Conservation Tillage Systems in the Southeast

Cover Crop Influence on Subsequent Crop

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Cover crops directly influence subsequent cash crops, such as when a legume provides nitrogen to the crop as it decomposes. They also indirectly influence subsequent cash crops by increasing soil organic matter, which affects water availability and nutrient cycling. In many cases, cover crops provide multiple benefits to cash crops that are enhanced as biomass production and years of cover crop use increase.

Effects on Soil Water

Cover crops use soil water while they are growing, which can negatively affect summer crop establishment if soil water is not replenished prior to planting. Short-term soil water depletion before cash crop planting may or may not be offset by soil water conservation later in the growing season. This is dependent on rainfall distribution in relation to cash crop development. Unger and Vigil [12] state that the “time of termination becomes more critical as the probability of precipitation decreases.” When soil moisture is depleted by a high-residue cover crop in the humid Southeast, a rainfall event can usually replenish soil water and can provide adequate water for cash crop establishment.

The most common practice to reduce the risk of early-season soil water depletion by cover crops is to desiccate the cover well ahead of planting the cash crop. For example, Munawar [9] and Wagger and Mengel [14] report that early-season soil water depletion can be reduced by killing the cover crop a minimum of two to three weeks before planting the cash crop. Once terminated, residues conserve soil moisture through reduced evaporation losses and increased infiltration. These factors increase the effectiveness of a rainfall or irrigation event by increasing efficiency of water use, thus reducing water requirements for the growing season. In irrigated systems, increased water-use efficiency can reduce the number of irrigation events and the total amount of water applied, resulting in reduced costs for diesel fuel or electrical power and preservation of water resources. These effects are most prevalent during short-term drought situations and would not be as effective under a prolonged drought situation.

Table 5.6 illustrates differences between three tillage systems following a simulated 2-inch rain. Conservation tillage combined with residue resulted in the highest infiltration amounts, which is equivalent to approximately one week of cotton water demand during the peak water use period. In contrast, because of runoff- and evaporation-related water losses in the conventional tillage system, the amount of water available only met approximately two days of cotton water demand under the same growing conditions.

Soil Temperature Fluctuations

In high-residue conservation systems, cover crop residues will reduce the amount of solar radiation reaching the soil surface. This results in cooler soils in the spring that are slower to warm up compared with conventionally tilled soils. Cover crop residues reduce daily fluctuations of soil temperature and reduce the difference between daily soil temperature maximums and minimums. The cooler soil temperatures benefit cash crops throughout the summer but can delay spring planting. Starter fertilizer applied at planting of a summer crop can sometimes offset the negative effects of cool, wet soil and delayed planting, but the cold and wet soils are the more critical factors affecting germination and early-season growth.

Delay Planting

The effect of reduced soil temperatures on crop growth is greater in northern areas of a crop's adapted zone. Residue removal from the zone of seed placement—by using row cleaners, for example, or in strip-till systems—will increase soil temperature in the seed zone and decrease the amount of residue that comes in contact with the seed. This results in better seed-soil contact and fewer allelopathic effects from residue on the developing seedling.

To optimize plant growth, summer crops should be planted according to soil temperature rather than calendar date. A delay in planting to let the soil warm up, especially with favorable growing degree days in the post-planting forecast, can eliminate associated stand establishment issues. For example, soil temperatures for cotton should be 65 degrees at seed-placement depth by 8 a.m., with the possibility of accumulating at least 50 growing degree days following planting to help ensure a good stand. A soil thermometer is easily obtained, practical and inexpensive. Use it in conjunction with local recommendations to guide planting dates for cash crops and to avoid cool, wet soil conditions that can persist with high-residue cover crops.

Starter Fertilizer

In nitrogen-limited soils, applying 25–50 pounds of nitrogen per acre as a starter fertilizer to cash crops following small-grain cover crops is a good management practice. Although yield increases from starter nitrogen applications are dependent on rainfall and crop, they occur frequently enough to justify the practice. Starter fertilizers can also benefit crops planted into high residue. Because soils beneath cover crop residue are typically cooler, nutrient availability is decreased. Early-season growth of the cash crop is almost always enhanced with starter fertilizers that contain nitrogen or a combination of nitrogen and phosphorus. Starter fertilizer promotes rapid canopy development, which reduces weed competition and may help offset the negative effects of cool, wet soils.

