(1) To physically support the growing plant

(2) To store and supply nutrients needed by the plant

(3) To store and supply water for the crop

(4) To allow for the exchange of air in the rooting zone

Soil quality refers to the Package of chemical, physical and biological factors which influence the ability of the soil to fulfill these functions. Tillage directly influences the soil physical properties of the soil, which impact on the chemical and biological characteristics of the soil.

Under reduced tillage, there are normally a number of factors which act in combination on the soil-plant system. Crop residue left on the soil surface reduces the contact between the soil and the raw organic matter. Presence of crop residue on the soil surface leads to a moderating effect on soil temperature, so the soil warms up more slowly in the spring (Carefoot et al.1990, Cox et al. 1990). The presence of the mulch and the increased snow trapping the standing stubble over winter leads to higher soil temperatures during the winter months (Gauer et al. 1980). The mulch reduces evaporation and may increase water retention, so soil moisture content is frequently higher under reduced tillage than under conventional tillage (Carefoot et al. 1990). Tillage also loosens and "fluffs" the soil, which increases pore space, breaks down soil aggregates and dries the soil. Removal of tillage leads to a denser, moister soil. The effects of crop residue remaining on the soil surface, lower soil temperature, increased soil moisture, and physical effects on the soil. The soil also allows for root penetration through the profile, which allows for nutrient and water uptake from the soil and physical support for the growing plant. A well-aggregated soil also resists surface crusting and is less subject to wind and water erosion than poorly aggregated soil.

Aggregation and aggregate stability tend to increase with increased organic matter content and may decrease with tillage, particularly if the tillage occurs at the wrong time. Aggregation tends to improve under a no-till system as compared to under conventional tillage (Table 1) (McFarland et al. 1990). Cultivation tends to reduce the number of soil macroaggregates (size >250 pm), which are stabilized by fungal hyphae and plant roots (Doran and Smith 1987,Tisdall and Oades 1982).

Table 1: Soil microaggregation at two depths in 1988 as influenced by cropping sequence and tillage (McFarland et al. 1990).

Means within a cropping sequence and depth f ollowed by the same letter do not differ at the 5% level of significance. Lack of letters indicates no significant difference.

Structure will interact to influence a wide range of chemical, biological and physical properties within the soil.

Physical properties of the soil which are influenced by tillage include aggregation, compaction, and organic matter content. These factors are interrelated and in turn influence other quality characteristics such as water-holding capacity of the soil, infiltration and internal drainage, aeration, microbial activity and soil erosivity. Overall, this directly influences crop growth, rooting, nutrient supply and sustainability of the soil resource.

AGGREGATION AND AGGREGATE STABILITY

The physical quality of the soil depends on the ability for the soil sand, silt, clay and organic matter particles to coalesce in stable crumbs, called aggregates. In a natural system, aggregation of a soil is influenced by the soil forming processes of parent material, climate, vegetation, relief, and time. However, agricultural practices and tillage in particular will have immediate effects on soil aggregation.

A well-aggregated soil is referred to as being "mellow" or "friable" and having good "tilth", while poorly aggregated soils may have problems with seedling emergence due to surface crusting, poor water infiltration, reduced aeration and water movement and restricted root growth due to natural compaction. A well-aggregated soil has an ample number of pores of varying size. The larger pores drain freely, allowing excess water to move from the soil surface deeper into the soil profile. After draining, these larger pores form passages for root growth and air exchange. The smaller pores tend to hold water in a continuous interconnected capillary system. This serves as both a system for the movement of nutrients and a reservoir of water which can be extracted by the plant roots during the growing season. The porous structure of

ORGANIC MATTER

Soil organic matter concentration is an important indicator of soil quality, as it influences soil fertility, soil tilth, aggregation, aeration, water infiltration and storage, and microbial activity. The steady state level of soil organic matter is determined by the physical, chemical and biological conditions in the soil which in turn influence! microbial activity. Tillage influences a number of factors which regulate biological activity and organic matter breakdown. These factors include water, temperature, air, position of organic material in the soil, and the physical environment for growth of roots and microorganisms. Organic matter loss from the soil is reduced by reducing tillage intensity and increasing the surface residue, or trash cover. Under a reduced tillage system, crop residues left on the soil surface reflect light and insulate the soil (Doran and Smith 1987). This reduces soil temperature during the early part of the growing season. Lower soil temperatures will slow microbial activity and reduce organic matter loss. The mulch also leads to increased water retention and reduced evaporative losses so soil moisture content generally increases. The higher soil moisture may either speed organic matter breakdown, in a relatively arid soil climate, or slow breakdown, if it leads to water logging and reduced aeration in a relatively moist soil climate. This is because biological activity of microorganisms is strongly influenced by the balance between water and oxygen. Aerobic microbial activity increases with water filled pore space to a maximum at 60%. Above this level, aerobic activity declines and anaerobic activity, such as denitrification, increases (Mielke et al. 1986). Therefore, optimum waterfilled pore space for biological activity is at about 60%. Under reduced tillage, the water-filled pore space may be higher than this, due to the higher water content and the increased proportion of smaller pore. This may reduce aeration and microbial decomposition of organic matter.

