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Summer Cover Crops for Organic Vegetable Production: Preliminary Results

 

By Dr. D.D. Treadwell Assistant Professor, and M. Alligood, Laboratory Technician, Horticultural Sciences Department

 

 

Project Overview

The benefits of crop rotation include improved soil physical properties, increased nutrient availability to subsequent crops, and reduced pest incidence (Liebman and Ohno, 1998). In cropping systems, rotation serves to increase biodiversity. Biodiversity is important in agricultural systems because it is the foundation for many pest-predator interactions, efficient nutrient cycling, and effective weed management. There are several recent reports that organic farming methods contribute to greater biodiversity than conventional systems (Bengtsson et al., 2005; Fuller et al., 2005; and Hole et al., 2005). The subtropical climate of Florida is challenging for vegetable growers, and few production recommendations from Land Grant Universities exist for organic vegetable producers in this climate. To examine the effect of rotation and biodiversity on production of organic spring and fall vegetable crops, a three year project was funded by the USDA Integrated Organic Program on nutrients (Treadwell), weeds (Carlene Chase, PI), nematodes (Robert McSorley), insects (Oscar Liburd), and plant diseases (Lawrence Datnoff). To assess how treatments would respond in a tropical climate, this project was repeated in the US Virgin Islands in St. Criox (Ramon Arancibia and Stuart Weiss) on a certified organic farm.

 

Materials and Methods

This project is aimed at developing holistic, integrated strategies for nutrient and pest management in organic vegetable systems. Systems research has been defined by Drinkwater (2002) and further explored in Land Grant University research by Mueller et al. (2002). Employing this experimental model, a two year field trial was initiated in the Organic Unit at the Plant Science Research and Education Center in Citra , Florida in 2006. Several different organic vegetable production systems, as treatments, differ by the number of crops grown in a single plot in time and space. The systems ranged from simple (monocultures) to complex (bicultures). There are 14 unique systems in all, arranged in a randomized complete block design and replicated four times. Each plot is 12 m long by 12 m wide and beds are 1.8 m on center. Plots are separated by a 12 m buffer. A summary of systems is presented in Table 1.

The experiment is located on a Candler sand (Hyperthermic, uncoated Lamellic Quartzipsamments). This series consists of > 94% sand to a depth of > 200 cm and has low available water holding capacity, low organic matter and low inherent fertility. The Candler series is commonly distributed on uplands in peninsular Florida . Typical agronomic use includes pasture, citrus, tomato and watermelon production.

Prior to beginning the experiment, the study area was planted in cowpea ‘White Acre' at 56 kg ha -1 on May 17, 2006 and terminated June 22. Following soil incorporation, immature compost composed of spent mushroom waste from a commercial mushroom facility was applied at 22,400 kg ha -1 and incorporated to a depth of 20 cm. Summer cover crops were planted on July 27. Monocultures of sorghum sudangrass ‘Brown-Midrib' and pearl millet ‘Tifleaf3' were drilled in 18 cm rows, 5 cm-deep at 44.8 kg ha -1 and 28 kg ha -1 , respectively. Sunn hemp and velvetbean ‘Georgia Bush' monocultures were established by broadcasting seed at 44.8 kg ha -1 and 112 kg ha -1 , respectively. Mixtures of sorghum sudangrass and velvetbean were established using 2/3 of the monoculture rate of each species (30 kg ha -1 and 74.6 kg ha -1 ; respectively), and mixtures of pearl millet and sunn hemp were established with a two-thirds rate of pearl millet (18.6 kg ha -1 ) and a half rate of sunn hemp (22.4 kg ha -1 ). All legume seed was inoculant with the appropriate inoculant prior to planting. On Oct. 2, cover crops were flail-mowed and incorporated in preparation for cash crop establishment.

