Monday, January 28, 2013

Nitrogen Sources for Organic Crop Production

Nitrogen is generally the most difficult nutrient to manage for organic crop production. Cover crops and composts can contribute substantial N for crops, but it is challenging to synchronize N release from these materials with the plant demand. Various commercial organic N fertilizers 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. This article was first written by me and Dr. Tim Hartz (Univ. Calif, Davis) for the magazine Better Crops

Nitrogen is the plant nutrient that is often most limiting to efficient and profitable crop production. Inadequate supply of available N frequently results in plants that have slow growth, depressed protein levels, poor yield of low quality produce, and inefficient 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 than any other. This brief review does not address all the important aspects related to N management, but covers the major sources of N for organic crop production and their behavior in soil. An extensive list of references is available at this website:

Although Earth’s atmosphere contains 78% N gas (N2), most organisms cannot directly use this resource due to the stability of the compound. Breaking the strong chemical bond in N2 gas requires either the input of energy (to manufacture fertilizer) or specialized nitrogenase enzymes. Since the use of manufactured N fertilizer is not allowed for organic production, these materials are not specifically addressed here.
The nitrogen cycle

There are many biological and chemical processes that cause first-year recovery by plants to generally be less than 50% of the applied N. Low N efficiency can also be caused by imbalance of other essential plant nutrients. Management of N is also made difficult due to uncertainties related to weather events following fertilization. Where N recovery is low, it is important to consider where the unrecovered N may be going and the potential environmental and economic risks associated with these losses (Figure 1).

Almost all non-legume plants obtain N from the soil in the form of ammonium (NH4+) or nitrate (NO3-). Some organic N-containing compounds can be acquired by roots in small amounts, but these are not a major source of plant nutrition. Ammonium is the preferred inorganic source of N for some plants (especially grasses), but nitrification processes typically oxidize this N form to NO3-. Many other crops grow best with predominantly NO3- nutrition. In most warm, well-aerated soils, 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 soil test 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, field 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 inorganic forms) before they can be used by plants. Environmental factors such as soil temperature, pH, moisture, and management practices such as tillage intensity all impact the rate of N availability from organic sources.

A major factor for using organic N sources involves knowing both the amount of N applied and the rate of N release from the organic material. Nitrogen availability coefficients are used to estimate the fraction of total N that will be available for crop uptake during the first growing season (called plant-available N or PAN). The N availability coefficient can vary widely, based on the nature of the material, management practices (such as placement), and environmental factors (such as season of the year). Examples of PAN coefficients are shown in Table 1. Major processes of the N cycle are described in Table 2.

Mineralization of Organic Matter
When the crop’s N supply comes exclusively from sources such as soil organic matter, cover crops, and composts, a thorough understanding of mineralization is essential to avoid a deficiency or surplus of available N. Mineralization is not consistent through the year and crop N demand should be matched with nutrient release from mineralization. Mineralization rates are dependent on environmental factors (such as temperature and soil moisture), the properties of the organic material (such as C:N ratio, lignin content), and placement of the material. Many excellent references discuss this process in detail. Failure to synchronize N mineralization with crop uptake can lead to plant nutrient deficiencies, excessive soil N beyond the growing season, and the potential for excessive NO3leaching (Figure 2).

Synchronizing nutrient release with plant demand is a challenge with organic materials. Rapid release from organic sources with a low C:N ratio may supply nutrients more rapidly than the plant's demand (A).  An organic material with a high C:N ratio may not release nutrients sufficiently rapid to meet the needs of growing plants (B) 

Composts: Generally, composts contain relatively low concentrations of N, P, and K. They typically decompose slowly and behave as a slow-release source of N over many months or years since the rapidly decomposable compounds have been previously degraded during the composting process. Composts can be made from on-farm materials, but they are also widely available from municipal and commercial sources.

These composts vary in quality and tend to have low immediate nutritional value, but provide valuable sources of stable organic matter. Since plastic, trash, and industrial waste may also turn up in selected municipal composts, some organic certification programs do not allow their use. Commercially composted manure is widely available from a variety of primary organic materials.

