Winter cover crops in a vegetable cropping system: Impacts on nitrate leaching, soil water, crop yield, pests and management costs

Published source details Wyland L.J., Jackson L.E., Chaney W.E., Klonsky K., Koike S.T. & Kimple B. (1996) Winter cover crops in a vegetable cropping system: Impacts on nitrate leaching, soil water, crop yield, pests and management costs. Agriculture, Ecosystems & Environment, 59, 1-17.
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This study is summarised as evidence for the following.

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

Crop production: Grow cover crops in arable fields

A replicated, randomized, controlled study in 1992–1993 in an irrigated broccoli field in the Salinas Valley, California, USA, found higher broccoli yields in plots with winter cover crops, compared to bare fallows. Crop yield: Higher broccoli yields were found in plots with cover crops, compared to bare soil, in one of four comparisons (the first broccoli harvest, with phacelia as the winter cover crop: 957 vs 830 g dry weight/m²). Methods: There were three plots for winter cover crops (half Phacelia tanacetifolia and half Secale cereale Merced rye, sown in November 1992 and mown in March 1993) and three control plots with bare soil in winter. All plots (252 x 24 m) were tilled in March 1993 (15 cm depth), and the cover crops were incorporated into the soil. Two broccoli crops were grown on raised beds (first crop: April–August 1993; second crop: August–November 1993). All plots were irrigated (440–450 mm/crop, subsurface drip irrigation) and fertilized (41–42 g N/m²/crop). Broccoli biomass was measured in two 1 m² areas/plot.
Soil: Grow cover crops in arable fields

A replicated, randomized, controlled study in 1992–1993 in an irrigated broccoli field in the Salinas Valley, California, USA, found more mineralizable nitrogen and more microbial biomass in soils with winter cover crops, compared to soils without winter cover crops, but found inconsistent effects on nitrate. Nutrients: More mineralizable nitrogen was found in soils with cover crops, compared to bare soils, in five of 14 comparisons (8.5–17 vs 3–9 µg NH₄-N/g dry soil, 0–15 cm depth), but there were similar amounts of ammonium in soils with or without cover crops (0.5–4.2 µg NH₄-N/g dry soil, 0–15 cm depth). Less nitrate was found in soils with cover crops, in three of nine comparisons at the end of the cover-cropping season (March–April: 0–7 vs 3–10 µg NO₃-N/g dry soil, 0–15 cm depth), but more was found in one of nine comparisons (4 vs 2 µg). Soil organisms: More microbial biomass (measured as carbon) was found in soils with cover crops, compared to bare soils, in seven of 14 comparisons (150–500 vs 70–150 µg C/g dry soil, 0–15 cm depth), and more microbial biomass (measured as nitrogen) was found in two of 14 comparisons (11–30 vs 5–12 µg N/g dry soil). Methods: There were three plots for winter cover crops (half Phacelia tanacetifolia phacelia and half Secale cereale Merced rye, sown in November 1992 and mown in March 1993) and three control plots with bare soil in winter. All plots (252 x 24 m) were tilled in March 1993 (15 cm depth), and the cover crops were incorporated into the soil. Two broccoli crops were grown on raised beds (first crop: April–August 1993; second crop: August–November 1993). All plots were irrigated (440–450 mm/crop, subsurface drip irrigation) and fertilized (41–42 g N/m²/crop). Soil samples were collected 16 times in November 1992–August 1993, including nine samples in March–April, when the cover crops were incorporated (0–75 cm depth, 6 cm diameter, four cores/plot).
Pest regulation: Grow cover crops in arable fields

A replicated, randomized, controlled study in 1992–1993 in an irrigated broccoli field in the Salinas Valley, California, USA, found more mites, but similar numbers of other pests and diseases, in soils with winter cover crops, compared to bare soils. Crop damage: Similar numbers of diseased seedlings were found in plots with or without cover crops (numbers of seedlings not reported). Pest numbers: More mites were found in plots with cover crops, compared to bare soils, in four of 28 comparisons (90–220 vs 30–150 mites/sample), but similar numbers of centipedes and springtails were found (0.3–15 springtails/sample; numbers of centipedes not reported). Similar amounts of disease-causing fungus were found in soils with or without cover crops (numbers of Sclerotina minor sclerotia not reported). Methods: There were three plots for winter cover crops (half Phacelia tanacetifolia and half Secale cereale Merced rye, sown in November 1992 and mown in March 1993) and three control plots with bare soil in winter. All plots (252 x 24 m) were tilled in March 1993 (15 cm depth), and the cover crops were incorporated into the soil. Two broccoli crops were grown on raised beds (first crop: April–August 1993; second crop: August–November 1993). All plots were irrigated (440–450 mm/crop, subsurface drip irrigation) and fertilized (41–42 g N/m²/crop). Pests were measured in soil samples (0–15 cm depth, 14 samples in March–November 1993). Broccoli diseases were measured in ten 2 m²areas/plot.
Water: Grow cover crops in arable fields

A replicated, randomized, controlled study in 1992–1993 in an irrigated broccoli field in the Salinas Valley, California, USA, found that less nitrate was leached from soils with winter cover crops, compared to bare soils, but cover crops had inconsistent effects on water availability. Water availability: Less water was found in soils with cover crops, compared to bare soils, in two of 16 comparisons (in March: 6–7% vs 8% soil moisture). Less water was lost through drainage from soils with cover crops, compared to bare soils (3.6–3.9 vs 6 mm/cm²). Nutrients: Less nitrate was leached from soils with cover crops, compared to bare soils (measured with ion-exchange resin bags or estimated from soil nitrate concentrations and drainage volumes: 7–9 vs 24–28 g NO₃-N/m²; measured with suction samplers: 155 vs 281 g NO₃-N/m²). Methods: There were three plots for winter cover crops (half Phacelia tanacetifolia and half Secale cereale Merced rye, sown in November 1992 and mown in March 1993) and three control plots with bare soil in winter. All plots (252 x 24 m) were tilled in March 1993 (15 cm depth), and the cover crops were incorporated into the soil. Two broccoli crops were grown on raised beds (first crop: April–August 1993; second crop: August–November 1993). All plots were irrigated (440–450 mm/crop, subsurface drip irrigation) and fertilized (41–42 g N/m²/crop). Soil samples were collected 16 times in November 1992–August 1993, including nine samples in March–April, when the cover crops were incorporated (0–75 cm depth, 6 cm diameter, four cores/plot). Leaching was measured with buried ion-exchange resin bags (60 cm depth, 10 g resin, excavated in March 1993) and suction samplers (60 cm depth, measured weekly in December 1992–March 1993).