Advancing Sustainable Agriculture in the Pacific Northwest

Conservation Tillage Systems

Information Resource

CONSERVATION TILLAGE HANDBOOK SERIES
Chapter 1 — Erosion Impacts, No. 12, Summer 1990


Conservation Farming and Sustainability

Don Wysocki

Recently, sustainability and sustainable agriculture have become topics of much discussion. These terms can have different meanings to different people. There are no universally accepted definitions for these concepts. Fortunately, precise definitions are not essential to understanding agricultural problems and discussing solutions.

In a general sense, sustainable means to maintain, endure or improve over the long term. In a farming context, a sustainable system is one that is capable of producing adequate amounts of food or fiber at a profit without environmental degradation indefinitely. Farming systems that are not productive or profitable, or that degrade the environment, are not sustainable. Unfortunately, there are trade-offs between short-term profitability and production and long-term productivity and environmental concerns. Agriculture has been faced with these trade-offs since it first began. The goal of a sustainable agriculture is to develop farming systems and strategies that align short-term profitability with long-term productivity and environmental integrity.

The STEEP (Solutions To Environmental and Economic Problems) program was initiated in 1975 to address soil erosion and related crop production problems in the Pacific Northwest. Although the word" sustainability" was not yet in vogue when STEEP began, concern for soil productivity and the environment were major factors that led to creation of the STEEP program, More recently other factors have been recognized as threats to sustainability and STEEP researchers have addressed many of these problems during the course of the STEEP program. This article discusses three threats to sustainability, estimates their importance and describes practices to solve them,

Threats to Sustainability

Many factors influence the sustainability of a farming system, However, most of us would probably agree that a sustainable farming system must be economically profitable, agronomically productive and environmentally prudent. A sustainable farming system should strive to preserve or improve profitability, productivity and environmental integrity. A system that neglects any of these cannot be considered sustainable. To simplify this discussion let's recognize that achieving sustainability must be done within economic constraints. With the assumption that profitability must always be positive, let's discuss three threats to sustainability.

Soil Erosion

Erosion is a threat to sustainability because it threatens both soil productivity and environmental integrity. Erosion directly decreases productivity by reducing soil depth and preferentially removing topsoil, which contain much of the soil organic matter, Indirectly, the loss of soil organic matter and topsoil lowers water holding capacity, reduces infiltration, adversely affects soil tilth and decreases soil fertility. Soil organic matter is the reservoir for plant nutrients, particularly nitrogen and sulfur, In a recent publication, STEEP researchers Paul Rasmussen and Harold Collins, USDA.ARS soil scientist and soil microbiologist, respectively, and Richard Smiley, OSU plant pathologist, all at Pendleton, OR, estimated that topsoil contains 80 percent of a soil's native fertility (Rasmussen et al. 1989). Erosion not only diminishes the productivity of the soil but also makes soil more susceptible to further erosion, A significant amount of STEEP research has addressed the effect of erosion on soil productivity. This research is summarized in Chapter 1 of the PAW Conservation Tillage Handbook, available through county extension offices.

Erosion has many off-site impacts that adversely affect the environment. Water erosion carries soil to streams, lakes and other water bodies. Eroded soil contributes to siltation and sedimentation problems and also contains attached nutrients and possibly pesticides, These diminish water quality and pose a risk to aquatic plants, fish and aquatic life. Soil lost by wind erosion decreases air quality and can be a health risk. Soil erosion threatens both soil productivity and environmental integrity,

Organic Matter Decline

Soil organic matter is associated with many of the characteristics that are desirable for maintaining and improving plant growth and soil productivity, A trend that has generally been observed on most cultivated soils is the decline of soil organic matter with increasing time of cultivation. Several STEEP researchers have observed this on cultivated soils in the dryland PNW. For example, Rasmussen has documented this decline on long-term plots in a wheat-fallow rotation at the Pendleton Ag Research Center. The estimated soil organic matter content before cultivation (ea. 1880) in the top foot of soil was 2.8 percent for the Walla Walla soils at Pendleton. By 1930, organic matter had declined to approximately 2,0 percent in the top foot of soil.