Manure: The chemical, physical, and biological properties of fresh manure vary tremendously due to specific animal feeding and manure management practices. The manure N is present in both organic and inorganic forms. Nitrogen is unstable in fresh manure because ammonia (NH3) can be readily lost through volatilization. Application of fresh manure or slurry on the soil surface can result in volatilization losses as high as 50% of the total N in some situations. The combination of wet organic matter and NO3- in some manure can also facilitate significant denitrification losses. The organic N-containing compounds in manure become available for plant uptake following mineralization by soil microorganisms, while the inorganic N fraction is immediately available. Figure 3 shows the wide range in N mineralization of manure applied to soil. 
Nitrogen mineralization from 107 individual dairy manure samples after 8 weeks of mineralization.  On average, 13% of the organic N was mineralized, but 19 samples had net mineralization.  Net N mineralization from the remaining 88 samples ranged from zero to 55% (from Van Kessel and Reeves, 2002).
Determining the correct application rate of manure and compost to supply adequate PAN during the growing season can be diffi cult. Begin by having manures and composts regularly analyzed for nutrient content since there is considerable variability. The PAN will always be smaller than the total N in the manure since some loss occurs through volatilization with spreading, and only a portion of the organic N will be available to the plants during the growing season following application. The remaining organic N will slowly mineralize in later years.

When manures and composts are applied at the rate to meet the N requirement of crops, the amount of P and K added is generally in excess of plant requirement. Over time, P can build up to concentrations that can pose an environmental risk since runoff from P-enriched fields can stimulate the growth of undesirable organisms in surface water. Excessive soil K can cause nutrient imbalances, especially in forages. The long-term use of P and K-enriched manures to provide the major source of N must be monitored to avoid these problems.
Manure spreader in the field

Manures and composts can be challenging to uniformly apply to the field due to their bulky nature and inherent variability. Application of raw manure may bring up concerns related to food safety, such as potential pathogens, hormones, and medications. The use of raw manure is restricted for some organic uses and growers should check with the certifying agency before using.

Cover Crops: A wide variety of plant species (most commonly grasses and legumes) are planted during the period between cash crops or in the inter-row space in orchards and
vineyards. They can help reduce soil erosion, reduce soil NO3- leaching, and contribute organic matter and nutrients to subsequent crops after they decompose. Leguminous cover crops will also supply additional N through biological N2 fixation. The amount of N contained in a cover crop depends on the plant species, the stage of growth, soil factors, and the effectiveness of the rhizobial association. Leguminous cover crops commonly contain between 50 and 200 lb N/A in their biomass.

Cover crops require mineralization before N becomes plant available. The rate of N mineralization is determined by a variety of factors, including the composition of the crop (such as the C:N ratio and lignin content) and the environment (such as the soil temperature and moisture). As with other organic N sources, it can be a challenge to match the N mineralization from the cover crop to the nutritional requirement of the cash crop. It is sometimes necessary to add supplemental N to crops following cover crops to prevent temporary N deficiency.

Commercial Organic Fertilizers

Alfalfa pellets can be used as a nitrogen fertilizer
Plant Products   
Alfalfa meal (4% N), cottonseed meal (6% N), corn gluten (9% N), and soybean meal (7% N) are all examples of plant products that are sometimes used as N sources for organic production. These products are also used as protein-rich animal feeds. They require microbial mineralization before the N is available for crop uptake. Mineralization of these N-rich materials is generally rapid. 

Alfalfa meal is sold as fertilizer

Animal Byproducts

Blood Meal: Derived from slaughterhouse waste (generally cattle), dried powdered blood contains approximately 12% N and rapidly mineralizes to plant-available forms. It
is completely soluble and suitable for distribution through irrigation systems.

