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NORTH AMERICA Sources for Organic Crop Production By Robert Mikkelsen and T.K. Hartz Nitrogen is generally the most difficult to manage for organic crop production. Cover crops and can contribute substantial N for crops, but it is challenging to synchronize N release from these materials with the demand. Various commercial organic N are available, but their costs may be prohibitive in many situations. Careful management of organic N sources is required to meet crop requirements, while avoiding undesirable N losses to the environment. itrogen is the plant nutrient that is often most limiting to effi cient and profi table crop production. Inadequate Nsupply of available N frequently results in that have slow growth, depressed levels, poor of low quality produce, and ineffi cient water use. Nitrogen-stressed plants often have greater disease susceptibility compared with properly nourished plants. However, excessive N can be detrimental for crop growth and quality, in addition to causing undesirable environmental impacts. For these reasons, more research has been conducted on managing this plant nutrient Composts and can provide a valuable source of organic matter, than any other. This brief review does not address all the im- but predicting the rate of N release to plants is not easy. portant aspects related to N management, but covers the major sources of N for organic crop production and their behavior in There are many biological and chemical processes that . An extensive list of references is available at this website: cause fi rst-year recovery by plants to generally be less than >www.ipni.net/organic/references<. 50% of the applied N. Low N effi ciency can also be caused by

Although Earth’s atmosphere contains 78% N gas (N2), imbalance of other essential plant . Management of N most organisms cannot directly use this resource due to the is also made diffi cult due to uncertainties related to weather stability of the compound. Breaking the strong chemical bond events following fertilization. Where N recovery is low, it is

in N2 gas requires either the input of energy (to manufacture important to consider where the unrecovered N may be going ) or specialized nitrogenase enzymes. Since the use of and the potential environmental and economic risks associated manufactured N fertilizer is not allowed for organic production, with these losses (Figure 1). these materials are not specifi cally addressed here. Almost all non- plants obtain N from the soil in + - the form of (NH4 ) or (NO3 ). Some organic N-containing compounds can be acquired by roots in small amounts, but these are not a major source of . Ammonium is the preferred inorganic source of N for some plants (especially grasses), but nitrifi cation processes typically - oxidize this N form to NO3 . Many other crops grow best with - predominantly NO3 nutrition. In most warm, -aerated , - the NO3 concentration may be at least 10 times greater than + the NH4 concentration. Unlike other plant nutrients (like P and K), there is no universal or widely used to predict the amount of supplemental N required to meet the crop’s need. Instead, the need for N supplementation is typically based on yield - expectations, fi eld history, and measurement of residual NO3 . Nutrients in commercial fertilizers are generally soluble, so their availability to plants is quite predictable. However, most organic N sources require mineralization (conversion to inor- ganic forms) before they can be used by plants. Environmental factors such as soil temperature, pH, moisture, and manage- ment practices such as intensity all impact the rate of N availability from organic sources. A major factor for using organic N sources involves knowing Figure 1. Where does the added N go? Organic N is converted to both the amount of N applied and the rate of N release from the various inorganic N forms prior to plant uptake. Careful management reduces unwanted N loss to the environ- Abbreviations and notes for this article: N = nitrogen; P = ; ment. K = ; C = carbon. Better Crops/Vol. 92 (2008, No. 4) Crops/Vol. Better 16 Better Crops/Vol. 92 (2008, No. 4) 17 leach- - 3

