Assessing Soil Quality in Intensive Organic Management Systems

Assessing Soil Quality in Intensive Organic Management Systems

Summary

We are evaluating the effects of cover crop strategies, amendment types, and tillage systems on soil quality in organic vegetable production and tree fruit production. In the short term, amendment had the greatest effect on soil physical and chemical properties in the vegetable crop experiment, while cover crop had a greater effect on biological properties. Spader tillage improved enzyme activities, infiltration rate, and compaction compared with conventional tillage. Improved soil quality indicators did not lead to short term yield improvement. In the orchard system, wood chips provided greater overall soil quality and production benefits than living mulches or cultivation.

Objectives/Performance Targets

1. 1. Compare the effects of selected organic row crop and orchard management systems on soil quality as measured by field and laboratory tests and field observations.
   2. Evaluate the NRCS Soil Conditioning Index as a tool for soil quality assessments.
   3. Develop underseeded cover crop guidelines for humid Northwest environments.
   4. Share findings with organic vegetable and fruit producers and other producers interested in sustainable practices in the Northwest.

Accomplishments/Milestones

Objective 1. Compare the effects of selected organic row crop and orchard management systems on soil quality as measured by field and laboratory tests and field observations.

This project includes soil quality measurements in an organic vegetable crop systems experiment at WSU Puyallup (established 2003), two orchard weed and soil management experiments established in Wentachee in 2004 and 2005, and six on-farm sites in western Washington. Five of the cooperating farmers grow organic vegetables, while the sixth farmer grows organic vegetables and small fruit (raspberry, strawberry, and blueberry). One farm also has small poultry flocks for egg and meat production.

Vegetable crop systems experiment.
Outline of procedures. The organic systems experiment at Puyallup compares 12 combinations of organic management systems, consisting of three cover crop treatments, two tillage treatments, and two soil amendment treatments arranged in a split-split plot design, with cover crops as the main plots. The cover crop treatments include 1) a traditional fall-seeded cereal rye-hairy vetch mix, 2) relay-intercropped hairy vetch planted into the standing cash crop, and 3) a short-term ryegrass-clover pasture that is rotated with cash crops in alternating years. The tillage treatments include conventional (plow, disc, rototill as needed), and modified (spader) tillage. Soil amendments are broiler litter compost applied at 1.5 dry tons/acre (low carbon) and on-farm mixed compost applied at 14 dry tons/acre (high carbon). The crop rotation for the first three years was snap bean (2003), spinach following summer cover (2004), and winter squash (2005). Beginning in 2006, 4 rotation crops will be grown in each plot each year, by dividing the plots into narrow beds, typical of those used by local organic mixed vegetable growers. The rotation across beds will be: 1) broccoli, 2) double crop of snap bean and fall spinach, and 3) winter squash.

The plots with the pasture cover crop are in cash crops each odd-numbered growing season (2003, 05, 07 etc.) and in pasture at all other times. Beginning in 2006 we will raise Cornish Cross pastured poultry during the growing season of the pasture years.

Soil quality measurements include soil physical properties (compaction, bulk density, infiltration, and aggregate stability); biological indicators (b-glucosidase and dehydrogenase enzymes, nematodes, collembola, microbial community assessment via phospholipid fatty acid analysis, substrate induced respiration to estimate total, fungal, and bacterial activity); and chemical properties (total and particulate organic matter carbon, and basic soil nutrients). Most samples were collected pre-plant (2 to 3 weeks following incorporation of cover crops and amendments) and mid-season (at the time of canopy closure of the squash crop). A few measurements were made post-harvest.

In 2006 we will add available water to our measurements. These measurements are more extensive than those originally proposed, and are supported both through this SARE project and other funding sources.

Results.
Bulk density. Soils with low bulk density are generally more porous and contain more organic matter than soils with high bulk density. Bulk density measured in July 2004 and July 2005 was slightly, but significantly lower in plots receiving the high C amendment (on-farm compost) compared with the low C amendment. Mean bulk density in 2005 was 1.03 g/cm3 for the low C treatment and 0.98 g/cm3 in the high C treatment. Tillage or cover crop did not affect bulk density in 2005.

Soil compaction (0 to 40 cm depth) was measured twice in 2005 using a recording cone penetrometer. Soils were consistently less compact at the 15 to 30 cm depth in the plots tilled with the spader, probably because of the greater depth of tillage done by the spader. These results are consistent with what we observed in 2004. Amendment and cover crop did not affect soil compaction.