Guano: Seabird guano (8 to 12% N) is derived from natural deposits of excrement and remains of birds living along extremely arid sea coasts. Guano was historically a very important N source before industrial processes for making fertilizer were developed. Many of the major guano deposits are now exhausted. Guano is also harvested from caves where large bat populations roost. It can be applied directly to soil or dissolved in water to make a liquid fertilizer.
This bat guano contains 10 percent nitrogen

Feather Meal: Feather meal (14 to 16% N), a by- product of the poultry industry, contains as much as 70 to 90% protein. It is mostly present as non-soluble keratin stabilized by highly resistant disulfide bonds. When treated with pressurized steam and animal-derived enzymes, the feather-based protein becomes a good source of available N for crop nutrition. Much of the feather N is not initially soluble, but it mineralizes relatively quickly under conditions favorable for plant growth.

Pelletizing the feather meal makes handling and application more convenient. Unprocessed feathers usually have a delayed N release, but can also be an excellent N source if the difficulty in uniformly applying low density feathers to the soil can be overcome.

Fish Meal and Fish Emulsion: Non-edible fish (such as menhaden) are cooked and pressed to separate the solid and liquid fractions. The solids are used as fish meal (10 to 14% N) for fertilizer and animal feed. The valuable fish oil is removed from the liquid fraction and the remaining solution is thickened into fish emulsion (2 to 5% N). Additional processing is often performed to prevent premature decomposition. The odor from fish meal products may be unpleasant in a closed environment such as a greenhouse. Mineralization of fish-based products is generally rapid. Fish products that are fortified with urea to boost the N concentration are not allowed for organic production.

These high-N animal byproducts have relatively rapid N mineralization. At typical summer soil temperatures, more than half of the organic N may mineralize within 2 weeks of application (Figure 4).

Nitrogen mineralization of four common organic N fertilizers at four soil temperatures.  Mineralization of N expressed as percent of added organic N (Hartz and Johnstone, 2006).
Seaweed Fertilizers
Seaweed-based products are typically derived from kelp species (Ascophyllum). 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 generally used in high-value cropping situations where economics may be favorable, or for reasons other than plant nutrition.

Sodium Nitrate
Sodium nitrate (NaNO3, 16% N) is mined from naturally occurring deposits in Chile and Peru, the location of the driest desert on earth where NO3 - salts accumulate over time. Sodium nitrate is generally granulated and readily soluble when added to soil. The intended use of NaNO3 in organic agriculture is typically to meet the N demand during critical plant growth stages and not to meet the entire nutritional need of the crop. In the U.S.A., the use of NaNO3 is limited to no more than 20% of the crop N requirement. In some countries, the use of NaNO3 is restricted.

Summing Up

Choosing the “best” source of N for organic crop production is difficult since nutrient ratios, PAN, mineralization rates, local access, ease of application, and cost all need to be considered. Computer-based tools are available to help with these choices. For example, Oregon State University has an “Organic Fertilizer Calculator” program that allows comparison of various materials to best meet the fertility needs of a soil. Similar programs are also available elsewhere.

Each organic N source has unique characteristics that require special management to gain the most benefit for plant health and economic production, while minimizing undesirable environmental losses. Commercial organic sources tend to be more costly to purchase than inorganic N sources, but many local or on-farm N sources may also be available. Some locally available N sources may contain low concentrations of N, requiring transportation and handling of large volumes of material. Cover crops are useful, but may be problematic to fit into a specific cropping system, depending on the length of growing season and rotational practices. As our understanding of soil N and organic matter improves, better N management will benefit all crop producers and the environment.


Sources for Further Information:

Andrews, N. and J. Foster. 2007. Oregon State Univ. EM 8936-E.

Baldwin, K.R. and J.T. Greenfield. 2006.
Production - Composting.pdf

Gaskell, M. and R. Smith. 2007. Hort Technology 17:431-441.

Hartz, T.K. and P.R. Johnstone. 2006. Hort Technology 16:39-42.

Sullivan, D.M. 2008. Oregon State University EM-8954E. http://extension.

Van Kessel, J.S., and J.B. Reeves III. 2002. Biol. Fertil. Soils. 36:118-123.

Various authors.

