“Understanding the basics of how nutrients are added to and released from soil organic matter will help the farmer in choosing crop sequences and amendments to optimize organic crop fertility.”
Soil organic matter and clay particles hold large stores of plant nutrients. These reservoirs, however, are not all available to the crop. In an organic crop rotation, the grower manages soil organic matter and nutrient availability by incorporating different crop residues, cycling among crops with different nutrient needs, using cover crops, and adding organic soil amendments. Most crops deplete soil nutrients during their growth cycle. Some of these nutrients leave the farm as harvested products, and the rest return to the soil as crop residues. The nutrients in residues may or may not be available to the next crop. Crop roots and residues improve soil fertility by stimulating soil microbial communities and improving soil aggregation. This improved soil physical environment facilitates water infiltration, water holding, aeration, and, ultimately, root growth and plant nutrient foraging. This section will review different ways that crop rotations affect soil fertility.
Understanding the basics of how nutrients are added to and released from soil organic matter will help the farmer in choosing crop sequences and amendments to optimize organic crop fertility. Certain fractions of soil organic matter contribute to plant nutrition more than other fractions. To effectively plan organic crop rotations to meet crop nutrient needs, several factors should be considered. Legume crops, which capture atmospheric nitrogen and “fix” it into forms available to plants, can be used strategically in rotations to meet the needs of nitrogen-demanding crops. Cover crops used after a cash crop capture surplus plant-available nutrients and conserve these for following crops. Cash crops themselves vary in their nutrient demands (see Appendix 1); considering their needs helps make the most efficient use of the available soil nutrients in a rotation. Finally, other types of organic amendments, such as compost and manures or approved mineral fertilizers, can supplement nutrients at targeted times during a rotation. Each of these topics is discussed in the sections below.
The Basics: How Nutrients Are Released from Soil Organic Matter
Levels of soil organic matter range from about 0.4 percent to 10 percent in mineral soils in temperate regions. While organic matter is a relatively small fraction of the soil, it has large effects on soil structure and soil fertility. Soil organic matter contains an estimated 95 percent of soil nitrogen (N) and 40 percent of soil phosphorus (P), and with the right levels and conditions it may provide all of the N and P needs of a crop. Estimates of total nitrogen in a soil with 3 percent organic matter range from 2,000 to 4,000 pounds per acre; estimates of phosphorus range from 100 to 300 pounds per acre. Soil microorganisms release these nutrients when they consume organic matter and subsequently die. The rate of this nutrient release is affected by the availability of carbon sources (energy for the soil microbes), soil temperature, soil moisture, tillage, types and numbers of soil organisms, and quality of the soil organic matter.
A portion (10–20 percent) of the total soil organic matter has been termed the “active” fraction and is most easily decomposed by soil organisms. This active fraction is replenished primarily by additions of organic matter (cover crops, crop residues, manures, compost). Soil organisms, which make up another 10–20 percent of soil organic matter, decompose this active organic matter. Upon death, these organisms release their nutrients to plants. The remaining soil organic matter is humus. The humus is more slowly digested by soil organisms and therefore is not a large source of available nutrients. Humus is very important, however, because it provides cation exchange sites, which hold nutrients in the soil and thus maintain their availability to plants.
Organic matter amendments to soil decompose at different rates, and this affects how quickly nutrients become available to crops. Several factors affect the rate of decomposition of organic amendments, including the carbon-to-nitrogen ratio of the amendment, soil type, temperature and moisture conditions, and the crop being grown. Green manures, which are part of the more active organic matter fraction, decompose readily, liberating nutrients relatively quickly. Composts have more stable, humic organic matter, and decompose more slowly. As a result, most composts release nutrients to crops more slowly than green manures.
Organic matter decomposition is enhanced in the area immediately around roots (the rhizosphere). Roots release organic compounds, such as carbohydrates, amino acids, and vitamins, into the soil, stimulating growth of microorganisms in this zone. Many of these organisms decompose organic matter, resulting in nutrient release to the crop. Very little research has been done to determine which plant varieties or species best support these nutrient-releasing microorganisms. In the future, such information may help identify crop varieties well adapted to organic systems.
When cover crops are regularly part of a rotation, their residues increase soil organic matter. The organic matter feeds the growth of microbes, which increases the release of N as they die and decompose. Thus, integrating cover crops into a crop rotation at specific points can help enhance nutrient cycling and conservation.
Nitrogen Contributions from Legume Cover and Cash Crops
Legumes may be present in a rotation as a harvested crop (for example, alfalfa) or as a green manure (for example, vetch or clover). Legumes are of special interest in organic crop rotations because of their ability to add nitrogen to the system. Specialized bacteria (Rhizobium spp.) associated with the roots of legumes convert atmospheric nitrogen (N2 gas) into plant-available nitrogen. The amount of N fixed by this association between bacteria and legumes varies with plant species and variety, soil type, climate, crop management, and length of time the crop is grown. When used strategically in a rotation, legumes provide N to the subsequent crop. The amount of N that a legume crop contributes to following crops depends on the amount of N fixed, the maturity of the legume when it is killed or incorporated into the soil, whether the entire plant or only the root system remains in the field, and the environmental conditions that govern the rate of decomposition. As a result, estimates of the amount of N contributions by legumes to subsequent crops range from 50 to over 200 pounds per acre (see Appendix 1).
