Summer Cover Crops for Tomato Production in South Florida

Yuncong Li, Herbert Bryan, Renuka Rao, Nolan Heckert, Teresa Olczyk
University of Florida, IFAS

Cover crops have become an integral part of vegetable production practices in south Florida for weed control, disease and insect suppression, and retaining nutrients during the heavy summer rains. A wide variety of plants can be used as cover crops in south Florida. Some are legumes that fix nitrogen and some are nematode resistant crops. The objective of this study was to select the best cover crop(s) for vegetable production in south Florida. We have evaluated ten cover crops, and eight of them were legumes in 1997. In 1998, only sunn hemp (Crotalaria juncea L.) sorghum-Sudan, sesbania (Sesbania exaltata Raf.), and aeschynomene (Aeschynomene evenia) were evaluated. Sunn hemp and aeschynomene were superior to other cover crops tested and reduced weed cover to 1% or less in 1997 and 1998. Sunn hemp produced 7,700 -10,300 lb dry weight per acre and fixed or retained up to 248 lb N/ac, while sorghum-Sudan retained only 27-33 lb N/ac. All legumes contributed more nitrogen compared to nonlegumes, such as sorghum Sudan, a common cover crop in Florida. Tomato plants from sunn hemp plots produced more marketable and extra large fruit than those from sesbania and control (weeds). Marketable fruit yields from the fumigation treatment were significantly higher than those from non fumigation treatment. The higher marketable tomato yield from fertilizer treatments with or without cover crops indicated that fertilizer application is still critical for cover crop and tomato production system.

Summer cover crops are used for weed control (Li, 1998), disease and insect suppression (McSorley et al., 1994; Stansly et al., 1999) during the vegetable off-season (May to September) in south Florida. Cover crops are also important to improve soil physical properties, increase soil organic carbon, conserve soil water, reduce surface runoff and recycle nutrients during the heavy summer rains (Hubbell and Sartain, 1980; Reeves, 1994; Mansoer et al., 1997). Planting summer cover crops have become an integral part of vegetable production practices in south Florida. Moreover the use of cover crops continues to increase as farmers seek to improve soil quality, reduce chemical input and replace methyl bromide. A wide variety of plants can be used as cover crops during late spring and summer in south Florida. Sunn hemp (Crotalaria juncea L.) and aeschynomene (Aeschynomene evenia) provide great ground coverage (Mansoer et al., 1997; Li, 1998, 1999). Nematode resistant cover crops such as 'Iron&Clay' cowpea (Vigna unguiculata L.) reduce nematodes for successive crops (Stansly et al., 1999). Obviously, legumes contribute more nitrogen by N fixation compared to nonlegumes such as sorghum-Sudan, which is a common cover crop in this area. In order to select the best cover crops for vegetable production in south Florida, we conducted experiments to evaluate suitability of eleven potential cover crops in south Florida and the effects of four cover crops on tomato growth and yield.

The experiments were conducted during 1997-1999 at the Tropical Research and Education Center, University of Florida, Homestead, FL. The first study was initiated in the summer 1997 to evaluate ten cover crops: 'Tropic Sun' sunn hemp, 'Sxl7' sorghumSudan hybrid, 'Americana' aeschynomene, 'PABU2222' soybean (Glycine max L.), 'Grey Stripped' sunflower (Helianthus annuus L. ), 'Calif. Blackeye No.5' cowpea (Vigna unguiculata L.), 'Lewis' guar (Cyamopsis tetragonolobus L),, 7120-mixed' peanut (Arachis stenosperma L ), 'Savanna Stylo' (Stylosanthes quionesis), and 'Florida Balsawood' (Ricinus communis L.). The experimental design was a randomized complete block with three replications. Plots without cover crops were used as controls. Cover crops were seeded by hand in rows 14 inches apart in plot 18 ft wide and 16 ft long on July 14, 1997. Seeding rates ranged from 45-100 lb/ac (Table 1). Sunn hemp and cowpea were inoculated with cowpea type rhizobium and other legumes were inoculated with appropriate rhizobium. Sorghum-Sudan and sunflower were fertilized with 326 lb 10N-0P-12K-8Mg per acre while no fertilizer was applied to the other cover crops. Plant height, fresh weight and dry weight of cover crops and weeds were evaluated three months after seeding. Plant shoots and roots were separated, washed, dried and analyzed for N contents. The cover crops were cut and tilled on Oct. 22, 1997.