 A pdf version of this article is available here:

Wednesday, January 23, 2013

Managing Potassium for Organic Crop Production

An adequate K supply is essential for both organic and conventional crop production. Potassium

is involved in many plant physiological reactions, including osmoregulation, protein synthesis, enzyme activation, and photosynthate translocation. The K balance on many farms is negative, where more K is removed in harvested crops than is returned again to the soil. An overview of commonly used K fertilizers for organic production is provided.
Adequate nutrients
are required for high-
yielding crops

Potassium is an essential nutrient for plant growth, but it often receives less attention than N and P in many crop production systems. Many regions of the U.S.A. and all of the Canadian provinces remove more K during harvest than is returned to the soil in fertilizer and manure (Figure 1). In the U.S.A., an average of only 3 units of K is replaced as fertilizer and manure for every 4 units of K removed in crops, resulting in a depletion of nutrients from the soil and increasing occurrences of deficiency in many places.
The annual K balance in agricultural soil is quite negative in many states.  This represents a depletion of the soil resource.

Potassium is the soil cation required in the largest amount by plants, regardless of nutrient management philosophy.

Large amounts of K are required to maintain plant health and vigor. Some specific roles of K in the plant include osmoregulation, internal cation/anion balance, enzyme activation, proper water relations, photosynthate translocation, and protein synthesis. Tolerance of external stress, such as frost, drought, heat, and high light intensity is enhanced with proper K nutrition. Stresses from disease and insect damage are also reduced with an adequate supply of K. Although there are no known harmful effects of K to the environment or to human health, the consequences of inadequate K can be severe for crop growth and efficient utilization of other nutrients, such as N and P. Maintenance of adequate K is essential for both organic and conventional crop production. More information and an extensive list of references are available at the website:

The global cycle for potassium

Supplemental K is sometimes called “potash”, a term that comes from an early production technique where K was leached from wood ashes and concentrated by evaporating the leachate in large iron pots. Clearly this practice is no longer practical and is not environmentally sustainable. This potash collection method depended on the tree roots to acquire soil K, which was then recovered after the wood was harvested and burned. Most K fertilizer, whether used in organic or conventional agriculture, comes from ancient marine salts, deposited as inland seas evaporated. This natural geological process is still visible in places such as the Great Salt Lake and the Dead Sea.

Organic Crop Production
The basic principles of plant nutrition are the same, whatever the production system used. Both organic and conventional production systems have many common objectives and generally work with the same basic global resources. While specific nutrient management techniques and options may vary between the two systems, the fundamental processes supporting soil fertility and plant nutrition do not change.

In general, the objectives of organic plant nutrition are to (i) work within natural systems and cycles, (ii) maintain or increase long-term soil fertility, (iii) use renewable resources as much as possible, and (iv) produce food that is safe, wholesome, and nutritious.

Which Organic Standards to Follow?
The use of approved nutrient sources is governed by a variety of regional, national, and international oversight organizations. Each organization maintains somewhat different standards and allows different materials to be used in their organic production systems as they individually interpret the intent of organic agricultural principles. As a result, a grower seeking advice on permissible organic materials should first know where the agricultural produce will be sold in order to meet the requirements of that market.

Drip irrigation for
sweet potatoes

In general, regulations for mined K sources specify that they must not be processed, purified, or altered from their original form. However, there is disagreement between different certifying bodies over what specific materials can be used. Unfortunately, some of these restrictions on certain nutrient materials do not have solid scientific justification and their inclusion or exclusion on various lists should not be viewed as one material being more or less “safe” than another fertilizer material.

Using On-Farm Resources
There are many variations possible for successful K management in organic production systems. The largest differences occur on farms that produce both livestock and crops compared with farms that strictly produce crops for off-farm sale. In the mixed livestock/crop systems, the nutrition of the animals generally takes first priority and the residual manure is returned to surrounding cropland. In these cases, imported K in feed and bedding frequently exceeds the output in milk and meat products, sometimes leading to an accumulation of K in the surrounding fields that receive manure. Large losses of K may occur on these farms during manure storage and composting. Since excreted K mostly goes into urine, if this fraction is not effectively recovered it will not be returned to the field with the solid portion of the manure.

