Critical nutrient levels for canola in Western Australia

Page last updated: Monday, 7 October 2019 - 9:45am

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Macronutrients in canola

Phosphorus

Interpreting soil and tissue tests

Table 2 Predicted percentage loss of seed yield due to the severity of phosphorus deficiency in canola (Source R. Brennan & D. Sharma, DPIRD)

Yield loss

>25%

15-25%

5-15%

<5%

Phosphorous 0-10cm soil test (mg/kg)(Colwell P)

<13

13-16

16-22

>22

Phosphorous in whole tops at seedling stage (%)

<0.27

0.27-0.35

0.35-0.4

>0.4

Phosphorous in whole tops at rosette stage (%)

<0.18

0.18-0.23

0.23-0.27

>0.27

  • Critical levels can be affected by pH, with less phosphorus available at pH below 4.8 and above 7.5
  • Critical soil extractable phosphorus levels are affected by the phosphorus buffering index (PBI). Higher PBI soils require more phosphorus to be applied due to the strong adsorption of phosphorus on soil particles. Phosphorus soil decision tools consider PBI within the decision making system.
  • High rates of phosphorus fertiliser near the seed can cause toxicity and germination problems, so higher rates are best banded away from the seed.
  • Generally it is not effective or economic to correct phosphorous deficiency during growing season.

Phosphorus in soils and plants

WA Wheatbelt soils were naturally low in phosphorus before clearing for agriculture. Continual use of fertiliser has built up levels of phosphorous in wheatbelt soils, however canola yield losses caused by phosphorus deficiency are still common.

Plant requirement for phosphorus is particularly high during the early stages of growth. Phosphorus supplied by the seed is depleted very rapidly, after which seedlings are dependent on the developing root system to maintain supply from the soil.

Early phosphorus deficiency is often transitory while the root system is developing and is compounded by dry soil soon after the emergence of canola seedlings. Often deficiency symptoms disappear as the root system develops or when topsoil re-wets following rainfall. Ensuring the developing seedling can access sufficient phosphorus is important because even transitory deficiency can result in lost yield.

Phosphorus availability in soil

Most phosphorus in fertiliser is water-soluble; however phosphorus is highly immobile within soils and reacts with other elements in the soil solution (precipitation) and on the surface of soil particles (adsorption). These reactions reduce the amount of soluble phosphorus available for root uptake by growing crops.

The ability of a particular soil to react and adsorb soluble phosphorus is measured as the phosphorus buffering index (PBI). For soils with the same level of extractable phosphorus (Colwell P), deficiency is most likely to occur on soils with high PBI, such as Darling Range forest gravels, compared to sand (low PBI).

As a guide, soils considered to have low-moderate phosphorus buffering index have a PBI less than 8. Soils consider to have medium buffereing index have a PBI between 9 and 15. Soils considered to have high to very high  phosphorus buffering index have a PBI above 16.

In soils with a long history of phosphorus fertiliser, phosphorus adsorption sites gradually become saturated meaning a higher proportion of applied fertiliser remains in solution and available to developing crops for longer. Phosphorous adsorbed to soil particles can also return to the soil solution and become available to following crops, meaning it has good residual value.

Phosphorus availability is also linked to soil pH, with solubility decreasing in both acidic and alkaline soils. The ideal soil pH range for canola is between pH 5.5-7.5 and phosphorus availability is high within this range. If soil pH is outside of this range, phosphorus solubility decreases, reducing availability to plants.

Growing canola on soils with a pH outside this ideal range can have other consequences related to phosphorus nutrition. Canola is particularly sensitive to aluminium toxicity, a common problem on wheatbelt soils with pH below 4.8. This can result in root pruning and reduced root exploration of the soil which can markedly increase phosphorus deficiency. Application of lime to acidic soils can raise their pH, limiting aluminium toxicity and increasing phosphorus solubility.

Diagnosis and management

The leaf turns bronze before dying
The leaf turns bronze before dying

For information about diagnosing phosphorus deficiency and to view more images of phosphorus deficiency symptoms in canola, see DPIRD’s guide to Diagnosing phosphorus deficiency in canola.

