Edward J. Deibert

Professor, soil science Department, North Dakota State University. ( http://.soilsci.ndsu.nodak.edu ). Paper prepared fcr 19th Annual Manitoba-North Dakota zero Tillage Workshop. Jan 27-29, 1997. Brandon, Manitoba. Manitoba-North Dakota zero Tillage Farmer's Association

Soil quality appears to be one of the new "buzz words" which has gained attention due to the increased interest in (1) degradation of our three main natural resources (soil, air, water) , (2) improvement of soil tilth and (3) maintaining the sustainability of the soil for future generations. In each of these areas the focus has been on the key attributes of soil quality which include the chemical, physical and biological properties of the soil.

The basic processes (physical, chemical and biological) , areas where soil property deterioration occurs under each process and factors or activities (agricultural, industrial, urban) responsible for degradation of the soil are covered in a paper by Lal et al. (1989). Soil tilth, often described as the physical condition of the soil related to ease of tillage, seedling emergence or root growth, has steadily declined as a result of intensive tillage and/or monoculture management practices. A review by Karlen et al. (1990) addresses some problems related to soil tilth and stresses the need to understand the multiple facets of the soil in order to manipulate the soil properties for optimum conditions.

Sustainability of our soils has gained wide attention due to the previous efforts by Leopold (1947) who emphasized soils from ecological aspects, Rodale (1983) who focused on the biological or organic manipulation of soils, philosophical ideas and emphasis on a regenerative soil building system by Kirschenmann (1988,1991) in the Northern Plains and Parr et al. (1992) who recently suggested that soil quality impacts not only soil productivity but human-animal health, food quality-safety and environmental quality in a sustainable system.

SOIL QUALITY INDICATORS

Soil quality is a term that is often used interchangeably with soil health. Health of the soil is determined by measuring certain properties, much like going to the doctor when one is sick. The doctor evaluates your vital signs to see what is not normal. Determining the health of your soil requires measuring soil properties to see if they have changed over time. Farmers have the ability to measure and record soil properties at any point in time, then measure that properties at another time to see what changes have occurred, positive or negative. The doctor evaluates your health and recommends changes or prescribes medicine to improve your quality of life. The farmer measures his soil properties and infers the quality of the soil. If the quality of the soil is declining, changes in management are required. A difference between human health and soil health is the doctor usually sends you a bigger bill. Thus soil quality may have many meanings depending on the health of one or more of the most important soil properties.

The Science Society of America has defined soil quality as the capacity of a soil to function within natural or managed ecosystems, to sustain plant and animal productivity, maintain or enhance water and air quality, and support human health and habitation. Doran and Parkin (1994) have defined soil quality as the capacity of a soil to function within ecosystem boundaries to sustain biological productivity, maintain environmental quality and promote plant and animal health.

Arshad and Coen (1992) indicated that soil quality can be expressed in terms of the sustaining capability of a soil to accept, store and recycle water, minerals and energy for production of crops at optimum levels while preserving a healthy environment. Quality of the soil depends on climate, landform, hydrology and management techniques employed. Janzen et al. (1992) said that soil quality must be expressed in terms of the soil to sustain plant growth or productivity. Productivity is a function of two interactive sets of variables: soil quality (intrinsic soil characteristics) and landscape quality (extrinsic factors such as climate. Soil health can be determined by the evaluation of soil properties that are subject to change when compared under the same landscape features. A list of some intrinsic and extrinsic factors are shown in Figure 1.