Infiltration measurements were made at planting and mid season in 2005 using a simple single ring infiltrometer with falling head. Infiltration rate was greater with the high C amendment and spader tillage, but the effects were transient in 2005. By mid season, infiltration rates had declined, and there were no differences among treatments.

Aggregate stability. No differences in aggregate stability have been observed among treatments in 2004 or 2005. We have evaluated aggregate stability as mean aggregate diameter and % water stable aggregates. Because biological activity plays an important role in the creation of stable aggregates, we would expect differences in aggregate stability to develop more slowly than for many of the other physical measurements. Mean aggregate diameter over all treatments was 2.37 mm in 2005.

Total C and particulate organic matter (POM) C were both greater in the plots receiving the high C amendment, indicating the short-term effect of amendment addition on soil C when large amounts of amendments are applied. POM levels were 4.52 g/kg soil in High C vs.3.49 g/kg in Low C in 2004, and 6.60 vs. 4.49 g/kg in 2005.

b-glucosidase and dehydrogenase. These enzymes were the most responsive biological indicators in our study. B-glucosidase activity was affected by cover crop all three dates sampled (pre-plant and mid-season 2005, as well as mid-season 2004), with the relay vetch plots tending to have the greatest activity and the post-harvest rye vetch the lowest. This is probably a result of the different qualities of the cover crop residues, with the most succulent residue being the relay vetch. Dehydrogenase activity was lowest for the post-harvest cover crop in 2004, which was the time when there were the greatest differences in the quality of the cover crops. The high C amendment increased dehydrogenase activity in the 2004 and 2005 mid season samples, while spader tillage increased dehydrogenase in the pre-plant 2005 samples. Spader tillage also increased b-glucosidase on one date, suggesting that the lower intensity of the spader may have some effect on soil biological activity. More data are needed to evaluate this hypothesis.

PLFA and Biolog plates. We used phospholipid fatty acid (PLFA) analysis and Biolog plates to evaluate the effects of the different systems on microbial community structure. Principal component analysis of the PLFA data showed a separation among cover crops mid-season 2004, when differences in the cover crop environment were greatest, but no substantial differences were seen on other dates. Effects of treatment on Biolog activities were small. Together, these data suggest that that there is little short term difference in biological community structure among the organic systems.

Nematodes and collembola. Analysis of nematodes and collembola allows examination of the soil food web at a higher trophic level. Extractions and measurements were begun in 2005. Preliminary data show non-significant trends toward higher nematode (p=0.09) and collembola (p=0.11) in treatments receiving the low C amendment (broiler litter), but no effect of tillage and cover crop.

Winter squash yield was not affected by treatment in 2005. Mean yield was 18.8 tons/acre for Table Ace and 13.0 tons/acre for Delicata.

Conlusions. The amendments provided a larger volume of C (especially the compost), but it is less labile than C provided by the cover crops. Tillage affected the depth of incorporation of amendments and the degree of disturbance of the soil. The spader tills deeper than conventional tillage and it incorporates residues more completely, but we believe that is nonetheless less disruptive than conventional tillage because the tillage action is slower, and there are fewer passes over the field.

Amendments had the greatest effects on the biochemical (total and POM C) and physical measurements. Effects on the biological indicators were less, with only dehydrogenase clearly responding to amendment. The cover crops did not supply enough biomass to affect the C measurements, but they had a greater effect on the biological measurements than the amendments did. Cover crops appeared to have a long-term effect on b-glucosidase, and transient effects on dehydrogenase and PLFA.

In two cases with the enzymes, higher activity was observed with modified tillage, which suggests the reduced disruption may have a positive effect on some biological factors.

Work remaining. In 2006 we will repeat the series of physical, biological, and biochemical measurements described above. In addition we will begin an evaluation of plant available water (using sand table and pressure plates) on samples collected mid-season. Nematode ecology will be evaluated using Structure and Enrichment Indices. We will prepare a manuscript focused on short term effects of different organic management systems on soil quality, and present results at workshops, field days, and farm walks focused toward organic and small-acreage farmers.

On-farm cooperators.

Results. In 2005 we measured bulk density, aggregate stability, infiltration rate, penetrometer resistance, and texture at multiple points on each of the six cooperating farms. At two of the farms we made measurements at two different farm sites under their management. We also collected composite samples for soil organic matter, pH, Bray P and exchangeable K. Each sampling location was logged into a GPS system so that we can return to the same points in subsequent years. Aggregate stability, texture, and nutrient analyses are in progress.

