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. Amendment had the greatest effect on soil organic matter, while cover crop had the greatest effect on bulk density and compaction in the short run in the vegetable systems experiment. Improved soil quality indicators did not lead to short-term yield improvement. Experiments on relay cover crops for vegetable systems have given promising results, with hairy vetch, woolypod vetch, and possibly red clover as suitable relay cover crops where irrigation water is available.

Objectives/Performance Targets

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); an orchard mulch experiment (established 1999) and an orchard weed management experiment (established 2004), both in Wentachee; and three on-farm sites in western Washington. Two of the cooperating farmers grow organic vegetables, while the third farmer grows organic vegetables and small fruit (raspberry, strawberry, and blueberry).

Vegetable crop systems experiment.

The organic systems experiment 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 are a post-harvest cereal rye-hairy vetch planted in the fall, relay interseeded hairy vetch, and a one-year temporary ryegrass-clover pasture (ley) that is rotated with cash crops. 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 is snap bean (2003), spinach following summer cover (2004), winter squash (2005), potato (2006), broccoli (2007), and snap bean (2008). The pasture treatment is cropped to vegetables in 2003, 2005, and 2007 and pasture the other years. The summer cover in 2004 was buckwheat in the relay treatment (2 successive crops grown between mid-May and late-July) and rye-vetch in the fall cover crop treatment (fall rye-vetch mowed in April and June with residue left as a mulch). Appendix Table 1 shows the timing of key field activities in each of the cover crop treatments in 2004.

Soil test measurements (organic matter, pH, and exchangeable K) for the systems experiment are summarized in Appendix Table 2. These samples were collected in April 2004, approximately one year after the first addition of organic amendments, but before any field activities were done in 2004. Although statistical analyses have not been completed on these data, several trends are apparent. The greatest effect is from the soil amendments, with the high-carbon treatment having greater soil organic matter and potassium even though one year had passed since application. The cover crops appear to have affected organic matter, while tillage had no effect (data not shown). Bray phosphorus was excessively high in all plots, probably the result of repeated poultry manure applications to the field before 1990.

We measured aggregate stability twice in 2004: 1) on 21-24 June the relay treatment was in buckwheat and the other treatments in rye-vetch stubble and pasture, and 2) on 22 September when the relay and fall treatments were planted to spinach. We saw no significant differences among treatments, which is not surprising, considering the short duration of the experiment to date. Weighted mean aggregate diameter averaged across all treatments was similar in June (1.94 mm) and September (1.84 mm).

Soil compaction (0 to 40 cm depth) was measured in June and September using a recording cone penetrometer. Trends among treatments were similar during both measurement periods, with overall greater resistance to penetration in the 0 to 30 cm depth in June. September penetrometer data are shown in Appendix Figure 1. Compaction tended to be greatest in the pasture plots. In the relay plots, conventional tillage had greater September compaction than spader-tillage in the 15 to 30 cm zone. The relay plots had the most extensive tillage in 2004, which may explain why the tillage difference was apparent in that treatment. Soil amendment had no effect on compaction at any depth at either sampling date (data not shown).

Bulk density was determined in June, when the only plots that had received tillage in 2004 were the relay plots. Bulk density was greatest in the pasture treatment (1.24 g/mL) followed by fall rye-vetch (1.14 g/mL) and relay (1.01 g/mL). This difference may be an artifact of the cropping cycle, since the relay plots had been tilled a month earlier and the pasture plots had received the most traffic (from harvesting and removing grass) and had the driest soil. The high carbon plots had slightly, although statistically significant, lower bulk density than the low carbon plots (1.117 vs 1.143 mg/L). There was no difference in bulk density between tillage treatments.

We used Biolog measurements on samples collected in August (three weeks after incorporating amendments and two weeks after planting early spinach) as a preliminary microbial community analysis. Amendment had a significant effect on Biolog measurements, with greater total absorbance for the high carbon amendment (46.2) compared with the low carbon amendment (43.1). Plots tilled by the spader had a greater number of wells with an absorbance > 1, although total absorbance was not affected by tillage. Fatty acid methyl ester (FAME) analyses were run on the same samples, but the data have not been analyzed yet.

