MONDAY, APRIL 22, 2024. BY STAN GRANT, VITICULTURIST.
The topic of soil erosion sometimes evokes images of dust storms during the 1930’s “Dust Bowl” in the south-central United States (Figure 1). Fortunately, instances of severe erosion of viticultural soils are infrequent. Instead, erosion often goes unnoticed as it slowly steals an especially valuable asset, the vineyard topsoil.
Figure 1. A Midwest dust storm during the 1930’s. (Photo Source: timesunion.com)
Recognizing the importance of conserving soils to sustain agriculture, the U.S. government established the USDA Soil Conservation Service (SCS) in 1935 expressly to reduce soil erosion. (The Natural Resource Conservation Service (NRCS), which has a broader conservation mission, superseded the SCS in 1994). In this article, we will, likewise, underscore the significance of soil erosion, as well as consider how soils erode and how to limit it.
Agents of Soil Erosion
Erosion can occur when one of two fluids, air or water, forcefully passes over or strikes a soil surface. The threshold wind velocity for soil particle detachment is about 13 miles per hour at 1 foot above the soil surface. With winds of this or greater velocities, soil particles, and small aggregates may slide along the soil surface or bounce a short distance into the air after other soil particles strike them. These modes of action are surface creep and saltation, respectively.
Usually of greater concern are the fine soil particles (< 0.1 mm diameter) lifted into the air and suspended in the wind. The net effect of suspension is the movement of fine soil particles downwind and possibly, to neighboring land. The very finest of these particles (< 0.01 mm diameter) may stay suspended for extended periods of time, becoming the pollutant known as PM (particulate matter) 10.
Figure 2. The Wind-Erosion Equation (WEE) is for predicting annual soil loss due to wind erosion (B) and the Universal Soil Loss Equation (USLE) for predicting annual soil loss due to sheet and rill erosion (A).
Soil surfaces that are expansive, bare, smooth, loose, and dry are especially vulnerable to wind erosion (Figure 2A). Actually, drought and soil dryness were principal contributors to Dust Bowl events. Cultivation accelerates the erosion of vulnerable soils and accordingly, an expansive dust cloud behind a tractor pulling a disk on a windy day is visible evidence of imprudent farming.
Water is the other principal agent of soil erosion. Raindrops and rainwater intercepted and falling from vines displace soil particles as they strike unprotected soil surfaces, while an overland flow of sheeting rainwater carries displaced soil particles downslope, and possibly, downstream.
Figure 3. Sheet and rill erosion on a sloped vineyard tractor row (A). Sheet, rill, and gully erosion on the slope and sediment deposits at the base of a levy adjacent to a vineyard (B). (Photo Source: Progressive Viticulture, LLC©)
Where sheeting water runoff collects in surface irregularities, the increased water volume, velocity, and turbulence can intensify its erosive force to create small, eroded channels called rills (Figure 3A). Rills may, under more extreme conditions, become larger channels called gullies (Figure 3B). Gullies also form in places where numerous rills come together.
The rate of rainfall, the infiltration capacity of the soil, and the steepness and length of the slope are among the factors determining the severity of soil erosion by water (Figure 2B). In addition to being prone to erosion by water, many hillside vineyard soils are shallow and can little tolerate soil loss.
Although seldom considered as such, land leveling operations prior to vineyard planting are, in effect, artificial soil erosion. During these activities, topsoil is cut at one location and deposited elsewhere. Left behind is a less favorable root zone and a bare surface susceptible to erosion.
Consequences of Soil Erosion
The soil nearest the surface is usually the most structured, permeable, microbial-active, and fertile part of a vineyard root zone (Figure 4). And it took many years for it to become so. Therefore, discernable surface soil erosion represents a reduction of root zone function with long-term implications.
Figure 4. The dark color of this topsoil is evidence of its abundant organic matter, while the abundant roots are indicative of its favorability as a root zone. (Photo Source: Progressive Viticulture, LLC©)
Perhaps, less obvious, soils repeatedly subjected to erosion, especially wind erosion, become coarser over time. Correspondingly, their capacities for holding water are reduced. At the same time, the lost finer particles, which had greater chemical activities than the remaining coarser particles, represent a loss of soil fertility and buffering capacity.
In the worst instances, erosion removed the entire surface soil, leaving only subsoil for a vineyard root zone. Previously situated under protective topsoil, the atmosphere and vegetation of the overlying vineyard have had little impact on subsoils. As a result, subsoils normally contain negligible organic matter and correspondingly, very little, if any, microbial activity.
Some of the products of soil microbial communities are directly beneficial to grapevine roots, but perhaps more important are their indirect effects. These include binding soil particles into aggregates for enhanced porosity and the weathering of minerals to release the nutrients they contain. The absence of microbial activity leaves most subsoils poorly structured, slowly permeable to air, water, and elongating roots, and low in readily available mineral nutrients.