Under reduced tillage, the raw organic matter left on the soil surface is less accessible to soil microorganisms and the microclimate is less favorable for decomposition than conditions below the soil surface. In addition, tillage breaks down soil aggregates. Organic matter that is occluded within macroaggregates is protected from decomposition. Tillage exposes this "protected" organic matter and leads to its decomposition (Doran and Smith

1987).

Due to the combination of all of these factors, intensive tillage generally speeds the breakdown of organic matter, leading to reduction in soil organic matter. This is why, in the past, tillage was used to enhance the release of nutrients for crop production, leading in part to the popularity of surnmerfallow prior to the advent of chemical fertilizers. Reduced tillage will slow, or even reverse, the loss of organic matter (Havlin et. al. 1990). For example, in an experiment comparing zero- and conventional-tillage, after 10 years of no-till, surface C and N content of the soil were about 25% higher than that plots that had been conventionally tilled. Both the quality and the quantity of organic matter were greater under no-tillage than under conventional tillage (Table 2.) (Arshad et al. 1990). Organic matter content in the surface 75 cm of soils was found to be higher in no-tilled as compared to conventionally tilled soils in studies reported by Mielke et al. (1986), but in the 75 to 150 cm depth, organic matter content was higher in the tilled soils.

Table 2. General characteristics of soils under different tillage systems.

COMPACTION

Compaction in a soil can be caused by implement traffic or by natural settling of the soil. The soil particles become more tightly packed, leading to less room for air and water movement, water storage and root growth. Measurements of compaction include penetration resistance, which is the amount of force required to push a standard diameter rod into the soil, and bulk density, the mass of a specific volume of soil. Both of these measurements tend to indicate how closely the soil particles are packed together.

Problems with excess compaction include restricted root growth and reduced movement of air and water within the soil. The reduced air and water movement occurs because compaction reduces the number and size of pores within the soil. Under reduced tillage, there is usually less total pore space, with more fine pores and fewer air-filled pores (Ehlers 1973, Van Ouwerkerk and Boone 1970). Cox et al. (1990) observed that corn growth was reduced under a no-till system and indicated that the lower yields were partly due to restricted rooting caused by the increased mechanical resistance in the soil, which led to higher water stress early in the growing season in no-till.

Under no-tillage, the channels and pores from root growth and insect and worm activity in previous years are not disturbed by the tillage process. The pore system is left more intact, leaving continuous macropores, through which the roots may grow, or which may act as channels for air and water movement. Water and air movement may be better at a similar bulk density under zero tillage as compared to conventional tillage due to the network of continuous macropores. Although both penetration resistance and bulk density in the surface layers of soil tend to increase under reduced tillage, the negative effect on root growth may not be as severe under no-tillage as it would be at an equivalent compaction measurement under conventional tillage system.

There tends to be a strong inverse relationship between bulk density of a soil and soil organic matter (Mielke et al. 1986). As the length of time that a soil has been cultivated increases, soil organic matter decreases and soil bulk density increases. When a soil is placed under no-tillage management, the surface soil layers tend to become more compacted (Ehlers et al. 1983, Grant and Lafond 1991). This is particularly true under coarse-textured soils such as sands or sandy loams, which can attain a higher, maximum density than can finer-textured soils (Marshall and Holmes 1979). On clay or clay-loam soils, swelling and shrinking from the clay component can help to alleviate compaction problems. However, in the deeper soil zones, compaction is generally no greater under reduced as compared to conventional tillage, and may be lower (Table 3) (Grant and Lafond 1991, Doran and Smith 1987 Mielke et al. 1986).

Table 3. Effect of tillage on penetration resistance (KPa) after winter wheat production, averaged over three soil depths and 3 crop rotations (Grant and Lafond 1991).

