A System Approach for Improved Integration of Green Manure in Commercial Vegetable Production Systems

2003 Annual Report for LS02-140

Project Type: Research and Education
Funds awarded in 2002: $171,800.00
Projected End Date: 12/31/2005
Region: Southern
State: Florida
Principal Investigator:
Johannes Scholberg
Agronomy Department, University of Florida

A System Approach for Improved Integration of Green Manure in Commercial Vegetable Production Systems


We have completed our second year of this program and based on previous results we were able to fine-tune some of our cropping systems. It is concluded that use of summer cover crops in S-Florida seems to provide the greatest short-term benefits since summer cover crops are directly followed by fall vegetable crops and use of summer cover crops can reduce N requirements by 50% or more. At northern locations we opted to follow summer cover crops with leguminous winter cover crops. On more fine-textured soils leguminous-based winter cropping systems seem to perform satisfactory. However, most leguminous winter cover crops are poorly adapted to Florida sandy soils. Due to their slow initial growth a substantial fraction of N accumulated in the sunn hemp residue is not effectively used. Overall N benefits from cover crop based systems for these systems were on the order of 30-67 kg N. During subsequent years we aim to use a mixture of fast growing non-leguminous crops with a slower growing leguminous winter cover crops. Use of such mixtures would ensure in more robust winter cover crop systems that would be better adapted to a wider range of soil en climatic conditions yet their carbon to nitrogen ratio would be fairly low and thus allow for rapid decomposition prior to establishment of a following spring vegetable crop. We anticipate that over time long-term benefits of cover crops use will be come more articulated.

Objectives/Performance Targets

A1) Determine which (combination of) green manure crop(s) when used with minimum
tillage will result in optimal nitrogen supply to subsequent vegetable crops;
A2) Assess the amount of supplemental N fertilizer required for optimal yields for green
manure based production systems and compare their yields with conventional systems;
A3) Determine nitrogen uptake efficiencies and N leaching for green manure-based cropping
systems in comparison with conventional vegetable cropping systems; and
A4) Develop a regional research and outreach program for improved integration of green
manure crops in commercial vegetable systems in close collaboration with local growers.
B1) Evaluate the long-term effects of included treatment combinations on soil quality and the
abundance and diversity of weed, arthropod and nematode populations;
B2) Improve the exchange and integration of information on the use of green manure crops in
commercial production systems in the SE region;
B3) Development of web-based nutrient management systems that will allow producers to
make more efficient use of organic nutrient sources such as green manure crops; and
B4) Use of the Decision Support System for Agrotechnology Transfer (DSSAT) model for risk
assessment (environmental and economical) of above cropping systems for a number
of farming systems and locations throughout the southeastern US.


In Citra, N-Florida, Sunn hemp produced between 7 MT/ha of dry weight in 12 weeks during the 2003 growing season compared to 8 and 12 Mt/ha during 2001 and 2002. The decline in productivity is related to increased incidence of Verticillium a soil-born disease, which at the end of the growing season affected 50-81% of the plants compared to 1-10% during the second year. Although sunn hemp clearly outperformed any other covercrop by far, alternating it with another less productive summer cover crop will be required on sandy soils to prevent disease built up. We therefore propose to develop a more dynamics crop rotation starting the summer of 2004 including cowpea and velvet bean. During the 2002/2003 cropping season we used cahaba white vetch, since this crop was recommended by Dr. Phattak. However, it appears that this crop is poorly adapted to Florida sandy soils and its production ranged from 1-3 MT in 20 weeks. Use of sunnhemp reduced inorganic nitrogen fertilizer requirements of sweet corn by 20-30% and reduced weed growth by 78%. However, on the poor sandy soils corn yields in the absence of fertilizer were invariably very low (< 1-2 Mt/ha) and with the use of cover crops supplementary N-fertilizer rates of 130-160 kg N may be required to attain maximum yield compared to 266 kg N required to maximize yield of non-covercrop treatments. In Tifton, Georgia, Sunn hemp produced 7 MT/ha in 10 weeks and Kahaba white vetch produced 4 Mt in 20 weeks. Double covercrop amended systems resulted in marketable corn yields of 8.1 Mt/ha compared to 1.5 Mt/ha for control treatments and N benefits from cover crops were on the order of 50-70 kg N. Benefits and yield response to fertilizer were greatest when low supplementary N-fertilizer rates were used. In Georgia, Verticillium wilt also occured, but during the second year it only affected less then 1% of the plants. On-farm work in S-Florida showed that Sunn hemp produced 4 MT/ha in two months. The participating grower indicated that it was undesirable for plants to become excessively large since plants would get too “woody”. She stated that this could potentially interfere with bed preparation of mulched vegetable cropping system. Keeping the cover crop small by mowing, resulted in smaller plants with less stems, yet proportionally more leaves. Growing Sunn hemp prior to tomato or pepper, reduced inorganic nitrogen fertilizer requirements of these vegetable crops by 33-67%, and resulted in similar or higher yields compared to the conventional treatment. The greater efficiency of the Sunn hemp residue in replacing inorganic N-fertilizer in S-Florida compared to N-Florida was related to tomato and peppers being grown directly after Sunn hemp. In N-Florida and Georgia, most vegetable crops are grown during the spring season, so substantial amount of nitrogen released from summer cover crops may be lost during the 4-5 months prior to planting. Based on sampling of sunn hemp it was determined that 4-6 weeks after the senescence less then 60% of the weight and less then 20% of the N were retained in the residue itself. This underlines that following a summer crop with a relatively rapid growing winter covercrop will increase its benefits via improved retention of previously fixed N and other nutrients. Because of the inconsistent performance of Kahaba white vetch as a winter cover crop in N-Florida we substituted this crop with radish aiming to more efficiently utilize residual soil N from sunn hemp residue. One of the considerations to include this crop was its rapid initial growth and its very deep root system, which should enhance the recycling of nutrients from greater soil depths, and could also enhance soil structure at greater soil depths. However, germination of radish was erratic due to the presence crop residue on the soil surface and we planted a mixture of hairy vetch and winter rye instead. This appeared to be an excellent mixture and initial results appear to be very promising (final growth data are still pending). Soil analysis of the upper 6 inches of the soil showed that total soil nitrogen and organic matter were slightly higher in cover crop treatments compared to conventional treatments. We will continue to monitor soil properties and we expect that differences will become more pronounced over time, especially in the topsoil layer (0-3 inch). Soil analysis also showed increased root-knot and lesion nematodes numbers after a sweet corn crop. Use of Sunn-hemp, on the other hand, reduced nematode populations compared to the conventional treatment. Results from suction lysimeters in the N-Florida location were hampered by technical problems during 2003. We will repeat this work during the spring of this year and also use alternative methods such as soil coring and resin traps to complement this work. These approaches should provide us more detailed information on nitrogen release and leaching patterns from crop residues and/or inorganic fertilizer over time. These results will be essential for the development of decision support system that will assist growers to better match nitrogen supply with crop nutrient requirements for cover crop based production systems. Laura Avilla, a graduate student originating from Costa Rica, joined our program in August of 2004 and has assumed responsibility of managing field trials in Gainesville. During the third year we will focus more on the economic and environmental aspects of the use of cover crops. Kari Reno, an undergraduate student at the School of Natural Resource and Environment at the Univ. of Florida will do an internship with our group during the summer of 2004 and will participate in environmental quality assessment studies. The detailed information on biomass and nitrogen accumulation and subsequent the breakdown of crop residues will allow us to calibrate decision support systems that will be used to assess the environmental and production benefits of cover crops for a wider range of conditions. A post-doctoral researcher will start working on the development of computer modules this summer that will allow growers to assess N availability from covercrops for different climatic and management conditions. Development of this software should stream-line integration of covercrops in commercial production since it will allow growers to improve their understanding how much N is release over different time periods.

