Jeffrey P. Mitchell

A comparison of soil quality indexing methods for vegetable production systems in Northern California

Abstract Consultants, farm advisors, resource conservationists, and other land managers may benefit from decision tools that help identify the most sustainable management practices. Indices of soil quality (SQIs) can provide this service. Various methods were tested for choosing a minimum data set (MDS), transforming the indicators, and calculating indices using data from alternative vegetable production systems being evaluated near Davis, California. The MDS components were chosen using expert opinion (EO) or principal components analysis (PCA) as a data reduction technique. Multiple regressions of the MDS indicators (as independent variables) against indicators representing management goals (as iterative dependent variables) showed no significant differences between the EO and PCA selection techniques in their abilities to explain variability within each sustainable management goal. Linear and non-linear scoring techniques were also compared for MDS indicators. The non-linear scoring method was determined to be more representative of system function than the linear method. Finally, indicator scores were combined using either an additive index, a weighted additive index, or a decision support system. For almost all indexing combinations, the organic system received significantly higher SQI values than the low input or conventional treatments. The efficacy of the indices was tested by comparisons with individual indicators, variables representative of management goals, and another multivariate technique for decision making that used all available data rather than a subset (MDS). Comparison with the comprehensive multivariate technique showed results similar to all of the indexing combinations except the additive and weighted indices using the linearly scored, EO-selected MDS. This suggests that a small number of carefully chosen soil quality indicators, when used in a simple, non-linearly scored index, can adequately provide information needed for selection of best management practices.

On-Farm Assessment of Soil Quality in California's Central Valley

Program, 1990; Mitchell et al., 1999). Mitchell et al. (1999) also reported a perceived decline in soil quality The high-value, large-scale crop production systems in the San among producers. As a result of these concerns, many Joaquin Valley (SJV) of California typically entail intensive tillage and large fertilizer and water inputs but few C additions to the soil. SJV producers have begun to question the long-term Such practices often contribute to a decline in soil quality. Our objecsustainability of their intensively managed agricultural tive for this participatory study was to examine the effects of supplesystems. mental C management practices (SCMPs) on various soil quality To help farmers in the SJV evaluate the soil quality indicators. To increase farmer participation, we conducted the study effects of alternative soil management practices, the West on farms using a variety of SCMPs, including cover crops, compost and Side On-Farm Demonstration Project (WSD) was conmanure amendments, and several different crop rotations common ducted from 1995 to 1998. This participatory research to the region. The SCMPs significantly changed a number of soil and extension program originally included 11 large-scale properties, including soil organic matter (SOM); total Kjeldahl N; SJV row-crop producers, University of California Coopmicrobial biomass C and N; exchangeable K; Olsen P; and extractable erative Extension researchers, USDA Natural Resources Fe, Mn, and Zn. A comparison including previously established, adjacent organic, conventional, and transitional fields in addition to the Conservation Service (NRCS) conservationists, USDAtreatment fields at one farm revealed significant differences in 16 of ARS scientists, and private-sector consultants. 18 soil quality indicators. A soil quality index computed for this farm Developing science-based guidelines to quantify imscored the established organic system significantly higher than the pacts of routinely used organic inputs in this region was conventional system. Our results suggest that significant changes in identified as an important priority among the project’s several soil quality indicators occur with a variety of SCMPs. This is farmer participants (Mitchell and Goodell, 1996). A especially noteworthy considering the intensive tillage, irrigation, and brief, written survey of 15 participants, conducted durhot, semiarid environment of the SJV, California, where increases in ing a routine project meeting, invited input about their SOM and related soil properties are generally not expected in a 3-yr interest in an indexing tool to evaluate soil quality (sensu study. Andrews and Carroll, 2001; Karlen et al., 1998). Fourteen of the respondents indicated that a soil quality assessment tool would be useful to compare manageW Fresno County in the San Joaquin Valley ment alternatives (one blank response) (S.S. Andrews, (SJV) of California is one of the world’s most J.P. Mitchell, and D.L. Karlen, unpublished data, 1999). productive agricultural regions. Farmers in this area Based on that level of participatory support, our project produce more than one-third of the county’s annual

