Urea, at 46% nitrogen (N), is the most concentrated solid nitrogen fertiliser. This means there is less to freight and handle. It is also one of the most economical nitrogen fertilisers. These factors have seen urea become the world’s most popular nitrogen fertiliser.
Urea can be applied on its own, or in blends with products such as DAP and Muriate of Potash.
It can be applied pre-plant, or in side and top-dressing programs. There are limits to how much urea can be safely applied at planting without harming germinating seeds or young seedlings.
Urea is soluble, and can also be applied in fertigation programs and as foliar sprays.
Urea is also used as a non-protein nitrogen supplement for ruminants, e.g. sheep and cattle. Urea allows ruminants to make better use of low protein roughages, e.g. dry grass, during dry weather and the onset of drought.
MANUFACTURE AND SUPPLY
Urea is a naturally occurring soluble organic compound. It is found in some moulds and fungi and in urine. The dark green growth responses in urine patches in poorly growing pasture and forage crops are often attributable to a response to nitrogen compounds such as urea.
Urea is manufactured synthetically in large quantities around the world for use as a nitrogen fertiliser and in industry, e.g. resins and glues.
The first step in the manufacture of urea is to react natural gas, atmospheric nitrogen and water together at high temperature and pressure to produce ammonia (NH3) and carbon dioxide (CO2).
These gases are then reacted at high temperature and pressure to produce urea, CO(NH2)2.
Incitec Pivot Fertilisers manufactures granulated urea in Brisbane, using natural gas from south west Queensland. This product is sold into northern New South Wales and southern and central Queensland.
To supplement local production, urea is imported through various ports along the eastern and southern seaboards.
USE OF UREA AS A SOIL-APPLIED FERTILISER
Soil Transformations and Root Uptake
Although urea can be taken up directly by some plants, the nitrogen in urea is more readily available to plants after mineralisation in the soil to nitrate (NO3-).
Firstly, urea is converted to ammonium (NH4+) compounds by the action of urease, an enzyme present in the soil. This process is known as ammonification and is usually completed within a few days of urea application.
Bacteria then convert the ammonium to nitrate. This process is called nitrification and is usually completed within a few weeks of application. Being a biological process, nitrification is moisture and temperature dependent. It occurs most quickly under warm moist soil conditions.
Nitrogen Loss Mechanisms
Of the essential plant nutrients, nitrogen is the only element that exists as a gas (N2) in its natural state. Not surprisingly, nitrogen is one of the more mobile soil nutrients, and one of the most subject to loss.
Once transformed to the ammonium and nitrate forms, urea like other nitrogen fertilisers is subject to loss in water or to the atmosphere.
Ammonia Volatilization occurs when urea is surface-applied without incorporation. After application, urea dissolves in water and in the presence of urease, forms ammonium ions (NH4+). If there are insufficient adsorption sites at the soil surface for the ammonium ions, ammonia (NH3) gas can form as the soil dries out, e.g. in the heat of the day following overnight dew. Such losses are greatest in alkaline (high pH) soils, in which hydroxyl (OH-) ions are present in high concentrations.
Once in the soil, ammonium (NH4+) ions are attracted and tightly held (adsorbed) on the surface of clay and humus particles. As such, they are resistant to loss.
Loss from within the soil can occur once the ammonium is converted to nitrate:
Leaching is the vertical, downward movement of water containing dissolved salts, deeper into the soil, beyond the reach of plant roots. Nitrate is not strongly sorbed onto soil colloids and is subject to leaching. This is most likely to occur on sandy soils, under high rainfall conditions, and where excessive irrigation water is applied.
Denitrification describes the process in which nitrates (and nitrites) are reduced to the gases NO, N2O and N2, which are then lost to the atmosphere. It occurs under waterlogged or anaerobic soil conditions. Soil bacteria, deprived of their normal source of oxygen (soil air), take oxygen from nitrate (NO3-), reducing it to gaseous forms (NOx). Denitrification is most likely to occur on poorly drained heavy textured clay soils, and where conditions favour soil microbial activity, i.e. high temperatures and soil organic matter.