Nutrient Release Plant Uptake Deficit Potential Time Time

Plant Uptake Potential Loss Potential Nutrient Release

A B

Nutrient Nutrient Synchronizing nutrient release with plant demand is a release from or- Rapid challenge with organic materials. C:N ratio may supply nutrients ganic sources with a low demand (A). An organic more rapidly than the plant’s release nutrients with a high C:N ratio may not material sufficiently plants (B). the need of growing meet rapid to Failure to synchronize N mineralization with crop uptake Failure to synchronize N mineralization When the crop’s N supply comes exclusively from sources N supply comes When the crop’s Generally, composts contain relatively low composts contain relatively low Composts: Generally, is not con- surplus of available N. Mineralization ciency or PAN). The N availability coeffi cient can vary widely, based can vary widely, cient coeffi The N availability PAN). can lead to plant nutrient defi ciencies, excessive soil N beyond can lead to plant nutrient defi for excessive NO the growing season, and the potential such as soil organic matter, cover crops, and composts, a thor- cover matter, such as soil organic a of mineralization is essential to avoid ough understanding defi year and crop N demand should be matchedsistent through the rates from mineralization. Mineralization with nutrient release factors (such as temperature are dependent on environmental (such the properties of the organic material and soil moisture), content), and placement of the material. as C:N ratio, lignin process in detail. Many excellent references discuss this Figure 2. organic material. Nitrogen availability coeffi to are used cients coeffi availability Nitrogen material. organic for crop available will be total N that of the fraction estimate N plant-available season (called rst growing the fi uptake during or (such as management practices of the material, on the nature as season of the factors (such and environmental placement), 1. Table cients are shown in coeffi of PAN year). Examples . 2 the N cycle are described in Table Major processes of of Organic Matter Mineralization concentrations of N, P, and K. They typically decompose and K. They typically decompose concentrations of N, P, source of N over many slowly and behave as a slow-release compounds months or years since the rapidly decomposable the composting process.have been previously degraded during materials, but they are Composts can be made from on- and commercial sources. also widely available from municipal ing (Figure 2). . Deni- 2 ) to be ) to 2 with 2 O and N 2 , N 2 to and then to and then to nitrite to + Surface applied 4 Broadcast Irrigated cations within the expanded + 4 form the and can be readily lost to 3 0.6 0.4 – 0.6 0.5 – 0.0500 0.03 Soil to gases such as NO to - from soil, , or manures is primar- from soil, compost, 3 3 Fraction of N available during the firstN available of Fraction year incorporated , the precursor to most other manufactured N most, the precursor other to 3 Certain and clay minerals (such as mica, illite, The process of combining atmospheric N Immobilization occurs when soil microorganisms assimi- When oxygen is in shortWhen oxygen bacteria supply in the soil, many The release of inorganic N from organic matter (, (proteins, The release of inorganic N from organic matter The loss of NH The 2-step bacterial oxidation of NH oxidation bacterial The 2-step nures and application methods (plant-available N). (plant-available methods and application nures -containing materials on the soil surface. materials -containing 3 changes to the gaseous NH changes to . This process requires oxygen and is most and rapid under conditions oxygen . This process requires - + 3 4 O, are potential gases. greenhouse O, are potential 2 Major processes in the N Cycle. 2. Major Table Table 1. Table First-year coefficients availability N for different ma- Layer Layer Scraped swine manureScraped dairy manureSwine lagoon effluent 0.6Dairy lagoon effluent 0.6Compost 0.8 0.4 0.8 0.4 0.5 0.5 – – 0.5 0.5 Poultry atmosphere from soil or organic materials. High temperatures and dry- High temperatures atmosphere from soil or organic materials. reactions, especially speed the volatilization to ing conditions also tend from NH favorable for crop growth. favorable : are capable of reducing NO Volatilization: trification results in a loss of plant available N and byproducts, such as trification N and byproducts, results in a loss of plant available N ily a function of the chemical environment. In an alkaline environment, environment, In an alkaline ily a function of the chemical environment. NH converted to plant-available forms and plant-available of N. Some free-living bacteria converted to actinorhizal plants are also capable of biological N fixation. This is the in unfertilized mechanism of supplying N major soils. Industrial Fixation: Manure type NO fertilizers. Ammonium Fixation: are capable of trapping NH ) Symbiotic relationship between bacterial bacterial Biological Fixation: relationship between Symbiotic dinitrogen gas (N allows of leguminous plants and a variety (C:N >25:1) Greenfield. . on organic 2006. Composting Source: Baldwin, K.R. and J.T. Production - Composting.pdf http://www.cefs.ncsu.edu/PDFs/Organic authors: http://www.soil.ncsu.edu/about/publications.php#AnimalWaste Various (C:N of 15:1 to 20:1) (C:N of 15:1 to Compost clay layers. This phenomenon also can occur with K. The extent of this of with K. The extent clay layers. This phenomenon also can occur significant, to depending negligible considerable, from process varies on the clay . Immobilization: The C:N ratio for plant uptake. inorganic N, making it unavailable late of predictor an absolute, is a good, but not of added organic materials (C:N ratio >25:1) or if mineralization N immobilization is likely whether (C:N ratio <20:1). is likely Mineralization: soil by decomposition their amino sugars, and nucleic acids) following of mineralization is influenced numerous microorganisms. The rate by making it difficult and management factors, accu- environmental to predict in the field.rately : hydrogen to form to NH hydrogen These composts vary in quality and tend to have low immedi- Manures and composts can be challenging to uniformly ate nutritional value, but provide valuable sources of stable apply to the fi eld due to their bulky nature and inherent vari- organic matter. Since , trash, and industrial waste may ability. Application of raw manure may bring up concerns also turn up in selected municipal composts, some organic related to safety, such as potential pathogens, hormones, certifi cation programs do not allow their use. Commercially and medications. The use of raw manure is restricted for some composted manure is widely available from a variety of primary organic uses and growers should check with the certifying organic materials. agency before using. Manure: The chemical, physical, and biological properties Cover Crops: A wide variety of plant species (most com- of fresh manure vary tremendously due to specifi c animal feed- monly grasses and ) are planted during the period ing and manure management practices. The manure N is pres- between cash crops or in the inter-row space in and ent in both organic and inorganic forms. Nitrogen is unstable . They can help reduce soil , reduce soil - in fresh manure because (NH3) can be readily lost NO3 , and contribute organic matter and nutrients through volatilization. Application of fresh manure or slurry to subsequent crops after they decompose. Leguminous cover