Impacts of cropping systems on soil nitrogen storage and loss

3 objectives were to (i) facilitate information exchange billion agricultural output, making it the highest reveamong farmers, consultants, and researchers regarding nue-producing county in the USA (California Dep. of these soil management practices; (ii) monitor and evaluFood and Agric., 1997). Dominant crop rotations inate on-farm, side-by-side comparisons of various SCMPs; clude annual crops (Mitchell et al., 1999) such as proand (iii) demonstrate the use of a soil quality index cessing tomato (Lycopersicon esculentum L.), cotton (SQI) for the region. (Gossypium hirsutum L.), onion (Allium cepa L.), garlic (A. sativum L.), cantaloupe (Cucumis melo L. var. reticulatus Naud.), wheat (Triticum aestivum L.), sugarbeet MATERIALS AND METHODS (Beta vulgaris L.), and lettuce (Lactuca sativa L.). Site Descriptions The intense production practices used in this region Side-by-side comparisons of conventional and organicinclude frequent and intensive tillage, irrigation, and based production systems were established on 11 farms in extensive use of fertilizers and pesticides but few addiautumn 1995. The farms were located in the western SJV tions of organic amendments to the soil (Mitchell et al., between Mendota and Huron, CA. At the beginning of the 1999). These intensive practices have raised concerns project, we randomly designated adjacent fields at each farm about resource management and water consumption as to receive either conventional or alternative treatments. The well as environmental concerns such as fugitive dust, fields varied in size but generally ranged from 30 to 60 ha ground water quality, and food safety (SJV Drainage Abbreviations: BD, bulk density; CEC, cation exchange capacity; S.S. Andrews and D.L. Karlen, USDA-ARS, Natl. Soil Tilth Lab., EC, electrical conductivity; MBN, microbial biomass nitrogen; MDS, Ames, IA 50011; J.P. Mitchell and T.K. Hartz, Dep. of Vegetable minimum data set; NRCS, Natural Resources Conservation Service; Crops and Weed Sci., and W.R. Horwath, G.S. Pettygrove, and K.M. PC, principal component; PCA, principal component analysis; PMN, Scow, Dep. of Soils and Biogeochem., Univ. of California, Davis, CA potentially mineralizable nitrogen; SAFS, Sustainable Agriculture 95616; R. Mancinelli, Dep. of Crop Prod., Univ. of Tuscia, 01100, Farming Systems (Project); SAR, sodium adsorption ratio; SCMPs, Viterbo, Italy; and D.S. Munk, Univ. of California Coop. Ext., 1720 supplemental carbon management practices; SJV, San Joaquin Valley; S. Maple Ave., Fresno, CA 93702. Received 22 May 2000. *CorreSOM, soil organic matter; SQI, soil quality index; TKN, total Kjeldahl sponding author (andrews@nstl.gov). nitrogen; WSA, water-stable aggregates; WSD, West Side On-Farm Demonstration Project; x-K, exchangeable potassium. Published in Agron. J. 94:12–23 (2002).