Preventing Nitrogen Losses
Under unfavourable conditions, losses of nitrogen from urea (and other nitrogen fertilisers) can be as high as 80%.
Such losses can be minimised if the loss mechanisms are properly understood; and local soil, climatic and managerial factors are taken into consideration. Gaseous losses of ammonia to the atmosphere (volatilisation) can be suppressed by soil coverings of 3 cm. This can be achieved by:
- placing urea into the soil, e.g. annual crops;
- applying urea with the irrigation water (fertigation); or
- irrigating the urea in soon after application (preferably on the same day).
This may not always be possible, e.g. with raingrown pasture, forage or tree crops. In these cases, keep an eye on the weather, and apply urea when rain is forecast.
Alternatively, a less volatile nitrogen fertiliser may be used, i.e. products containing ammonium nitrate (Nitram and Cal-am).
In risk situations, leaching and denitrification losses can be minimised by applying nitrogen close to the time it is needed, or split-applying it during the main growing season.
There is great flexibility in how and when to apply urea. The best time will vary with the crop, soil type, locality and the application equipment available to do the job.
Brief comments on some of the alternatives follow:
Pre-Plant (in Annual Crops)
In vegetables, and quite often in grain crops, urea is normally applied at the time of the last cultivation before planting. It should be applied into, or be incorporated into the soil.
Provided nitrogen losses through leaching (most likely to occur on sandy soils in high rainfall areas) or denitrification (from water-logging) are not likely to occur, urea can be applied earlier than this, i.e. several months before planting. This may even provide advantages in rain grown crops on heavy clay soils where soil moisture is conserved during the fallow period. Early applied nitrogen will be converted to nitrate and carried into the subsoil by rain, where it and the conserved moisture will be utilised simultaneously during the growing season. Nitrogen applied just before or at planting may be left stranded in dry topsoil, where it cannot be utilised by crop roots, in the event that little rain is received during the growing season.
Weeds should not be allowed to grow during the fallow period after applying urea as they will rob the crop of both moisture and nitrogen.
If applied pre-plant into formed beds, ensure the fertiliser band is offset from the position of the planting line, i.e. it is not placed directly below the intended position of the seed, so as to avoid root burn.
Planting (Annual Crops)
In wide row crops, urea should be applied away from the seed or transplants in the inter-row space. Where raised beds or mounds are used, urea should be placed into the side of the hill, rather than in the centre of the inter-row space. This allows earlier root access and improves nutrient recovery by plants. The soil in the centre of the inter-row space is more likely to be compacted, and this is the area in which leaching and denitrification is most likely to occur in the event of wet weather. There is no limit to the amount of urea applied in this way, as there is if the urea is banded near or with the seed at planting.
It is recommended that urea not be used in NPK planting mixtures in crops planted at wide row spacings. A small amount of starter nitrogen is all that is required at planting, e.g. as DAP or Granam, with the balance being applied before and/or after planting, e.g. as urea. Too much nitrogen at planting can set the crop back, or even cause it to fail to emerge.
Urea can be blended with MAP or DAP for use at sowing in grain crops planted at narrow (18 – 20 cm) row spacings, the fertiliser and seed being applied through the same delivery hose. There are limits to the amount of nitrogen (or urea) that can be safely applied in direct contact with the seed in this way.
Generally, it is recommended that the combined nitrogen rate (from all blend ingredients) should not exceed 20 kg/ha N (equivalent to 45 kg/ha of urea) at planting in winter cereals (wheat, barley, oats) sown at narrow (18-20 cm) row spacings under good planting conditions.
As fertiliser burn is more likely to occur on sandy soils, the suggested maximum safe rates of urea that can be sown with the seed of winter cereals is varied according to the soil type in district recommendations for northern NSW and Qld., as illustrated in the following table:
Depending on circumstances, it is still possible that fertiliser burn may occur when these guidelines are adhered to. Your Incitec Pivot Dealer or Agent can help you with more precise recommendations derived from the “Seed Safe” Program.
NOTE: Urea should not be banded with crop seeds when soil moisture levels are low and barely adequate for germination, nor should it be banded with the seed of sensitive species such as canola and lucerne. In general, the smaller the seed, the more likely it is to be harmed by fertiliser placed in direct contact with it, although some large-seeded species are also susceptible to fertiliser burn, e.g. French bean.