Nitrogen Scavenging and Conservation by Nonlegume Winter Cover Crops
Winter-hardy grains and grasses have extensive root systems that are more efficient than legumes at scavenging soil nitrates in the fall, thereby reducing late fall and winter leaching of nitrogen (75). In the northeastern US, small grains (rye and wheat) are the most common winter-hardy cover crops used by vegetable growers, since harvests of cash crops often extend into late summer and fall. Once incorporated in the following spring, these cover crops will release captured N and other nutrients to subsequent crops, but at a slower rate than from legume cover crops because of the slower decomposition of grain residues.
In some cases, such as when heavy crop or cover crop residues with high carbon-to-nitrogen ratios (30:1 or higher) are tilled into the soil, soil N may become unavailable to plants (immobilized) in the short run because it is taken up by soil microorganisms as they feed on the carbon-rich residues. Seeding a legume cover crop with small grains (for example, hairy vetch with cereal rye) can reduce N immobilization by providing additional N to microorganisms during decomposition of residues. Alternatively, delaying the planting of a cash crop for about two weeks after incorporation of residues generally allows sufficient time for the cycling of N through microorganisms and then back into the soil. Incorporating nonlegume cover crops while they are still young and leafy also reduces problems with N immobilization.
One important consideration when using overwintering cover crops is their potential to deplete soil water. Although cover crops can improve water infiltration and soil water-holding capacity, the short-term depletion of soil water in the early spring can reduce yields of subsequent cash crops in dry springs. In this situation, cover crops may need to be incorporated early to conserve soil water, or irrigation may be required. The opposite is also true—cover crops can help dry up wet fields in the spring.
Winter-killed cover crops (species vary by climate) also capture significant amounts of soil nitrogen (up to 50–90 lbs/acre) in the fall (102) prior to being killed by low temperatures. The amount of soil N captured is related to the N that is available, the time of planting, and the total growth of the cover crop prior to being killed. Researchers observed that Brassica cover crops grew more in the fall and, as a result, captured more N than an oat cover crop (102). Across species, however, the fall-planted, winter-killed cover crops reduced soil nitrate levels in the fall and increased levels in the spring, compared to soil left bare over the winter. Thus, excess soil nitrogen from the end of one season was captured and conserved for the following season’s crop. Note that while Brassica cover crops are good at capturing nutrients, they may host diseases (clubroot) and insects (flea beetle) that attack other Brassica species in the rotation.
|TABLE 3.2: Rooting depth and lateral spread of roots for several crops|
|Crop||Estimated rooting depth (inches)||Lateral rooting spread (inches)|
|Source: Adapted from reference 42: A. A. Hanson, Practical Handbook of Agricultural Science (Boca Raton, FL: Taylor & Francis Group, LLC 1990).|
Differences in Crop Nutrient Uptake
Crop nutrient uptake varies due to many factors, including rooting depth and breadth; variety; and environmental factors, including soil tilth. Generally, crops may be characterized as having low, medium, or high nutrient demands based on their nutrient uptake efficiency (table 3.1). Different varieties within any crop may be more or less efficient at taking up nutrients. Those crops with a high nutrient demand (predominately N) require higher levels of those nutrients to be present in the soil solution. This high demand could be related to large vegetative plant growth prior to fruit set (in the case of corn and tomatoes) or due to poor foraging ability of the crop’s roots (in the case of lettuce). Green manures and soil fertility amendments have the most benefit when they target the crops with high nutrient demands. On inherently fertile soils, crops with low nutrient requirements often achieve good yields from residual soil fertility alone.
Crop rooting depth can have important implications for nutrient availability as well as soil physical characteristics. Crop rotations that integrate deep-rooting crops with less nutrient-efficient crops can help cycle nutrients in the soil profile. The deep-rooted crops listed in table 3.2 absorb nutrients from deep in the soil and move them to the plant’s top growth. As crop residues are returned to the surface soil, these newly “mined” nutrients are potentially available to future crops. Deep-rooted crops also create channels into the soil that later can improve water infiltration. Although most of the listed crops are typical of grain rotations, the data are also relevant to vegetable producers, since grain and forage crops are integrated into vegetable rotations as cover crops.
Compost, Micronutrient, and Rock Powder Applications for Crop Nutrition
Soil tests may suggest the need for additional inputs of particular nutrients. In some cases, soils are naturally low in nutrients; in other cases, export of nutrients in crops has led to soil depletion. Organic soil amendments such as composts, trace element mixes, plant and animal meals, and rock powders can be used to meet some of these needs. Many organic soil amendments become available only slowly; in some cases application to the previous cover crop improves availability to the cash crop. Since some of these amendments can be expensive, they should be applied strategically within a rotation. Prior to the application of any of these materials, adjust soil pH to the desired range for the majority of crops within the rotation (generally 6.2 to 6.8). High or low pH will reduce the availability of phosphorus and many micronutrients.