The second cover crop experiment was conducted during 1998-1999. 'Tropic Sun' sunn hemp, sorghum-Sudan hybrid, 'Americana' aeschynomene were selected for the experiment based on first year's results. Another summer-annual legume, sesbania (Sesbania exaltata Raf.), was also included in the second year's experiment. The experimental design was a randomized complete block with four replications. Plots without cover crops were used as controls. Cover crops were inoculated with appropriate rhizobium and seeded with a grain drill at the effective rate of 20 lb/ac on July 3, 1998. Actual seeding rates were determined based on effective seed rate and seed germination rate. No fertilizer was applied to any cover crop during the summer. Plant height, population, fresh and dry biomass were measured 90 days after seeding. Subsamples were also collected for nutritional analysis. The cover crops were cut and tilled on Oct. 13, 1999.

The cover crop plots were disked four times and turned with 3-bottom plow before tomato beds were prepared and mulched on December 17, 1998. Each cover crop plot (18 ft x 50 ft) was split into subplots (6 ft x 50 ft) as treatment 1 (dry fertilizer with fumigation), treatment 2 (dry fertilizer without fumigation), and treatment 3 (no dry fertilizer and no fumigation). Granular dry fertilizer (1000 lb 6N-P₂O₅-12K₂0 per acre) was banded and then rototilled in the bed for treatment 1 and 2. On December 31, 1998 "Sanibel" tomato plants were transplanted 20 inches apart in each bed placed 6 ft apart from the center. All plots were fertigated with total 100 lb N/ac as a liquid fertilizer (4N-0P₂0₅-8K₂0) beginning a month after transplantation. Tomatoes were harvested three times at mature-green to pink color stages from 10 plants in each plot. Fruit number and fruit weight for extra large, large, medium and small sizes from each plot were recorded. Data were analyzed by analysis of variance and means were compared using Duncan's Multiple Range Test, 5 % level.

The sunn hemp was superior to other cover crops tested from both experiments during 1997 and 1998. Seeds germinated very well and seedlings rapidly produced a thick ground cover (Tables 1 and 2). It produced a total of 7,700-9,900 lb dry biomass (roots and shoots) per acre 3 months after seeding. As a legume, sum hemp fixed and retained up to 248 lb N/ac. Plants grew as high as 113 inches in the summer, 1998. Sunn hemp has been widely used as green manure for crop production in the tropics (National Academy of Sciences, 1979). Mansoer et al. (1997) reported that 'Tropic Sun' sunn hemp produced 5360 lb dry biomass per acre 9-12 weeks after planting during the fall in Alabama.

Another cover crop that performed well in trials is 'Americana' commercial aeschynomene (Florida), a warm-season legume forage and well adapted to calcareous soils. It reached heights of 14.8 and 46 inches for 1997 and 1998 seasons, respectively. Aeschynomene has been reported to be resistant to most root-knot nematodes. Aeschynomene evenia, a newly available species was used for this trial. It produced 2,600 lb dry biomass per acre which was less than that of sorghum-Sudan grown in 1997 and produced 4,500 lb dry biomass in 1998 which was 21.6% higher than that of sorghum Sudan. Aeschynomene fixed 56-108 lb N per acre compared to 27-33 lb N/ac retained by sorghum-Sudan. Aeschynomene also had a very good ground cover (about 95% of area) (Table 1).

'Lewis' guar, also called "cluster bean", is a legume often grown as a vegetable and is well adapted to tropical climates. Bryan et al. (1992) tested six cultivars in south Florida and obtained relatively high pod yield. They also reported that guar had high levels of tolerance to many diseases and pests. However, the growth of guar in this trial was poor, and this may have been caused by late planting date. Soybean and cowpea used for this trial are subtropical legumes and grow well in south USA. They are made into hay, silage or seeds and are also used as cover crops in tropical countries. 'Calif. Blackeye No. 5' is often used as a vegetable in summer planting in Florida. However, with heavy rainfall (El Nino year) foliage was lost prematurely due to disease problems. Soybean provided better yield and ground cover than cowpea. Both crops did not generate enough biomass, ground cover and nitrogen. However, cowpea or soybean could be mixed in planting with sunn hemp or sorghum to increase biomass. Tests on mixed planting of cover crops are in progress in south Florida.