The plots at Pendleton are on less than 4 percent slope, therefore water erosion has been negligible and the decline is almost entirely due to oxidation. Oxidation is a natural chemical and microbial decomposition process that converts carbon in soil organic matter to carbon dioxide. The process is enhanced by cultivation.

Since 1931 various management options have been studied to assess their effect on soil organic matter (Fig. 1). Addition of 10 tons of m6nure/acre every other year (an option not available or practical for most producers) has stabilized or slightly increased organic matter content. Organic matter has continued to decline for other treatments. Loss of organic matter by oxidation has exactly the same effect as loss of organic matter through erosion. Soil fertility, water holding capacity, infiltration and tilth are all diminished. Obviously the decline of organic matter by oxidation is a threat to both productivity and environmental integrity.

Soil pH Decline

Several STEEP researchers have observed declines in soil pH over the past several decades. Among these are Robert Mahler and Robert McDole, University of Idaho soil scientist and extension soils specialist, respectively, and Rasmussen at Pendleton. Soil pH decline or acidification is a natural process, however, it is accelerated by certain cultural practices. A principle contributor to accelerated pH decline has been the use of ammonium-based fertilizers. All common nitrogen fertilizers including anhydrous ammonia, aqua ammonia, ammonium nitrate, ammonium sulfate and urea have this base. These fertilizers are acid forming and lower soil pH proportional to the amount applied. In a wheat-fallow rotation at the Pendleton Ag Research Center, nitrogen fertilization over the past 50 years has progressively lowered soil pH (Table 1).

Eventually, the addition of nitrogen fertilizers will result in lowering soil pH to the extent that crop growth will be adversely affected. Mahler and McDole have found that soil pH levels lower than 5.3 can reduce cereal yields and pH levels below 5.6 can reduce yields of legumes. High rainfall areas of northern Idaho and eastern Washington have reached these levels and areas of eastern Oregon are approaching them as well.

Table 1. The effect of nitrogen fertilizer application on soil pH under a wheat-fallow rotation, Pendleton OR (Rasmussen et al. 1989).

 

Soil Depth Cumulative N application, 1940-83 (lb/acre)
493 728 986 1,221 1,714 2,207
(inches) pH
0 to 6 6.1 6.1 5.8 5.7 5.6 5.4
6 to 12 6.5 6.5 6.4 6.4 7.3 6.2
12 to 18 7.3 7.3 7.1 7.2 7.2 7.1

 

 

Fig. 1. Change In soil organic matter In the top foot of soil from 1981 to 1986, including residue management effects from 1931 to 1986. Wheat-fallow rotation, conventional moldboard tillage, Pendleton, OR (Rasmussen et al. 19S9).

Correcting soil pH requires the addition of lime, This has been a common agricultural practice in humid regions for many years. In the dryland areas of Washington, Idaho and Oregon this practice is in its infancy and the infrastructure, lime sources and equipment for application are not well developed. Correction of soil pH may require the addition of 1 to 2 tons of lime per acre, Fortunately, the corrective effects of liming are long-lasting. Depending on soil, rainfall and cropping conditions, lime may be required once every 5, 10 or even 20 years.

The most immediate effect of declining soil pH is on productivity. As soil pH drops, yields will decline, Lower yields maybe the direct result of reduced pH on plant growth or may be the indirect result of other factors. Richard Smiley, plant pathologist and STEEP researcher at Pendleton, has found that the severity and amount of strawbreaker foot rot increases with declining soil pH. At Washington State University, STEEP researcher Tim Murray has shown that the incidence of Cephalosporium stripe increases as the soil becomes more acid, In his studies the disease was noticeably increased when the soil pH was less than 6.0, Declining soil pH has also been found to affect microbial activity. This may influence slow nutrient cycling and decomposition of residues in the soil.