Side-dressing Row Crops
Urea can be side-dressed into the inter-row space in crops planted in wide rows. It is best to place the urea into the soil, unless it can be irrigated in soon afterwards, e.g. by overhead sprinklers, in which case it can be placed on the soil. Where the seedbed is mounded, apply the urea into or on the side of the hill, rather than the centre of the inter-row space. To avoid root pruning, be careful not to band the urea into the soil too close to the young plants.
Leaf or foliar burn may occur where urea is applied through a spinner-broadcaster. This may set the crop back, and affect the quality of leafy vegetables. Foliar burn can be minimised with an overhead irrigation immediately after applying the urea to wash off any fertiliser that lodges on the leaves.
Top-dressing Winter Cereals and Pasture
It is not practical to apply urea into the soil in grain crops planted at narrow (18 – 20 cm) row spacings or pasture.
If irrigation is available, water the urea into the soil as soon as possible after applying the urea, preferably on the same day.
In raingrown crops top-dress urea when the prospects of follow-up rain are good, i.e. rain is forecast.
If rain is not received soon afterwards or irrigation is not applied, nitrogen can be lost to the atmosphere through ammonia volatilization. In short pasture, 5 – 10mm of rain in the one fall should be sufficient to wash the urea into the soil.
Preventing Urea Poisoning in Grazing Animals
In raingrown pasture or forage crops, some urea may lodge on the foliage. If ingested by grazing animals, this may cause urea poisoning.
In rotationally grazed pasture and forage crops, remove the stock before applying urea, and do not re-admit until after regrowth occurs (or rain is received or irrigation applied, to wash urea from the plant leaves). It is also advisable that this is done to reduce the risk of nitrate poisoning. Nitrate levels in new regrowth can be high, and it is best to wait three weeks after applying nitrogen fertiliser before grazing.
To reduce the risk of urea poisoning in set-stocked raingrown pasture:
- Take care when filling applicators and spreaders to avoid spillages and not over-fill fertiliser boxes so urea spills from them;
- Do not allow stock access to urea by leaving open bags or uncovered applicators containing urea in the paddock;
- Use good quality urea with a minimal level of fines. Fines may build up on the spreading mechanism, and not be distributed evenly over the field, dislodging in lumps that remain on the foliage. Fines are more likely to lodge on and remain on the leaves of the grass, while prills or granules are more likely to fall through the pasture canopy.
In plant sugarcane, urea is side-dressed into the drill before closing-in, and covered with soil.
In ratoon cane, urea is applied after harvest, either split stool, or drilled into the soil either side of the crop row. Application to the centre of the inter-row space should be avoided, as it takes longer for the cane roots to grow to this area. The soil in the centre of the inter-row space may also be compacted due to vehicular movement, restricting root growth. Furthermore, denitrification and leaching are more likely to occur from the centre of the inter-row if water ponds in this area.
Where trash blanketing is practised, it is best to place the urea through the trash into the soil. Cane trash has very high urease activity, and volatilization losses from urea applied to it can be substantial unless overhead irrigation, e.g. travelling irrigator or water winch, is applied or rain is received soon afterwards. 15 – 20mm in the one application or fall should be enough to wash most of the urea through the trash into the soil. This will depend on the thickness and stage of decomposition of the trash.
In raingrown situations where urea can not be applied into the soil or irrigated after application, volatilization losses can be minimised by applying urea:
- when rain is forecast;
- delaying application until the cane is 50cm high.
Alternatively, a less volatile nitrogen fertiliser such as Incitec Nitram or Cal-am can be used.
Utilization of fertiliser nutrients is improved if the fertiliser is applied to the area in which the tree roots are concentrated and most actively taking up nutrients, i.e. beneath the drip-line or edge of the canopy.
Urea can be spread around each tree, starting 30cm from the trunk to just outside the canopy. Ensure the fertiliser is spread evenly to avoid root burn.
Alternatively, the fertiliser can be spread in a broad band under the drip line along both sides of each row of trees.