Impacts and Contributions/Outcomes

Corey Cherr, the graduate student participating in SARE program, presented our results at the Florida Agricultural Conference and Trade Show (FACTS) in Lakeland on April 30, 2003 and at two scientific presentations during the national annual meetings of the American Society of Agronomy/Crop Science Society of America/Soil Science Society of America in Denver on November 4, 2003. Johan Scholberg presented our initial research findings for a invited guest seminar lecture for Doctor of Plant Medicine Program at the University of Florida. The DMP program is a unique program that trains students to become crop consultants and work directly with farmers. On February 2nd, we presented results from our program to the Junior Science, Engineering and Humanities Symposium (JSEHS) at the University of Florida to participating high school students to increase their awareness of the importance of sustainable agriculture via improved use of cover crops in conventional production systems. During the second week of April in 2003 research trials was shown to 25 students participating in the southern regional student activities subdivision of the ASA/CSSA/SSSA. On January 23, 2004 we hosted a field tour showing to 40 growers participating in the Southern Sustainable Agricultural Working group and developed a brochure that outlines our initial results. On March 12, 2004 we had our second annual group meeting and during a small conference participants presented and shared results and we discussed program priorities for this coming year. On March 15, Johan Scholberg outlined research to a vegetable group meeting of 40 growers in Immokalee, S. Florida. It appears that this program has generated a lot of excellent information and we start learning a lot about what cover crops systems work where and why. Participation and feedback from growers and grower’s organization will remain essential for us to fine-tune the program and in this manner we anticipate to further increase the program impact and general awareness of growers, students and scientists of our cover crop program.


Robert McSorley

[email protected]
Professor Nematology
Entomology and Nematology Dept., Univ. of Florida
Natural Area Drive PO Box 110620
Gainesville, FL 32611-0620
Office Phone: 3523921901
Sharad Phatak

[email protected]
Professor of Horticulture
Department of Horticulture, University of Georgia
P.O. Box 748
Tifton, GA 31793-0748
Office Phone: 2293863355
Marty Mesh

[email protected]
Executive Director FOG
Florida Certified Organic Growers & Consumer Inc.
PO Box 12311
Gainesville, FL 32604
Office Phone: 3523776345
Website: http://www.foginfo.org/
Nancy Roe

[email protected]
Director of Farming Systems Research Inc.
Farming Systems Research Inc.
5609 Lakeview Mews Dr.
Boynton Beach, FL 33437
Office Phone: 5616382755
Website: www.vegies4u.com
Odemari Mbuya

[email protected]
Assistant professor
Florida Agricultural and Mechanical University
College of Engineering Sciences Technology and
Agriculture Center for Water Quality
Tallahassee, FL 32307-4100
Office Phone: 8505993383