Declining soil pH can have adverse environmental impacts. If plant vigor is reduced because of low soil pH levels there may be less plant and residue cover to protect soil from erosion. If declining soil pH favors certain diseases then adjustments in tillage or residue management practices or rotations may become necessary to regain control of the diseases. These may be practices or rotations that leave less residue for erosion protection.

Prioritizing Threats to Sustainability

Soil erosion, oxidation of soil organic matter and declining soil pH are all threats to sustainability in the dryland PNW. We need to deal with these threats on a priority basis, In certain landscapes, erosion will be the greatest threat to sustainability, while in others the most immediate problem may be oxidation of organic matter or pH decline.

To develop a perspective, compare soil organic matter loss by erosion and oxidation. Assume a soil averages 2 percent organic matter in the top foot and has an erosion rate of 5 tons/acre/year (erosion rates are considerably higher on many soils). After 50 years, 250 tons of soil (approximately 1,7 inches) will be lost. Organic matter lost by erosion would be 0.02 X 250 tons = 5 tons or 10,000 pounds. Using the rate of oxidation for a farming system using 40 pounds N/acre, without burning the stubble, organic matter has declined from 2.0 percent to about 1.7 percent over the past 50 years (Fig. 1). Assuming an acre furrow slice of 1,000 tons, the amount of organic matter lost by oxidation has been 0.003X 1,000 tons soil/acre furrow slice = 3 tons or 6,000 pounds. Both are significant losses, but a low rate of erosion removes more organic matter than typically lost through oxidation.

Although this comparison is very rough, it shows the potential threat of erosion. In most areas of the PNW, sustainability is most seriously threatened by wind or water erosion. Erosion also has many off-site impacts that contribute to environmental degradation elsewhere, Oxidation of organic matter dries not contribute directly to off-site effects.

Fortunately, conservation tillage systems that reduce erosion also appear to favor retention of soil organic matter, In long-term plots at Pendleton, organic matter decline has been less on tillage systems that leave more residue at or near the surface (Table 2). This indicates that residue management to control erosion may also offset the loss of soil organic matter through oxidation. Systems that include burning return the lowest amounts of crop residue to the soil and consequently have had the greatest decline in soil organic matter.

Decline of soil pH presents little threat to sustainability if properly treated. In areas where pH has declined to critical levels priority should be given to correcting this problem. Fortunately low soil pH is corrected by the infrequent additions of lime. Costs maybe considerable at the time of application, but can be worked into a management system over the long-term to maintain crop productivity and effective erosion control.

Summary

Among the threats to sustainability in the dryland PNW are erosion, soil organic matter oxidation and declining soil pH. Farming systems in the region must address these problems, We must recognize and prioritize these problems for various landscapes in the region. In general, erosion is the most extensive threat followed by loss of organic matter through oxidation and then by declining soil pH. Through the efforts of growers, conservationists, STEEP researchers and others, conservation farming systems have evolved to deal with erosion and crop production problems. These systems should be the basis for addressing the larger issue of sustainability. Proper soil, crop, pest and residue management systems coupled with the judicious use of fertilizers and other agricultural chemicals are the most sustainable systems we now have.

Table 2. Effect of primary tillage on SOIL organic matter In a wheat. fallow rotation near Pendleton, OR (modified from Rasmussen et, al 1989).

 

Soil Depth Primary Tillage1 , 1940-86
Plow Disk Sweep
(inches) (Organic Matter %)
0-3 2.1 2.8 2.8
3-6 2.1 2.2 2.2
6-9 1.9 1.9 1.9
9-12 1.3 1.5 1.4
0-9 2.0 2.3 2.3

Literature Cited

Rasmussen, P. E,, R. W. Smiley and H. P. Collins. 1989. Long-term management effects on soil productivity and crop yield in semiarid regions of eastern Oregon. Agricultural Experiment Station Bulletin 675, Oregon State University, Corvallis, 57 p.

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Hans Kok, WSU/UI Extension Conservation Tillage Specialist, UI Ag Science 231, PO Box 442339, Moscow, ID 83844 USA (208)885-5971
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