Decomposition and Nutrient Release Dynamics of Cover Crop Materials

Final Report for SW97-045

Project Type: Research and Education
Funds awarded in 1997: $41,064.00
Projected End Date: 12/31/2002
Matching Non-Federal Funds: $2,125.00
Region: Western
State: California
Principal Investigator:
Dr. Jeff Mitchell
University of California, Davis
Expand All

Project Information


Replicated field studies were conducted in conjunction with the West Side On-Farm Demonstration Project during the 1998 – 99 and 1999 – 2000 winters in Five Points, CA to determine growth and decomposition characteristics of barley, vetch, Phacelia and a barley/vetch mixture. Biomass production of these cover crops from October to mid-March ranged from 8,142 lbs/acre for the barley/vetch mixture, to 4281 lbs/acre for vetch. By six weeks following incorporation or burial, 50% of the original biomass of each cover crop had decomposed, and by 16 weeks following burial, more than 80% of the biomass had decomposed. These results indicate rapid breakdown of each of these materials during spring and summer in this region.

Project Objectives:
  • The objectives of this research were:

    1. To compare potential cover crops or cover crop mixtures in terms of dry matter production and total nutrient content

    2. To monitor the loss of weight and the percentage of nutrients remaining as indicators of cover crop decomposition

    3. To compare the estimated amount of nutrients released from cover crops by the bag method vs soil sampling, and

    4. To showcase this work by holding periodic field data update meetings for interested people including local participating farmers in the West Side On-Farm Demonstration Project


Winter cover cropping is an alternative agricultural practice that has received much attention as a means for ameliorating soil physical properties, adding carbon and nitrogen to the soil, and for contributing to pollution reduction. The use of off-season winter cover crops as green manures can provide many benefits to annual cropping systems including surface cover to reduce rainfall runoff (Joyce at al., 2002), root channels or pores to increase water infiltration (Joyce et al., 2002), and scavenging and cycling of residual nitrate (Jackson et al., 1993) and thereby preventing leaching losses during rainy periods. Keisling et al. (1994) showed that hydraulic conductivity and bulk density were significantly improved as a result of winter cover cropping. The use of cover crops has also been shown to be effective in stabilizing soil aggregates and improving porosity. As sediment and nutrient-bearing runoff decreases, surface water quality would, in theory, improve. Winter cover cropping, however, has not been widely adopted in many of California’s highly productive row crop production valleys largely because producers do not have adequate experience with and information on many aspects of cover crop management, and the relative costs and benefits associated with the use of cover crops in these regions of traditionally high value crop production (Andrews et al., 2001).

The West Side of the San Joaquin Valley, from Los Banos in the north and Kettleman City in the south, is one of the world’s most productive agricultural regions. The leading crops of this region include processing tomatoes, cotton, onions, garlic, cantaloupe and lettuce that are grown on over 570,000 acres annually. During the last 30 years, land use patterns on the West Side have changed considerably. Over 60% of the acreage in this area was typically planted to wheat, barley and safflower in 1965, whereas in 1994, these crops were grown on less than 7% of the area. The intensification in the production of high-value crops has led to fewer additions of organic mater to the soil, more aggressive tillage operations and a reported decline in soil quality. In the fall of 1995, a group of 12 West Side farmers in conjunction with a team of University of California extension advisors, specialists and researchers, as well as other public and private agency consultants, initiated an extensive on-farm demonstration project of soil building practices and pest management options in this intensive row crop region. This project, Extending biologically integrated farming practices within the San Joaquin Valley’s West Side, evaluated sixteen on-farm demonstration comparisons of biologically soil building / pest management systems with conventionally managed systems. In each of the biologically integrated parcels, cover crops and composted organic materials were integrated into rotations wherever appropriate, whereas in the conventionally managed parcels mineral fertilizer applications are made. Key soil physical, chemical and biological attributes were monitored at each site. These soil property data were then used to develop and test a soil quality index for the region (Andrews et al., 2001).

