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Farmnote 27/96 [Reviewed September 2006]

By Mel Mason, Senior Research Officer, Plant Research and Development Services, South Perth

Summary

How to calculate the cost per unit of nitrogen of various nitrogen sources (3 tables). Discusses effect of soil acidity; take-all; soil deficiencies of manganese, molybdenum sulphur and zinc; volatilisation; leaching; time of application; evenness of spreading on the efficiency of various fertilisers.

The efficiency of the nitrogen and the cost are perhaps the main consideration when comparing fertiliser sources. However, the efficiency of the nitrogen varies with different sources in some situations.

The commonly used nitrogen fertilisers in Western Australia for broadscale farming are set out in Table 1.

Table 1. Nitrogen fertilisers commonly used in broadscale farming in Western Australia

Cost per unit of nitrogen

For fertilisers which supply nitrogen and no phosphorus, the cost per unit of nitrogen applied should be considered rather than the cost of a tonne of the actual fertiliser (see Table 2). The price at the time of purchase should be used.

Table 2. Comparison of costs of fertilisers supplying nitrogen only

To find the price per unit of nitrogen, multiply the cost of one tonne of the fertiliser by 100 and divide by the percentage nitrogen in the source. For example, for urea:

For nitrogen-phosphorus fertilisers and combinations of fertilisers, actual costs have to be worked out. Table 3 sets out examples of comparative costs for two nitrogen-phosphorus fertilisers compared with urea plus superphosphate. Similar comparisons with any of the nitrogen-phosphorus fertilisers and fertiliser combinations could be made.

Table 3. Comparison of costs of fertilisers supplying nitrogen and phosphorus

However, the ratio of nitrogen and phosphorus in nitrogen-phosphorus fertilisers is fixed and may not be the best ratio for the site. A combination of fertilisers is often needed, though there is now a good range of fertilisers supplying nitrogen and phosphorus in various ratios.

Agriculture Western Australia's N-P Decide model can calculate the optimum economic rate of application of each fertiliser or combination of fertilisers and the optimum economic combination for each situation. The actual optimum economic rates of nitrogen and phosphorus for the different combinations will differ because of price differences.

Access to the N-P Decide model is through Agriculture Western AustraliaÃ-s district offices.

Other considerations relate to specific situations which affect the efficiency of fertilisers.

Soil acidity

Apart from calcium ammonium nitrate and Summit Sustain®, all the nitrogen fertilisers commonly used in Western Australia have the potential to acidify the soil. Agras, ammonium sulphate and other ammonium sulphate based fertilisers such as Summit Cereal® have a greater acidifying effect than fertilisers such as urea or DAP. You should take this into account when selecting a source of nitrogen, particularly where the soil already has a low pH.

Take-all

Where the disease take-all occurs, the use of acidifying ammonium fertilisers such as CSBP Agras No. 1®; or Summit Cereal®; can give some degree of control of the disease, resulting in higher yields (see Farmnote No. 5/94 'Take-all disease of cereals).

Manganese deficiency

The use of acidifying fertilisers such as CSBP Agras No. 1®;, ammonium sulphate or Summit Cereal®; can reduce manganese deficiency. The acidifying effect of these fertilisers increases the availability to the plant of manganese already in the soil. However, if manganese deficiency occurs over the paddock with each crop sown, use manganese superphosphate or Agras and manganese with the crop.

Molybdenum

Soil acidity reduces the availability of molybdenum. Take particular care on acid soils liable to molybdenum deficiency. Ensure that there is an adequate molybdenum supply if you are applying acidifying fertilisers such as Agras, ammonium sulphate or Summit Cereal®;. See Farmnote No. 25/88 'Copper, zinc and molybdenum fertilisers for new land' and Bulletin 4196 'The wheat book', page 71.

However, avoid unnecessary applications of molybdenum, since too much molybdenum in herbage can cause copper deficiency in sheep and cattle. Such deficiency may not be simply overcome by the addition of extra copper fertiliser (see Farmnote No. 15/94 'Copper deficiency in sheep and cattle).

Sulphur

Sulphur is also an essential nutrient for crops. In the past, the sulphur needs of crops have been met by superphosphate that has been applied as a source of phosphate. Superphosphate contains 11.5 per cent sulphur. Sulphur is also supplied in many other nitrogen fertilisers (Table 1). Sulphur can also be applied as gypsum (14 per cent) or elemental sulphur (99 per cent). The latter is insoluble and not readily available to plants in the short term. Consider the sulphur content of fertilisers in situations where sulphur deficiency has occurred or is likely to occur.

However, fertilisers such as DAP (1.7 per cent S) and the triple superphosphate (1 per cent S) are very low in sulphur. With the increased use of such fertilisers as sources of phosphorus, there are fewer additions of sulphur and an increased likelihood of sulphur deficiency.

If you have not used sulphur-containing fertilisers for three to five years or more, check the crop visually for signs of sulphur deficiency or by tissue testing. Sulphur deficiency causes the plants to become yellow or pale as with nitrogen deficiency. However, whereas nitrogen deficiency causes initial yellowing of the oldest leaves, sulphur deficient plants often have green older leaves, with yellowing of the younger tissue.

Symptoms and treatments for sulphur deficiency are described in Farmnote Nos. 3/87, 64/85 and 61/91 - see Further reading.

Sulphur deficiency can usually be avoided by using fertilisers containing sulphur every three to five years. In easily leached, sandy soils, sulphur may have to be applied more often, especially for canola crops.