Crop rotations are a central part of organic production systems. While this practice can be helpful for supplying N when legume crops are included and may also reduce K leaching losses, rotations alone do not supply any additional K to the farm. Plant roots have been shown to enhance soil mineral weathering by depleting rhizosphere K and causing a shift in the K equilibrium. This shift can speed natural processes and enhance the rate of clay transformations. Subsoil K reserves may be important for some crop rotation systems where deeprooted plants can extract K which may be subsequently used by shallow-rooted crops. While rotational crops may influence the availability of existing soil K, the removal of any plant material from the field continually depletes the soil nutrient supply and ultimately reduces long-term productivity.

Plant-available K is usually measured in the topsoil, but some deep-rooted plant species can take up considerable amounts of K from the subsoil. The contribution of subsoil K to the plant K requirement depends on the amount of plant available K in the top and subsoil, potential root-limiting factors, and the root distribution pattern of the specific crop. Soil testing done near the soil surface will not account for this subsoil contribution to the K supply.
Alfalfa removes large amounts
of potassium in each harvest

Potassium Balance
Since off-farm sales will always lead to a removal of K and additional loss of K through leaching and runoff is inevitable, the potential of a cropping management system to replenish the K reserve is important. The use of farm budgets is useful for describing the nutrient flow within a farming system and to assist with nutrient planning for long-term rotations and mixed farming systems. Depending on a variety of factors, the on-farm budgets of N, P, and K on organic farms have been shown to range from a surplus to a deficit.
The demand for K by various crops has been well established by measuring the K concentration in the harvested portion of the crop (Table 1). However, much less attention has been paid to the rate at which K must be supplied to growing plants. Both the total amount required (quantity) and the rate of supply (intensity) are equally important. This concept is important for all crop growth, but requires special attention when using low-solubility nutrient sources that may provide an adequate amount of total K, but not at a rate sufficiently rapid to meet peak-demand periods of plant growth.

Potassium Release from Soil Minerals
The most common mineral sources of K in soils are feldspars and micas...soil minerals remaining from the primary parent material. Weathering of these primary minerals produces
a range of secondary minerals that may also serve as a source of K in soil. These minerals include micaceous clays such as illite and vermiculite.

Crushed rocks and minerals have been evaluated as K sources in many field and greenhouse experiments. In general, plants are able to gain a very limited amount of K from minerals applied as biotite, phlogopite, muscovite, and nepheline. Feldspar K is not plant available without additional treatment or weathering.

The rate of K release from minerals is influenced by factors such as soil pH, temperature, moisture, microbial activity, the reactive surface area, and the type of vegetation. Therefore, a mineral that is somewhat effective as a K source in one condition may be ineffective in another environment.

Some soil minerals may act as a sink for removing K from solution. When K is adsorbed in the interlayer sites of illite, vermiculite and other smectite clays, the clay layers collapse and trap the K within the mineral lattice. This fixation process is relatively fast, while the release of this interlayer K is very slow. Non-exchangeable K should not be confused with mineral K, since non-exchangeable K is held between adjacent tetrahedral layers of clay, instead of being covalently bonded in mineral crystal structures.

Potassium Sources for Organic Production
Regular applications of soluble K, regardless of the source, will increase the concentration of K in the soil solution and the proportion of K on the cation exchange sites. All of the commonly used soluble K sources (including manures, composts, and green manures) contain this nutrient in the simple cationic K+ form. Most soluble inorganic fertilizers and organic manures are virtually interchangeable as sources of K for plant nutrition. When using readily available forms of K, the overall goal of replacing the harvested K is generally more important than minor differences in the behavior of the K source. Any differences in plant performance are usually due to the accompanying anions, such as chloride (Cl-) or sulfate (SO42-) or the organic matter that may accompany the added K.