While there is considerable dogma about the difficulty of increasing organic matter and humus contents of soils such as those of the irrigated, arid and hot Central Valley, very little is actually known about basic mechanisms and rates of organic matter decomposition under customary crop management conditions of this region. If farmers are to invest wisely in organic soil inputs, as a possible means of conditioning their soils and efficiently cycling nutrients, they need more information than is currently available on these aspects of organic amendments.

There is not a unique technique to measure decomposition of organic materials in the field. Some of the direct methods include the use of wire screening containers (Falconer et al., 1993), wooden boxes covered with wire mesh (Attiwil, 1968), core sampling (Weary and Merriam, 1978), tethered leaves (Vitousek et al., 1994), or litter bags (Singh and Gupta, 1977). Among the different methods, the litter bag technique has been commonly used in a number of terrestrial ecosystems. Although this approach may underestimate the rate of decomposition because of the restriction to the entry of large invertebrates, scavengers or decomposers (Singh and Gupta, 1977); Wieder and Lang, 1982), or from contamination by roots, leaves, and mosses entering the bags (Swift et al., 1979), the litter bag technique presents more advantages over other methods. The litter bags minimize losses due to fragmentation (Singh and Gupta, 1977), and it is generally assumed that the results will reflect trends that are characteristics of unconfined litter. Selection of the litter bag mesh size depends partially on the size range of soil fauna, but it is often a compromise between providing maximum access and minimizing residue losses (Swift et al., 1979). In previous studies, the use of mesh sizes from 0.5 to 5 mm resulted in similar decomposition values (Macauley, 1975). Basically, 1 mm openings allow access to all microorganisms and invertebrates except earthworms. The use of the litter bag technique to monitor cover crop decomposition is suitable for cases in which the cover crop is used as a surface mulch since the litter bags can be filled with the same amount of material used as mulch in the surrounding surface. Extrapolation of this technique to incorporated cover crops is also possible. However, results obtained from this application may be slightly biased because not all plant material in the bag will be in direct contact with the surrounding soil. To our knowledge, this WRSARE study represented the first atempt in California to make use of this application of the litter bag technique to characterize decomposition of incorporated cover crops.


Materials and methods:

These studies were conducted during 1998 – 99 and 1999 – 2000 in two different 2.7 acre fields at the University of California West Side Research and Extension Center in Five Points, CA, located about 40 miles southwest of Fresno, CA. In August and September prior to each experiment, the fields were planted with a sorghum/sudan hybrid crop to create more uniform water and fertility initial conditions for the start of the cover crop work. The sorghum/sudan was mowed, baled and removed in late September. The fields were then leveled and 40” beds were prepared. On October 26, 1998 and October 21, 1999, the four cover crops were planted in 0.09 acre (14 ft X 280 ft) plots replicated 6 times. Plant biomass determinations were made when the cover crops were chopped using a Buffalo Rolling Stalk Chopper which cuts and crimps the cover crop into 7 – 8 inch pieces in mid-March of the following year of each winter growth period.

After the cover crops had been chopped, samples were taken randomly from each plot, spread out and air dried for one week using circulating fans in a greenhouse. The chopped cover crops remaining in the field were also dried for one week before being incorporated into the soil to an average depth of about 8 inches using a disk harrow. Disking was done twice to simulate spring land preparation practices of farmers who use cover crops in the San Joaquin Valley. Four 40” beds per plot were then prepared using a furrow sweep “lister” implement. About thirty 40 gram subsamples of each cover crop were weighed, dried for four days at about 120F in a forced air drier, and reweighed to determine moisture content. About 20 grams of the air-dried material was placed into a pre-weighed 7 X 7” plastic mesh bag and then weighed to determine the exact amount of cover crop in each bag before being closed using stainless steel staples. In 1999, about 600 of these mesh “litter” bags were prepared for each cover crop, and in 2000, about 1000 bags were used. These bags were then placed at a depth of about 8 inches into the prepared field beds so as to match cover crop materials in the litter bags with cover crop materials incorporated into the field plots. Six fallow (no cover crop) plots were included in the field plot design randomization. Sprinkler irrigations were applied about every two weeks to replenish water evaporated from the soil surface so as to simulate soil water conditions in irrigated fields At 1, 2, 4, 8, 12, and 16 week intervals following burial, 5 – 8 bags per plot were retrieved and stored at about 34F for about one to two days before being opened. The cover crop materials remaining in each bag was then carefully cleaned by hand removing adhered soil, air dried in paper bags and then weighed. The percentage of cover crop material remaining in the bags at each retrieval time was then averaged according to cover crop type and this was plotted for each year. Three soil samples per plot were taken and composited in conjunction with cover crop burial and each of the litter bag retrieval dates at 0 – 1 and 1 to 2 ft for determinations of nitrate and ammonium.