AERATION

Adequate air movement within the soil is important in crop production. Crop roots require oxygen for respiration to provide the energy for root growth and nutrient uptake. Carbon dioxide is formed during respiration and must be

removed from the roots. Therefore, If aeration is restricted, root growth and nutrient uptake will be restricted. Nitrogen, ammonia, water vapor and fumigants also move as gases in the soil and are thus influenced by tillage (Marshall and Holmes 1979). Microorganisms as well as plants are influenced by aeration of the soil, with some flourishing where oxygen supply is adequate and others preferring anaerobic or oxygen-free conditions.

As was mentioned in the discussion on compaction, the surface soil under no-tillage may have a lower total pore space, smaller pores and fewer air-filled pores under conventional tillage. This leads to poorer aeration in the surface soil (Table 4) (Mielke et al. 1986). This would be of more concern under conditions where the soil tends to be relatively moist. In arid agricultural areas, such as the Canadian prairies and the U.S. Great Plains, aeration may be restricted in depressional areas, during the spring, after rainfall events, or on heavy textured soils which remain wet for a long period of time. However, it may be that the network of root and worm channels which are left undisturbed under no-tillage situations may aid in aeration, counteracting some of the potential problems of eliminating tillage.

Table 4. Air permeability of notified and slowed soils under wheat cropping (Mielke et al. 1986).

WATER STORAGE AND MOVEMENT

Infiltration of water into a soil is affected by five major, interrelated factors (Meek et al. 1990):

1. Bulk density
2. Aggregation
3. Surface sealing
4. Macropores
5. Pans, or restrictive layers within the soil

Mielke et al. (1986) reported that water movement within the soil was lower in no-tilled than conventionally tilled soils. This is related to the reduced porosity in the soils. However, in some soils no-till systems may increase infiltration rate over time, by maintaining a continuous system of macropores, which encourage water movement. Macropores play a major role in water movement, and serve as channels for root growth and for solute movement (Ankeny et al.1990). No-till may also reduce surface sealing by maintaining trash cover on the surface. This would reduce crusting and enhance water infiltration. Starr (1990) observed that infiltration in a plowed system continuously decreased during the growing season, likely du e to increasing bulk density, destruction of aggregates and puddling at the soil surface. This did not occur under reduced tillage, likely because the soil surface had more protection from raindrop impact because of the crop residues and soil aggregates may have been more stable due to higher organic matter content. Although bulk density increased under reduced tillage, there was not a coincident decrease in infiltration, likely due to the presence of macropores from root and worm channels. Ankeny et al. (1990) found that tillage itself had little effect on infiltration rates, but that wheel traffic from implements reduced infiltration more on conventionally than no-tilled soils (Fig. 1).

The amount of water that is held in the soil, is influenced by the total storage capacity of the soil, and by the proportion of that total capacity that is filled. You may have a large fuel tank on your truck, but you still need to fill it up before you can go anywhere. The water-holding capacity of a soil is related to the total pore space, and tends to be lower under no-till than conventionally tilled systems. However, in a situation where water is limited through the growing season, as normally is the case in the prairies-Great Plains area, the amount of recharge may be more important on most soils than the water-holding capacity.

Reduced tillage systems have a greater ability to conserve moisture as compared to conventional tillage systems. Standing stubble increases the depth of snow available for spring moisture recharge and also reduces surface sealing. Water movement into the soil may be increased under no-till since root and earthworm channels are retained. These beneficial effects of no-till tend to remain through the growing season. In contrast, while tillage may increase infiltration briefly, the beneficial effects are usually temporary and infiltration may decline as the soil settles and crusting occurs. In addition, the cooler soil temperatures found under trash covers reduce moisture loss by evaporation. As well as preserving moisture or crop growth, reduced evaporation will help prevent an increase in surface salinity, as the salts will not be drawn to the soil surface by the evaporative movement of water. So, although the total amount of water that can be held by the soil under a reduced tillage system may be similar or lower than that under conventional tillage, the likelihood of it being recharged is greater under reduced tillage.

Numerous studies, including those by Lafond (1991) at Indian Head have shown that soil moisture and efficiency of moisture use tended to be higher under reduced tillage systems than under conventional tillage (Table 5). This is especially true during early season growth, before the crop canopy has been fully established. The higher soil moisture curing stand establishment may be particularly important. In an environment like the prairie-Great Plains region, the increased efficiency of moisture use is a definite advantage for no-tillage systems.

Table 5. The effects of tillage systems on total spring soil water (cm) averaged over 4 years.