Starter fertilizers should be placed near the seeding row in a narrow band. Starters can be applied at the soil surface, but their effectiveness is increased if placed below the soil surface. The typical recommendation for placement of starter fertilizers is a two-by-two placement, meaning 2 inches to the side and 2 inches below the seed. Banding starters on the soil surface near the row is nearly as effective, especially if row cleaners are used when planting. Fertilizer materials may be liquid or solid. Take care not to over apply or place starter fertilizer too close to the seed as this could damage seedlings.

Cash Crop Fertilizer Management

Legume cover crops can add significant amounts of fixed nitrogen to a cropping system. The nitrogen content of legume cover crops and the amount of nitrogen available to subsequent crops is affected by:

  • legume species and adaptation to specific soil and climatic conditions
  • planting date
  • residual soil nitrogen
  • time of termination

Early establishment of legume cover crops (i.e., early planting, interseeding or natural reseeding) results in greater biomass production and nitrogen production. The nitrogen content of legume cover crops is optimal at the flowering stage, as much of the nitrogen in the plant is transferred to seed after this date. Typically, legume cover crops are terminated when about 50 percent of the legume cover is blooming. Legumes contribute 15–200 pounds of nitrogen per acre, with typical values of 50–90 pounds of nitrogen per acre. In North Carolina, delaying the kill date of crimson clover two weeks beyond 50 percent bloom, and hairy vetch two weeks beyond 25 percent bloom, increased the biomass of clover by 41 percent and vetch by 61 percent. Corresponding increases in nitrogen content were 23 percent for clover and 41 percent for vetch [13].

In almost all cases, legumes will begin releasing nitrogen as soon as they are terminated. Residue from young plants will have a low C:N ratio, which promotes quicker release of nitrogen. If there is not a cash crop actively growing soon afterward, that nitrogen could be lost and unavailable to the crop. Residue from mature legumes has a higher C:N ratio and is more resistant to decomposition, so the potential to synchronize nutrient release with cash crop uptake is greater.

Unless they are terminated when very young (before joint stage), grass cover crops typically have high C:N ratios, so they do not provide much nitrogen to the following crop and can actually consume nitrogen during the decomposition process. As a result, nitrogen rates for cash crops following a high-residue cereal cover crop should be increased 25–30 pounds of nitrogen per acre above the standard nitrogen fertilizer recommendations for the respective cash crop. The additional nitrogen should be applied early in the season, usually at planting. Over time, the use of high-residue cereal cover crops will increase organic matter content and may reduce nitrogen requirements in future growing seasons.

Mixtures or cocktails with both legume and grass components can help offset nitrogen immobilization by a mature grass cover crop or help reduce the likelihood of nitrogen loss following termination of a pure legume cover crop.

Disadvantages and Concerns

Despite the many positive attributes associated with cover crops, many growers are wary of high residue levels. They have concerns about field operations in the residue, soil moisture at planting and subsequent cash crop establishment. Cash crop establishment can be complicated if growers are unfamiliar with adjustments needed for planting equipment and how to manage high-residue systems. Possible causes of establishment problems:

  • residue interference with planter operations, resulting in poor seed-soil contact
  • soil water depletion
  • wet soils due to residue cover
  • reduction in soil temperature from residue cover
  • allelopathic effects of residues
  • increased levels of soil-borne pathogens
  • increased predation by insects and other pests

One of the easiest ways to prevent potential problems is to desiccate the cover crop at least two to four weeks before planting the cash crop. However, terminating early will reduce the amount of biomass produced.

Cover crops may also reduce nitrogen-fertilizer efficiency in conservation systems, depending on the method of application. Surface applications of urea-containing fertilizers to soils with large amounts of residue can result in large losses of nitrogen. When applied on top of the cover crop residue, urea and urea-ammonium nitrate solutions volatilize more than ammonium nitrate and subsequently lose more nitrogen to the atmosphere. This is because urease, an enzyme present in soils and organic residues, reacts with urea and makes it unstable. This unstable form can quickly convert to ammonia and carbon dioxide and be lost to the atmosphere. Injecting urea-containing fertilizers into the soil eliminates volatilization losses. Banding urea-containing fertilizers reduces losses compared to broadcast applications because banding minimizes fertilizer and residue contact while increasing fertilizer and soil contact.

Download the tables from Chapter 5.