Research results and discussion:

Cover crop biomass production was actually monitored monthly during each growth season, however, only data for the last harvest, representing the final amount produced at the time the cover crops were terminated are shown (Table 1).

Table 1

Cover crop biomass March 30, 1999

Total biomass % Weed biomass

Barley 6928 + 844 2.2
Barley/vetch 8142 + 204 2.5
Phacelia 5567 + 346 1.6
Vetch 4281 + 860 8.2

Cover crop decomposition characteristics for each of the four cover crops tested in this study for 1999 and 2000 are shown in Figures 1 and 2. In 1999, only 42 – 48% of the original biomass of the vetch, phacelia, and barley/vetch cover crops was present in the litter bags two weeks after burial. 60% of the barley cover crop remained at two weeks after incorporation. By 8 weeks after burial, only 10 – 20% of the original material remained in the litter bags for all cover crops, and by 16 weeks, less than 10% of the original material remained.

Figure 1

In 2000, similar, but slightly more gradual patterns of decomposition were seen (Figure 2). Two weeks after burial, percentages of original biomass remaining in the litter bags randed from 45 for vetch, to 72% for barley. By 8 weeks after burial, about 20% of the vetch remained, while about 35% of the barley remained. By 16 weeks, less than 20% of the original material of all cover crops remained.

Figure 2

To our knowledge, this is the first study to evaluate decomposition characteristics of green manure cover crops in California’s Central Valley using the litter bag technique. We therefore cannot directly compare the findings of this work with other studies. Our results, however, seem to both confirm “common knowledge” regarding organic matter decomposition in this region, which suggests that residues turn over or “disappear” quite rapidly, and also they document rather strikingly rapid decomposition rates of materials with quite different C:N ratios. An analysis is underway to evaluate soil NO3- and NH4+ for the sixteen week sampling period following cover crop incorporation in each year of the study.

An important practical outcome of these studies is the obvious fact that each of these cover crops decomposes virtually entirely within a single summer season. This observation indicates that persistence of incorporated residues through a typical summer crop period will not likely be an obstacle or impediment to cultivations or harvesting. This finding may allay concerns that growers who have not used cover crops previously may have had about “extra” management and expense associated with their use, and may dispel fears that cover crops may somehow contaminate or lower the quality of machine-harvested crops such as processing tomatoes (i.e increase MOT (material other than tomato)).

Research conclusions:

The findings of this work contribute important information to our growing, but still quite limited body of knowledge related to cover crop use in the San Joaquin Valley. While the preliminary findings of this work have already been presented at a number of relevant venues, I would expect that they will become rather important components of the general knowledge base for cover crops in this region. The fact that the results of these studies (as well as of the additional third year of data that are also very similar to the first two years) were as consistent as they were suggests or indicates that these data may become fundamental to our understanding of organic matter decomposition in this area. I also expect that these findings will serve as points of departure for our growing research program on reduced, or conservation tillage production approaches in which cover crop materials are not incorporated into the soil as they were in this project, but rather deliberately left on the soil surface undisturbed.

