Management Practices Affecting the Nutrient Provision Capacity of Cover Crops

Cover cropping is an effective practice for soil health management. One of the many benefits of cover cropping is that it can be a significant source of nutrients to the subsequent cash crops. This article explains how cover crop species and the timing and method of termination influence the pulse of nutrients released from the residues and their availability to the subsequent cash crop.

Cover Cropping to Improve Soil Health

Soil health is defined as “the capacity of soil to function as a vital living system to sustain plant and animal productivity”.1 Soil is considered healthy if it has the following characteristics:2

  • Sufficient nutrient supply
  • Good soil tilth
  • Sufficient root depth
  • Good internal drainage
  • Reduced population of plant pathogens and insect pests
  • Resistance and resilience to degradation
  • Low weed pressure
  • Free of chemicals and toxins that may harm the crop

When soil health is poor, long-term reductions in agricultural productivity may occur. Therefore, building healthy and resilient soil is important for sustainable production and is a focus of many soil conservation programs in the United States. Practices such as crop rotation, reduced tillage, cover crops, and organic amendments of biochar and compost are widely implemented for soil health management.3

Among these soil health management practices, cover cropping is often recommended and adopted. Cover crops are unharvested crops planted in rotation between cash crops. Worldwide, research has demonstrated the benefits of cover crops including improved soil structure, prevention of soil erosion, reduced soil compaction and water loss,4 increased soil organic matter content,5 suppression of weed growth,6 and provision of plant nutrients.7 These benefits often lead to fewer resource inputs (e.g., lower costs).8 For more information on selecting cover crops for specific benefits, consult the Southern Cover Crops Council’s Cover Crop Resource Guide. South Carolina farmers are still exploring the practical, economical, and environmental impacts of integrating cover crops into their farming systems. This lag in adoption, despite the demonstrated benefits of cover crops, is primarily due to the lack of relevant research detailing optimal management practices for better agronomic and environmental outcomes.

Types of Cover Crops

Cover crops are diverse and differ in their functional traits and services (table 1):

  • Legumes (e.g., crimson clover, hairy vetch, etc.) fix atmospheric nitrogen (N), potentially reducing N fertilizer inputs for the subsequent cash crop.
  • Grains (e.g., rye, oats, etc.) efficiently scavenge nutrients, accumulating greater biomass, enhancing weed management.
  • Brassicas (e.g., mustard and radish) reduce soil compaction, increasing water and nutrient movement within soils, via their extensive root system.

Such diversity, paired with varied climatic and edaphic (i.e., the physical, chemical, and biological properties of soil) factors, makes cover cropping benefits site-specific or, at a minimum, regional-specific.9 One concern with cover cropping relates to their nutrient provision capacities and the extent to which those nutrients are available to the following cash crops. Cover crop selection, along with the timing and method of cover crop termination, can help to address this issue.

Cover Crop Residue Decomposition and Nutrient Release

Decomposition is the process of breaking down complex organic materials (e.g., protein and lipids) into inorganic forms of nutrients (e.g., nitrate and ammonium) that can be readily absorbed by plants. Soil microbes regulate the decomposition of organic materials. Cover crop termination makes their residues accessible for microbial decomposition. The timing of termination affects both the residue decomposition rate and the associated nutrient release pattern. In general, cover crops with a relatively low carbon (C) to N (C:N) ratio in tissue (below 24:1) decompose faster than crops with a higher ratio (above 24:1) (table 1).10,11 For example, legume cover crops (e.g., crimson clover and pea) and brassica (e.g., forage radish) generally have lower C:N ratios (15-30:1) than grain cover crops (e.g., cereal rye, 35–80:1) (table 1), and therefore decompose faster after termination (figure 1). If cover crop termination is delayed, cover crop plants can accumulate more hemicellulose and lignin, which have higher C, but lower N content, resulting in plant residues with a slower decomposition rate. Cover crop selection and the timing of termination are important, but appropriate killing method and residue management also need to be considered. Mechanical method (such as rolling with a crimper and mowing with a flail mower) along with chemical method (use of chemicals such as herbicides) are common practices for the residue management.12 Rolling the residues with a roller-crimper can ensure better residual quality for mulching purpose which is difficult to achieve with flail mowing.13 Mowing with a flail mower results in small pieces of material that can cause rapid decomposition and is not as persistent as mulch.14 Proper killing of cover crop residues with the use of herbicides (glyphosate, 2,4-D, etc.) are also gaining popularity with the ease and ability to kill cover crops across varied growth stages.15