Spinach yield at harvest was affected by amendment and previous cover crop, but not by tillage (Appendix Figure 2). We had two varieties of spinach (Koala and Spaulding) in the early (Aug 5) planting and Spaulding alone in the late (Aug 20) planting. Spinach yield was significantly greater following the fall rye-vetch cover crop than the relay vetch-summer buckwheat rotation for both varieties and both planting dates. The reason for this difference is not clear; the decomposing buckwheat may have affected the early planting, but this is less likely for the late planting. Amendment also affected yield, with the low C amendment (chicken litter, labeled C in Appendix Figure 2) producing greater yield of Spaulding at both planting dates. Koala was not affected by amendment. The amendment affected the stand density of Spaulding, and this stand density effect appears to be the cause of the yield difference. The reason for the stand difference is not clear, but it does not appear to be caused by physical differences in the seedbed.

In 2005 we will measure penetrometer resistance, bulk density, and aggregate stability in the spring before incorporating the cover crops, and again in mid to late summer when all plots are in winter squash. We will sample for organic matter, pH, P, and K in spring before application of amendments and incorporation of cover crops. We have a graduate student on other funding who is working on light fraction soil C measurements, and we plan to collect samples for light fraction C before cover crop incorporation, one month after incorporation, and in late summer. We will also begin infiltration measurements in the summer of 2005, using a single ring infiltrometer.

On-farm cooperators.

We measured bulk density, aggregate stability, and penetrometer resistance at multiple locations on each of the cooperating farms. We collected composite samples for soil organic matter, pH, Bray P, and exchangeable K. Mean bulk density and aggregate stability values are compared across all sites (including the WSU experiment) in Appendix Table 3. Penetrometer resistance is shown in Appendix Figure 3. Although we could not make statistical comparisons, the following trends were apparent.

Bulk density: FC < MEF < TB < WSU

Organic matter: FC > MEF = TB >= WSU

Penetrometer: FC < TB (vegetables) < WSU = MEF < TB (raspberry)

Aggregate stability: FC > WSU > TB(raspberry) > MEF > TB (vegetable)

From the point of view of physical properties, the soil at FC had the highest quality, with considerable variation among the other sites, depending on the particular property measured. Soil chemical properties are shown in Appendix Table 4. The nutrient picture is different from the physical properties, with farm FC being borderline deficient in P and K, and MEF and TB having excessive levels of those nutrients.

A number of factors affect these differences, including current management, past management, and soil texture and landscape position. During 2005 we will investigate these differences among the sites, determine soil texture at all sites, and also identify paired conventional farms for at least two of our organic sites, where we will do similar soil measurements. We will identify all sampling areas via GPS.

Orchard mulches and weed management experiments.

The sub-objective of these experiments is to evaluate soil quality differences related to in-row weed control methods in orchards including mechanical tillage, wood chip mulch, living mulch, or control treatments applied to the weed strip. Soil infiltration rate and resistance were measured in 2004 to help set a baseline for monitoring the soil quality impacts of repeated mechanical tillage compared to untilled or mulched soil.

The orchard mulch trial was established in 1999 in a 5 yr-old Red Delicious/M26 block. Spring applied treatments included the control (no till, mow), wood chip mulch, shredded paper mulch, chopped alfalfa hay and living clover mulch with 5 replicates. These were all applied to the weed strip in the tree rows. Fall-planted 1999 living mulch treatments included mustard, winter rye, and white clover. In spring of 2002, mint slugs and a bentgrass living mulch were applied to the mustard and rye plots. In May 2003, grass hay was applied to the alfalfa hay plots and an insectary mix living mulch replaced white clover. Plots were inadvertently tilled using the Wonder Weeder in spring 2004 and so were not maintained.