Understandably, subsoil conditions restrict grapevine growth and productivity, particularly where subsoils are shallow. In addition, subsoils are more prone to compaction and erosion than the topsoil that formerly covered them.
Controlling Soil Erosion
Erosion, being a force of nature, is unstoppable. We can, however, take steps to protect our soils and minimize surface soil loss to tolerable levels. A multifaceted management approach is most effective for conserving vineyard topsoil. Such combined measures are particularly important for soils exposed and vulnerable during vineyard development or redevelopment. Approaches for controlling erosion are described below.
Reducing or Eliminating Erosive Forces Before They Enter a Vineyard
The first line of defense against soil erosion lies on the perimeter of vineyards. Hedgerows of trees and shrubs on the upwind side of a vineyard deflect and reduce the velocity of incoming winds, thereby reducing their erosive force (Figure 5A). Such vegetative windbreaks are present in parts of the Salinas Valley, where high winds regularly occur in the afternoon during the growing season.
Figure 5. A hedge row of trees serves as a vineyard windbreak (A) and a culvert pipe outlet for delivering collected overland flow into a vegetated ditch for diversion away from vineyards (B). (Photo Source: Progressive Viticulture, LLC©)
On the vineyard periphery, divert water flowing toward it from adjacent lands. Doing so may simply require a vegetated embankment on the vineyard edge, but in other instances may involve engineered structures, like lined ditches and culverts, that capture, concentrate, and reroute overland flow (Figure 5B).
Reducing Erosive Forces Within Vineyards
Cover crops across headlands and other non-farmed areas, and between tractor rows provide a second line of defense against soil erosion. Tall cover crops deflect wind and significantly reduce its velocity near soil surfaces, but even short cover crops provide some degree of protection from wind erosion (Figure 6). Sod-based cover crops, either continuous, tilled, or rotated with another cover crop type, reduce sheet and rill erosion.
Figure 6. A tall, dense cover crop that provides substantial protection for vineyard soils against both wind and rainwater erosion. (Photo Source: Progressive Viticulture, LLC©)
Standing cover crops break the fall of raindrops, thereby decreasing their impact on soil surfaces. In addition, cover crop roots, especially the dense fibrous roots of grass cover crops, bind soil particles together, enhancing both water infiltration and erosion resistance. At the same time, the bottoms of cover crop stems impede and disrupt sheeting runoff water, which decreases its velocity and erosive force. Due to these combined benefits, cover crops are especially important for soil erosion control in sloped vineyards.
For sites with very steep slopes, terracing reduces slope length, decreases the velocity of overland flow, and facilitates water collection for delivery outside of the vineyard. To be effective, terraces follow the contour of the slope, and the fill portion of the terrace gently slopes (≤ 2%) away from the vine row on its outer edge towards the terrace cut on its inner edge. Terrace surfaces, especially the steeper cut surface, benefit from the erosion protection of a cover crop or mulch.
Securing Surface Soils
Cover crops can have one other soil conservation advantage for vineyards. Cover crop residues incorporated to some extent into a surface soil add organic matter, which stimulates soil microbes and their production of compounds that bind soil particles into aggregates.
Soil aggregation has two soil conservation benefits. First, it increases a soil’s resistance to the erosional forces of both rain and wind. Second, it increases soil permeability, thereby increasing the rate of rainwater infiltration and decreasing the volume of runoff.
Incorporating organic soil amendments, either compost or manure, prior to planting a cover crop can accelerate soil aggregation, as well as provide nutrients to support cover crop establishment and growth. When not applying organic amendments, broadcast apply a starter fertilizer prior to seeding to promote rapid cover crop growth and establishment of erosion protection. For cover crops that include legumes, include phosphorus as well as nitrogen in the starter fertilizer. In practice, the actions listed here to enhance soil aggregation constitute the last line of defense against soil erosion.
To maximize the soil conservation benefits of cover crops, delay cultivation until late spring, after the threat of severe weather. Importantly, to retain soil organic matter, minimize tillage or at least, tillage passes. Annually alternating tilled tractor rows with mowed cover crop tractor rows is another option. Keep in mind a rough and slightly trashy soil surface following cultivation is more resistant to wind and water erosion than a smooth and completely clean soil surface.
For vineyards that require cover crop mowing to minimize the risk of grapevine frost damage, mowed cover crop residues, when sufficiently bulky, act as protective mulch that reduces soil erosion. In fact, some hillside vineyards regularly use anchored straw mulches on headlands and other non-farmed areas to effectively inhibit sheet and rill erosion either alone or as a protective shield for soils with newly planted cover crops. Net covers, fiber and chemical binders, and crimping, rolling, or punching into the soil are methods for anchoring mulches.