By virtue of our conducting this project in conjunction with the West Side On-Farm Demonstration Project, we have had an opportunity to intensify the focus of this broader program and produce relevant science-based information that may result in increased understanding of cover crop management in the West Side of the San Joaquin Valley. The West Side On-Farm Demonstration Project, which has involved twelve large-scale farmers in the Huron to Mendota region of western Fresno County, has enabled these farmers to become considerably more familiar with alternative management practices such as winter cover cropping and impacts of these practices on soil properties and soil quality (Andrews et al., 2001). This project, sponsored by WRSARE, has thus provided new information on an important management option for soil carbon sequestration in this very important production region.

Participation Summary

Research Outcomes

No research outcomes

Education and Outreach

Participation Summary:

Education and outreach methods and analyses:

I expect to submit the following two manuscripts for consideration for publication by September 1, 2002.

Mitchell, J.P., T.K. Hartz, G.S. Pettygrove and W.R. Horwath. Decomposition and nutrient release dynamics of cover crop materials. Agronomy Journal.

Mitchell, J.P., T.K. Hartz, G.S. Pettygrove and W.R. Horwath. Cover crops for the San Joaquin Valley’s West Side. California Agriculture.

The first of these manuscripts will summarize the basic cover crop decomposition and soil nitrogen data that have been compiled in this WRSARE project. Discussion will also evaluate the litter bag technique for this application and relate our findings to other “litter” decomposition work. The California Agriculture article will present our basic time-course cover crop growth data, our soil water monitoring by neutron probe for each cover crop during the winter growth period, and chronicle in a more general way, broad results of the West Side On-Farm Demonstration Project.

Once the full body of data that this project has produced is synthesized, it will become a part of our knowledge base for alternative soil management systems for this region and can then be readily inserted as appropriate into our ongoing extension education programs. Though the formal funding of the West Side On-Farm Demonstration Project has expired, we still are very much involved with the core group of farmers that made up this effort, and the WRSARE project information will be further presented and discussed with these farmers in both formal group and individual meeting formats.

Data and information resulting from this project have been incorporated into the following presentations.

Assessing and improving soil quality in the Central Valley: What have we learned from on-farm studies during the last three years? Biologically Integrated Vineyard Systems Group. Invited presentation. Brooks Ranch, Fresno, CA. March 2, 1999. 25 participants.

Cover crop decomposition and nutrient release dynamics project and results of the West Side BIFS Project soil quality monitoring. UC West Side Research and Extension Center, Five Points, CA. August 5, 1999. 50 participants.

Impacts of alternative management on soil quality in the BIFS and SAFS Projects. Presentation, focus group and handouts to West Side BIFS Project participants. UC West Side Research and Extension Center, Five Points, CA. October 2, 2002. 18 participants.

Management strategies to sequester soil carbon. 2001 California Plant and Soil Conference. California Chapter of the American Society of Agronomy and California Plant Health Association. Radisson Hotel, Fresno, CA. February 8, 2001. 80 participants.

Management strategies for soil organic matter. San Joaquin Valley Fertility and Management Short Course. UC West Side Research and Extension Center, Five Points, CA. March 14, 2001. 46 participants.

Use of organic amendments in soil nutrient and quality management. Evaluations and impacts of on-farm studies in California’s Central Valley. In Soil fertility issues and practices. Concurrent session of Partnerships for sustaining California agriculture. Heidrick Ag History Center. Invited presentation. Woodland, CA. March 28, 2001. 30 participants.

Education and Outreach Outcomes

Recommendations for education and outreach:

Areas needing additional study

This project has investigated one aspect of the use of cover crops in this region of California's Central Valley and has provided information on the short-term fate of cover crop materials in production systems in this area. Clearly, however, much more work is needed in order to follow and better understand longer-term impacts of repeated integration of these organic materials in these cropping systems. It will be important to relate these longer-term monitoring efforts on potential changes in soil properties, crop productivity and possible, off-farm nutrient, soil and water losses. It will also be quite useful to follow up on this initial study by evaluating the fate of cover crop materials that are not incorporated into the soil but rather, left on the surface as in conservation tillage production. In a variety of follow-up studies, we are currently actively pursuing these issues.

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.