A new orchard weed control trial was set out in an 8-yr old block of Gala/M26 in 2004. Treatments include control wood chip mulch, Weed Badger (3x), Wonder Weeder (2x), Wonder Weeder (3x), and Wonder Weeder (4x) with 5 replicates.

Soil resistance measurements were taken in the new tillage trial using a cone penetrometer with readings from 0-42.5 cm depth, at 1.5 cm intervals. Nine measurements were made per plot. We were not able to take valid measurements in the long term mulch trial due to a number of gopher tunnels and spring tillage.

There were no differences in soil resistance (KPa) among treatments at the surface or below 180 mm soil depth (Appendix Table 5). Differences did show up between 60mm and 150mm. The undisturbed control plots (mowed) showed higher soil resistance (1116 kpa) than all the other treatments at the 60mm depth including the undisturbed wood chip mulch. At 90 and 120 mm, soil resistance in the control plots was again higher than for the wood chip mulch, but similar to the tillage treatments. Our hypothesis was that the wood chip mulch would improve soil physical properties and the tillage would degrade them relative to the control.

Soil infiltration rate was measured in 2001 and 2004 in the long-term mulch trial, and in 2004 for the new trial. Three measurements per plot were taken. Six-inch diameter rings were filled with water to a 1” ponded depth and time of infiltration recorded.

In the tillage trial, measurements were taken in early August, after Weed Badger and Wonder Weeder (3x) plots had been tilled 3 times. Wonder Weeder plots 2x had been tilled twice. Initial and follow-up (T1 and T2) infiltration rates were measured with Time 1 at ambient soil moisture near the end of an irrigation cycle when the soil was drier; and Time 2 following T1 to simulate soil moisture at field capacity. Infiltration in the Weed Badger 3x plots was significantly slower than in the other treatments (Appendix Table 6). Soil resistance data was not the obvious explanation for this, as results were similar to other treatments. Measurements on the wood chip plots were not valid. In the long-term mulch trial only T1 measurements were made in 2001. Infiltration in the paper mulch plots was faster than for the control, mustard, and rye plots but similar to wood chip and clover treatments. This difference can’t really be explained at this point. None of the plots had been disturbed since being established in 1999. In 2004, there were no significant differences among treatments (Appendix Table 7). However, these plots (except clover) had been disturbed by early spring tillage in 2004, and their treatment management had lapsed some in the previous year. In addition, soil samples were collected from the Wonder Weeder plots and will be used for soil carbon analysis pending a decision on what test(s) will be most useful.

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

We have not made progress on this objective to date besides obtaining and testing the SCI spreadsheet and having farmers collect field activity data. This winter we will begin inputting data from our experiments and the cooperating farms into the SCI spreadsheet.

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

We are comparing the establishment, winter cover, weed density, and nitrogen effects of different types of cover crops relay planted into snap bean and sweet corn. This experiment was begun in 2003 under another funding source.

In 2004 cover crops were interseeded into sweet corn (Blaze) and snap bean (Jade) 35 days after planting the cash crops. The experiment includes four experimental treatments and two control treatments, replicated four times in a randomized complete block design. Each plot measures 10 ft by 30 ft, and the soil is a fine sandy loam. The experimental treatments differ slightly in the bean and corn plots. Experimental treatments in the beans include:

Hairy vetch planted at a rate of 50 lb/acre in single rows between the bean rows using a Planet Jr. planter

Woolypod vetch planted in the same manner

Hairy vetch broadcast by hand (75 lb/acre) and incorporated using light tillage

Red clover broadcast by hand (30 lb/acre) and incorporated using light tillage

Fall seeded rye-hairy vetch control

No cover crop control

All cover crops were broadcast in the corn, because the corn plants were too tall to survive equipment traffic at the time of planting. Treatments 1 and 2 were broadcast at 50 lb/acre, and Treatments 3-6 remained the same as for beans. Crops were irrigated as needed and managed organically. Weed management was done with shallow cultivation (basket and tine weeding) before cover crop planting, and one hand weeding, and one hand spot weeding between cover crop planting and harvest. Dates of planting, harvesting, mowing, and incorporation of cover crops and cash crops are summarized in Appendix Table 8. This table also includes dates for the 2003 experiment.

