BIG N Technical Points
What are the likely short-term effects of BIG N on soil micro-organisms?
The short-term effects are those that occur in the period between application and the time the ammonia in the application band is totally converted to plant available nitrate (NO3-)
The high concentration of ammonia in the retention band, which is not immediately fixed onto the soil, reduces the population of all bacteria, fungi and nematode and actinomycete species measured
This effect declined over time as a result of the conversion of ammonia to nitrate by unaffected bacteria outside or at the periphery of the application band (Blue and Eno, 1954)
Ten days after the application, ammonia tolerant organisms (e.g. the bacteria Nitrosomonas sp.) increased in number until the ammonia treated soil had 6 to 25 times more bacteria than the soil outside the application band.
To determine the potential for short-term effects on the microbial population, it is important to know the size of the ammonia bands in the soil.
Experimental work conducted to determine the size of ammonia application retention zone has found the radius to be 5 - 7.5 cm, across a wide range of soil types.
Given a band radius of 5 cm, applied in 35 cm rows, approximately 8 - 10% of the top 30 cm of soil would be affected by ammonia.
Therefore, 90% of the soil microbes are therefore unaffected by an ammonia application in the short term.
To have a significant affect on a major proportion of the microbes in the surface 30 cm of soil, about 1,500 kg/ha of ammonia would need to be applied evenly to the soil in a single application. This rate is far in excess of the 40 - 200 kg/ha/annum applied in a cropping system.
What are the likely long-term effects of BIG N on soil micro-organisms?
It is likely that any change to the microbial population of the soil as a result of applying ammonia will be as a result of two factors:
- Not enough nitrogen being applied to the soil to provide the energy or protein to sustain the current microbial population and/or diversity
- Changes to the soil chemistry (e.g. pH) that favour a different balance of species.
Any ammonium-forming fertiliser (legumes, manure, urea, BIG N®, MAP & DAP) will have a similar effect for the same rate of nitrogen input.
Given that there appears to be little short term effect, long term effects are unlikely. This is confirmed by a long term study of the effects of different nitrogen sources on soil structure and chemistry conducted by Darusman et al. (1991) and others in the USA.
After twenty years of the annual application of ammonia, no major productivity limiting changes to the soil could be detected from any form of nitrogen used.
The short and long-term effects of BIG N on soil structure
BIG N does have the potential to increase compaction, as does urea and other highly alkaline compounds, such as lime, if applied to the soil in sufficient concentration
The conditions under which a significant proportion of the soil could be affected are unlikely to occur in an agricultural enterprise
The proportion of the soil affected and the amount of time that potential for damage exists indicates that normal commercial applications are unlikely to directly contribute to soil compaction
If minor damage to soil structure did occur, it is unlikely to persist from one season to the next or be cumulative over time.
There is no evidence that the long-term use of BIG N has contributed to an increase in compaction in cropping soils.
Earthworm populations in the spring after anhydrous ammonia application
How does BIG N stay in the soil?
The ammonia molecule (NH3) is highly reactive and ionises to ammonium (NH4+) readily in the presence of hydrogen ion donors (e.g. water)
As a result, the ammonium ion gains a strong positive charge and is attracted to negatively charged substances (e.g. clay or organic matter). Soils with a high clay and organic matter content therefore have a large capacity to retain ammonia.
The first process by which ammonia is retained is physical adsorption.
- This reaction is characterised by easy reversibility. The high concentration of ammonia in the soil atmosphere and in the soil water forces more ammonia attach to weak adsorption sites than when it is not present.
- When the concentration of ammonia gas in the injection site becomes low, physically adsorbed ammonia reverts back to the gaseous phase, and diffuses through the soil until it is chemically adsorbed or lost to the atmosphere.
The second process is chemical adsorption.
- Chemical adsorption of ammonia by the soil is characterised by difficulty in reversing the reaction because of the strength of the chemical bond that is formed.
- As a result, these sites are important in retaining ammonia so it is not converted back to the gaseous phase and lost to the atmosphere.
- Clay minerals and organic matter are the most numerous sites for chemical adsorption.
The ability of the soil to retain ammonia in the long term is more related to the factors that provide chemical adsorption, such as clay or organic matter content, than soil moisture.
What are the important factors in retaining BIG N in the soil?
Moisture is not critical for the retention of ammonia in the soil as the clay and organic matter provide the long-term ammonia retention sites. Dry soils retain at least as much ammonia as moist soil in the long-term.
However, the moisture content is a highly important consideration in providing the proper soil physical conditions to ensure rapid and complete sealing of the injection channel and as a short term reservoir for retention of ammonia (initial capacity).