Zinc

Several fertilisers contain zinc as an impurity: Agras No. 1®; (600 parts per million - ppm), Agrich®; and Agstar®; (400 ppm), double Phos (600 ppm). This zinc impurity can often maintain crop yields in marginal zinc situations. However, it will not replace the application of zinc at recommended rates, which should be done on susceptible soils every 10 years. Details are given in Farmnote Nos. 47/87, 21/86, 33/88 and 25/88 (crops and pastures on new land) - see Further reading.

Copper and zinc have been added to many fertilisers. The content of these trace elements should be checked when deciding on fertilisers to be used in potentially deficient areas.

Volatilisation

Urea forms an alkaline zone around each granule as it breaks down. At this higher pH, the urea changes into ammonia gas (which contains nitrogen). If the urea is covered by soil, most of this ammonia will be absorbed by the soil. However, if the urea is on the soil surface, much of the nitrogen supplied by the urea can be lost to the atmosphere as ammonia. This process is known as volatilisation.

Volatilisation will occur only with urea on most of Western Australian light soils, because these light soils are acidic. However, losses can occur with the other ammonium sources if they are topdressed on to alkaline soils such as the Dongara sands.

Losses by volatilisation will vary according to conditions at the time. Losses can be avoided if the urea is covered by soil soon after application or washed into the soil by a good rain following application. Maximum loss will occur when the urea is topdressed on moist, light soil and application is followed by an extended warm dry period.

Volatilisation losses from urea in the field will generally range from 0 to 20 per cent of the nitrogen applied. Where early application is advisable, avoid most of the loss by topdressing the urea before sowing and covering it during the seeding operation. Deep banding of urea will also avoid this loss.

Leaching

Except on very poor sandy soils, little ammonium nitrogen is leached. However, nitrate nitrogen is very susceptible to leaching and urea can be leached while it remains as urea. On most soils, the urea will be completely converted to ammonium nitrogen within a week, with 90 per cent being converted in two to three days.

Ammonium nitrogen is converted by special bacteria to nitrate nitrogen by a process called nitrification. The rate of this conversion depends on several factors, including soil moisture and soil pH. The process is slow on low pH soils and rapid on alkaline soils.

Because of the greater acidifying effect of fertilisers such as Agras No. 1, Summit Cereal and ammonium sulphate, the ammonium nitrogen in these sources is less rapidly nitrified to nitrate than with less acidifying sources such as urea, DAP or Summit Sustain®;.

The longer the nitrogen stays in the ammonium form, the less susceptible it is to leaching. However, any loss from leaching depends on the amount of nitrate present during leaching rains.

On the other hand, if the topsoil dries, the ammonium nitrogen that remains in this zone will not be available to the plant until the topsoil is rewetted, while nitrate nitrogen may be available because it has moved downward into a moist soil zone.

Time of application

Nitrogen-phosphorus fertilisers such as Agras No. 1®;, DAP and Summit Cereal®; are usually applied at sowing, drilled with the seed, because phosphorus is needed in a band close to the seed at establishment.

Urea and other nitrogen-only sources should be applied within four weeks after sowing. In higher rainfall areas, where leaching is more likely, do not apply them before four weeks unless a machine is unlikely to get on the land later. In that case, apply the nitrogen earlier.

Nitrogen is needed early in the life of the crop because the main response is through increased tillering, which is determined early. If application is delayed beyond four weeks after sowing, there is less chance of getting a profitable response.

The time of application is less critical where there is a reasonable supply of soil nitrogen than where fertility is very low. This is because the soil nitrogen supply may be enough to produce the tillers and set up the yield potential, while the nitrogen fertiliser is only needed to help realise that potential by ensuring survival of ear-bearing tillers.

Evenness of spreading

If any nitrogen fertilisers are topdressed, it is important to get an even spread. Spinner type spreaders often result in uneven distribution of fertiliser with more than the recommended rate in some places and less, or none, in other places. The overall response will be less than with even spreading, because the increased yield in the strips receiving high fertiliser rates will be less than the decreased yield in the strips getting lower rates of fertiliser.

It is important to get even distribution of fertiliser, even if it means using a combine to topdress.

Also consider -

When you are choosing between nitrogen-phosphorus fertiliser sources, also consider:

• the ease of handling and storage;
• the more flexible timing of application of nitrogen-only sources (for example, urea); and
• the rate of fertiliser that can be drilled in contact with the seed without a harmful effect on plant numbers and grain yield. Do not drill urea in contact with cereal seed, either alone or in mixtures, at rates greater than 30 kg/ha. No urea should be placed with canola seed. Canola germination is very susceptible to the soluble nitrogen fertilisers and especially to urea.

If there is doubt about the need for other nutrients such as sulphur and zinc and if you cannot check this easily, use sources containing these nutrients as an insurance, particularly if there is little difference in the cost of nitrogen and phosphorus supplied by the chosen fertilisers.

Further reading

• Farmnote No. 41/84 'Choosing sources of nitrogen for cereals' (Agdex 110/541).
• Farmnote No. 64/85 'Sulphur deficiency in subterranean clover' Agdex (131/632).
• Farmnote No. 21/86 'Zinc deficiency in wheat'(Agdex 112/632).
• Farmnote No. 3/87 'Sulphur deficiency in cereals'(Agdex 110/632).
• Farmnote No. 47/87 'Zinc deficiency in cereals with DAP' (Agdex 110/632).
• Farmnote No. 25/88 'Copper, zinc and molybdenum fertilizers for new land' (Agdex 541).
• Farmnote No. 33/88 'Zinc deficiency in oats' (Agdex 113/632).
• Farmnote No. 5/94 'Take-all disease of cereals' (Agdex 110/633).
• Farmnote No. 15/94 'Copper deficiency in sheep and cattle' (Agdex 400/652).