Before planting the cash crops in the spring, soils were sampled to a depth of 1 foot for nitrate-N. Although some of the cover crop treatments from the previous year were supplying N, we fertilized with an organic source of N so that no plots were limited by nutrients. In this way we could evaluate if competition from the current cover crop affected yield of the cash crop.

Cash crops were hand harvested and fresh yield determined. After harvest of the cash crop, the residue was mowed and left on the surface. Cover crop stand densities were measured in November by averaging stand densities determined by two observers in three representative 0.25 m2 quadrats in each plot.

Biomass produced by the 2003 cover crops by early April 2004 is shown in Appendix Table 9. Hairy vetch had the greatest overall biomass, with woolypod vetch also producing substantial biomass. Both hairy vetch and woolypod vetch established a good ground cover by late October 2003. Common vetch failed to become established as a relay crop. Crimson clover also fared poorly, with too little biomass to measure. Red clover produced low amounts of biomass. Spring soil nitrate N was greatest under hairy vetch. Based on the 2003 results, we modified the treatments in 2004, eliminating common vetch and crimson clover, broadcasting red clover, and adding a broadcast hairy vetch treatment.

The 2004 cash crop yields were not affected by the cover crops, indicating that competition by the cover crop was not significant. Because our system is irrigated, the risk of competition for water was less. Mean yield of snap bean was 6.6 ton/acre fresh weight, and sweet corn was 11.9 ton/acre fresh weight.

Cover crop stand in November 2004 was greater in the bean experiment than in the corn experiment. (Appendix Table 10). This was not the case in 2003, when cover crop stands were similar in the corn and bean experiments. In 2004 the corn residue was mowed sooner, and would have been greener. Also, rainy weather following mowing made the mowed residue heavier and more compact and more likely to smother the cover crop. Broadcast red clover compared favorably with the vetches in emerging from the corn residue..

Results to date suggest that relay planted cover crops can be a viable option for organic growers for some crops. Hairy vetch, woolypod vetch, and possibly red clover show promise in our environment. Challenges include weed management and shading by cash crops and their residues.

The cover crop treatments for 2005 will be the same as those used in 2004. Corn and beans will be the cash crops again, with the two crops rotated to where the other was planted in 2004. The interval between planting the cash crops and planting the relay cover crops will be reduced by 5 to 7 days in 2005 to allow for drilling seed into the corn. Cover crop biomass, weed biomass, and weed species will be determined in representative 0.25 m2 quadrats (minimum 2 per plot) at the time of cover crop incorporation. In-row and between-row weed counts will be made in the cash crop just before the last cultivation and before harvest. We will also measure cover crop biomass tissue N concentration and N uptake at the time of incorporation. We will sample soil for nitrate N approximately two and eight weeks (pre-side dress nitrate test) after incorporation, and determine cash crop yield at harvest.

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

We held a winter cover crops field day at the WSU experimental site on February 19, 2004, and a fall field day on September 20 focused on cover cropping and the organic systems experiment. In addition, we participated in three summer farm walks on organic farms in western Washington, where we presented soil quality information related to the practices on the focus farms. Total field day attendance was 100, with an additional 75 at the farm walks. We also have cover crop research information on our web site at: http://www.puyallup.wsu.edu/soilmgmt/Abstracts&Pubs/IF-Sum-CvrCrp/IF_Sum_CoverCrop.htm

In 2005 we will hold a summer field day focused on soil quality and organic systems, and develop and hold a soil testing workshop for organic farmers, in cooperation with Oregon State University and other funding sources. We will present information on relay cover cropping at the Western Washington Hort Society meetings in January and post 2004 research results on our web site this winter. We will continue to participate in summer farm walks, and we will prepare summaries of this project for WSU and farmer based publications and web sites.

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.

Collaborators: