Organic Broadcaster

Reduced tillage, cover crops, manures help build soil health

By Matt Ruark, University of Wisconsin-Madison

Soil health is the term used to represent all of the biological, chemical, and physical properties that are important for soil to function both agronomically and ecologically. Organic farmers are, of course, well aware of soil health, as this is a main consideration in their farming operation, and USDA regulations requires operations to “maintain or improve the natural resources of the operation, including soil and water quality.” In this piece, I will focus primarily on the carbon and nitrogen aspects of soil health and the management practices that can affect them.

The two main concepts in building carbon and nitrogen in soil involve the total pool (referred to as carbon or nitrogen sequestration) and the biologically active pools. Many aspects of soil health are driven by inherent soil factors, such as clay content, the parent material of the soil, or the landscape position. The good news is that many of these factors can be improved through changes in management practices.

Impact of Tillage
The long-term storage of carbon and nitrogen in your soil is dependent on the ability of the soil to form aggregates. We can think of aggregates as the very, very small chunks of soil that are an agglomeration of soil organic matter in various states of decay, roots and root exudates (glomalin), microorganisms (dead or alive), silt and clay. The carbon and nitrogen within these aggregates are physically protected from further decay, leading to stabilization or increase of soil organic matter levels.

Tillage, by nature, is the physical disruption of soil and thus physically breaks apart soil aggregates, exposing previously protected carbon and nitrogen. The aggregate formation process must begin again. If soils are continually disrupted through multiple tillage passes each growing season, the aggregate formation process has little chance to restart.

Research from the Wisconsin Integrated Cropping Systems Trial (WICST), near Arlington, Wis. has shown that soils under organic cropping systems that rely heavily on tillage for weed control have less aggregates as a percentage of their soil compared to reduced-tillage grain systems. It should be noted that this difference in soil aggregation was not necessarily bad, as organic grain yields were similar to somewhat greater than those in the conventional system. However, there may be further benefits to the soil and cropping system if we reduce tillage in our organic cropping systems.

Animal & Green Manures
Soil aggregates can be built up through the use of animal and green manures. For example, at the WICST trial, an organic corn-alfalfa-alfalfa rotation, which was tilled multiple times during the corn production phase, still had greater soil aggregation compared to a reduced-till grain system where there were fewer tillage passes. How was this possible? First, the crop rotation included alfalfa, which has a lot of root biomass. Root biomass is a key driver of soil carbon building and soil aggregate formation. Second, there was not tillage in every year. Third, the system also had an application of liquid dairy manure two out of every three years. The combination of these factors, led to more aggregates and more overall carbon in the soil.

Biologically active carbon and nitrogen pools represent the carbon and nitrogen that will be made available to microorganisms and the plant during the growing season. This is an important part of total soil carbon and nitrogen because it represents the food source for soil organisms and a nitrogen source for the crop. Building biologically active carbon and nitrogen pools can also happen in the presence of tillage, but require additional carbon inputs, such as animal or green manures.

One type of biologically active carbon measure is particulate organic matter (POM). The POM is a portion of the total soil organic matter (SOM) that is labile, meaning it will likely be readily decomposed during the growing season. Overall, organic fields tend to have greater POM concentrations compared to conventionally managed systems. This is likely a result of the use of compost, manures, and cover crops. However, there is a tillage aspect to this as well.

At the WICST, the organic grain cropping system did not have more POM compared to conventional grain cropping systems, even though it had composted manure applications (but only once every three years) and as stated earlier, had many tillage passes per year. But, the organic corn-alfalfa-alfalfa rotation had greater POM compared to the conventional corn-alfalfa-alfalfa-alfalfa rotation. This is quite interesting because the organic system had more frequent tillage because of the shorter rotation, but had more frequent application of liquid dairy manure. So, across these rotations, manure had a greater impact than tillage on increasing the biologically active carbon pool. The WICST trial also has a pasture system, which resulted in the highest POM levels of any system, further highlighting the ecosystem benefits of pastureland.

The use of cover crops, as a green manure or for erosion control, has also been shown to be an effective management practice for increasing biologically active carbon and nitrogen pools. This has been linked with the diversity in below-ground biomass – meaning, there are more types of plant roots growing on a parcel of land. Long-term research out of Iowa shows us that over time (~10 years) use of cover crops will lead to greater nitrogen supply from the soil (i.e., an increase in the biologically active nitrogen pool).