Fertigation is the application of fertiliser dissolved in irrigation water. This provides a simple and effective way to apply fertiliser, without the need for mechanical application. The tractor can be left in the shed. Fertigation allows the season’s nitrogen fertiliser requirements to be split into a number of smaller applications. Urea is soluble and is particularly suited to application in this way.
Overall or cumulative nitrogen fertiliser rates often remain unchanged, but they may need to be:
- Increased if improved irrigation and farm management practices allow higher yields to be achieved;
- Reduced if plant recovery of applied nitrogen is improved, and nutrient losses are reduced.
An Agritopic on “Fertigation” can be obtained from your Incitec Pivot Fertiliser distributor.
FOLIAR APPLICATION OF UREA
Use of Foliar Sprays
With few exceptions, it is impossible to feed crops their complete nitrogen requirements through the foliage without burning the leaves. Foliar sprays are therefore used to supplement, but not replace soil applications of nitrogen. They can also be used where a quick response to nitrogen is required.
Urea is absorbed directly by the leaves and the active bark of trees and plants. Responses to urea applied in this way are extremely quick, as most of the urea will have entered the plant within 2 hours of application, and absorption is almost complete within 2 days.
Tree and field crops with a large leaf surface area for urea absorption are the most suited to foliar applications. Citrus is an example. In southern Australia, foliar sprays are often used as the principal means of applying nitrogen in citrus orchards.
High volume urea sprays are also used in pineapples in southern Queensland. Pineapples are grown in wide beds, so soil application is not very practical. Nitrogen and potassium are applied in solution through boom sprays. Much of the spray runs off the foliage to the soil, and ultimately taken up by the roots, rather than enter the plants through the foliage.
In most other crops, only limited amounts of nitrogen can be applied directly to the foliage. Urea sprays are generally only used during periods of high crop nutrient demand.
Foliar applications also offer a means of overcoming temporary nitrogen deficiency when plant growth and root uptake of nutrients is limited by external factors e.g. waterlogging or post irrigation stress in flood irrigated crops on heavy clay soils.
Biuret is an impurity formed during the manufacture of urea. Its chemical formula is
NH2CONHCONH2 while that of urea is NH2CONH2
At the levels typically present in urea, biuret is not harmful to plants or soil organisms when applied to the soil. It is broken down to ammonium (as urea is), and becomes part of the nitrogen cycle.
Biuret, however, is a cumulative toxin when applied to foliage. It is not translocated or broken down and metabolized after absorption by the leaves. Consequently, repeat applications of urea with a high biuret content can result in biuret toxicity. Symptoms of biuret toxicity are tip yellowing and leaf burn. This will be most evident in the older leaves. Toxicity is most likely to occur in long-lived perennial and evergreen tree crops where several applications commonly occur, e.g. citrus and pineapple, rather than in annuals, e.g. vegetables and cereals.
Incitec Pivot Granulated Urea contains a maximum of 1.4% biuret. A low biuret urea is also available, i.e. Liquifert Lo-Bi, which has a maximum biuret content of 0.45%. Liquifert Lo-Bi should be used where urea is foliar-applied on a regular basis and/or at high rates, particularly in high value horticultural crops. There is no need to use Liquifert Lo-Bi in preference to normal grades of urea for soil application.
In short-season annual crops, e.g. grain, cotton and vegetables, where a single foliar application of urea is to be applied, normal grades of urea can usually be used without risk of biuret toxicity. In perennial tree crops and vines, and where repeated foliar applications of urea are to be made, biuret toxicity is more likely to occur. The extra cost incurred in using a low biuret grade of urea, i.e. Liquifert Lo-Bi, is more likely to be justified in these circumstances.
Spray Concentrations and Rates
Specific local advice should be sought on spray programs for citrus and pineapple.
In other crops, urea is typically applied at rates up to 10 kg/ha per application, the dilution rate in water depending on the spray volume. The spray volume is usually not critical and merely improves the uniformity of coverage, up to the point of run-off. The water itself evaporates soon after application.
Urea can be applied to winter cereals at 20 - 30 kg/ha, e.g. a 20 - 30% solution at 100 L/ha. This may cause some leaf scorch, but should have no lasting effects.