There is no general evidence that potassium sulfate (K2SO4) is more effective than potassium chloride (KCl) as a source of plant-available K, and both SO4 2- and Cl- provide essential nutrients that are required for plant health. Chloride is sometimes disparaged as being harmful to soil, but there is no evidence for this claim at typical rates of application. It has a well-documented role in improving plant health and prevention of a variety of plant diseases. Chloride-derived salinity was the same as sulfate-based salinity on its effect on common soil microbes (e.g. Li et al., 2006) and the addition of K decreased the harmful effects of salinity on soil microbial activity (Okur et al., 2002).

Approved and Restricted Potassium Sources
The National Organic Program in the U.S. and the Canadian General Standards Board classifies products as either allowed, restricted, or prohibited for use in organic production. Allowed products are permitted for organic production when applied as directed on the label. Restricted materials can only be applied for certain uses and under specific conditions. Prohibited products may never be used for organic production. The properties and value of these materials as sources of plant nutrients vary considerably. The following K sources are used sometimes for organic production.

Greensand is the name commonly applied to a sandy rock or sediment containing a high percentage of the green mineral glauconite. Because of its K content (up to 5% K), greensand has been marketed for over 100 years as a natural fertilizer and soil conditioner. The very slow K release rate of greensand is touted to minimize the possibility of plant damage by fertilizer “burn”, while the mineral’s moisture retention may aid soil conditioning. However, the K release rate is too slow to provide any significant nutritional benefit to plants at realistic application rates. Soluble K is generally <0.1% of the total K present. Deposits of greensand are found in several states (including Arkansas and Texas), but the only active greensand mine in North America is located in New Jersey.

Greensand is not very
effective at supplying
plant-available potassium
Langbeinite (Potassium- magnesium sulfate)
This material (K2SO4•MgSO4) is allowed as a nutrient source if it is used in the raw, crushed form without any further refinement or purification. Several excellent sources of this approved product are available for use with organic crop production. Langbeinite typically contains 18% K, 11% Mg, and 22% S in forms readily available for plant uptake. The major source of langbeinite in North America is from underground deposits in New Mexico.

Lanbeinite is a very effective
source of potassium
(as well as magnesium and sulfate)
Manure and Compost
Since these organic materials are extremely variable (based on their raw materials and their handling), they also contain highly variable K concentrations. Composted organic matter is generally allowed as a nutrient source. Raw manures have restrictions on the timing of their use, but the details depend on the certifying agency. The K in these organic materials is largely available for plant uptake, similar to approved inorganic sources. Repeated applications of large amounts of manure can result in K accumulation in the soil, which may lead to luxury consumption of K by the plant.
A chemical analysis of the manure or compost composition is necessary in order to use these resources for maximum benefit. It may be helpful to consider where the compost or manure K is coming from, since neither composting nor animal digestion produces any nutrients.

Potassium Sulfate
When K2SO4 is derived from natural sources, it is allowed for organic crop production. Much of the current production of organically approved K2SO4 in North America comes from the Great Salt Lake in Utah. It may not undergo further processing or purification after mining or evaporation, other than crushing and sieving. This product is not allowed in some European countries without special permission from the certifying agency. It generally contains
approximately 40% K and 17% S.

Rock Powders
Mined rocks, including ballast, biotite, mica, feldspars, granite and greensand are allowed without restriction. Tremendous variability exists in the K release rate from these mineral sources. Some of them are wholly unsuitable as K sources for plant nutrition due to their limited solubility and their heavy and bulky nature, while others may have value over long periods of time. In general, a smaller particle size translates to a greater surface area, reactivity, and weathering rate. Obtain information for specific rock materials before using.

Since sea water contains an average of 0.4 g K/L, seaweed may accumulate up to several percent K. When harvested, seaweed biomass can be used directly as a K source or the soluble K may be extracted. These K sources are readily soluble and typically contain less that 2% K. While seaweed-derived products are excellent K sources, their low K content and high transportation costs can make it problematic for field-scale use, especially far from the harvesting area.

Sylvinite (Potassium Chloride)
KCl is restricted in the USDA standards unless it is from a mined source (such as sylvinite) and undergoes no further processing. It must be applied in a manner that minimizes Cl accumulation in the soil. Generally, KCl should only be used after consultation with the certifying agency. The Canadian GSB has included KCl on the “Permitted Substances List” for organic food production systems. Unprocessed sylvinite often contains approximately 17% K.