Field operations pertinent to nutrient management are summarized in Table 2. To date, soils were analyzed on three occasions: a preplant soil sample by replication prior to planting cover crops, and two sample events during the course of the trial: 1) immediately following cover crop termination (but prior to cover crop incorporation) and 2) immediately following the final vegetable harvest. In the summer prior to cover crop planting, a total of 24 soil cores from each replication were collected using a manually operated 1.9 cm (inner radius) soil probe to a depth of 20 cm, combined and mixed in the field and submitted to Waters Agricultural Laboratories, Inc. (Camilla, GA) for nutrient composition and corresponding fertilizer recommendations for fertility. For the two samples that occurred during the trial, 24 cores were collected from each plot and submitted for analysis in the same manner described above.

Two subsamples of above ground cover crop biomass were reserved for total carbon and nitrogen (N) content by combustion analysis (TruSpec CN, Leco, St. Joseph , MI ). Subsamples were dried in a forced air oven at a temperature of 120 C, ground with a 40 mm mesh Wiley mill. Approximately 1 liter of ground sample was collected for each subsample.

Preliminary analysis was performed using analysis of variance (ANOVA GLM, SAS V8, Cary , NC ) using cover crop species as a main effect. When treatment differences were significant, means were separated using Duncan 's least significant differences at alpha = 0.05. Future analysis will consider crop effects as well as rotational effects.

 

Preliminary Results and Discussion

Pretrial Soil Test. Results of the pre-trial soil sample are presented in Table 3. pH values for most of the plot area in the site were 5.6, therefore lime was recommended at a rate of 2240 kg ha -1 . Soil samples taken January 22 indicated the pH of most of the area had increased to 7.25, and no differences of soil parameters were observed among blocks.

Cover Crop Biomass. Dry weights of cover crops ranged from 893-5049 kg ha -1 and weights were significantly different by species. In general, grass cover crops produced more dry weight than legumes. Among monocultures, grass species pearl millet ‘Tifleaf3' and sorghum sudangrass performed similarly, producing 4547 kg ha -1 and 4115 kg ha -1 respectively. Among bicultures, sorghum sudangrass and velvetbean in combination produced dry weight similar to the grass monocultures (4471 kg ha -1 ). Velvetbean was a poor performer (893 kg ha -1 ) due to poor germination and slow growth and development compared to other cover crops.

Nutrients from Cover Crops. Following cover crop termination, nitrogen contribution was greatest in the sunn hemp (Table 4). No differences were observed in the N content of remaining cover crops. Of the 90 kg N ha -1 that was potentially available following sunn hemp, approximately 40% of that N was predicted to remain in the soil profile and be mineralized for uptake by the fall vegetable crop. Among all cover crops, this ranged from 12 kg ha -1 in velvetbean plots to 32 kg ha -1 in sunn hemp plots. In this particular soil series, the soil is very sandy and we are not confident 40% is an accurate estimate. The amount of N mineralized and retained could be substantially less, depending on soil microbial activity, precipitation, temperature, and amount of tillage. Vegetables were scheduled for planting two weeks after cover crop termination. Although cover crop nitrogen content can be estimated with previously published data on nitrogen averages multiplied by the dry weight produced, it is only an estimate. Soil nitrogen content is the most reliable method to determine residual plant available nitrogen. However, the short time window between cover crop termination and vegetable planting is a limitation for growers who wish to use soil N test results from a licensed laboratory to determine the amount of nitrogen to add to soil.

As a general rule, cover crop residues with C:N ratios less than 25:1 will decompose and result in the generation of NO 3 -N if there is sufficient soil to residue contact and the soil environment is favorable for soil bacteria. The carbon to nitrogen ratios were near 40:1 for all cover crop treatments containing a grass species (either in monoculture or in a biculture). Legume monocultures had C:N ratios of 12:1 (velvetbean) and 14:1 (sunn hemp) indicating a rapid generation of plant available N could be available quickly after termination and incorporation. In subtropical and tropical climates, these N transformations may occur too quickly to be utilized by subsequent crops; in sandy soils this N can be quickly lost to groundwater (Cherr et al., 2006). Mixing legumes and grasses is one strategy to reduce the risk of N loss. In the absence of readily available soil N, increasing C:N ratios slows the rate of decomposition by reducing the rate at which soil microbial metabolic processes can occur.