Peanut (Arachis stenosperma L.) is a warm season/tropical perennial legume native of South America. It has been reported to grow well in south Florida. However, it takes a long time to establish and cover the ground. It produced 1200 lb dry biomass per acre 3 months after seeding. Even though it contributed 26 lb N per acre through N fixation, it is not recommended as summer cover crop for vegetable production. However, it could be evaluated as a cover crop for tropical fruit groves in this area. Perennial peanut species are being evaluated as living mulches for vegetables.

'Grey stripped' sunflower grew very well and produced shiny yellow flowers. However, the extremely wet summer resulted in premature defoliation. 'Savanna Stylo' (Stylosanthes quionesis), and 'Florida Balsawood' (Ricinus communis L.) were also planted in this trial, but these crops were not harvested because of very poor germination and slow growth without fertilizer. Sesbania (Sesbania exaltata) grows widely in Florida along ditches, roadsides, disturbed sites or riverbanks (Hall and Vandivar, 1991). It was included in the second year's trial because of large biomass production and nitrogen fixing ability. However, it did not germinate well and growth rates were very slow in calcareous soils. It may also have been susceptible to rootknot nematodes. Velvet bean and hairy indigo were not included in this trial but have been observed as cover crops in 1968 by Dr. Herbert Bryan, TREC, Homestead. Both crops grew very poorly in the calcareous marl soils in south Florida.

Tomatoes plants grown in sunn hemp plots produced significantly higher early (760 cartons/ac) and total extra large fruit (884 cartons/ac) and total marketable yield (2237 cartons/ac) compared to control and sesbania treatment (Table 3). The higher biomass (9,900 lb/ac) and nitrogen fixation (248 lb N/ac) by sunn hemp may be the major factors for the higher yields. Plant residues improved soil physical properties and increased soil organic carbon and mineralization of residues also increased nutrient availability. There were no significant differences for early marketable yield and average fruit size among cover crop treatments. The order of impact of cover crop on tomato growth could be given as sunn hemp > aeschynomene > sorghum-Sudan > sesbania = control. Sunn hemp was also reported as an ideal crop in rotation with Nematode susceptible crops because it does not harbor nematodes (Li, 1999).

After cover crops were tilled, cover crop plots were split into dry fertilizer (1000 lb 6N-6P₂0₅- 12K₂0 per acre ) and no dry fertilizer treatments. Tomatoes with dry fertilizer application produced twice as much total marketable yield as those with no dry fertilizer treatment (Table 3). Total extra large fruit weight from a dry fertilizer treatment was higher than that from zero dry fertilizer treatment. The results indicate that fertilizer application still is critical for cover crop and tomato production system. High decomposition rates of cover crop residues during early tomato season could cause losses of fixed nitrogen by cover crops through volatilization or leaching. In Alabama, N release from sunn hemp residue was 50% during the first 4 week following mowing (Manseor et al, 1997). Laboratory and field incubation experiments are in progress to determine mineralization rates of cover crops under south Florida condition.

The trial also included fumigation and non fumigation treatments. Fumigation significantly increased extra large fruit yield and total marketable yield (Table 3). However, differences between fumigation treatments were much smaller than that from fertilizer treatments. There were no significant interactions between cover crops and fumigation treatments. Another field investigation is underway to determine impact of cover crops on nematodes in south Florida.

In conclusion, both sunn hemp and aeschynomene are legumes and well adapted in calcareous soils. They produced higher biomass, better ground coverage and contributed more nitrogen to subsequently grown crops than the sorghum-Sudan, which is a common cover crop grown in this area. Tomato growth following cover crops also proved that sunn hemp and aeschynomene are better than currently used cover crops for vegetable production in south Florida.