Wood Ash
Ash from hardwood trees served as one of the earliest sources of K for building soil fertility. This highly variable material is composed of the elements initially present in the wood which were not volatilized when burned. Wood ash is an alkaline material, with a pH ranging from 9 to 13, and has a liming effect of between 8 and 90% of the total neutralizing value of commercial limestone. In terms of commercial fertilizer, average wood ash would have an analysis of approximately 0% N, 1% P, and 4% K. The use of ash derived from manures, biosolids, coal, and some substances is prohibited for organic production. Check with the certifying organization prior to applying ash to soil.

Growers using organic production practices, like all growers, have need for an adequate supply of soil K to sustain healthy and high-yielding crops. There are many excellent sources of K that are available for replacing the nutrients removed from the soil in harvested crops. Failure to maintain adequate K in the rootzone will result in poor water use efficiency, greater pest problems, decreased harvest quality, and reduced yields. Regular soil testing for K is the key for establishing the requirement for fertilization. If a need for supplemental K exists, organic producers generally should first consider locally available K resources and supplement with mineral sources. The expense of transporting and applying low nutrient content amendments must also be considered.

A pdf version of this article appeared in the IPNI magazine "Better Crops".  Click here for a copy. 

An expanded version of this article appeared here:
Mikkelsen, R.L. 2007. Managing potassium for organic crop production. HortTechnology 17:455-460. 

Thursday, January 3, 2013

Biuret in Urea Fertilizers

In the past, urea manufacturing processes sometimes resulted in fertilizers with elevated biuret concentrations. In high concentrations, biuret interferes with internal N metabolism and hinders protein formation in plants. Biuret is degraded by many soil microorganisms, but the rate is relatively slow. Modern urea manufacturing typically results in biuret concentrations less than 1.0 to 1.3%, which does not pose problems for most uses. There are some plant species that appear to be especially sensitive to biuret, so “low-biuret” urea should be used for foliar application in these situations.

This blog post is from an IPNI article that appeared in Better Crops.  This original version is available here.

Urea has become the leading form of N fertilizer worldwide. Urea, a naturally occurring compound, can also be made by reacting carbon dioxide with ammonia at high temperature and pressure. Its high N content (46% N) makes urea economical to produce, transport, and deliver to the farm.
        Two concerns are sometimes expressed by growers using urea as a N source for crop nutrition. First, when urea remains on the soil surface, a portion of the applied N may be lost through NH3 volatilization…thereby diminishing its fertilizer value. When urea is first applied to soil, it generally reacts quickly with soil enzymes (urease) to convert to NH4 + then to NH3 (Figure 1) which may be lost as a gas. Considerable effort has been made to understand this NH3 loss pathway, resulting in urea coatings (such as controlled-release fertilizers), additives (such as urease inhibitors), and management practices that can substantially reduce these losses.
A second concern related to urea fertilization is potential biuret toxicity for growing crops. When molten urea is heated near or above its melting point (132 ºC or 270 ºF) during manufacturing, several different compounds can be formed…including biuret (Figure 2). Biuret can be toxic to plants at elevated concentrations, whether applied to soil or foliage. Although modern urea manufacturing methods now consistently result in low biuret concentrations, questions still arise regarding potential hazards associated with biuret. 
Biuret in Soils
     Many years ago, researchers found that plant growth was reduced or completely eliminated following high applications of biuret to soils, and this growth suppression often persisted for a period of many weeks. Although the ability to degrade biuret is widespread among soil microorganisms, microbial growth is only half as fast with biuret as a N source as it is with urea. The presence of biuret also decreases the rate of nitrification in soil.

