffic to the extent possible, especially when soils are wet. Ensure vehicles stay as close to the middle of the rows as possible. Decreasing tire pressure is one way to distribute force over a wider surface area, or purchase wider tires for orchard equipment, if available. If sprays need to go on at critical times in an orchard when soils are wet, consider an aerial air application. Do not allow vehicles to stray close to tree rows. Allow resident winter vegetation or sow cover crops during fall in between tree rows, to reduce soil compaction, as well as stabilize the soil and allow for earlier orchard entry in wet conditions.
Adding soil amendments like gypsum will not improve infiltration in compacted orchards. Calcium can, however, improve infiltration in orchards affected by high sodium or when the irrigation water is extremely low in any dissolved salts, such as canal water. The reduction in infiltration is different in sodium-affected soils than compacted soils; sodium particles deflocculate clay particles, and calcium helps them focculate while also displacing sodium from the soil. If you are trying to address poor infiltration in your orchard, thoroughly investigate the cause of poor infiltration. Test your water, soils, and plants, and also dig down into the soil to investigate the structure.
More resources:
https://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs142p2_053260.pdf
http://calag.ucanr.edu/archive/?type=pdf&article=ca.v043n04p23