INFILTRATION AND SOIL WATER STORAGE UNDER WINTER COVER CROPPING IN CALIFORNIA’S SACRAMENTO VALLEY

Abstract Organic and low-input cropping systems that use more C inputs are alternatives to conventional systems for sustaining long-term soil fertility. An understanding of the impacts of these cropping systems on N balance (N applied minus N removed in harvested plant material), storage and loss is necessary to improve long-term soil fertility and minimize the risk of environmental pollution. An evaluation of 4-year rotations of organic (N from legumes and composted manures), low-input (N from legumes and reduced amounts of synthetic fertilizers), and conventional (conv-4, N from synthetic fertilizers) and a conventional 2-year rotation (conv-2, N from synthetic fertilizers) on N balance, storage and loss was conducted from 1989 to 1998. Compared to the conv-2 system, the organic and conv-4 systems showed 119 and 8% greater cumulative N balances, respectively, over the duration of the study. However, N balance in the low-input system was 19% less than in conv-2 system. After 10 years of differential management, total N in the top 15 cm of soil was 1.46 g kg−1 in the organic, 1.26 g kg−1 in the low-input, 1.13 g kg−1 in the conv-4, and 1.1 g kg−1 in the conv-2 system. Compared to the conv-2 system, cumulative N losses for the organic, low-input and conv-4 systems were lower by 80, 92, and 10%, respectively. These findings suggest that organic and low-input cropping systems that add C to soil have the potential for storing N and making it available for future crop use, while minimizing the risk of environmental pollution.

Simulating greenhouse gas budgets of four California cropping systems under conventional and alternative management

Winter cover cropping on agricultural fields may improve rainfall infiltration and enhance soil water storage in areas such as California’s Sacramento Valley, where the majority of precipitation occurs in the winter over a relatively short period of time in a series of heavy rainfall events. Enhanced soil water storage within the root zone on cover–cropped fields may benefit a grower by reducing the demand for surface water deliveries to meet the irrigation needs of subsequent crops. A study was conducted in the winters of 1998–1999 and 1999–2000 to determine a field’s ability to conserve water for subsequent crops and to evaluate the effects of soil physical conditions on the water balance for three 4–year rotation farming systems within the Sustainable Agriculture Farming Systems (SAFS) Project at the University of California, Davis. Rainfall, runoff, and soil water content data was collected on two treatments using a winter cover crop and one treatment maintained fallow during the winter. Runoff and soil water content measurements were significantly affected by farming systems. Cumulative event runoff from 10.67 m 2 infiltration test areas was consistently higher on the fallow treatment than on the cover–cropped treatments. Winter 1999–2000 field water content measurements from 0–1.05 m depth were significantly higher in the cover–cropped systems than in the fallow treatment after field capacity had been reached. A hydrologic model was developed using the measured data and lysimeter data for evaporation and evapotranspiration to track daily water budget components (i.e., runoff, infiltration, evaporation, evapotranspiration, and soil water storage) and to assess changes in surface hydraulic conductivity. Model simulations showed that optimized hydraulic conductivity decreased for all treatments with successive runoff, but was less pronounced in cover–cropped plots. The study indicated that cover cropping can improve soil water storage for subsequent crops if the cover crop is destroyed before the additional soil water is lost as evapotranspiration.

Conservation tillage for organic agriculture: Evolution toward hybrid systems in the western USA

Despite the importance of agriculture in Californias Central Valley, the potential of alternative management practices to reduce soil greenhouse gas (GHG) emissions has been poorly studied in California. This study aims at (1) calibrating and validating DAYCENT, an ecosystem model, for conventional and alternative cropping systems in Californias Central Valley, (2) estimating CO2, N2O, and CH4 soil fluxes from these systems, and (3) quantifying the uncertainty around model predictions induced by variability in the input data. The alternative practices considered were cover cropping, organic practices, and conservation tillage. These practices were compared with conventional agricultural management. The crops considered were beans, corn, cotton, safflower, sunflower, tomato, and wheat. Four field sites, for which at least five years of measured data were available, were used to calibrate and validate the DAYCENT model. The model was able to predict 86-94% of the measured variation in crop yields and 69-87% of the measured variation in soil organic carbon (SOC) contents. A Monte Carlo analysis showed that the predicted variability of SOC contents, crop yields, and N2O fluxes was generally smaller than the measured variability of these parameters, in particular for N2O fluxes. Conservation tillage had the smallest potential to reduce GHG emissions among the alternative practices evaluated, with a significant reduction of the net soil GHG fluxes in two of the three sites of 336 +/- 47 and 550 +/- 123 kg CO2-eq x ha(-1) x yr(-1) (mean +/- SE). Cover cropping had a larger potential, with net soil GHG flux reductions of 752 +/- 10, 1072 +/- 272, and 2201 +/- 82 kg CO2-eq x ha(-1) x yr(-1). Organic practices had the greatest potential for soil GHG flux reduction, with 4577 +/- 272 kg CO2-eq x ha(-1) x yr(-1). Annual differences in weather or management conditions contributed more to the variance in annual GHG emissions than soil variability did. We concluded that the DAYCENT model was successful at predicting GHG emissions of different alternative management systems in California, but that a sound error analysis must accompany the predictions to understand the risks and potentials of GHG mitigation through adoption of alternative practices.

Management of Aphid-Borne Viruses and Bemisia argentifolii (Homoptera: Aleyrodidae) in Zucchini Squash by Using UV Reflective Plastic and Wheat Straw Mulches

Organic farming has been historically dependent on conventional tillage operations to convert perennial pasture leys to annual crop rotations, incorporate crop residues, compost and cover crops, as well as to mechanically kill existing vegetation. Conventional tillage, however, has long been known to lead to soil degradation and erosion. A recently developed no-till organic production system that uses a roller–crimper technology to mechanically kill cover crops was evaluated in two states in the western United States. In Washington, pumpkins (Cucurbita spp.) grown in a no-till roller–crimper (NT-RC) system produced yields 80% of conventional tillage, but with fewer weeds. However, in California on-farm research trials in organic cotton (Gossypium barbadense L.), tomato (Lycopersicon esculentum Mill.), eggplant (Solanum melongena L.) and cowpea (Vigna unguiculata (L.) Walp.), the no-till system produced virtual crop failure, or yields less than 20% of the standard production method. The major problems associated with rolled cover crops in California included reduced crop seedling emergence, planter impediment with excessive residue, lack of moisture and delay in transplanting of vegetable crops due to continued growth of cover crops, in-season crop competition from cover crop regrowth and impracticability of using cultivators. Further, excessive dry residue during summer in California can present the risk of fire. In both California and Oregon, considerable success has been demonstrated with zone tillage (strip tillage) in conventionally produced field and vegetable crops. In a replicated Oregon trial, the organic strip tillage treatment produced 85% of the broccoli (Brassica oleracea L.) yield compared to a conventional tillage treatment. Our studies suggest that the zone tillage concept may offer opportunities to overcome many of the agronomic challenges facing no-till.

Soil management effects on the nonlimiting water range

Abstract Plastic UV reflective mulch (metalized mulch) and wheat straw mulch delayed colonization by Bemisia argentifolii Bellows & Perring and the incidence of aphid-borne viruses in zucchini squash. No insecticides were used in either mulch treatment. The mulches were compared with a preplant treatment of imidacloprid and an untreated, unmulched control. In 2000, yield of marketable fruit in the plastic and straw mulched plots was approximately twice that from the imidacloprid plot. In 2001, yield from the straw mulch plots was twice that of the imidacloprid and plastic mulch plots. Yields from both mulched plots and from the imidacloprid plots ranged from 3 to 12 times higher than those from the control plots. The mulches were more effective than a preplant application of imidacloprid in reducing the incidence of both B. argentifolii and aphid-borne viruses. Plants growing over the plastic mulch and the straw mulch grew more rapidly and reached a larger size, as determined by plant dry weight, than did those growing over bare soil, with or without imidacloprid. The spectral quantum flux from the plastic averaged between 80 and 90% of ambient spectral quantum flux values in the UV (300–400-nm) range. Spectral quantum flux values of wheat straw were similar to those of the reflective mulch and ambient near 300 nm but were virtually identical to bare soil beyond 320 nm. The metalized mulch reflected 94% of the incoming photosynthetically active radiation (PAR) in the 400–700-nm range compared with ambient, whereas the straw mulch reflected 85%. Bare soil reflected only 41% of incoming PAR compared with ambient.