Seedling Damage
     When urea with elevated biuret is placed adjacent to seeds, toxicity may result to the geminating plant. Some of this damage is due to the NH3 evolved from the urea during normal hydrolysis, but biuret may make the harsh condition more severe.
        The extent of biuret damage to seedlings depends on the crop, the biuret concentration, and the fertilizer placement. Neither urea nor urea which contains biuret should be placed directly with a seed during planting. If the fertilizer is separated from the seed by a small volume of soil, toxicity problems are greatly diminished. Amending the urea with a small amount of urease inhibitor will also reduce these adverse affects.

Soil Application of Biuret
     Many studies have been done to determine the maximum biuret concentration tolerated by crops. The specific crop sensitivity depends on many factors such as the plant species, soil properties, the method and timing of fertilizer application, and both the concentration and total amount of biuret applied.
          The soil properties on which the biuret-exposed crop is grown are important in determining potential toxicity. Biuret is not retained in soil and is easily leached. Plants are generally less sensitive to biuret when it is applied to soils containing appreciable amounts of clay or organic matter, or of low pH.

         The specific toxic agent associated with biuret in the root zone is not known. It has been considered that cyanuric acid or nitrite may accumulate in the soil following biuret application and contribute to plant toxicity. Although these compounds can be injurious to plants, biuret by itself is also harmful.
        Many crops can tolerate large amounts of biuret applied with urea if it is not in direct contact with the seed. A general guideline for safe use of urea applied to soil would permit a maximum 2% biuret in urea. Many crops are not adversely affected until biuret concentrations greatly exceed this level, which is greater than the 1.0% biuret commonly found in most urea currently produced in North America. There are a few plant species (such as citrus and pineapple) that do not tolerate elevated levels of biuret.
        Foliar application of urea can be extremely beneficial in some circumstances for plants. Several cereal, vegetable, and perennial crops respond favorably to foliar applications of urea with increased growth, yield, and quality. These benefits can include boosting grain N concentrations, reducing N losses through leaching and denitrification, and supplying N when root uptake is limited. However, foliar-applied nutrients may be directly absorbed by plants (without the buffering effects of the soil), so careful attention must be paid to this practice to do it properly.

        Following foliar application of urea containing 0.5% biuret to potatoes, visual symptoms of yellow leaves, upward leaf rolling, and necrotic leaf margins have been noted. Application of urea and biuret on oranges resulted in damaged leaves, where the apical portion of the leaf was the most sensitive to biuret (see photo). These yellow leaves never regained their normal color, although the new flush of growth appeared normal.

Because biuret is not rapidly metabolized by plants, repeated spray applications of urea and biuret may have a cumulative effect, especially with perennial crops.

Effects of Biuret on Plant Metabolism
     Plants are not able to rapidly metabolize biuret. In one experiment, biuret still remained in the leaves of orange trees eight months after foliar application. Soil-applied biuret similarly accumulates in plants for long periods of time. The exact mechanism of biuret damage to plants is still uncertain, but the harmful effects of high concentrations have been well documented.
          When present in elevated concentrations, biuret interferes with normal protein synthesis and internal N metabolism in the plant. Lower N concentrations are typically found in biuret-damaged leaves than in healthy urea-treated leaves. Biuret also disrupts normal activity of many important plant enzymes…increasing some enzymes and decreasing others… compared with healthy leaves.

       Although biuret in urea can be damaging to plants when present in high concentrations, modern manufacturing processes have greatly reduced the severity of this problem. Early urea fertilizer manufacturing facilities often produced urea containing more than 5% biuret. Foliar application of urea solutions containing 1% biuret is acceptable for many common agronomic crops. However, for foliar fertilization of some sensitive crops, urea with especially low concentrations of biuret (less than 0.3 % biuret) may be required. If the sensitivity of a specific crop to biuret in foliar sprays is not known, it is advisable to start with low-biuret urea until the sensitivity has been determined.

       The modern N fertilizer industry produces urea that is remarkably safe, consistent, and effective for enhancing plant growth. Urea has many properties that make it the most commonly used N fertilizer in the world. Biuret toxicity problems are generally rare, but special attention should be made for fertilization of especially sensitive crops.

I published a summary of specific crop sensitivity to biuret in the journal Fertilizer Research.  Click here to see that paper.