Under acidic conditions, clay minerals are the most attractive sites, while under alkaline conditions organic matter is more effective.
Ammonia adsorption will proceed to pH 10.5, as long as sufficient hydrogen ions are present. Soil pH as a single factor appears to be of secondary importance in the ammonia retention mechanism.
Application depth, soil moisture and soil physical condition factors are more important in most circumstances when retaining BIG N in the soil. The cation exchange complex plays an important part in the mechanism of ammonia retention. It is a measure of the soil’s ability to hold positively charged ions (NH4+)
Because of the numerous other reactions that the cation exchange complex is involved in, it has been found in general to be an unreliable tool when used by itself to assess a soils ability to retain ammonia.
The effect of placement of ammonia is important only when it is related to soil tilth, soil texture and soil moisture. As application conditions move away from the optimum in each of the above soil components, then deeper placement is an option to improve retention.
In many ways BIG N application is an art more than a science. Given a small amount of basic information and a little experience, many farmers have been able to successfully apply BIG N under a wide range of soil conditions.
The ammonia retention zone
Nitrification begins at the outside edge of an ammonia band where the ammonia concentration and pH are favourable for the functioning of the nitrifying micro-organisms
As the concentration of ammonia and pH decrease, the rate of nitrification increases towards the centre of the application band until the ammonium is completely nitrified or a pH low enough to be limiting is reached
Nitrification of banded ammonia is dependent on microbial activity. Reduction in initial numbers slows nitrification – ‘slow release’. There is NO data to suggest any detrimental long term effect on soil microbial activity.
Some selected properties of ammonia
Ammonia (NH3) is a colourless gas under standard conditions of temperature and pressure. It has a molecular weight of 17.03 and represents the -3 valance state of nitrogen.
Ammonia is soluble in water, methanol, ethanol, ether, turpentine and other organic solvents. Liquid ammonia dissolves many inorganic salts and alkali earth metals, iodine, phosphorus, sulphur most alcohols and other organic compounds.
Anhydrous ammonia does not affect common metals, but moist ammonia will rapidly react with copper, silver, zinc and alloys and other metals in the presence of oxygen.
Liquid ammonia does not dissolve sulphates, sulphites, carbonates, phosphates or most metallic sulphides. Table 1 summarises some properties of ammonia.
Ammonia, strictly speaking, is not a poison. It has no cumulative toxic effect, however it has a powerful corrosive action on tissue. Exposure to atmospheric concentrations above 0.5% (by volume) will cause death by suffocation with minutes. The human nose can detect concentrations as low as 0.005% (by volume).
It is highly unlikely that a human can be exposed to sufficient quantities of ‘external’ ammonia long enough for its metabolic toxicity (poisoning of internal body functions) to manifest. The body’s biochemical mechanisms for removal of ammonia are extraordinarily rapid and efficient.
Secondly, the reaction of sensitive organs such as skin, eyes and lungs is sufficiently severe that the effects would drive the victim away long before the symptoms of metabolic toxicity could become evident.
For more complete information about acute anhydrous ammonia exposure contact National Poisons Information Centre 131126 or obtain a BIG N MSDS from www.incitecpivot.com.au
Cold Flo and Conventional Systems
When applying BIG N conventionally, the ammonia is released from the regulator/flow divider directly into the soil
As a result of the pressure drop between the regulator and the soil, a proportion of the ammonia changes from liquid to gas creating a high pressure product at the point of application that is around 50% vapour and 50% liquid product
Cold Flo® is a system where the product is taken from the outlet side of the flow dividerand fed into a super-cooler to maintain the product as a higher proportion of liquid (~85%) at low energy at the point of release in the soil.
The combination of high liquid content and low pressure with Cold Flo allows ammonia to be more effectively retained in the soil under a wider range of soil conditions.
The increased residence time as liquid ammonia provides more time for a mechanical covering device (harrows, press-wheels) to close the application furrow before flashing off of liquid to vapour
If mechanical covering is not used, losses can be greater than for conventional application.
How long after applying BIG N do I have to wait before I can sow?
After the application of BIG N there is a period of time that ammonia vapour moves through the soil before becoming attached to clay of organic matter as ammonium and nitrifying to nitrate ready for uptake.
While the vapour is unattached, it can cause disruption to germinating seeds if placed in close proximity. The amount of ammonia vapour and how long it remains in the soil is dependent on soil texture, soil moisture, application rate and seed bed utilisation %.
Generally if the seed is placed at least 5 cm to the side of the edge of the application, band sowing can be undertaken immediately after application.
