Long-Term effects of compost and cover crops on soil phosphorus in two California agroecosystems

Published source details Maltais-Landry G., Scow K., Brennan E. & Vitousek P. (2015) Long-Term effects of compost and cover crops on soil phosphorus in two California agroecosystems. Soil Science Society of America Journal, 79, 688-697.
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This study is summarised as evidence for the following.

	Action	Category
	Soil: Add compost to the soil Action Link
	Soil: Grow cover crops in arable fields Action Link

Soil: Add compost to the soil

A replicated, randomized, controlled study in 2003–2011 in farmland the Salinas Valley, California, USA, found no difference in nutrients or soil organisms between soils with or without added compost. Nutrients: No difference in phosphorus was found between soils with or without added compost (497 vs 456 mg total phosphorus/kg soil). Soil organisms: No difference in microbial biomass (measured as phosphorus) was found between soils with or without added compost (2.0 vs 1.7 mg chloroform-extractable phosphorus/kg soil). Methods: Composted yard waste (15 Mg/ha/year) was added to four treatment plots (240 m²), but not four control plots. Lettuce and broccoli were grown in rotation (two crops/year). Soil samples were collected in soil cores (20 cores/plot, 0–30 cm depth) in 2011.
Soil: Grow cover crops in arable fields

A replicated, randomized, controlled study in 1993–2011 in arable farmland in Davis and the Salinas Valley, California, USA, found more soil organisms, but no difference in nutrients, in soils with winter cover crops, compared to soils without cover crops. Nutrients: No difference was found in phosphorus, or the change in phosphorus over time, in soils with or without cover crops (2011: 519 vs 517 mg total phosphorus/kg soil; 1994–2011: 23 vs 21 mg less total phosphorus/kg soil; experiment in Davis). Soil organisms: More microbial biomass (measured as phosphorus) was found in soils with cover crops, compared to soils without cover crops (1.4 vs 1 mg phosphorus/kg soil; experiment in Davis). Implementation options: No differences in phosphorus or microbial biomass (measured as phosphorus) were found between soils with different species of cover crops (513–535 mg total phosphorus/kg soil; 3.1–3.7 mg microbial phosphorus/kg soil), or in soils with cover crops grown every year, compared to every four years (513 vs 497 total; 3.8 vs 2.0 microbial; experiment in the Salinas Valley). Methods: In one experiment (in Davis), nitrogen-fixing cover crops (peas, vetch, and/or fava beans) were grown in six treatment plots, but not in six control plots. Wheat was grown in rotation with cover crops (once every two years) or in rotation with fallows. In another experiment (in the Salinas Valley), there were four plots (240 m²) for each of four treatments (legume-rye, mustard, or rye cover crops grown every year, or legume-rye cover crops grown every four years). Lettuce and broccoli were grown in rotation (two crops/year). Soil samples were collected in soil cores (20 cores/plot; 0–30 cm depth) in 2011. Soil cores were also collected in 1993 (number of samples not reported).