BIOLOGICAL ACTIVITY

Microbial activity in the soil can be limited by poor aeration and flooding or by too little water. Aeration will be a limitation more frequently in reduced tillage systems than under conventional tillage, since reduced tillage systems tend to have less total porosity and a greater proportion of water-filled pores than do no-tilled soils. Also, no-tilled soils tend to have a higher water content than conventionally tilled soils, which would again decrease air-filled pore space. Dry conditions would limit microbial activity more frequently under conventional tillage than under no-till, since conventionally tilled soils tend to dry out more quickly. Since moisture is usually deficient, rather than excessive on the arid soils in the U.S. Great Plains and Canadian prairies, biological activity is likely to be improved under reduced tillage as compared to conventional tillage systems (Power 1990). In studies done in Saskatchewan, microbial biomass, or the amount of microorganisms present in the soil, increased under reduced tillage as compared to conventional tillage (Carter and Rennie 1982). Although tillage causes a short-term increase in microbial population, this dies back as the food supply for the organisms decreases and the soil dries. During the growing season, the microbial population in the surface 10 cm under reduced tillage is usually higher than under conventional tillage (Doran 1980). Doran (1980) attributes the higher microbial population to the higher moisture level available under reduced tillage.

SOIL ERODIBILITY

Unlike other changes which tillage causes to the soil, erosion loss is a permanent change, which causes a decrease in soil productivity. It has been estimated that a loss of one inch of topsoil can reduce wheat yields by 1.5 to 3.4 bushels per acre (Sparrow 1984). Simulated erosion studies, where the topsoil was removed mechanically, have shown that fertilizer can overcome the decrease in productivity of up to a 20 cm loss of topsoil on a clay soil. On a sandy soil, however, no amount of fertilizer could replace the productivity loss caused by topsoil removal (Kenyon 1987).

Soil loss can occur during a rainstorm, particularly when the soil is already wet and when the rainfall is intense, such as during a thundershower. Minimum tillage can reduce erosion, by maintaining a greater trash cover, which physically protects the soil, increases infiltration and decreases the speed of volume of surface run off. The increased aggregation commonly noted under reduced tillage reduces the sensitivity of the soil to erosion losses and can reduce crusting which can decreased the volume of surface run-off, lowering water erosion risk (Blevins et al. 1910). Rainfall erosion studies in Manitoba include a comparison between minimum and conventional tillage. No-till losses, although not included in this study, could be somewhere between alfalfa and minimum tillage. For example, for one rainstorm in 1990, 1.7 inches of rain resulted in a soil loss of 0.05 T ha^-1 for alfalfa, 1.81 T ha^-1 for minimum tillage wheat, 2.06 T ha^-1 for conventionally tillage wheat, 10.37 kg ha^-1 and 13.16 kg ha^-1 for corn (Shaykewich 1991). This was on a highly erodable sandy loam soil.

Soil loss which occurs during the snow melt period can also be a factor. Studies in the Peace River area indicate fall tillage can cause soil loss to be nearly 190% higher in the spring when snow melt occurs (Chanasyk and Woytowich 1987). No-tillage could almost eliminate these losses.

Sandy loams and loamy soils, which are the most prone to water erosion, are also the soils most prone to soil loss by wind. The critical period for erosion control is early to mid spring, prior to and just following planting. Until the stand is established, the soil surface is left unprotected from the winds. Standing stubble serves to physically protect the soil from the wind, while the beneficial effect of no-tillage on soil aggregation leaves the soil itself in a form less sensitive to erosion.

SUMMARY

The effect of reduced tillage on soil quality depends largely on the soil and climatic conditions in which the system is operating. Reducing tillage intensity generally has the beneficial effect of increasing soil aggregation and soil organic matter content, while reducing soil erodibility. Soil moisture storage generally increases under reduced tillage, which is commonly beneficial in the Canadian prairies and the Great Plains region of the United States, where moisture restrictions frequently limit crop yield. Compaction frequently increases under reduced tillage, which may lead to problems with root penetration, aeration and water movement. However, in some cases compaction decreases with reduced tillage due to the reduction in traffic. The negative effects of compaction may be reduced because root and worm macropores were retained when tillage was eliminated. As with compaction, water infiltration showed variable results, sometimes increasing and sometimes decreasing with reduced tillage. Reduced tillage may have an advantage in terms of infiltration where poor soil aggregation leads to problems with surface crusting and excessive settling of the soil during the growing season. Although reduced aeration and the lower soil temperatures observed under no-tillage may reduce biological activity, the higher soil moisture and decreased death loss from soil mixing may increase activity on the dry soils of the prairies.

In general, where soil moisture tends to be limited, reduced tillage is likely to have an overall beneficial effect on soil quality and could enhance the sustainability of soil productivity over time.

REFERENCES

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Fig. 1.