The effect of cover crops and fertilization with ammonium nitrate on corky root of lettuce

Published source details van Bruggen A.H.C., Brown P.R., Shennon C. & Greathead A.S. (1990) The effect of cover crops and fertilization with ammonium nitrate on corky root of lettuce. Plant Disease, 74, 584-589.
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This study is summarised as evidence for the following.

	Action	Category
	Soil: Grow cover crops in arable fields Action Link
	Crop production: Grow cover crops in arable fields Action Link
	Pest regulation: Grow cover crops in arable fields Action Link
	Water: Grow cover crops in arable fields Action Link

Soil: Grow cover crops in arable fields

A replicated, randomized, controlled study in 1986–1988 in an irrigated lettuce field in the Salinas Valley, California, USA, found less ammonium in plots with winter cover crops, compared to winter fallows. Nutrients: Less ammonium was found in soils with cover crops, compared to fallows, in at least one of eight comparisons (after harvesting the spring crop, in plots that were side-dressed with fertilizer: 4.4 vs 5.2 ppm NH₄-N). Similar amounts of nitrate were found in soils with or without cover crops (in March 1998: 11–27 vs 29 ppm NO₃-N). Methods: There were six plots (10.7 x 1.1 m raised beds) for each of two winter cover crops (broad beans or rye) and six control plots (bare fallow, maintained with herbicide). The cover crops were seeded in November 1986–1987, irrigated until emergence, and chopped, disked, and chisel ploughed in spring (25–30 cm depth). Lettuces were planted in May and July 1987 and March and August 1988, and they were harvested in July and October 1987 and June and October 1988. The lettuces were irrigated (1–2 cm every 2–3 days until emergence, then 2 cm/week), and some lettuce plots were fertilized (110–220 kg N/ha in total; up to 110 kg N/ha as side-dressing). Soil samples were collected in March, June, August, and September 1988 (0–22 cm depth, 6 cm diameter, four samples/plot).
Crop production: Grow cover crops in arable fields

A replicated, randomized, controlled study in 1986–1988 in an irrigated lettuce field in the Salinas Valley, California, USA, found similar lettuce yields in plots with or without winter cover crops. Crop yield: Similar lettuce yields were found in plots with cover crops or fallows (210–664 g fresh weight/head). Methods: There were six plots (10.7 x 1.1 m raised beds) for each of two cover crops (broad beans or rye), and there were six control plots (bare fallow, maintained with herbicide). The cover crops were seeded in November 1986–1987, irrigated until emergence, and chopped, disked, and chisel ploughed in spring (25–30 cm depth). Lettuces were planted in May and July 1987 and March and August 1988, and were harvested in July and October 1987 and June and October 1988. The lettuces were irrigated (1–2 cm every 2–3 days until emergence, then 2 cm/week). Head weight was measured in 25 plants (autumn 1987) or all plants in 3 x 3 m quadrats (other harvests) in each plot.
Pest regulation: Grow cover crops in arable fields

A replicated, randomized, controlled study in 1986–1988 in an irrigated lettuce field in the Salinas Valley, California, USA, found less-severely diseased lettuces in plots with winter cover crops, compared to winter fallows. Crop damage: Less-severely diseased lettuces were found in plots with cover crops, compared to fallows, in one of four harvests (autumn 1988: data reported as disease scores, based on taproot damage by corky root disease). Implementation options: Less-severely diseased lettuces were found in plots that were cover cropped with Secale cereale rye, compared to Vicia faba broad beans (data reported as disease scores). Methods: There were six plots (10.7 x 1.1 m raised beds) for each of two winter cover crops (broad beans or rye) and six control plots (bare fallow, maintained with herbicide). The cover crops were seeded in November 1986–1987, irrigated until emergence, and chopped, disked, and chisel ploughed in spring (25–30 cm depth). Lettuces were planted in May and July 1987 and March and August 1988, and they were harvested in July and October 1987 and June and October 1988. The lettuces were irrigated (1–2 cm every 2–3 days until emergence, then 2 cm/week). The severity of corky root disease was measured in 10 roots/plot at harvest.
Water: Grow cover crops in arable fields

A replicated, randomized, controlled study in 1986–1988 in an irrigated lettuce field in the Salinas Valley, California, USA, found less water in soils with winter cover crops, compared to winter fallows. Water availability: Less water was found in soils with cover crops, compared to fallows, in two of five comparisons (75 and 90 cm depth: 14–24 vs 16–20% soil moisture content). Implementation options: Less water was found in soils that were cover cropped with Secale cereale rye, compared to Vicia faba broad beans, in one of five comparisons (90 cm depth: 30% less water). Methods: There were six plots (10.7 x 1.1 m raised beds) for each of two winter cover crops (broad beans or rye) and there were six control plots (bare fallow, maintained with herbicide). The cover crops were seeded in November 1986–1987, irrigated until emergence, and chopped, disked, and chisel ploughed in spring (25–30 cm depth). Lettuces were planted in May and July 1987 and March and August 1988, and they were harvested in July and October 1987 and June and October 1988. The lettuces were irrigated (1–2 cm every 2–3 days until emergence, then 2 cm/week). Soil moisture was measured with a hydroprobe at five depths (25, 56, 75, 90, and 106 cm), 13 and 16 weeks after the cover crops were seeded (three measurements/depth).