Table 1. Common cover crops used in the southeastern United States with their carbon to nitrogen (C:N) ratios by cropping season.16,17 To determine the plant C:N ratio, collect the plant tissue samples from the field at the time of termination, dry to a constant weight, and send to a certified commercial lab for nutrient analyses.

Legumes  C:N Ratio Cropping Season
Crimson clover (Trifolium incarnatum) Low: 15-20:1 Winter
Field peas (Pisum sativum) Low: 15-20:1 Winter
Hairy vetch (Vicia villosa) Low: 15-20:1 Winter
Red clover (Trifolium pratense) Low: 15-20:1 Winter
Berseem clover (Trifolium alexandrinum) Low: 15-20:1 Summer
Cowpea (Vigna unguiculata) Low: 15-20:1 Summer
Soybean (Glycine max) Low: 15-20:1 Summer
Sun hemp (Crotalaria juncea) Low: 10-20:1 Summer
Grains C:N Ratio Cropping Season
Cereal rye (Secale cereale) High: 35-80:1 Winter
Wheat (Triticum aestivum) High: 60-80:1 Winter
Oats (Avena sativa) Low: 15-25:1 Winter
Barley (Hordeum vulgare) High: 50-80:1 Winter
Buckwheat (Fagopyrum esculentum) High: 25-50:1 Summer
Sudangrass (Sorghum sudanese) High: 30-50:1 Summer
Annual ryegrass (Lolium multiflorum) Medium: 20-30:1 Summer
Brassica C:N Ration Cropping Season
Mustard (Brassica juncea) Low: 10-20:1 Winter
Forage Radish (Raphanus sativus) Medium: 15-30:1 Winter
Decomposition patterns (% Biomass remaining) of rye and Austrian winter pea residues

Figure 1. Decomposition patterns (% Biomass remaining) of rye and Austrian winter pea residues in sandy loam soils at Clemson University Pee Dee Research and Education Center after simultaneous termination with herbicide (glyphosate) on May 10, 2018. Cover crops with a lower carbon to nitrogen (C:N) ratio (Austrian winter pea) decompose faster than those with a higher C:N ratio (rye). Image credit: Pratima Poudel, Clemson University.

Synchronization of Cover Crop Nutrient Release and Cash Crop Uptake

Synchronization of nutrient release from residues with the nutrient demands of the subsequent cash crop is necessary for the maximization of nutrient benefits of cover cropping. This could possibly be achieved with the selection of the right cover crop, method, and timing of termination. If cover crops are terminated too early, the nutrients released as residues decompose will likely be lost with percolating water, since the subsequent cash crop roots will not be mature enough to capture the released nutrients.18 If cover crops are terminated too late, the nutrients released may lag behind the demands of the subsequent cash crop,19 and the cover crops may also compete with cash crops for soil moisture.20,21 Excess residues, due to late termination, can also obstruct seed germination and seedling growth of subsequent cash crops.22 Other factors, such as rainfall and temperature, may also contribute to the lack of synchronization of nutrient release by cover crops to cash crop nutrient uptake. This could influence microbial activity and affect decomposition rates. Rates of nutrient release from residues may also decline due to the type of residue remaining.

Cover cropping can have many positive impacts on soil health, including improving soil fertility. To maximize nutrient benefit, management practices such as cover crop selection (e.g., legumes vs. non-legumes), timing (e.g., early vs. late), and the method of termination (e.g., mechanical or chemical) are important. Optimizing management factors to synchronize the nutrient release from cover crop residue with subsequent cash crop demands may lead to lower demand for synthetic fertilizer. Please contact the authors or your local Extension Agent for advice on the method and timing of cover crop termination based on the cash crop to be grown.

References Cited

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