Comparison of Multi-Species Cover Crops after a Small Grain Crop in a Transition to Organic Row Crop Rotation

Commodities

Practices

Without the use of synthetic fertility in organic agriculture, it is essential to improve nutrient cycling within the soil. Nutrient availability is critical for crop health, insect and disease suppression and ultimately crop productivity. A robust microbial consortium is important for creating availability of nutrients in the soil. The use of high amounts of chemistry and tillage, characteristic of conventional agriculture, are detrimental to ecologically balanced soil microbiota. Regeneration of soil biology is critical.

Problem 1a- Due to prior chemical use and tillage, soil nutrient cycling is limited.

Cover crops are known to improve microbe abundance in the soil; however different species have various microbe associations.

Problem 1b- Nutrient cycling cannot be boosted significantly by simple single species cover crop options.

Adding to this complexity is that excess cover crop residue creates field challenges for equipment. In central Iowa, almost all organic row crop farmers primarily use a tine weeder for weed control.

Excessive surface residue will create piles and plug equipment. This narrows cover crop selection to species that will produce a lower amounts of lignified material and decompose over winter.

Problem 1c- High lignin cover crop species will cause equipment related issues/ frustrations in season.

Much of the agricultural land in the U.S. has been severely degraded resulting in loss of soil, soil structure and microbiological abundance. The role of cover crops is essential to effectively regenerate soil and to properly steward the land. Cover crops are recognized for a multitude of benefits; however, for the purposes of this project I am focusing on the nutrient cycling benefits as well as the extent to which the cover crop mixes decompose.

One of the rationalizations for the use of complex cover crop mixes is to maintain or increase an ecologically balanced soil microbiota (Diverse Cover Crop Mixes for Good Soil Health, Paul R. Salon, USDA-NRCS, Big Flats Plant Materials Center). It has been reported that differences in cover crop species root exudates can affect heterogeneity and quantity of soil microorganisms in the rhizosphere. Also, vegetation influences soil microorganism communities through the abundance, quality, and distribution of residue and root exudates. So, the consensus is that comingling many different cover crop families will build microbiome heterogeneity in the soil.

Nutrient cycling is the process of nutrients being transformed, reused, and moved through the soil system. Soil microbiome abundance improves nutrient cycling because different microbes perform different jobs, and a heterogeneous community can collectively carry out all the transformations needed to keep nutrients moving through the system. The solution to improved nutrient cycling is to properly manage complex multi-species cover crops to build a heterogeneous microbiome.

While monitoring biological indicators are best predictors of nutrient cycling, it will still be important to run chemical testing to understand changes in nutrient availability before and after cover crop growth. So, both biological and chemical soil testing are key measurements in this project.

The cover crop mixtures within this project need to be complex, but what is complex. Cover crop species are grouped into six major categories: 1) cool season grasses 2) cool season legumes 3) cool season broadleaves 4) warm season grasses 5) warm season legumes 6) warm season broadleaves. A mixture should combine species from as many categories as practically possible based on planting season and goals. Previous SARE study (GNC12-154) had one treatment where only cover crops were used during a organic transition period. The results of this study did not find an increase in microbial biomass or soil organic matter. A possible reason is that the cover crop heterogeneity used was low. The summer cover was only one species (either sorghum-sudan grass or sunn hemp) and the winter mix was crimson clover, hairy vetch, rye and Austrian winter pea (2 species - 3 legumes and 1 grass). Part of my strategy will be designing the cover crop mixes to encompass the 6 categories. In addition, my goal is to plant these mixtures in August and allow them to winter kill so both warm and cool season varieties will get ample growing season to perform.

Lastly, the cover crop mixtures need to be designed to select species that will winter kill, cover the ground over-winter, but do not provide significant lignin material that will cause equipment issues the following spring. Sales agronomists representing the seed suppliers will be consulted on this selection.

Four treatments will be compared during this trial:

• Multi-species cover crop 1
• Multi-species cover crop 2
• Monocrop cover crop
• Fallow or no cover crop planted

Project Objectives:

1. Design and test 2 cover crop mixtures that have the potential to improve soil health in a summer planted organic context.
2. Collect quantitative data that allows us to differentiate the health of the soil between treatments.
3. Effectively communicate project results via a Practical Farmers of Iowa annual conference presentation.