Soil Nutrients at Cover Crop Termination. Concentrations of nutrients were different among cover crop species only for potassium (Table 5). Velvetbean and fallow treatments had more potassium than remaining cover crop treatments, presumably due to the lack of or low biomass production in these plots. Phosphorus concentrations were sufficient for crop growth, N content was insufficient, and additional N was needed for the fall vegetable crop. Future nutrient management efforts will concentrate on improving the cation saturation balance ratio. Our most recent soil sample collected in January indicated our calcium levels increased dramatically from our September concentrations. This was likely due to the high concentrations of Ca in our irrigation water, and not the lime application. Water samples are routinely collected by the staff at the research station and during periods of dry weather the calcium concentration increases. Solutions for this are being explored. In addition, we continue to time our tillage events to minimize soil disturbance as much as possible to optimize soil organic matter content. Understanding the best cover crop – crop rotation to improve soil quality with the appropriate external inputs will be critical to grower adoption of organic methods.

 

Table 1. Research Design, Plant Science Research and Education Center, University of Florida

System		2006			2007
		Summer	Fall	Spring	Summer	Fall	Spring
Control
Control	1	Fallow	broccoli	sweet corn	Fallow	squash	pepper
Control	2	Fallow	squash	pepper	Fallow	broccoli	sweet corn
Simple Systems
1A	3	Sorghum Sudan	broccoli	sweet corn	Pearl Millet	squash	pepper
1A	4	Pearl Millet	squash	pepper	Sorghum Sudan	broccoli	sweet corn
1B	5	Sorghum Sudan	squash	pepper	Pearl Millet	broccoli	sweet corn
1B	6	Pearl Millet	broccoli	sweet corn	Sorghum Sudan	squash	pepper
2A	7	Velvetbean	broccoli	sweet corn	Sunn hemp	squash	pepper
2A	8	Sunn hemp	squash	pepper	Velvetbean	broccoli	sweet corn
2B	9	Velvetbean	squash	pepper	Sunn hemp	broccoli	sweet corn
2B	10	Sunn hemp	broccoli	sweet corn	Velvetbean	squash	pepper
Complex Systems
3A	11	Sorghum Sudan & Velvetbean	broccoli & crimson clover	sweet corn& beans	Sunn hemp & Pearl Millet	squash/rye-vetch	pepper & beans
3A	12	Sunn hemp & Pearl Millet	squash/rye-vetch	pepper & beans	Sorghum Sudan & Velvetbean	broccoli & crimson clover	sweet corn & beans
3B	13	Sorghum Sudan & Velvetbean	squash/rye-vetch	pepper & beans	Sunn hemp & Pearl Millet	broccoli & crimson clover	sweet corn & beans
3B	14	Sunn hemp & Pearl Millet	broccoli & crimson clover	sweet corn& beans	Sorghum Sudan & Velvetbean	squash/rye-vetch	pepper & beans

 

Table 2. 2006 Field Operations Pertinent to Soil and Nutrient Management

Date	Operation	Rate of Application
17 May 2006	Pre-trial planting of Cowpea ‘Iron Clay' prior to summer cover crops.	Inoculated seed planted to 56 kg ha -1
13 July 2006	Apply mushroom compost	22,400 kg ha -1
7 Aug 2006	Soil sample Pre-trial by replication
5 Oct 2006	Dolomitic lime incorporated to 15 cm with a rototiller in plot areas and in border areas to 30 cm with an offset disk harrow.	2,240 kg ha -1
27 Sept 2006	Cover Crop Biomass sample collection	N/A
29 Sept 2006	Soil sample # 1 Following cover crop termination	N/A

 

  Table 3. Pre-trial Soil Analysis by Blocks in Citra, FL, July 2006.