Plant tissue testing can be used to confirm suspected phosphorus deficiency. Use whole-shoots and compare paired good and poor plant samples where possible to help diagnosis.

Soil testing before seeding is used to determine soil phosphorus status and “Colwell P” is the standard test used in Western Australia. There are a range of fertiliser decision tools and software applications used by fertiliser companies, agronomists and farmers to interpret soil test results and develop phosphorus fertilisation strategies. These tools consider the PBI of the soil, the soil pH and Colwell P of the soil when determining phosphorus fertiliser requirement which provides a clearer picture than looking at Colwell P alone.

As a guide, approximately 4kg of phosphorus is removed in each tonne of harvested canola seed and this can be used to estimate the amount of phosphorus needed for maintenance of soil phosphorus levels.

A single application of a compound fertiliser is generally drilled at seeding. These fertiliser products usually contain both nitrogen and phosphorus and are made using either MAP (mono-ammonium phosphate) or DAP (di-ammonium phosphate).

Avoid phosphorus toxicity at seeding

Care must be taken not to place high fertiliser levels close to the canola seed as this can cause germination problems or seedling toxicity. This is more likely on very sandy soil types and at high fertiliser rates or on wider row spacing. Banding fertiliser away from the seed can prevent toxicity.

Although phosphorous fertiliser is sometimes top-dressed, this is less effective at supplying young crops because phosphorus is immobile in soils and crop requirement is particularly high during early plant growth.

Remedial action

Phosphorus deficiency usually occurs during early crop development, so it is very important to ensure phosphorus is adequately supplied at sowing. Top dressing phosphorus post seeding is less than half as effective in correcting a deficiency due to its immobility in soils, except on some sandy soils with very low phosphorus buffering index (PBI).

If phosphorus deficiency has been diagnosed by plant tissue testing, it is generally not economic to correct the deficiency during that growing season. Foliar application is ineffective as the amount of phosphorus applied in the spray is small.

Sulfur

Interpreting soil and tissue tests

Table 3 Predicted percentage loss of seed yield due to the severity of sulfur deficiency in canola (source R. Brennan & D. Sharma, DPIRD)

Yield loss

>25%

15-25%

5-15%

<5%

Sulfur 0-10cm soil test (mg/kg) (in KCI)

<5

5-6

6-8

>8

Sulfur in whole tops of young plant (%)

<0.2

0.2-0.35

0.3-0.5

>0.5

  • Canola needs more sulfur than wheat
  • Mineralisation of soil organic matter can supply significant amounts of sulfate
  • Deeper soil testing provides a better picture of the sulfur status of a soil than top soil alone
  • Young canola crops sometimes grow through mild deficiency without yield penalty where sulfur levels increase down the soil profile

Sulfur in soils and plants

Canola has a very high requirement for sulfur when compared with cereal and grain-legume crops

Plant available sulfate sulfur comes from applied fertiliser, or the breakdown of soil organic matter. Sulfate is highly soluble so the majority remains available to plants in the soil solution. However a small amount can be adsorbed on soil constituents. On very sandy soils the soluble sulfate is leached from the root-zone early in the growing season leaving young crops sulfur deficient.

Soil type will determine the ability of the soil to supply sulfur to a developing crop. Soil types with high levels of organic carbon can supply significant amounts of plant available sulfur during the growing season through mineralisation. Also, in some WA soil types including Esperance sandplain, sulfur levels can increase down the soil profile because it is retained by iron, aluminium and clay in the subsoil. On these soils, sulfur deficiency is occasionally seen in young canola plants but the roots frequently access sulfur down the soil profile and plants recover without any detrimental effect on crop yield. This phenomenon can reduce the reliability of early plant tissue tests as a predictor of sulfur supply. It also reduces the reliability of soil tests to 0-10cm without deeper soil testing.