Infiltration rates varied widely among sites and appeared to be affected by soil texture, current and past management, and length of time in organic production. Two farm sites (ND and NP) had higher bulk density and infiltration rates than the other sites, but otherwise infiltration and bulk density were not correlated with each other. Penetration resistance varied within farms, but two general patterns emerged among farms. Farms N and F tended to have less compaction than the other farms in the shallower and deeper parts of the profile, but more compaction in the middle of the profile.

Work remaining. In 2006 we will repeat on-farm measurements at all of the partner farms. We will also evaluate differences among farms in light of soil texture, length of organic production, management practices, and the Soil Conditioning Index. We will add at least two conventionally managed farms, a task not done in 2005. The conventional farms will be paired with organic farms based on soil series and texture. We will evaluate soil biological measurements from the systems experiment at WSU Puyallup to determine which would be most suitable for on-farm use.

Orchard mulches and weed management experiments.

The objective of these experiments is to evaluate soil quality differences related to in-row weed control methods in orchards including mechanical tillage (different rates and types), wood chip mulch, living mulch or control treatments applied to the weed strip. Soil infiltration rate and resistance to penetration were measured to monitor the soil quality impacts of repeated mechanical tillage compared to untilled or mulched soil. Our overarching hypothesis for these two trials is that the wood chip mulch improves soil physical properties, and tillage degrades them relative to a bare or mowed control. Additionally, we hypothesized that a living mulch planting would also increase soil quality over tillage or control treatments.

Procedures:
1. Tillage Comparison Trial. In April 2004, a 2-year trial was set out in an 8-yr old block of Gala/M26 in transition to organic certification at the Wenatchee Valley College teaching orchard near East Wenatchee, WA. Treatments included control (no tillage, mowing to keep weeds down), wood chip mulch (applied 6” thick), Cultivator Y (3x), Cultivator Z (2x), Cultivator Z (3x), and Cultivator Z (4x), with 5 replicates, where “x” denotes the number of times plots were tilled. In Year 2 Cultivator Y treatment was tilled once only, in early August. Cultivator Y is a hydraulically driven unit with a vertical axis cultivating head. Cultivator Z is a ground-driven rolling cultivator with a spring blade that works in between the trees. Infiltration and soil penetration resistance data were collected in both years, and in 2005 soil samples were taken for pending organic matter and microbial tests. Resistance data were collected using a Rimik® recording cone penetrometer with readings from 0-425 mm depth, at 15 mm intervals, and nine measurements were made per plot. Infiltration was collected using brass rings in 2004, and Mini Disk Infiltrometers (Decagon, Inc. Pullman, WA) in 2005.

2. Integrated mulch trial. In April 2005, a trial was set up in a new apple (Pinata/M7) block to test the following understory treatments during orchard establishment: bare ground control, bare ground with Brassica meal, wood chip mulch, cultivation, Sandwich system (tillage each side of the tree line and a 45-cm strip of living mulch in the tree line), and a Living Mulch (LM) treatment filling the 150 cm wide tree row. The living mulch and sandwich had separate legume and non-legume treatments to test the nitrogen impacts. The LM and Sandwich legume treatments are a species mix of Mt. Barker subclover, black medic, burr medic, birdsfoot trefoil, and bentgrass. The Sandwich non-legume treatment is transplanted sweet woodruff and Corsican mint, while the living mulch non-legume mix contains sweet alyssum, five spot, mother of thyme and bentgrass. Experimental design is a randomized complete block with 5 replicates. Tillage rates were uniform for all tilled plots, and as needed throughout the growing season. Soil measurements in 2005 included volumetric soil moisture using both a Moisture Meter® and Hydrosense® meter (Decagon, Pullman WA), resistance, and nutrient and carbon status. Resistance data were collected as in the tillage trial. Infiltration data were collected with Mini-Disk® infiltrometers (Decagon). Soil analysis is pending.

Results:
Infiltration. In 2004, water infiltration was significantly slower under Cultivator Y for both the ambient and field capacity runs using a single ring infiltrometer. This suggested possible compaction, but at all depths, soil resistance with Cultivator Y was similar to the control. Moreover, this difference was not apparent in 2005 using the disk infiltrometer, perhaps because Cultivator Y was only used one time in comparison to 3 times in 2004. There was significantly faster infiltration under the control treatment in 2005 at the lower tension (0.5cm) than the other treatments, but no differences at the 2 cm tension. This suggests that the control treatment may have led to more macropore development.

