The U.S. Department of Agriculture (USDA) NOP soil fertility and crop nutrient management practice standard (§205.203) requires agriculture production systems 1) to “select and implement tillage and cultivation practices that maintain or improve the physical, chemical, and biological condition of soil and minimize soil erosion”; 2) to “manage crop nutrients and soil fertility through rotations, cover crops, and the application of plant and animal materials”; and 3) to “manage plant and animal materials to maintain or improve soil organic matter content in a manner that does not contribute to contamination of crops, soil, or water by plant nutrients, pathogenic organisms, heavy metals, or residues of prohibited substances” (USDA Agricultural Marketing Service, 2012). Many studies support opinions held by producers that increases in SOM enhance physical and chemical soil properties and management practices that favor the aggradation of SOM result in plant growth and productivity benefits (Kimble, 2007). Merwin et al. (1994) suggested comparisons between organic, or integrated orchard management systems, and conventional orchard management practices may reveal measurable differences in SOM levels. Qualitative characterizations of conventionally managed orchard soils reflect lower soil quality for the soil ecosystem than for organic or integrated systems (Reganold et al., 2001), whereas greater soil quality in organic and integrated systems is attributed to organic residue additions to the soil surface through the use of various GMS. Such GMS provide continuous additions of SOM to the soil and likely have a positive effect on overall soil quality in organic orchards.
The addition of SOM to mineral soil is a recognized beneficial practice from the standpoint of increasing the capacity of the soil to store plant-available water (Bhogal et al., 2009; Hudson, 1994; Jordán et al., 2010; Mulumba and Lal, 2007). Organic constituents from crop residues, manures, and composts tend to absorb water as water moves through a soil profile, thus enhancing the soil’s water-holding capacity.
Likewise, applications of organic material, including applied mulches, incorporation of green manure crops, livestock manure, and compost, increase soil porosity and reduce soil bulk density (BD) and potential compaction (Celik et al., 2010; Jordán et al., 2010; Soane, 1990; Stock and Downes, 2008). Soil BD is related to other soil physical properties and has been described as an indicator of soil and environmental quality (Arshad and Coen, 1992; Doran et al., 1996; Lal and Kimble, 2001). Decreases in soil BD may be achieved as soil aggregation improves, where the beneficial aspects of well-aggregated soils include protection of SOM within the aggregate (Tisdall and Oades, 1982), increased diversity of the soil microbial community (Fliebach et al., 2006), enhanced soil, air, and water movement (Deurer et al., 2009), and a reduction of surface crusting, runoff, and soil erosion (Carter, 2002; Karlen et al., 1992; Kemper and Rosenau, 1986).
Surface crusting and erosion may be reduced or eliminated in orchards with application of non-living groundcover mulches. Mulches useful for organic apple (Malus × domestica Borkh.) production include woodchips, municipal green compost, shredded paper, and mow-blow green mulch (Rom et al., 2010). Other researchers have used hay-straw and living groundcovers, including white clover (Trifolium repens L.) and red clover (Trifolium pratense L.), all of which contribute to SOM reserves (Granatstein and Mullinix, 2008; Merwin et al., 1994, 1995, 1999; Sanchez et al., 2003) and favor formation of water-stable soil aggregates.
An important constituent for binding aggregates into water-stable forms and supporting maintenance and growth of a healthy soil food web is carbon (C). Soil aggregates are instrumental in storing and protecting C mineralized from decomposed residues. Carbon in soil aggregates exists in a variety of forms, from decomposing, labile particulate organic matter bound into aggregates by fine roots and hyphae to stable, humidified plant residues occluded within small microaggregates and unavailable to the soil microbial community (Kay, 1998; Tisdall and Oades, 1982). Furthermore, polysaccharides and mucilages exuded from soil micro-organisms may also be tightly adsorbed onto mineral soil particles, thereby strengthening aggregate fracture zones and decreasing the potential for slaking (Kay, 1998).
Tisdall and Oades (1982) categorized soil aggregates as either macroaggregates (i.e., greater than 0.25 mm diameter) or microaggregates (i.e., less than 0.25 mm diameter). Macroaggregates are usually bound by plant roots and fungal hyphae and tend to decline in number as SOM declines (Jastrow and Miller, 1991; Karlen et al., 1992; Tisdall and Oades, 1982). Microaggregates are fixed by polysaccharides and organo-mineral complexes to form relatively stable structures largely unaffected by soil management practices (Six et al., 2004; Tisdall and Oades, 1982).
The porous nature of well-aggregated soils affects the rate at which water enters the soil profile (i.e., infiltration) and the extent to which water can enter a soil impacts the amount of water runoff. Greater runoff increases the erosion potential (Lado et al., 2004; Le Bissonnais, 1996; Stern et al., 1991; Wakindiki and Ben-Hur, 2002) and decreases the water that can be stored in the soil for plant growth (Merwin et al., 1994).
To reduce water droplet impact and promote the formation of stable aggregates, land management strategies that concentrate residues at the soil surface are often recommended, and greater water-stable aggregate content can improve infiltration by slowing crust formation during a rain or irrigation event (Albrecht and Sosne, 1944; Boyle et al., 1989; Freebairn et al., 1991; Lal, 1993; Le Bissonnais, 1996; Le Bissonnais and Arrouays, 1997). The arrangement of macroaggregates with respect to one another creates macropores (Deurer et al., 2009), and pore size and pore volume per volume of soil dictate the rate at which water can infiltrate into the soil (Arshad and Coen, 1992).
Studies have shown application of mulches in apple orchards increased infiltration rates (Goh et al., 2001; Granatstein and Mullinix, 2008) and soil aggregate stability (Deurer et al., 2008). Mulches provide additional benefit of permitting water infiltration into the soil and greater retention as a result of reductions in evaporation, thereby reducing irrigation requirements (Granatstein and Mullinix, 2008). After 6 years of orchard research, Merwin et al. (1994) documented decreased SOM and water infiltration in plots treated with pre-emergence herbicides and tillage as compared with those managed with living and inert mulches.
The southeastern United States has experienced increased interest in orchard establishment and fruit production, especially small-scale and organically managed farms. A considerable amount of information is available on the efficacy and suitability of using GMS systems as an orchard floor management tool in other, more northern regions of the United States, but no research exists that shows their effects on physical properties of the old, highly weathered soils of the Ozarks Highlands and similar regions. Furthermore, the impact of organic nutrient sources on organically managed orchard soils in the Ozark Highlands or south has not been documented. Therefore, the objectives of this study were 1) to evaluate the impact of groundcover management system and nutrient source on soil quality-related variables such as SOM, BD, plant-available water, water-stable aggregation, saturated hydraulic conductivity, and water infiltration in an organically managed apple orchard; and 2) to qualitatively compare soil quality in an organic apple orchard with those to an adjacent conventionally managed orchard on a highly weathered soil in the Ozark Highlands region of northwest Arkansas. It was hypothesized that use of GMS that increase SOM will also increase water-stable soil aggregation, which will lead to an overall increase in water infiltration and plant-available water. It was further hypothesized that organic nutrient sources will enhance SOM, water-stable aggregation, infiltration, and water-holding capacity and result in decreased BD.
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