When using cover crops for erosion control, specifically grass crops with high C:N ratio biomass, my recommendation is to use starter fertilizer that contains nitrogen. There will be some short-term immobilization of soil nitrogen by microorganisms as they decompose the grass residue and having a supplemental nitrogen supply near the seed will offset this effect.

Interseeding Cover Crops

Red clover is interseeded into corn at the University of Wisconsin Arlington Agricultural Research Station.
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There are many new and exciting innovations with interseeding cover crops. Interseeding refers to planting the cover crop into a standing cash or forage crop sometime during the growing season. This can be accomplished either early or late in the season.

Interseeding early in the growing season (e.g., V4-V6 growth stage for corn) has the benefit that the cover crop can be drill-seeded with a specialized or modified grain drill. (See Penn State Interseeder and Applicator.) Drilling cover crop seed has many advantages over broadcast seeding. These include a greater chance of seed establishment and plant growth, and lower seeding rates.

Interseeding may be the only way to get cover crops established consistently and successfully in full season crops (e.g., corn and soybean). The drawback to interseeding is that the cover crop functions as a weed in this scenario, competing with the main crop for nutrients, water, and if growth is large enough, sunlight. The cover crop itself can also function as a weed control method if a good stand is established, through smothering out of smaller plants. (Radish does a particularly good job of this.) The competition aspects of an interseeded cover crop will vary from system to system, and be of larger concern for short plants and of less concern for wider row spacings.

Interseeding has been shown to work nicely in corn, as by the time the cover crop gets established, the corn canopy is nearly fully developed, suppressing further growth of the cover crop. After corn harvest, a nice stand of cover crops exists, providing erosion control benefits (which is essential if the corn is harvested for silage or corn residue biomass is also harvested). The increased growth allows for greater root development (aiding with increases to below-ground C), or if the interseeded cover is a legume, functioning as a nitrogen source to the subsequent crop.

Research trials at the University of Wisconsin’s Arlington Research Station in 2014 and 2015 have demonstrated that rye, red clover, and radish all were successfully established when interseeded at the V5 growth stage and no yield loss in corn was measured. The rye and red clover will survive Wisconsin winters, so termination plans must be developed; radish will winter-kill in Wisconsin so there may be some advantages to that in terms of not needing an additional tillage pass in the spring. However, I am particularly excited about the success of red clover when interseeded, as this red clover has a well known nitrogen credit (i.e., can supply 50 or more pounds of N per acre to the subsequent crop) and has been shown to also improve yields of corn crops as well.

Successful but variable stand establishments have occurred with late planting of cover crops. In Wisconsin, aerial application via plane or helicopter has been a popular method for corn and soybean fields. The disadvantage to this method over interseeding is that seeding rates are higher and less growth of the cover crop occurs. In corn fields following aerial application of seed, I have observed that rye, barley radish, and turnip have all established in one way or another, but sometimes the growth is minimal.

For corn, the rule of thumb is that cover crops should be seeded when the corn is dried up to the ear and about 50 percent of the soil surface below the canopy receives sunlight.

Soybean fields tend to have greater cover crop growth compared to corn fields because of harvest date, but also if timed right, can have better conditions for growth. The rule of thumb for soybean fields is to seed between 50 percent senescence and 50 percent leaf drop. You need to have some sunlight reaching the ground, but should err on seeding too early—when there is too much leaf drop, the seeds will not contact the soil surface. Don’t expect even stand establishment in a soybean field as a field will not have perfectly even senecense and leaf drop, which will impact establishment and growth. Research out of Iowa has demonstrated no yield reductions when interseeding oats or rye into soybeans during August.

Overall, it is important to note that few controlled studies have been conducted on interseeding cover crops into grain crops and even fewer into vegetables. Many farmers are already using this approach and have fine-tuned their systems through trial and error over time.

Reduction or elimination of tillage in organic production systems remains a grand challenge, but there are gains to be made with cover cropping practices that may help eliminate the need for tillage at certain times of the year. Cover crops also help build biologically active carbon pools beneficial for nutrient supply and plant health. The ability to annually apply animal manures in conjunction with perennial crops further increases the ability to maintain and even build soil health on our farms.

Matt Ruark is an associate professor in the Department of Soil Science at the University of Wisconsin-Madison.

From the May | June 2016 Issue

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