Implementation of the Conservation Reserve Program (CRP) in the United States after the Food Security Act (FSA) of 1985 has provided a large number of sites for monitoring changes in soil properties on highly erodible land (HEL) that was place in grass for 10 years. A large number of these sites will be placed back into production during the next few years. This provides farmers, with CRP land, the opportunity to collect baseline data (chemical, physical and bilogical properties) on their soils and then evaluate in later years how their management practices have influenced the quality of the soil. A number of CRP sites have been selected and monitored by North Dakota scientists for soil quality with results in the process of being published (Rosek et al., 199

Walker and Wang (1992) also implicated that benchmark sites in Canada must be established and monitored. These sites should cover a range of agro-ecosystems that can be broken out by major soils, physiographic region or farming region. This closely resembles the grouping of tillage management regions (TNR) proposed by Alimaras et al. (1985). There are currently four publications titled "Crop Residue Management To Reduce Erosion and Improve soil quality for the" 1) Northern Great Plains (Moldenhauer and Black, 1994), 2) Southern Great Plains (Stewart and Moldenhauer, 1994) , 3) Northwest (Papendick and Moldenhauer, 1995) and 4) Southeast (Langdale and Moldenhauer, 1995). This separation suggests that since tillage, crop rotation or residue management systems differ across the country, a different set of criteria may be required to evaluate soil quality for each designated region. Going one step further, individual farmers within the same region may use different management systems and the evaluation of soil quality may be more site specific.

The recent introduction of precision or site specific farming across the country has given us another new soil management tool (See articles by Reetz, 1994; Dealer Progress, 1995; Armstrong,

1996 for basic information on operations of these systems). Yield monitors, global position systems (GPS) and the ability to produce map overlays (GIS) may be used to identify or monitor changes in soil quality at any position in the field or landscape position (See Figure 2).

There is or at least appears in the literature to be no clear acceptable set of criteria to assess changes in soil quality. The list can be quite large (See Figure 1) or small but consensus is that a minimum data set is required. Current emphasis is being placed on ways to select or determine which factor or factors will adequately assess changes in the soil environment. Reganold and Palmer (1996) suggested that the method used to make soil measurements, gravimetric (weight) or volumetric, may produce different results. Some have suggested that here is a need or at least an attempt to standardize the soil quality measurements (Hortensius and Welling, 1996). John Doran (1994) has proposed the use of a field kit that can be used to easily measure certain soil quality indicators. This kit was designed for on-farm use. The kit gives instructions and contains the required equipment to measured bulk density, soil water content, water-filled pore space, EC, pH, N03-N, infiltration, water holding capacity, and soil respiration. These soil quality measurement procedures are outlined in published articles by Cramer (1994a, 1994b, 1994c)

It is interesting to note that soil scientists have been collecting data on soil properties for a long time. This information was used to evaluate different tillage or management systems. Certain soil properties like organic matter, bulk density, nutrient levels, aggregation, etc. were used to assess soil tilth, compaction, erosion, etc. Measuring soil properties was especially important during the middle 1980's when emphasis was placed on switching to conservation tillage and residue management systems to control erosion and/or meet government program guidelines related to the environment.

The importance of the soil physical properties for comparing tillage systems was previously discussed by Deibert (1983). The emphasis has not changed, just the name, now referred to as soil quality or soil health. It seems that non soil scientists have finally taken an interest in what is going on below the soil surface, something they can no longer ignore when using a system approach. Farmers also have gone back to viewing the soil as having value with the soil needing the same respect and consideration as was always given to those management areas above the soil surface. As heard at one previous tillage meeting "farmers are once again wearing out the knees rather than the seat of their pants". Other scientists have finally realized that the nutrients that come from the soil and the processes that influence their availability can have a major impact on the quality of the seed that ends up in the food chain. The quality of the seed has already played a major role as it is distributed and sold in a global market.

It is nearly impossible, in the scope of this paper, to cover all the chemical, physical and biological factors that can be used as a tool to measure soil quality. The paper by Kennedy and Papendick (1995) covers the microbial characteristics of soil quality. A summary of how farmers view soil health and quality is covered by Romig et al. (1995). Organic matter, crop appearance, earthworms, erosion and tillage ease were the top five factors given by farmers for rating soil health. Cihacek et al. (1996) summarized the linkages between soil quality, essential nutrients and nutrition or health of plants, animals and humans. Recent papers discuss the influence of tillage systems and Conservation Reserve Program (CRP) on the different quality factors associated with chemical, physical and biological properties in the soil (Doran et al., 1996; Deibert, 1995; Karlen et al., 1994; Rosek et al. , l99-)