The application of nitrogen fertiliser can induce sulfur deficiency on soils where there is not enough plant available sulfur in the soil. Application of nitrogen can stimulate rapid crop growth, increasing demand for sulfur beyond the soils ability to supply. Subsoil constraints that impede root development, such as acidity, sodicity and hardpan can also induce sulfur deficiency.

Estimates for sulfur removal by canola crop can vary, but a survey conducted in Western Australian found each tonne of canola grain removed between 3.2-6.0 kg sulfur, compared with about 1.5-3kg for wheat.

Diagnosis and management

Serrated leaf blades that extend further down the petiole become thickened and roll inwards showing purple undersides
Serrated leaf blades that extend further down the petiole become thickened and roll inwards showing purple undersides

For additional information, and to view images of sulfur deficiency symptoms in canola, see DPIRD's guide to Diagnosing sulfur deficiency in canola.

Plant tissue testing can be used to confirm suspected sulfur deficiency. Use whole-shoots and compare paired good and poor plant samples where possible to help diagnosis. Both soil tests from 0-10cm and early plant tissue tests can over estimate sulfur deficiency and sampling to 30cm provides a better understanding of the distribution of sulfur within the soil profile. Table 3 (above) can help interpret soil or tissue tests through estimating yield loss due to inadequate sulfur.

Soil testing levels less than 7mg S/kg soil in the top 10cm of soil can indicate sulfur deficiency but deeper soil tests can also be helpful determining the sulfur status of soils.

Sulfur nutrition in canola crops is usually managed through the application of sulfur containing compound fertiliser at seeding. These fertilisers supply enough phosphorus for the entire crop, but only a starter amount of nitrogen and sulfur. Additional nitrogen and sulfur are then applied post emergence. Fertilisers with a range of nitrogen: sulfur ratios (N: S ratios)are available as either liquid or granular products. These are achieved by blending ammonium sulfate with urea or adding Sulfur to UAN (urea–ammonium nitrate) products such as Flexi N. Gypsum and ammonium sulfate are the most common sources of applied Sulfur. Gypsum contains approximately 16–18% sulfur, is relatively inexpensive, and unlike ammonium sulfate, it is not acidifying.

Crop constraints that restrict root development such as traffic pans, disease or soil acidity will worsen sulphur deficiency and final yield, even if sulphur is available further down the soil profile. Management strategies that overcome these constraints can help prevent sulfur deficiency. These can include liming, deep ripping and using appropriate rotations and seed treatments to reduce disease development.

Remedial action

Sulfur deficiency can be corrected through the post emergence application of sulfur containing fertilisers.  A range of liquid and granular products are available, although foliar sprays generally cannot supply enough sulphur to overcome a severe deficiency. Application of sulfur in the sulfate form using a product such as ammonium sulfate can overcome deficiency rapidly after the fertiliser has moved into the soil profile. 

Applying sulfur to crops that are mildly to moderately sulfur deficient does not always increase yield because canola crops sometimes access sulfate down the soil profile, as root systems develop. Transient sulfur deficiency does not usually reduce yield and additional soil testing to 30cm depth combined with farmer knowledge of individual soil types can help predict the ability of a particular soil to supply sulfur to crops.

Potassium

Interpreting soil and tissue tests

Table 4 Predicted percentage loss of seed yield due to the severity of potassium deficiency in canola (source R. Brennan & D. Sharma, DPIRD)

Yield loss

>25%

15-25%

5-15%

<5%

Potassium 0-10cm soil test (mg/kg) (Colwell K)

<29

29-36

36-46

>46

Potassium in whole tops at early seedling stage (%)

<3.8

3.9-4.5

4.5-5.5

>5.5

Potassium in whole tops at rosette stage (%)

<3

3-3.5

3.5-4.0

>4.0

  • Deeper soil testing provides a better picture of the potassium status of a soil than top soil alone.
  • Applying potassium to a mildly deficient crop early in the season may not be economically beneficial as developing root systems can sometimes access additional potassium deeper in the soil profile later in the season.

Potassium in soils and plants

Originally, potassium deficiency in WA was most likely to occur on deep white sands and deep grey sandy duplex soils, but is now also common in the sandy duplex soils and sandy gravels over clays.

Potassium is required for many plant processes including photosynthesis, ionic balance and water use efficiency. Potassium is required in greater amounts than phosphorus and is highly mobile within the plant and if K is in short supply it will move from old to new leaves. Because of this deficiency symptoms are frequently observed on oldest leaves. An adequate supply of potassium provides the plant with increased resistance to disease, frost and drought.

Fertiliser potassium is mobile within the soil so can be prone to leaching beyond the root zone, particularly in the deep sandier soils. Where there is a sub soil constraint thats limits root exploration such as low soil pH or a hard pan, plants will be unable to access leached potassium from deeper in the soil profile.

For canola, the Colwell potassium soil test in the 0-10 cm should be between >46 mg/kg for adequate supply. In sandy duplex soils, soil test potassium levels can fall below 40 mg/kg in the 0-10 cm layer, but potassium levels may rise in the clay subsoil so deeper soil testing is recommended to provide better information about a soil’s ability to supply potassium to canola.

Diagnosis and management

Bulging interveinal leaf tissue; interveinal discolouration and death spreads towards the midrib
Bulging interveinal leaf tissue; interveinal discolouration and death spreads towards the midrib

For additional information, and to view images of potassium deficiency symptoms in canola, see DPIRD’s guide to Diagnosing potassium deficiency in canola.

Potassium fertilisers can be drilled at seeding or applied post emergence, if soil test results are below critical levels. The drilled fertiliser products are usually a compound fertiliser containing potassium or a compound fertiliser blended with muriate of potash (MOP, KCl 50% K). Care needs to be taken when drilling potassium fertiliser with the seed as it can decrease seedling emergence under certain conditions.

MOP is usually used for post emergence application; however potassium sulfate (SOP, 42% K) can be used but is more expensive than MOP. Both products are effective at overcoming potassium deficiency.

For soil test levels near the critical Colwell K level, replacement of the quantity of potassium expected to be removed in harvest grain should be considered. This will prevent Colwell soil potassium levels declining to deficient levels over time. Canola grain usually contains about 9kg potassium/tonne.

Remedial action

Top-dressed MOP fertiliser will usually correct the deficiency and muriate of potash is the most widely used product, but the more expensive sulphate of potash can be used. Foliar sprays of potassium generally cannot supply enough potassium to overcome a severe deficiency and can also scorch crops.

Like sulphur, applying potassium to a mildly deficient crop early in the season may not be economically beneficial as developing root systems can sometimes access additional potassium deeper in the soil profile later in the season. Soil testing below 10cm depth and local knowledge of soil types can help predict expected benefit of applying potassium

Nitrogen

Interpreting tissue tests

Table 5 Predicted loss of seed yield due to the severity of nitrogen deficiency in canola (source C. Scanlon & D. Sharma, DPIRD)

Yield loss

>25%

15-25%

5-15%

<5%

Nitrogen in whole tops at seedling stage (%)

<2.7

2.7-3.0

3.0-4.0

>4.0

Nitrogen in whole tops at rosette stage (%)

<4

4-4.5

4.5-5.1

>5.1

  • Mineralisation of soil organic matter can supply significant amounts of nitrate so soil tests alone are a poor predictor of nitrogen supply.
  • The peak economic nitrogen application rate is lower than the rate for peak yield.
  • Nitrogen supply can affect grain quality characteristics such as oil content

Nitrogen in soils and plants

While nitrogen fertiliser is consistently the largest and most costly input for canola, it represents one of the highest returns on investment. Nitrogen fertiliser generally increases the yield of canola where seasonal conditions allow, although the oil concentration can be reduced. This means the peak economic nitrogen application rate can be lower than the rate for peak yield. High nitrogen canola crops may look fantastic but grain can be downgraded if oil percentage is low. Growers may target a $2 increase in yield for each $1 cost of fertiliser.

Nitrogen for crop growth can be supplied from residue organic nitrogen, soil organic nitrogen and applied fertiliser. Calculate fertiliser needs by assessing soil supply and crop demand and agronomists or fertiliser consultants can help interpret soil tests and formulate nitrogen fertiliser strategies. There are also numerous tools, apps and software products such as SYN, NULogic®and NBroadacre®

Residue organic nitrogen (RON) is generated from the breakdown of legume crop or pastures. Peak supply is in the first year after legume growth, dwindling over the second and third years after legumes. The amount of nitrogen supplied by this source depends on the amount of legume grown (t/ha) and proportion of legume in the pasture, biomass and harvest index of a legume crop and the number of years. This can range from 130kgN/ha for good density legume content pasture, grown in a long season with 400mm GS rainfall, to 260 after an extended pasture phase, or only 20kg/ha for poor density low legume content pasture short season 200mm GS rainfall, or 40kg/ha for a 1t/ha lupin crop.

Soil organic nitrogen (SON) is very closely related to the soil organic carbon (OC) %. Supply of mineralised nitrogen from organic carbon is very stable between years as soil microbes cycle the nitrogen when conditions favour microbial growth. The supply available to plants is reduced by gravel content, which effectively limits the amount of soil available to the plant.

Approximately 50kg/ha per 1% OC with less than 10% gravel, dropping to 30kg/ha for more than 30% gravel but up to 60kg/ha with summer rain. So for 2.5% organic carbon, no gravel summer rain scenario, up to 170kgN/ha but 0.8% OC, no gravel or summer rain, only 40kgN/ha supplied to crops.

Ammonia and nitrates on your soil test show organic nitrogen that has already mineralised for the season, so amounts are generally higher after summer rain. The mineralisation process continues all year and the nitrogen mineralised during the warm wet period in spring is a strong driver of the spring flush.

After adding up the expected supply from RON and SON, we can use fertiliser nitrogen to ensure the total N supply meets the likely N demand of the crop.

N demand= RON supply + SON supply + fertiliser supply

Diagnosis and management

Fewer and smaller flowering stems are produced
Fewer and smaller flowering stems are produced

For information about diagnosing nitrogen deficiency and to view more images of deficiency symptoms in canola, see DPIRD’s guide to Diagnosing nitrogen deficiency in canola.

Growers generally apply a starter dose of nitrogen as part of the compound fertiliser drilled at seeding. Post crop emergence, best practice is to apply nitrogen before bolting (approximately 8-10 weeks), as nitrogen uptake is most rapid during stem elongation. However, canola can continue nitrogen uptake past this time, unlike wheat. Later nitrogen top-ups generally have a similar response to applications before bolting; up to early flowering in low and medium rainfall zones and early pod fill in high rainfall zones. This gives farmers a secondary chance to apply nitrogen top-ups in response to a favourable season or as logistics (timing) permit or wet paddock conditions allow.

Remedial action

Post emergence, nitrogen fertiliser is usually top dressed in the form of urea or a blend containing sulphate or, a liquid fertiliser is applied as a foliar spray. Rainfall needs to fall after application before crop roots can access the nitrate although some foliar uptake occurs with foliar spraying. There is a risk of volatilisation loss from urea or nitrate sources of nitrogen. Loss is greatest from dry alkaline soils with dewy conditions, but rarely exceeds 3% per day.

The yield potential for canola is established during stem elongation and the budding stage, so ideally all nitrogen should be applied before this stage of growth (8-10 weeks) although yield responses have been measured from later applications.

Unlike cereals, canola does not ‘hay off’ when too much nitrogen has been applied, but nitrogen reduces oil concentration, particularly with late application.

Contact information

Mark Seymour
+61 (0)8 9083 1143
Martin Harries
+61 (0)8 9956 8553

Critical nutrient levels for canola in Western Australia

Authors

Ross Brennan
Andrew Blake
Darshan Sharma
Jackie Bucat
Mark Seymour
Martin Harries