In vegetable crops, a spray volume of 1 000 L/ha, containing 1 kg/100 L (1% w/v) urea will apply 10 kg/ha urea.
In tree crops, more water is required, i.e. higher spray volumes, so lower concentrations of urea are used.
The following table gives examples of urea concentrations commonly used in foliar sprays for a variety of crops.
Urea is the most commonly used nitrogen fertiliser for foliar sprays. Apart from being the most economical source of nitrogen, it is less likely to burn the foliage than other nitrogen fertilisers.
Urea is non-ionic and therefore causes minimum osmotic (salt) damage to the crop. Phytotoxicity (leaf burn and scorch) however, is possible, if the concentration or volume of urea solution is too high.
To minimise the risk of leaf burn, urea should be applied early in the morning and not during the heat of the day, or under hot dry conditions, i.e. urea sprays should ideally be made in cool conditions. Crops should not be wilting as in this condition absorption of urea into plants is severely impaired.
If there is no prior experience with urea sprays in the crop, or the crop is lush or growing conditions are harsh, it may be wise to test-spray a portion of the crop to check its sensitivity. If necessary, reduce the spray concentration, and test again.
Preparation of Spray Solutions
Urea sprays should be used soon after preparation, particularly where surface water from dams and streams is used. They should not be allowed to stand for extended periods, e.g. overnight, as the urea may be converted to other compounds that are much more likely to harm the foliage than urea itself. For example, ammonium isocyanate (NH4CNO) can be formed from urea through bacterial action.
If urease is present, ammonium carbonate may be formed. At high pH, ammonium carbonate may form free ammonia, which is toxic at low concentrations to foliage. Urea itself raises the pH of solutions to which it is added. If the water is of good quality and/or spray additives are acidic, urea is unlikely to have a detrimental effect. However, if the pH of the water is already high, urea may compound any associated problems, and increase the potential for ammonia toxicity.
A little sulphuric acid (battery acid) can be slowly added to the spray solution to reduce the pH. Liquifert P can also be used as a buffering agent with alkaline fertilisers such as urea
The addition of a non-ionic wetting agent to urea sprays greatly increases the degree of absorption by the plant parts sprayed.
Use of Urea with Micronutrient Sprays
The addition of a small amount of urea to micronutrient (trace element) sprays helps enhance uptake of the micronutrient. It does this by helping open the stomata (pores) in the leaves, through which the micronutrient enters the plant. As a guide, urea can be used at 1 kg/100 L (1% w/v) in field crops, 500 g/100 L (0.5% w/v) in vegetables, and 100 g/100 L (0.1% w/v) in tree crops. Higher rates are not needed, unless the crop needs extra nitrogen.
It should not be necessary to use low biuret urea, i.e. Liquifert Lo-Bi, with micronutrient sprays, as the urea application rate is low, and usually only one or a few sprays are required, i.e. micronutrient sprays are not applied repeatedly during the growing season.
NOTE: An Agritopic on “Foliar Fertilisers” can be obtained from your Incitec Pivot Fertiliser distributor, as well as Use Directions for “Liquifert N” and “Liquifert Lo-Bi”.
USE OF UREA AS A LIVESTOCK SUPPLEMENT FOR RUMINANTS
An Agritopic on “Stock Supplements” can be obtained from your Incitec Pivot Fertiliser distributor, in which the use of Urea as a livestock supplement is discussed in more detail.
Treatment of Urea Poisoning in Livestock
Death from urea poisoning is rapid (generally within 2 hours of ingestion of the urea). By the time symptoms appear (severe abdominal pain, shivering, excessive saliva, rapid breathing, unstable gait, bellowing and bloat), it is often too late.
To have any chance of being effective, treatment must be quick. Vinegar (acetic acid) needs to be administered at quite high dose rates (2 to 4 litres as a guide for cattle) repeating the treatment if necessary. The vinegar makes the ruminal contents more acidic and delays uptake of ammonia by the blood.