SOIL AGGREGATION AS A QUALITY INDICATOR

This paper will focus on soil aggregation because changes in this one property, due to modifications in management practices such as tillage or rotation, influence 1) soil tilth, 2) control or dictate soil erosion processes, 3) directly or indirectly influence nutrient release, physical restrictions or water availability and biological activity in the soil and 4) regulate plant growth processes affected by root growth. This soil property is also greatly changed when switching to conservation or notill systems, thus impacting soil quality. Soil is comprised of primary and secondary aggregates with primary aggregates molded together to form secondary aggregates which give soil good tilth and/or soil structure. Native gr~55 50115 have excellent ~oii ~ggreg~tiQn

Deibert et al. (1981) , Douglas and Gross (1982) and Weill et al. (1989) reported increased soil aggregation and/or stability with reduced tillage systems. Lynch and Bragg (1985) indicated that a well aggregated soil resulting from microbial activity allowed a variety of pore sizes. Small pores influence water holding capacity and large pores enhance infiltration. Although surface aggregates contained higher levels of organic C, Wiebe et al. (1994) reported lower aggregate stability of the 0.25-0.85 mm size fractions with zero tillage compared to short and long term conventional till. A comparison of long term conventional and alternative farming systems from documented farm fields in Iowa showed significantly higher aggregate stability of soils on alternative farms (Jordahl and Karlen, 1993).

Kirkby et al. (1996) proposed that the processes that control soil aggregation are determined by precipitation (Figure 3). In dryland areas where precipitation is 0 to 400 mm (0 to 16 inches) , soil aggregation processes are controlled by clay-salt interactions (30% aggregation) . At precipitation levels 300-700 mm( 12 to 28 inches) , aggregation is dominated by soil organic matter dynamics and/or microbial activity (30 to 60%). At precipitation from 500-900 mm ( 20 to 35 inches) , soil aggregation is dominated by earthworm dynamics. If this is true, under normal conventional tillage systems, low soil aggregation would be found in the Northern Great Plains (annual rainfall of 12 to 20 inches) . Most of the aggregation would be controlled by clays-salts with only a small amount due to microbial-organic dynamics and little by earthworm activity.

Earthworms have an enormous effect on the physical condition of the soil, especially aggregation (Lee, 1985). Earthworm populations are influenced by climate and management practices. Jamieson (1974) suggested that earthworms are not likely to be found in regions where annual precipitation is less than 600 mm (24 inches) . Utter et al. (1995) found earthworms in areas of North Dakota where annual precipitation is around 12 inches where notill practices were used. Deibert and Utter (1994) found that earthworm populations were also related to different management practices.

Deibert (1996) proposed that earthworms are a good indicator of soil quality because they impact other soil properties or soil quality indicators. This idea is significant because, as we move into conservation tillage systems like notill or zero-till, evaporation is reduced, infiltration is increased and water content of the soil increases with far less precipitation. The switch to a management practice that leaves residue on the surface causes a shift to the left (Compare Figure 4 with Figure 3) in the percent of aggregation created by microbial and earthworm activity in a given precipitation zone. By creating a better soil environment with conservation tillage systems, greater aggregation is achieved and thus a better quality soil is maintained in dryland areas of the Northern Great Plains.

MEASURING SOIL AGGREGATION

Size and stability of the secondary aggregates are important physical characteristics used to evaluate the soil. Detailed laboratory procedures are available for exactly measuring these characteristics. Quality of the soil can be inferred based on the status of the aggregates. Any change in either aggregate characteristic, positive or negative, due to changes in management will determine the direction the quality of the soil is going.