Nitrate leaching and soil nitrate content as affected by irrigation uniformity in a carrot field

Water, oxygen, temperature, and mechanical resistance (MR) are soil physical properties that directly affect plant growth. In this research, the nonlimiting water range (NLWR), defined as the range of soil water content in which neither water, oxygen, nor MR is limiting for plant growth, was measured on soil samples from three field experiments. Undisturbed soil core samples were collected from an experimental field on Wasco silt loam (coarse-loamy, mixed, nonacid, thermic Typic Torriorthents) amended with dairy manure (25 and 50 ton/ha), sludge composting biosolids/green waste (25 and 50 ton/ha), and a control treatment from Shafter Field Experiment Station, Kern County. Similar samples were collected from a Reiff loam (coarse-loamy, mixed, nonacid, thermic Mollic Xerofluvents) and Yolo silt loam (fine-silty, mixed, nonacid, thermic Mollic Xerofluvents) treated with conventional 4-year rotation, conventional 2-year rotation, low-input, and organic farming systems in the Sustainable Agricultural Farming System (SAFS) in Davis, CA. The third set of samples were from a fine sandy soil (mixed, thermic, Typic Haplargid) irrigated with four saline waters with EC values of 0.5, 4.0, 8.0, and 12.0 dS m−1 in a pistachio orchard located at Paramount Farms, Kern County, CA. The core samples were used to determine saturated hydraulic conductivity and water retention at −1500 kPa (wilting point) and −10 kPa (field capacity) to calculate the available water content and the NLWR. Oxygen diffusion rates (ODRs) were measured on the soil cores after they were equilibrated to field capacity, and MR was measured using an automated mini-penetrometer after the soil was equilibrated to −1500 kPa. The results showed that organic amendments did not significantly change the ODR, MR, NLWR, and saturated hydraulic conductivity (Ks). Mean values of MR and ODR were not significantly different in the four agricultural systems either, but soil under organic farming system had a relatively narrow NLWR because less water was retained at field capacity, indicating that the soil surface under this farming system has more large pores. The mean Ks was significantly higher in conventional 4- and 2-year rotations than in organic and low-input treatments. Increasing irrigation water salinity above 4 dS m−1 progressively decreased the saturated hydraulic conductivity but had relatively little effect on other physical properties. In general, the treatment variables had relatively little effect on the NLWR, ODR, MR, or Ks of surface soils, except for a decrease in Ks when irrigated with waters with more than 4 dS m−1.

The sustainable agriculture farming system project in California 's Sacramento Valley

Abstract High value crops such as carrot planted in coarse soils of the Southern San Joaquin Valley in California are prime candidates for nitrate leaching through irrigation nonuniformity. A 2-year study was carried out to explore the impact of irrigation uniformity on nitrate leaching. Irrigation uniformity was measured using catchcans. Soil nitrate (NO 3 -N) and ammonium (NH 4 -N) contents were measured from soil sampled at different depths and times during two growing seasons. Nitrate leaching was determined using ion-exchange resin bags at 1-m depth sampled three times during each season. Although, soil NO 3 -N as well as seasonal irrigation was significantly higher along the lateral irrigation pipe than between the sprinklers, nitrate leaching was not significantly higher. As expected, soil nitrate content decreased as percolation increased for both years. Nitrate leaching, as estimated by anion-exchange resin bags, was positively correlated to soil NO 3 -N content but was not correlated to irrigation depth, irrigation uniformity, or deep percolation. Field variation in saturated hydraulic conductivity ( K s ), soil organic matter (OM), and soil water retention at field capacity had limited effect on NO 3 -N and NH 4 -N distributions in the profile and on nitrate leaching. The results of this experiment suggest that irrigation nonuniformity has less impact on nitrate movement than suggested by earlier studies.