Resistance. In 2004 in the tillage trial, the untilled control had highest soil resistance (kPa) at depths of 60, 90, and 120 mm. At 150mm, Cultivator Z 2x had significantly higher resistance. In 2005, there was no consistent effect of any treatment on soil resistance. Cultivator Y had significantly higher resistance over the Cultivator Z 2x and 3x treatments at depths of 60 and 90 mm, suggesting that Cultivator Y may contribute to greater soil compaction over Cultivator Z.

Soil resistance data in the IMM trial suggest wood chips generally had less resistance than all other treatments. There were significant treatment effects at soil depths of 30 and 300 mm. At 30 mm, the Cultivator Z treatment had significantly higher resistance over the wood chip mulch. At depths of 300 and 330 mm, the living much planting treatments had significantly higher resistance over the wood chip mulch. Interestingly, at 330 mm, the living mulch planting had a higher soil resistance than the control. This depth is likely below the rooting zone of the living mulch species.

Other soil quality measures. Wood chips were shown to provide benefits to fruit production, yield efficiency (data not shown), and may provide other soil quality benefits. In other trials at the tillage trial site, wood chips have been found to have increased moisture. Additionally, the IMM trial soil moisture data show that there is a trend of higher moisture content in the wood chip treatment than the tillage and living mulch treatments. By 2005 in the tillage comparison trial, the wood chip mulch appeared to have root and organic matter development at the surface where the wood chips were breaking down. Tests for soil nutrients, organic matter, and microbial status will be conducted this winter.

Objective 2. Evaluate the NRCS Soil Conditioning Index as a tool for soil quality assessments.

Procedure: We ran a series of scenarios based on field data (crop yield, cover crop biomass, amendment application rates, and field operations) for the different systems in the WSU Puyallup organic systems experiment. We evaluated 1-year (2005) and 2-year (2004-05) simulations for the 6 cover crop and amendment combinations, but did not compare tillage systems, because the Soil Conditioning Index (SCI) database does not contain field operation inputs for spaders.

Results: Preliminary results indicate a wide range of soil conditioning indices among the experimental treatments. One-year results ranged from –0.13 to +0.12, while 2-year results ranged from –0.02 to +0.36. The two-year results were higher because they included a year with more time spent in cover crops and less in cash crops. This range of values suggests that relatively large differences in soil quality may become apparent over time in the experiment. A problem with adapting the SCI to use with small-scale organic farms is that many of the common equipment and tillage operations on these farms are not part of the current SCI database. Also, it is clear that accurate record keeping is needed to gain confidence in the SCI results.

Work Remaining: In 2006 we will begin discussions with the SCI developers about including tillage and cover crop operations that are applicable to small scale organic vegetable production. We will refine the erosion factor for our own experimental site using RUSLE inputs, and expand the scenarios to three (2003-2005) and four (2003-2006) years. If we can include new tillage and cover crop operations in the model, we will then run simulations on selected partner farms, based on the quality of input and field operations records from those farms. In the long run, we will continue to run SCI simulations for our own systems experiment, and determine if relative changes in soil quality predicted by the model are supported by field measurements.

Objective 3. Develop underseeded cover crop guidelines for humid Northwest environments.

Relay planting or interseeding of legume cover crops is designed to overcome some of the drawbacks of legumes, such as slow establishment in the fall, poor winter ground cover, and poor weed suppression, while improving N accumulation. Relay-cropping presents its own challenges, however, including shading by the cash crop, competition with the cash crop, weed management, and effects on winter hardiness. We are determining which cash crops, cover crops, and conditions are most amenable to successful relay cropping of legumes.

Procedure: This experiment was begun in 2003 using other funding, and has been jointly funded by SARE and WSU since 2004. Legume cover crops are interseeded into snap bean (Jade) and sweet corn (Blaze) approximately 30 days after planting the cash crop. Following cash crop harvest, the residue is mowed and the cover crop must grow through the residue. Specific treatments have changed each year as we have narrowed the focus of our experiment based on previous years’ results. Measurements include cash crop fresh yield, cover crop stand at intervals during the winter and spring, cover crop biomass at the time of incorporation, and soil inorganic N three weeks after cover crop incorporation.

In 2005 we moved the location of the experiment to increase the size of the plots and alleys. New plot sizes are 30 ft by 15 ft with 20 ft alleys between each plot along the direction of planting. Each experiment has five treatments and three replicates.

