Managing Soil Food Webs for Enriched and Suppressive Soils: Effects of Cover Crop Diversity and Quality
Farmers and scientists are looking for alternatives to chemical control of plant pathogens due to heightened awareness of human health and environmental concerns. Many farmers are moving toward IPM and organic management. Organic farming is one of the fasted growing segments of US Agriculture. Certified organic cropland more than doubled between 1992 and 1997 and nearly doubled again for some crops between 1997 and 2003. In 2003 nearly 1.3 million acres of US cropland were dedicated to organics (ERS, 2004). Farmers need alternative strategies to manage plant parasites and pathogens.
Our primary aim is to provide farmers with management strategies that optimize soil food web services, specifically nutrient cycling and disease suppression.
Objective 1: Understand the effects of cover crop quality on the soil food web. Do cover crop carbon to nitrogen ratios influence the nutrient cycling and suppressive capacity of soils as indicated by the Structure Index (SI) and Enrichment Index (EI)?
Objective 2: Quantify the effects of increased cover crop diversity on the soil food web. Can complex cover crop mixtures increase disease suppressive and nutrient cycling capacity of soil food webs as indicated by the SI and EI?
Project activities were initiated in September 2005 at two field sites following our objectives to 1) Understand the effects of cover crop carbon to nitrogen ratio on the soil food web and 2) Quantify the effects of increased cover crop diversity on the soil food web.
Field site 1 is located at the UC Davis Student Farm. Cover crop treatments were planted in September 2005, consisting of two and four species cover crop mixtures and a no cover crop control. Treatments are as follows: 1) Cayuse Oats 60lbs/A +Triticale 118 60lbs/A , 2) Magnus Peas 80lbs/A + Purple/Lana vetch 40lbs/A, 3) Cayuse Oats 15 lbs/A + Triticale 118 15 lbs/A + Magnus Peas 60lbs/A +Purple/Lana Vetch 30 lbs/A, 4) Fallow control. Each treatment had five replications in 30 x 40ft blocks. Initial soil samples were taken in August 2005 and we assessed nematode taxa to perform faunal analyses. In May of 2006 we mowed the cover crop, strip tilled and transplanted tomatoes. At the time of cover crop mow down we took cover crop samples for biomass, carbon, and nitrogen levels; and soil samples for total nitrogen, total carbon, NO3, NH4, and nematode faunal analysis. The tomato crop was harvested in September 2006. At the time of harvest tomato plant biomass, red/ green tomato harvest weight and soil for nematode faunal analysis. Cover crops were planted for year two into undisturbed tomato beds on October 9th-11th, 2006. Cover crop treatments were adjusted as follows: 1) Cayuse Oats 60lbs/A + Wheat 60lbs/A, 2) Magnus Pea 75 lbs/A + Lana/Common vetch 50lbs/A, 3) Cayuse Oats 10lbs/A + Wheat 10lbs/A + Lana/Common Vetch 50lbs/A + Magnus Pea 75 lbs/A, 4) no cover crop control. The grain plus legume mix was planted with legumes alone every other row.
Field site 2 is located in the UC Davis Mesocosm Facility. The Mesocosm Facility provides plots isolated by 2 ft diameter barrels sunken into the ground, filled with a sand based medium and with watering frequency and duration controlled by a timer. Plots were planted in September 2005 to determine the affects of annual cover crop mixtures, annual monocrops, perennial grass mixtures and perennial grass monocrops. Treatments including a bare fallow control are replicated four times in a randomized complete block. Cover crops for year one include Lana Vetch, Magnus Pea, Triticale 118, and Cayuse Oats in monoculture and fours species mixtures. Native grasses include Poa secunda secunda, Vulpia microstachys, and Elymus Glaucus in monoculture and three species mixtures. In year two cover crop combinations were adjusted to increase the number of species in cover crop mixtures. Complementary species were added to the originals treatments: yellow clover, bell bean and wheat. Soil samples were taken for nematode faunal analysis in September 2005, May 2006 and September 2006. Cover crop and cash crop biomass were taken in May and September of 2006 respectively.
Strip tillage into mowed down cover crops presented a challenge in May of 2006. Beds were hard and uneven after setting the buried drip tape and the light strip tillage equipment available was not able to penetrate the thick layer of cover crop mulch blanketing the beds. Many adjustments to the coulters and residue managers increased the strip tiller’s effectiveness. But the coulters still only disturbed an inch to an inch and a half of soil. In order to plant tomato transplants we needed three to four inches of soft soil. The answer was weight. We strapped two 50 gallon drums on top of the strip tiller and filled them with water. This provided the extra pressure we needed to power through the thick cover crop mulch and hard bedtops.
Pests have presented a challenge during the second cover crop planting. As the beds have not been tilled in over a year, soil dwelling pests have not been disturbed. Two weeks after cover crop germination small vetch, pea and oats plants were full of holes. When we inspected the field at night, we saw hundreds of slugs. We treated the field with organically certified “Sluggo” and slug pressure was eliminated. Unfortunately, the cover crop was already significantly damaged and a supplementary replant was necessary. Rabbits have continued to be a pest problem throughout the fall. The project area is fenced but continual maintenance is necessary to ensure that rabbits have not tunneled under or found low places to jump over fences. Ground squirrels are also a problem in the area and had to be removed by flooding.