on the soil surface can result in volatilization losses as high crops will also supply additional N through biological N2 as 50% of the total N in some situations. The combination of fi xation. The amount of N contained in a depends - wet organic matter and NO3 in some manure can also facilitate on the plant species, the stage of growth, soil factors, and the signifi cant denitrifi cation losses. The organic N-containing effectiveness of the rhizobial association. Leguminous cover compounds in manure become available for plant uptake crops commonly contain between 50 and 200 lb N/A in their following mineralization by soil microorganisms, while the biomass. inorganic N fraction is immediately available. Figure 3 shows Cover crops require mineralization before N becomes plant the wide range in N mineralization of manure applied to soil. available. The rate of N mineralization is determined by a variety of factors, including the composition of the crop (such 18 as the C:N ratio and lignin content) and the environment (such 16 as the soil temperature and moisture). As with other organic N 14 sources, it can be a challenge to match the N mineralization 12 from the cover crop to the nutritional requirement of the cash 10 crop. It is sometimes necessary to add supplemental N to crops 8 following cover crops to prevent temporary N defi ciency. 6 Commercial Organic Fertilizers 4 2 Plant Products

Number of manure samples Number 0 Alfalfa meal (4% N), - -30 -20 -10 0 10 20 30 40 50 meal (6% N), corn gluten (9% Mineralization % of organic N N), and meal (7% N) are all examples of plant products that Figure 3. Nitrogen mineralization from 107 individual dairy ma- are sometimes used as N sources for nure samples after 8 weeks of incubation. On average, organic production. These products 13% of the organic N was mineralized, but 19 samples are also used as protein-rich animal Alfalfa pellets. had net immobilization. Net N mineralization from the feeds. They require microbial miner- remaining 88 samples ranged from zero to 55%. (from alization before the N is available for crop uptake. Mineraliza- Van Kessel and Reeves, 2002). tion of these N-rich materials is generally rapid. Determining the correct application rate of manure and compost to supply adequate PAN during the growing season Animal Byproducts can be diffi cult. Begin by having manures and composts regu- Blood Meal: Derived from slaughterhouse waste (gener- larly analyzed for nutrient content since there is considerable ally ), dried powdered blood contains approximately variability. The PAN will always be smaller than the total N 12% N and rapidly mineralizes to plant-available forms. It in the manure since some loss occurs through volatilization is completely soluble and suitable for distribution through with spreading, and only a portion of the organic N will be irrigation systems. available to the plants during the growing season following : guano (8 to 12% application. The remaining organic N will slowly mineralize N) is derived from natural deposits in later years. of excrement and remains of birds When manures and composts are applied at the rate to living along extremely arid meet the N requirement of crops, the amount of P and K added coasts. Guano was historically a very is generally in excess of plant requirement. Over time, P can important N source before industrial build up to concentrations that can pose an environmental risk processes for making fertilizer were guano. since runoff from P-enriched fi elds can stimulate the growth of developed. Many of the major guano undesirable organisms in . Excessive soil K can deposits are now exhausted. Guano is also harvested from caves cause nutrient imbalances, especially in forages. The long-term where large bat populations roost. It can be applied directly to use of P and K-enriched manures to provide the major source soil or dissolved in water to make a liquid fertilizer. of N must be monitored to avoid these problems. Feather Meal: Feather meal (14 to 16% N), a by- Better Crops/Vol. 92 (2008, No. 4) Crops/Vol. Better 18 Better Crops/Vol. 92 (2008, No. 4) 19