Bryan, H.H., K.R. Narayanan, and R.T. McMillan. 1992. Guar cultivar responses to nitrogen fertilizer rates on oolitic calcareous soils and diseases incidence in south Florida. Acta Horticulturae 318:271-280.

Hall, D.W. and VX Vandivar, 1991. Hemp Sesbania, Sesbania exaltata (Raf.) Cory, In Weeds in Florida, Fla. Coop. Ext. Serv. Circular SP 37.

Hubbell, D.H. and J.B. Sartain. 1980. Legumes - a possible alternative to fertilizer nitrogen. Fla. Coop. Ext. Serv. Circular SL-9.

Li, Y.C. 1998. Evaluation of Summer Cover Crops in South Florida. Vegetarian 98 (3):4.

McSorley, R., D.W. Dickson, J.A. De. Brito, R.C. Hochmuth. 1994. Tropical rotation crops influence nematode densities and vegetable yield. J. Nematology 26:308-314.

Mansoer, Z. D. W. Reeves, and C.W. Wood. 1997. Suitability of sunn hemp as an alternative late-summer legume cover crop. Soil Sci. Soc. Am. J. 61:246-253.

National Academy of Sciences. 1979. Tropical Legumes: Resources for the future. National Academy of Sciences, Washington, D.C.

Reeves, D.W. 1994. Cover crops and rotations. P 125-172. In J.L. Hatfield and B.A. Stewart (ed.) Advances in soil science: Crops residue management. Lewis Publ., Boca Raton, FL.

Stansly, R R. McSorley, and M.Ozores-Hampton. 1999. Management of Root-Knot Nematode in organic production. Citrus&Vegetable Magazine, March 1999.

Table 1. Average plant height, fresh and dry biomass, ground coverage and fixed nitrogen for eight cover crops grown on a calcareous soil in the summer of 1997.

Plant	Legume	Seeding Rate	Plant Height	Biomass^z		Ground coverage		Nitrogen in plant
Plant	Legume	Seeding Rate	Plant Height	Fresh	Dry	Weed	Crop	Shoot	Root	Total
		Lb/ac	Inches	------------%-----------		---------%---------		%	%	Lb/ac
Sunn hemp	yes	40	71.5	30.7	7.7	1.0	99.0	2.5	0.6	182
Sorghum-Sudan	no	60	29.9	11.4	3.0	1.7	85.0	0.9	1.1	27
Aeschynomene	yes	40	14.8	11.2	2.6	0.8	95.0	2.4	1.1	56
Soybean	yes	70	16.6	9.0	2.3	3.0	66.7	3.1	1.4	67
Sunflower	no	45	32.8	14.4	2.2	15.0	71.7	1.6	0.8	33
Cowpeas	yes	100	21.1	7.7	1.5	25.0	60.0	2.3	1.3	34
Guar	yes	40	18.9	7.2	1.4	27.0	61.7	2.1	1.3	27
Peanut	yes	80	5.5	6.9	1.2	7.7	75.0	2.3	1.3	26
Weeds ^Y	no			1.8	0.3	56.7	0	1.3	0.5	4

z Biomass was calculated by total weight of root and shoot.
^Y Weeds were harvested from control plots and height of weeds was not reported because of variation.

Table 2. Seeding rate, average plant height, fresh and dry biomass for four cover crops and weeds (control) grown on a calcareous during summer, 1998.

Plant	Seeding^z Rate	Plant Height			Biomass (30 days)		Biomass (90 days)		N in plant^y
Plant	Seeding^z Rate	30 days		90 days	Fresh	Dry	Fresh	Dry	Total
	Lb/ac	---------Inches--------			--------1000 lb/ac---------		---------1000 lb/ac----------		Lb/ac
Sunn hemp	20	88a^x	113a		40.8a	10.3a	33.4a	9.9a	248
Sorghum-Sudan	20	41b	46b		16.5b	3.7b	14.8b	3.7b	33
Aeschynomene	20	25c	42b		12.2bc	3.1bc	16.7b	4.5b	108
Sesbania	20	31bc	29c		5.4c	1.6c	4.2b	1.7b	--
Weeds^w	--	22c	24c		6.2c	2.0bc	5.2b	1.3b	17