Information on aggregates, either size or stability, can be determined without detailed procedures by six simple steps. First, select a time of year (spring or fall) , a specific landscape position or soil type in the field and a specific crop or surface residue. Designate this period as time zero (start or end of a tillage or rotation sequence) . Second, remove surface residue and collect soil from the surface 1 to 2 inches with a flat spade at 4 to 5 locations at the site and combine until 4 to 5 lbs have been collected. Third, spread out the soil in a low flat cardboard box (pop or beer flat) and remove large pieces of residue and rocks. Let the soil air dry (24 to 48 hours) , just like drying a sample for soil testing. Fourth, obtain some screen (8 to 10 mesh) and either make a gallon size wire basket out of screen with wire handle for determining aggregate stability or make a flat screen (8 to 10 inch square or round diameter) for determining aggregate size. Fifth, weigh out 1/2 lb (220 grams) of air dry soil. For determining aggregate stability, place weighed soil in wire basket, fill a five gallon bucket about 3/4 full of water. Place the basket with soil in th~ bucket with water and allow to settle slowly until the soil is covered with water. Move the basket up and down slowly for a set number of times (20 to 30 times) , being sure the soil is always under water. Transfer the soil (aggregates) left in the basket onto a drying cardboard flat and allow to air dry. For determining aggregate size, place the weighed soil on top of the flat screen. Then proceed to shake the screen back and forth a set number of times (50 to 6Q times) . Weigh the soil (aggregates) on top of the screen. Sixth, the percent aggregate stability or aggregate size is determined by dividing the amount of aggregates in the basket or on top of the screen by the dry weight of the original weighed soil sample. Best results can be obtained by repeating the sieving operation 2 to 3 more times. Keep a record of the results. Seventh, the first six steps in the procedure are repeated at another period in time under the same conditions (time, site, soil, residue) . Soil quality is improving if aggregation increases while soil quality is degrading if aggregation decreases. One can measure aggregation from a similar grassland site for comparison or as an aggregation goal to achieve for the best quality soil.

SUMMARY

Determining the health or quality of the soil is not a new idea, only the name has been changed to focus on an area that will appease the environmentalist, non-scientist or scientist who has finally realized that we need sustainable soils to maintain quality food for the future. Fluctuating temperatures and precipitation extremes will always alter the chemical, physical and biological properties of the soil. To understand the relationship between climatic variations, we must first understand how soil properties behave at a specific site under a set of tillage-management conditions. Soil properties or soil health factors used to determine quality of a soil are numerous. No one soil property can be used as a universal index of soil quality since some are more dynamic at one location while static at another location. A switch from conventional tillage to conservation will have a major impact on soil dynamics. Although aggregation is not an easy soil property to measure, it still remains one of the best indicators of soil quality. Farmers and researchers need to select an indicator like aggregation or at the least a property, whether chemical, physical or biological, that best indicates soil degradation processes at a specific site and then infer the quality of the soil at some point in time. As higher values are placed on our soils, the need to monitor changes in soil quality will become more important and easily achievable as we move into the next era of precision agriculture aimed at maintaining the sustainability of our soils.Figure 1. Soil And Landscape Quality Factors Influencing Soil Productivity (Deibert, modified from Janzen et al, 1992).

Physical	Climate
bulk density	precipitation
aggregation	temperature
resistance	growing season
water content/retention	heat units
infiltration rate	evaporation
hydraulic conductivity	humidity
temperature	wind
porosity	freeze-thaw cycles
particle size	wet-dry cycles
Chemical	Topography
organic carbon	percent slope
electrical conductivity	elevation
pH	slope length
cation exchange capacity	slope aspect
base saturation	shape
available nutrients	Hydrology
total nutrients	water storage
Biological	water runoff
respiration	drainage
enzyme activity	leaching
earthworms
micro fauna
decomposition
mineralization
microbial biomass

Figure 1. Soil and landscape quality factors influencing soil productivity (Deibert, modified from Janzen et al. 1992).

Figure 2. Geographical Information System (GIS) map overlays of quality indicators from Global Positioning System (GPS) sites on the landscape (Deibert).

Figure 3. Processes dominating soil aggregation in conventional tillage systems as influenced by annual precipitation (Deibert, modified from Kirkby et al., 1996)

Figure 4. Processes dominating soil aggregation in conservation tillage systems as influenced by annual precipitation (Deibert).

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