Dr. Charles Shapiro
Dr. Charles Wortmann
Dr. Richard Ferguson
Date of this Version
Williams, T. (2017). A Review of Cover Crops for Eastern Nebraska. MAS thesis. University of Nebraska-Lincoln.
The focus of this project is two fold, first to provide a short literature review of cover crops (CC) and then discuss the results from a field experiment that was designed to evaluate the nitrogen contribution from cover crop mixes to the subsequent corn crop. The literature review will focus on the aspects of CC that are related to the field of study. Cover crops, as defined by the United States Department of Agriculture (USDA) are crops that are agronomically sound and grown for the purpose of erosion control or other objectives related to conservation or soil improvement. Cover crops have many environmental and agronomic consequences that may be positive or negative depending on what the goal is for the cover crop. It is important to determine what the agronomic objective is for the cover crops chosen when deciding on a cover crop system. Some important consequences to consider when making a decision on the use of cover crops are the cost of seed, loss of economic production while the cover crop is growing, lower soil temperatures in the spring, and depletion of soil water at planting time (Jost 1998).
Cover crops and no-till systems are mutually beneficial. No-till systems typically take 7-9 yeas to transition from conventional farming. Using a cover crop with continuous long-term no-till shortens the time period to 2-4 years (Hoorman et al. 2009). No-till corn struggles because surface residue ties up nutrients and slows down decomposition of nutrients. Soil microbes and long-term soil organic matter can tie up N. Compaction, poor drainage, and cold, wet soils hinder the production of the corn crop (Hoorman et al. 2009). Cover crops can recycle nitrogen in the soil, increase the soil organic matter, improve soil structure, and increase water infiltration.
Cover crops can reduce runoff and erosion, enhance soil fertility, and suppress pests: including weeds, insects, and pathogens (Table 1). The reduction of water runoff and soil erosion by cover crops is based on the principle of amending soil structure, protecting the soil from raindrop impact, and reducing the speed and carrying capacity of water runoff by improving soil aggregation and enhancing soil structure (Eckert et al. 1991). Water infiltration is also increased due to the channels created by roots and enhanced earthworm activity. The use of appropriate cover crops may enhance the soil fertility by increasing the organic matter content, release of organic nitrogen (N), nutrient recycling, and provide crop residue to regulate temperature and conserve moisture. Pest management may also be another benefit from cover crops by preserving a balance between pests and predators, enhancing biological diversity, and preventing weed growth through ground cover. These are all aspects that are associated with cover crops. The literature review will look more into the soil fertility and N contribution from these different cover crop mixes.
Cover crop mixes that consist of grasses, legumes, and brassicas are used to harness the variety of positive consequences the cover crops provide. This project focuses primarily on the nitrogen (N) contribution from the cover crop mixes in order to boost the yield of two subsequent corn crops.
Nitrogen is one of the most important plant nutrients. It is continually cycled among plants, soil organisms, soil organic matter, water and the atmosphere (Figure 1). Nitrogen enters the soil from many different sources and leaves the root zone of the soil in many different ways. The balance between inputs and outputs and the various chemical changes of the nitrogen determine the amount of nitrogen available for plant growth.
Nitrogen fixation is an important part of the nitrogen cycle and cover crops can play a large role. Legumes fix atmospheric nitrogen (N2) by transforming it into ammonia (NH3) that converts to ammonium (NH4). All organisms use NH3 to manufacture amino acids, proteins and nucleic acids and other N-containing components necessary for life. Nitrogen fixation is done by bacteria, known as Rhizobium, in nodules on the roots of legumes. The NH3 produced in these nodules is absorbed by the host plant. The plant supplies the necessary nutrients and energy for the bacteria, thus a symbiotic relationship. These nutrients and energy are at the cost of the plant and any stress that reduces the plant activity will reduce nitrogen fixation. The N can then be returned to the soil when the plant dies and decomposes.
The amount of N in the cover crop does not always make it into the soil. Nitrogen can be lost to the atmosphere through denitrification and volatilization. Denitrification occurs when N is lost through the conversion of nitrate to gaseous forms of N. This often occurs when the soil is saturated and the bacteria use nitrate as an oxygen source. This is common in poorly drained soils. Volatilization is the loss of N through the conversion of ammonium to ammonia gas, which is released to the atmosphere. The volatilization increases at higher soil pH and when evaporative conditions are high. This is higher for manures and urea fertilizers that are applied on the surface and not incorporated (Cornell University Cooperative Extension Agronomy Fact Sheet #2)
Non-legumes can scavenge N from the soil profile. The scavenged N from the soil profile will prevent the N from leaching and make it available to be used by the following crop. The scavenged N is then available to the subsequent crop after the plant has been killed and decomposed. The timing of availability of the N to the subsequent crop is dependent on timing and method of killing the CC.