3 in or- 3 BCBC , 16% N) is is , 16% N) 3 accumu- - 3 is restricted. 3 (NaNO Sodium nitrate Choosing the “best” source of N for organic crop production is diffi cult since organic crop production is diffi mineralization rates, nutrient ratios, PAN, cost local access, ease of application, and all need to be considered. Computer-based is limited to no more than 20% of the crop is limited to no more some countries, the use N requirement. In of NaNO ganic is typically to meet the N ganic agriculture is plant growth stagesdemand during critical entire nutritional need and not to meet the the use of NaNO of the crop. In the U.S.A., late over time. Sodium nitrate is generally late over time. Sodium soluble when addedgranulated and readily use of NaNO to soil. The intended may be favorable, or for reasons other than than reasons other or for be favorable, may nutrition. plant Nitrate Sodium Summing Up mined from naturally occurring deposits in naturally occurring mined from of the driest Peru, the location and earth where NO on http://smallfarms.oregonstate.edu/organic-fertilizer-calculator Production - Composting.pdf oregonstate.edu/catalog/pdf/em/em8954-e.pdf php#AnimalWaste Each organic N source has unique characteristics that characteristics that Each organic N source has unique require special management to gain the most benefi t for plant most benefi require special management to gain the minimizing undesir- health and economic production, while organic sources tend able environmental losses. Commercial N sources, but to be more costly to purchase than inorganic also be available. Some many local or on-farm N sources may low concentrations of locally available N sources may contain of large volumes of N, requiring transportation and handling may be problematic to material. Cover crops are useful, but c , depending on the length of t into a specifi fi growing season and rotational practices. As our understanding of soil N and organic matter improves, better N management t all crop producers and the environment. will benefi tools are available to help with these choices. For example, tools are available to help with these Fertilizer Calculator”Oregon State University has an “Organic materials to best program that allows comparison of various programs are also meet the fertility needs of a soil. Similar available elsewhere. For additional references, visit: >www.ipni.net/organic/references<. Dr. Hartz is an Extension Specialist in the Department of Plant Sci- Dr. ences, University of California, Davis; e-mail [email protected]. located at Merced, Region Director, Mikkelsen is IPNI Western Dr. California; e-mail [email protected]. Sources for Further Information EM 8936-E. Univ. 2007. Oregon State Foster. and J. Andrews, N. Greenfield. 2006. http://www.cefs.ncsu.edu/PDFs/Organic K.R. and J.T. Baldwin, 17:431-441. M. and R. Smith. 2007. HortGaskell, 16:39-42. 2006. Hort Technology Johnstone. and P.R. Hartz, T.K. EM-8954E. http://extension. University 2008. Oregon State D.M. Sullivan, Soils. 36:118-123. III. 2002. Biol. Fertil. Reeves and J.B. J.S., Kessel, Van authors. http://www.soil.ncsu.edu/about/publications. Various Time of Incubation, weeks Time of Incubation, weeks Guano Blood Meal Blood 02468 02468

75 65 55 45 35 25 15 75 65 55 45 35 25 15

Net N mineralization, % mineralization, N Net Net N mineralization, % mineralization, N Net 59F 68F 77F 50F Time of Incubation, weeks Time of Incubation, weeks Feather Meal temperatures. Mineralization of N expressed as percent of added organic N. N expressed as percent of added organic N. Mineralization of temperatures. Source Hartz 2006. and Johnstone, Nitrogen mineralization of four common organic N fertilizers at four soil 02468 02468

ed with to boost the N concentration are not allowed

75 65 55 45 35 25 15

Seaweed-based products are typically derived from kelp Non-edible fi sh (such Fish Meal and : Non-edible fi These high-N animal byproducts have relatively rapid N 75 65 55 45 35 25 15 Net N mineralization, % mineralization, N Net Net N mineralization, % mineralization, N Net species (). Dried kelp contains approximately 1% N and 2% K, with small amounts of other plant nutrients. Due to their low nutritive content, kelp products are gener- ally used in high-value cropping situations where economics Seaweed Fertilizers product of the poultry , contains as much as 70 to 90% product of the poultry industry, keratin stabilized protein. It is mostly present as non-soluble de bonds. When treated with pres- by highly resistant disulfi the feather-based surized steam and animal-derived enzymes, N for crop nutrition.protein becomes a good source of available but it mineralizes Much of the feather N is not initially soluble, for plant growth. relatively quickly under conditions favorable and application Pelletizing the feather meal makes handling usually have a delayedmore convenient. Unprocessed feathers N source if the dif- N release, but can also be an excellent low density feathers to the soil culty in uniformly applying fi can be overcome. Figure 4. as ) are cooked and pressed to separate the solid as menhaden) are cooked and pressed sh meal (10 to used as fi and liquid fractions. The solids are sh oil is 14% N) for fertilizer and animal feed. The valuable fi removed from the liquid fraction and the remaining solution is sh emulsion (2 to 5% N). Additional process- thickened into fi ing is often performed to prevent premature decomposition. sh meal products may be unpleasant in a The odor from fi closed environment such as a greenhouse. Mineralization of is generally rapid. Fish products that are sh-based products fi fortifi for organic production. mineralization. At typical summer soil temperatures, more than half of the organic N may mineralize within 2 weeks of application (Figure 4).