Block	P	K	Mg	Ca	pH	CEC
	ppm
1	48.0a 1	29.0a	25.5a	398a	5.80a	4.45a
2	33.5c	25.0c	21.0c	402a	5.85a	3.85b
3	33.5c	23.5d	19.5d	196b	5.40b	4.40a
4	35.5b	27.0b	22.5b	219b	5.40b	4.30ab
Significance	0.01	0.01	0.01	0.01	0.01	NS

1 Values followed by the same letter within column are not different by Duncan 's LSD, P= 0.05.

 

Table 4. Above Ground Biomass Dry Weight (kg ha -1 ), Total Nitrogen (N) (kg ha -1 ) and Carbon (C) to Nitrogen Ratios (C:N) of Cover Crops Immediately Prior to Termination in Citra, FL, Sept 2006.

Cover Crop Species 1	Biomass Dry Weight (kg ha -1 )	Total N 2 (kg ha -1 )	C:N
F	0c 3	0c	0c
PM	4547a	57b	38:1a
SH	2840b	90a	14:1b
SH+PM	5049a	60b	38:1a
SS	4115ab	54b	38:1a
VB	893c	32b	12:1b
SS+VB	4471a	53b	41:1a
Significance	0.01	0.01	0.01

1 F=fallow, PM=pearl millet, SH=sunn hemp, SH+PM=sunn hemp + pearl millet, SS=sorghum sudangrass, VB=velvetbean, SS+VB=sorghum sudangrass + velvetbean.

2 Total N was calculated according to the following: Total N = N (%) * Biomass weight (kg)

3 Values followed by the same letter within column are not different by Duncan 's LSD, P= 0.05.

 

Table 5. Nutrient content in soils following cover crop termination for nitrate nitrogen (NO 3 -N), phosphorus (P), potassium (K), calcium (Ca) and magnesium (Mg).

Cover Crop Species 1	NO 3 -N	P	K
	ppm
F	3.36	45.8	18.0a 2
PM	3.31	40.2	12.8b
SH	3.41	35.7	14.8b
SH+PM	3.59	37.5	11.5b
SS	3.28	34.8	14.8b
VB	3.45	38.9	20.3a
SS+VB	3.34	37.3	12.6b
Significance	NS	NS	0.01

1 F=fallow, PM=pearl millet, SH=sunn hemp, SHPM=sunn hemp + pearl millet, SS=sorghum sudangrass, VB=velvetbean, SSVB=sorghum sudangrass + velvetbean.

2 Values followed by the same letter within column are not different by Duncan 's LSD, P= 0.05.

 

Literature Cited

Bengtsson, J., A. Ahnstrom, and C. Weibull. 2005. The effects of organic agriculture on biodiversity and abundance: A meta-analysis. J. Applied Ecol. 42:262-269.

Cherr, C.M., J.M.S. Scholberg, and R. McSorley. 2006. Green manure approaches to crop production: A synthesis. Agron. J. 98:302-319.

Drinkwater, L.E. 2002. Cropping systems research: Reconsidering agricultural experimental approaches. HortTechnology 12(3):355-361.

Fuller, R.J., L.R. Norton, R.E. Feber, P.J. Johnson, D.E. Chamberlain, A.C. Joys, F. Mathews, R.C. Stuart, M.C. Townsend, W.J. Manley, M.S. Wolfe, D.W. MacDonald, and L.G. Firbank. 2005. Benefits of organic farming to biodiversity among taxa. Biology Letters 1(4):431-434.

Hole, D.G., A.J.Perkins, J.D. Wilson, I.H. Alexander, and P.V. Grice. 2005. Does organic farming benefit biodiversity? Biol. Conserv. 122:113-130.

Liebman, M. and T. Ohno. 1998. Crop rotation and legume effects on weed emergence and growth: Applications for weed management. In J.L. Hatfield, D.D. Buhler, and B.A. Stewart (eds.)., Integrated Weed and Soil Management. Pp. 181-221. Ann Arbor Press, Chelsea , MI .

Muller, J.P., M.E. Barbercheck, M.Bell, C. Brownie, N.G. Creamer, A. Hitt, S. Hu, L. King, H.M. Linker, F.L. louws, S. Marlow, M. Marra, C.W. Raczkowski, D.J. Susko and M.G. Wagger. 2002. Development and implementation of a long-term agricultural systems study: Challenges and opportunities. HortTechnology 12(3):362-368.