If there is a possibility that the seed-line will follow less than 5 cm to the side or above the BIG N application band, then sowing should be delayed by at least 30 days where there is reasonable soil moisture.
Delay planting by up to 90 days if the application rate is high, in wide rows and soil moisture is low.
Can BIG N be applied in dry soils?
Ammonia moves further from the point of injection in coarse textured soils and soils low in moisture.
If the injection knife or disc causes sidewall smearing (when soils are wet), then ammonia may preferentially move back up the slot. A similar movement occurs if the soil breaks into clods at application and there are large air voids left in the soil.
Both of these conditions can result in greater ammonia concentration toward the soil surface and greater potential losses at or after application (this is the same if the injection point is near the soil surface).
Dry soil can hold ammonia. Ammonia dissolves readily in water, but it is held or retained in soil by clay and organic matter. The problem with dry soil and low moisture is that soil moisture is needed to temporarily hold the ammonia so it can become attached to clay or organic matter
If dry soils are cloddy and do not seal properly, the ammonia can be lost at injection, or seep through the large pores between clods after application
Proper depth of injection and good soil coverage are a must for application into dry soils. Wing sealers immediately above the outlet port on the knife can help close the knife track, limit the size of the retention zone, and reduce vertical movement of ammonia.
If soils are dry and in good physical condition, they hold more ammonia than soil that is moist.
- The soil remains dry up until planting (and limits nitrification)
- The ammonia is injected shallow or there is poor soil structure (ammonia placed near the seed location)
- The rate of application is high
- It is possible for ammonia damage to occur
- If BIG N cannot be injected deep enough, the soil is breaking into clods, the knife is smearing through the soil, there isn't good coverage of the knife track with loose soil, and ammonia is escaping. A pungent smell will be detected if ammonia is escaping. A white vapor is water vapour, not ammonia.
Stop and either change the way the equipment is working or is set up, or wait until the soil has better structure or moisture.
Soil engaging tools
BIG N application down tines are generally straight forward, with BIG N being plumbed at the rear of the shank and point to provide protection from wear and breakage
Whether the tube is split to either side of the point or installed as a single outlet behind the tine is dependent on factors such as soil type, tilth, moisture, speed of application, application depth, crop residue level, crop row spacing and application rate
With discs, the tube that conveys the anhydrous ammonia to the soil is generally mounted on a tine or rod that follows closely in the path of the furrow cut by the disc
Disc diameter and configuration is usually established through an assessment of the soil tilth, texture and moisture in association with stubble type, length and amount, and presence and type of weeds.
BIG N at sowing
For crops grown on wide row spacing, the inter-row space provides opportunity to place the BIG N at a safe distance from the seed furrow and mid-row banding
More care is needed where the BIG N is applied down the sowing tine or disc and released to one or both sides of the seed row
In this situation, the recommended minimum distance between the release point and the seed furrow is dependent on the product rate, seedbed utilisation %(SBU), soil texture and moisture
Research suggests that the minimum distance should be 50 mm between the seed furrow and the BIG N release points either side of the seed tube for an nitrogen rate of 200 kg/ha and 20 % SBU
As the rate is increased or the SBU decreased, the seed - fertiliser separation distance should be increased proportionally.
Application of BIG N at sowing can be achieved without sacrificing crop establishment by placing the product away from the seed at sufficient distance such that product will not affect crop establishment
The relatively high rates of application of nitrogen fertilisers in most cropping systems means that ammonia creates the greatest risk for toxicity among fertilisers commonly used
In a situation where fertiliser is placed with or in close proximity to germinating seed, it is not only the effect on the germination process that determines the success or failure of the crop to establish, but it is also the effect on the rate and magnitude of extension of the roots and shoots
Placement of high rates of ammonium fertilisers (particularly urea, DAP and BIG N) directly below the seed line of tap-rooted crops such as canola and cotton should be avoided.
BIG N application in irrigation water
The application of fertilisers though irrigation water appears to have inherent
advantages. Among these are :
- Labour and equipment savings
- Ammonia may be metered accurately and quite simply from nurse tanks directly into the water of most irrigation systems
- Nutrient supply can be regulated to coincide with nutrient demand
- Soil compaction can be reduced.
- BIG N application in furrow irrigation systems on clay soils should be restricted to situations where new lengths are less than 600 m
- Weather conditions - losses are minimised on cool, humid days with no wind
- Water quality - losses will be less if water pH is low
- Maintaining concentration of not over 110 mg/L of ammonia (110kg/ML)
Keep the exposed surface of flowing water to a minimum by reducing water turbulence and using narrow deep furrows.