Eroded Soil Remediation
Organic matter additions are essential for the remediation of eroded soils, especially for exposed subsoils. As mentioned above, added organic matter, whether from applied organic soil amendments, incorporated cover crop residues, or both, is the substrate necessary for microbial activity.
Paramount among beneficial microbial activities is their production and release of compounds, including polysaccharides, organic acids, phenolics, and amino acids, that either directly or indirectly benefit vine roots. Indirect benefits include improved tilth and fertility. Applying microbial inoculants can accelerate the growth of microbial populations and their beneficial activities, especially if accompanied by essential nutrients that may be in short supply in a newly exposed subsoil.
Added mineral amendments, such as gypsum, can increase the soluble mineral concentrations, which further enhances the remediation and condition of eroded soils. The time period for soil remediation depends on the frequency and intensity of both organic and inorganic (mineral) amendment additions.
Conclusions
Soil is the foundation for vineyard productivity and profitability. For that reason, erosional loss of soil is costly, and excessive soil erosion is a symptom of inadequate or improper soil management. Successful efforts to minimize erosion are multifaceted, involving consistent actions to curtail erosive forces before they enter the vineyard, to lessen erosive forces after they enter the vineyard, and to secure soil surfaces within the vineyard. Restoration of eroded soils entails adding resources in short supply, especially those supporting microbial activities.
Stan Grant served on the Lodi Winegrape Commission’s LODI RULES Committee for many years and greatly contributed to the program. Check out the LODI RULES for Sustainable Winegrowing Standards, especially Chapter 4: Soil Management, for more information on conserving, protecting and regenerating vineyard soils. lodigrowers.com/standards/
Further Reading
Bissonnais, YL, and Singer, MJ. Seal formation, runoff, and interrill erosion of seventeen California soils. Soil Science Society of America Journal. 57, 224-229. 1993.
Follet, RF, and Stewart, BA. Soil erosion and crop productivity. American Society of Agronomy. Madison, WI. 1985.
Francis, CJ. How to control a gully. U.S. Department of Agriculture Farmers’ Bulletin No. 2171. 1973.
Goldman, SJ, Jackson, K, and Bursztynsky, TA. Erosion and sediment control handbook. McGraw-Hill, New York. 1986.
Grant, S. Maximizing cover crop benefits through selection and management. Lodi Winegrape Commission Coffee Shop. (www.lodigrowers.com). October 12, 2015.
Grant, S. The ultimate goal of vineyard soil management: optimized root zone function. Lodi Winegrape Commission Coffee Shop. (www.lodigrowers.com). December 20, 2021.
Grant, S. There is more to vineyard floors. Lodi Winegrape Commission Coffee Shop. (www.lodigrowers.com). March 28, 2022.
Grant, S. Till or No-Till?. Lodi Winegrape Commission Coffee Shop. (www.lodigrowers.com). January 23, 2023.
Hillel, D. Introduction to soil physics. Academic Press, New York. 1982.
Ingels, CA, Bugg, RL, McGourty, GT, and Christensen, LP (Eds.). Cover cropping in vineyards. University of California Division of Agriculture and Natural Resources Publication 3338. 1998.
O’Geen, AT, Elkins, R, and Lewis, D. Erodibility of agricultural soil, with examples in Lake and Mendocino Counties. University of California Division of Agriculture and Natural Resources Publication 8194. 2006.
O’Geen, AT, Prichard, TL, Elkins, R, and Pettygrove, GS. Orchard floor management practices to reduce erosion and protect water quality. University of California Division of Agriculture and Natural Resources Publication 8202. 2006.
O’Geen, AT, and Schwankl, LJ. Understanding soil erosion in irrigated agriculture. University of California Division of Agriculture and Natural Resources Publication 8196. 2005.
Peterson, AE, and Swan, JB. Universal soil loss equation: past, present, and future. Soil Science Society of America, Madison, WI. 1979.
Shepard, H, and Grismer, M. Quantifying erosion rates for various vineyard management practices. Practical Winery and Vineyard. 29 (1), 50-54, 56-58, 60-62, 64. Jan/Feb 2007.
Stimson, D, and O’Connor, K. Multiple benefits in vineyard erosion control. Practical Winery and Vineyard. 27 (1), 62-70. Jan/Feb 2005.
Wischmeier, WH, and Smith, DD. Predicting rainfall erosional losses – A guide to conservation planning. Agriculture Handbook No. 537. USDA-SEA, Washington, DC. 1978.
Woodruff, NP, and Siddoway, FH. A wind erosion equation. Soil Science Society of American Proceedings. 29, 502-608. 1965.
Zoebeck, TM, and Van Piet, RS. Wind erosion. In Hatfield, JL; Sauer, TJ. (Eds.). Soil management: building a stable for agriculture. American Society of Agronomy and Soil Science Society of America, Madison, WI. 2011.
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