INCITEC PIVOT UREA FERTILISER PRODUCTS
Incitec Pivot Fertilisers markets several grades of Urea:
Granular Urea: This is the most commonly used urea fertiliser, and is the preferred product where urea is applied dry to the soil. The granules are harder and larger than prilled urea. Provided granular urea is of good quality, it normally handles and stores better than prilled grades of urea. It is also more suitable for use as a blend ingredient in mixed fertilisers, being more evenly matched in sizing with DAP, MAP and coarse crystalline grades of Potash.
Prilled Urea: Having a smaller particle size than granulated urea, Prilled urea dissolves more quickly. Prilled urea is imported into Mackay for addition to Biodunder, a liquid waste product from the Sarina ethanol distillery that is high in potassium. The nitrogen enriched Biodunder is then used as a liquid fertiliser in ratoon sugarcane. Prilled Urea is also available ex Mackay and Townsville for use in the preparation of stock licks for beef cattle in northern and western Queensland.
Liquifert N and Stockfeed Urea: These products are obtained by screening undersized granules from granular urea manufactured at Brisbane. Having a smaller particle size, they dissolve more quickly where urea solutions are required, e.g. fertigation programs, and in the preparation of stock licks.
Liquifert Lo-Bi: A low biuret grade of urea for use in foliar sprays in high value horticultural crops and where urea is sprayed repeatedly at high rates.
Granular urea is compatible in dry blends with Gran-am (ammonium sulphate), DAP, MAP and coarse potassium fertilisers, e.g. Incitec Muriate of Potash.
Do not mix Urea with Nitram (ammonium nitrate), Triple Super (triple superphosphate or TSP), and SuPerfect or Super (single superphosphate or SSP). These blends have a depressed Critical
Relative Humidity and absorb atmospheric moisture readily. A chemical reaction occurs when urea is mixed with TSP or SSP. Chemically bound water in the superphosphate is released causing the mixture to become wet and sticky. If stored for any length of time, the mixture will build up on drives and equipment, making application impossible.
Urea is compatible in solution with most other fertilisers.
Urea is compatible with most weedicides, insecticides and fungicides. However, their compatibility with urea should be checked before use. Urea solutions are alkaline, which may affect some products.
Urea will dissolve in its own weight of water, but becomes more difficult to dissolve as the concentration increases. As urea dissolves, the temperature of the solution falls, so a practical limit of 30% w/v (30 kg of urea in 100 litres of solution) is suggested.
If the urea has a small particle size, e.g. Liquifert N, it will dissolve more quickly, and require less agitation. Granular urea will take longer to dissolve on account of its larger particle size and hardness.
For reference, the solubility of urea in water, in saturated solutions, in kg urea per 100 kg or L of water is given below, although these concentrations are not realistically achievable on farm, i.e. without heating and agitation.
Urea is used in the manufacture of UAN (Urea Ammonium Nitrate) solutions, e.g. Easy N. The solubility of urea and ammonium nitrate are mutually enhanced in the presence of each other, allowing a concentrated nitrogen solution to be prepared.
Care of Equipment
Following the application of urea, machinery should be thoroughly cleaned by pouring sufficient water through it, to dissolve any urea particles or residues. Moving parts should be lubricated as recommended by the manufacturer.
Urea is relatively non-corrosive to metal piping. However, where urea is applied through an irrigation system, the system should be flushed with clean water after use. Irrigate with water only for some time after the urea has been applied. Metering devices should be dismantled and thoroughly cleaned.
The information contained in this publication is for use as a guide only. The use of fertilisers and supplements are not the only factors involved in producing a top yielding pasture or crop, and in livestock production. Local soil, climatic and other conditions should also be taken into account, as these could affect pasture or crop responses to applied fertiliser, and animal responses to supplements. Because conditions of use, suitability of product and application conditions are beyond our control,
Incitec Pivot Limited hereby expressly disclaims liability to any person, property or thing in respect of any of the consequences of anything done or omitted to be done by any person in reliance, whether wholly or in part, upon the whole or any part of the contents of this article.
Copyright, 2012 - All rights reserved.
Copying or reproduction in whole, or in part, by any means, or translation into a machine language without the written permission of Incitec Pivot Limited, is strictly prohibited.