Most composts contain relatively stable forms of organic matter and low levels of readily available nutrients. Some types, such as poultry compost, may contain high levels of nutrients compared to other organic fertility amendments, but not compared to commercial fertilizers. Good composts applied at specific points in a rotation can improve soil fertility in the long term by enhancing soil structure and tilth, improving soil water movement, and providing a slow-release fertility source. Usually, meeting the complete nitrogen needs of a crop by using only compost is difficult without also adding excessive phosphorus. Build-up of excessively high phosphorus levels can occur when composts based on animal manures are used at high rates (greater than 10 tons/acre) once or twice per year. Accumulation of excess P can damage neighboring bodies of water and stimulate weed growth (see “The Role of Crop Rotation in Weed Management”).
Micronutrients can be supplemented using foliar-type fertilizers, including seaweed extracts and borax (consult the Organic Materials Research Institute’s approved materials list for organically acceptable formulations, www.omri.org/OMRI_products_list.php). These can provide low levels of nitrogen, calcium, magnesium, boron, zinc, and iron. Foliar fertilizing must be managed carefully, since effectiveness depends on uptake of the micronutrients through the plant cuticle. Depending on application rates, environmental conditions, and plant maturity, foliar feeding can sometimes result in burning of leaves.
Rock powders (ground limestone, gypsum, granite dust, rock phosphate) and trace element mixes slowly release nutrients to plants. The more finely ground the powder, the sooner the minerals will be available to the crop due to a greater surface area of the powder available for microbial digestion and physical weathering. Like composts, rock powders cannot be used to provide immediate crop needs. They should be used as long-term sources of crop nutrients.
|TABLE 3.4: A sample nutrient budget for nitrogen and phosphorus from an organic vegetable rotation|
|Nitrogen Budget (lbs N/acre)||Phosphorus Budget (lbs P/acre)|
|N export||N input source||N input||
Cumulative N balance
P input source
Cumulative P balance
|2||Lettuce, spinach||8,000, 5,000||28||Compost||150||373||4||Compost||40||99|
|4||Lettuce, pepper||8,000, 30,000||60||Compost||150||726||8||Compost||40||195|
Putting It All Together: Nutrient Budgets During Crop Rotation
One strategy for reviewing the effects of a crop rotation on soil nutrients is to construct a nutrient budget. A nutrient budget can be complex or fairly simple, depending on its purpose. For simplicity, consider just soil N and P. Think of them as deposits in a soil fertility bank account. Most of these nutrients are tied up in long-term investments, in the form of organic matter. But a portion of the account is available for withdrawal. Assuming a soil is relatively fertile, the long-term goal is to maintain an approximately constant balance in the account, rather than to increase or decrease nutrient storage. As crops are removed, nutrients are withdrawn or exported from the system. As legumes, manures, composts, or other amendments are added to the soil, the nutrient bank balance increases. By examining rotations through time, a farmer can make general estimates of the increase or decrease in potentially available nutrients and change his or her management accordingly.
“Generally, the technique of using crop rotation for disease management is to grow non-host plants until the pathogen in the soil dies or its population is reduced to a level that will result in negligible crop damage.”
Consider the examples in tables 3.3 and 3.4. In the first example (table 3.3), periodic applications of manure to a long-term rotation resulted in moderate increases in soil nitrogen but did not help maintain soil phosphorous levels. Through each cycle of the five-year rotation, about 50 pounds of P was exported off the farm. Future crops may require an additional source of P. In the vegetable rotation (table 3.4), yearly compost additions led to a rapid buildup of soil nitrogen and phosphorous. Such high levels are not environmentally sound and may be prohibited in some states, depending on nutrient management regulations. Also notable in the vegetable rotation is the low level of nutrient export via these crops, compared with the agronomic crops (table 3.3). Excess nutrients in the vegetable rotation may leach out of or run off from the system eventually, even if cover crops or other cultural practices are used to minimize losses.
By reviewing the inputs and outputs of a rotation, general trends of nutrient accumulation or depletion can be detected. Although nutrient exports by crops or nutrient inputs via cover crops and other amendments can only be estimated (Appendix 1), these values and budgets will still point to potential problems in nutrient management within a crop rotation. This approach will not account for losses through leaching or soil erosion. It also does not include an estimate of the “starting” reserves of soil nutrients. History of management, inputs, and native soil organic matter levels in each field will all contribute to the starting reserve. With this information on the general trends of nutrient accumulation within a field, alternative rotations or different crops (including cover crops and green manures) may be considered to strategically capture, export, or contribute essential plant nutrients.
For further reading, see References 24, 42, 75, 92, 102, and 107.