Results:
Summary of 2003-04 experiment. Cover crops included hairy vetch, woolypod vetch, common vetch, red clover, and crimson clover. The hairy vetch and woolypod vetch were well established by late October. Red clover had a fair stand, but common vetch and crimson clover performed poorly and were dropped from the trial. Some winter damage occurred to the vetch in January during a week-long period of snow cover and cold temperatures (minimum 10 F with snow cover), but the vetch recovered by spring. Cover crops were harvested in early April 2004 and biomass was similar in the corn and bean experiments. Hairy vetch had the greatest biomass (2170 lb/acre dry weight) followed by woolypod vetch (1760 lb/acre) and red clover (720 lb/acre). Hairy vetch significantly increased early season inorganic N over the control treatment, but red clover did not. Later incorporation would likely have substantially increased biomass and N.

2004-05 experiment. Treatments included relay hairy vetch and red clover (both broadcast and drilled) and a post-harvest rye-vetch blend. Establishment of the relay cover crops was much poorer than the previous year. Heavy crop residue from the corn, followed by early rains that compacted the crop residue reduced the fall stand of the interseeded cover crops. Cover crops partially recovered during the fall, but cold night temperatures (10 consecutive nights between 20 and 25 F) and lack of insulation by snow in February reduced hairy vetch ground cover while red clover ground cover was not affected. This illustrates one of the potential problems with interseeded hairy vetch – reduced winter hardiness when planted in mid-summer. Red clover had the greatest biomass at incorporation (12 May), averaging 1980 lb/acre dry matter (mean of corn and bean experiments), while average hairy vetch yields were only 900 lb/acre. The post-harvest rye vetch (planted 4 October) yielded nearly 6000 lb/acre of dry matter. Because of the poor stand of the legume cover crops in the spring of 2005, the nitrogen contribution of these crops was small, as measured by soil nitrate N in late May.

2005-06 experiment. This year’s design makes a direct comparison of interseeded vs. post-harvest plantings of hairy vetch and red clover. Relay hairy vetch had complete ground cover by mid-October in the beans and partial ground cover in the corn. Both vetch and clover emerged well through the corn residue. Corn ear yield was less in 2005 than in 2004, and this may be indicate lower levels of residue as well, which could improve the stand of the cover crop by reducing the amount of smothering. Observations of the cover crops on 22 December suggest less stand reduction than the previous year, despite colder temperatures (22 consecutive subfreezing nights from late November through mid December, with lowest temperature of 15 F).

Work remaining. We will continue this experiment for at least through 2006-07, and are seeking additional funding from other sources to expand the crops evaluated. We hope to start at least one on-farm trial in 2006.

Objective 4. Share findings with organic vegetable and fruit producers and other producers interested in sustainable practices in the Northwest.

In August we held a field day at WSU Puyallup focused on organic agricultural systems, cover cropping, and soil quality. Attendance was 80. A 1-evening soils module was taught at three locations as part of WSU classes for small farms operators. Total attendance was 65. We led a farm walk focused on soil quality and soil testing for farmers from the Hmong community. Attendance was 20. We made a cover crops presentation at the annual meeting of the Western Washington Horticulture Association. In addition we presented posters at the National Small Farms conference and the American Society of Agronomy national meetings, which brought our work to a national audience. Information on our soil quality and organic systems research is available on our web site: http://www.puyallup.wsu.edu/soilmgmt/SusAg.htm The website is revised each January with current information, including portions of this report.

Work remaining. In 2006 we will host at least one field day at WSU Puyallup, participate in local farm walks, update the website, and prepare practical summaries for farmer-oriented publications, such as the Tilth newsletter and WSU Sustaining the Pacific Northwest newsletter.

Impacts and Contributions/Outcomes

Our work is leading to strategies and recommendations for relay planted cover crops in irrigated vegetable production systems. Organic growers at our field days have responded enthusiastically to our work on relay planted cover crops. Our research site gives growers an opportunity to compare different relay cover crop strategies. Growers are eager to obtain additional information from our research, to make informed decisions on adopting relay-planted cover crops.

Our soil quality research is still in early stages, so we do not have on-farm impacts to report yet. We have demonstrated that different organic management systems affect soil quality in different ways in the short run; we plan to continue this research to gain a better understanding of long-term effects. Even though the measurable effects of modified tillage using the spader are small at this point, growers are impressed by the quality of seedbed that it can produce after a single pass. The spader appears to be most suitable for small farms and light to medium textured soils.

Based on connections we have made with partner farmers, two farmers will be initiating on-farm experiments in the coming year; one is focused on weed management and the other is focused on nutrient management.

Collaborators: