The natural fertility of cropped agricultural soils is declining with time and grain growers need to continually review their management programs to ensure the sustainability of grain production. Pasture leys, legume rotations and fertilisers all can have important roles in maintaining the chemical, biological and physical fertility of soils.
With more frequent opportunity cropping, improved farming techniques and higher yielding varieties, paddock nutrition programs should be reviewed regularly. Paddock records, yield monitoring, remote sensing images, fertiliser test strips, crop inspection and monitoring, benchmarking water use efficiency, and soil, plant tissue and grain testing can assist in formulating efficient, sustainable cropping programs.
Whilse rotations with grain legumes and ley pastures have an important role to play in soil fertility, fertilisers remain the major source of nutrients. Nutrient input programs should aim to supply a balance of the required nutrients in the amounts needed to achieve a crop's yield potential. Nutrient removal in grain is the main pathway for fertility rundown, with high-yielding crops exporting large amounts of nutrients.
Common nutrient deficiencies in Queensland´s broadacre grain areas are nitrogen (N), phosphorus (P), potassium (K) and zinc (Zn), while sulfur (S), copper (Cu) and molybdenum (Mo) may be also be lacking in some soil types and growing areas.
Ultimately, nutrients removed from paddocks will need to be replaced at some point to sustain production. In irrigated cropping, large quantities of nutrients are removed and growers need to adopt a strategy of programmed nutrient replacement, but dryland growers should also consider this approach. Table 1 below gives the average nutrients removed by various crops in both grain and hay.
|Crop species & productivity||Yield kg/ha||N kg/ha||P kg/ha||K kg/ha||Ca kg/ha||Mg kg/ha||S kg/ha||Zn g/ha|
|Irrigated wheat grain||7000||125||24||35||3.5||10||3||200|
|Dryland wheat grain||2000||40||7||10||1.5||2.8||5.5||60|
|Dryland barley grain||2000||36||8||8||2||2||4||60|
|Dryland sorghum grain||3000||45||8||9||3||3||3||60|
|Irrigated maize grain||9000||120||23||27||1||12||11||150|
|Dryland maize grain||4000||50||12||12||0.5||5||5||70|
|Irrigated soybean grain||4000||275||25||74||-||11||12||150|
|Dryland soybean grain||2000||140||12||37||3||5.5||6||80|
|Irrigated peanut pods||5000||200||18||50||5||12||12||250|
|Dryland peanut pods||1500||55||5||14||1.5||4||3.5||75|
The yield potential of a crop will be limited by any nutrient the soil cannot adequately supply. Poor crop response to one nutrient can often be linked to a deficiency in another nutrient or other management technique. Crop response can also be linked to soil constraints such as acidity, sodicity or salinity, problems with beneficial soil microorganisms or pathogens such as nematodes.
To attain optimum yields, an adequate supply of each nutrient is necessary. However, it is important to realise that only a small proportion of the total amount of an element in the soil may be available for plant uptake at any one time. For nutrients to be readily available to plants, they must be present in the soil solution (the soil water), or easily exchanged from the surface of clay and organic matter particles in the root zone, and be supplied when and where the plant needs it.
Temperature and soil moisture content will affect the availability of nutrients to plants, and the availability of nutrients will also depend on soil pH, degree of exploration of root systems and various soil chemical reactions, which vary from soil to soil. Fertiliser may be applied in the top 5-10cm, but unless the soil remains moist the plant will not be able to access it. Movement of nutrients within the soil profile in low rainfall areas is generally low except in very sandy soils.
Lack of movement of nutrients combined with current farming methods (e.g. zero till) is resulting in stratification of nutrients, where concentrations of nutrients are building up in the surface of the soil where they are not always available to plants, depending on the seasonal conditions. Often, on the Western Downs and in Central Queensland, wheat is deep sown in moisture that is below the layer whereby nutrients have been placed or are stratified, which has implications for management and fertiliser practices. More detailed information on particular nutrients is discussed below.
Nitrogen supply and grain protein content
Nitrogen is a primary constituent of protein, so an adequate soil nitrogen supply is an essential ingredient for producing wheat with a high protein content. Grain protein is modified by the grain yield of the crop - increasing grain yield has a diluting effect on grain protein.
This is why in drier seasons or seasons of low grain yield, a larger proportion of the crop is of a high protein percentage, whereas, in wetter growing seasons, high yields can be produced but may be at a lower protein. This seasonal variation is why paddock grain yield, protein and rainfall records should be kept for a number of years to obtain a true indication of its nitrogen fertility.
Low grain protein, the signal of nitrogen deficiency
Grain protein can be used to indicate if current nitrogen fertiliser or rotation practices are providing adequate nitrogen to the crop to meet the requirements arising from the water supply the crop had access to. Profits may also suffer not only due to lost yield, but potentially downgrading to a lower classification.
|Wheat grain protein||Indicated nitrogen supply|
|Less than 11.5%||Acutely nitrogen deficient this season. Grain yield would almost certainly have been increased had more nitrogen fertiliser been applied; grain protein will increase if sufficient nitrogen is applied to achieve season yield.|
|11.5-12.5%||Probably nitrogen deficient this season. Nitrogen adequate to achieve season grain yield. Grain protein will most certainly increase with higher rates of nitrogen. High protein premiums for specialty classifications of wheat may justify higher rates of nitrogen to produce higher grain protein levels.|
|Higher than 12.5%||Nitrogen not deficient this season. Water supply probably limited grain yield. Applying additional nitrogen fertiliser will not increase yield but may increase protein. Applying nitrogen fertiliser to produce higher protein (13%) is only economical if high protein premiums exist for Australian prime hard wheat or other specialised markets.|
Nitrogen management should ensure wheat crops consistently produce a grain protein content of 11.5-12.5 per cent to achieve season yield potential. However, applying nitrogen fertiliser to produce grain protein content above 12.5 per cent may not be economical unless high premiums are available for high protein Australian prime hard (APH) wheat.
Soil testing for nitrogen
The approximate amount of nitrogen available in the soil can be determined by soil testing. Soil tests should be taken at various places in each paddock to a depth of at least 60 cm, but preferably 90 or 120cm, so the quantity of available nitrogen can be calculated. Test results are only an indication, so historical grain yield and protein levels from the paddock should also be used to determine nitrogen requirements. Table 3 indicates the approximate amount of available nitrogen needed at planting for a particular yield and protein.
|Grain protein (%)|
Factors affecting level of nitrogen required:
- seasonal yield and protein levels - If the expected yield is exceeded due to good climatic conditions, grain protein may fall below the protein targeted. If the yield is not achieved, for example due to moisture stress, then grain protein may be above the protein targeted
- planting date, variety and soil moisture - These must be considered in establishing seasonal target yield and protein levels. Early planting and good stored moisture at planting should indicate higher target yields. Using a tool like HowWet is a good approach to estimating plant available water
- SOI and seasonal rainfall prospects - With higher rainfall prospects consider increasing nitrogen rates. Using a cropping simulation tool such as Whopper Cropper can provide further information on yield and protein potentials
- level of soil fertility - Soils farmed consistently for 30 to 50 years (or more) will generally have a higher nitrogen requirement for optimum yield and protein levels than new country
- cropping history - Double cropping will normally require higher nitrogen rates
- zero tillage - Stubble retention combined with zero and reduced tillage has increased the yields and cropping frequency of our farming systems by storing more soil water. Higher nitrogen rates may be required to make efficient use of this extra stored soil water
- protein premiums - High premiums for APH high protein wheat have warranted an increase in nitrogen rates.
Calculation of profile nitrogen levels from soil test values
Most soil nitrogen test results are expressed in milligrams per kilogram (mg/kg) or parts per million (ppm). To make crop recommendations, it is necessary to convert nitrogen test results to kilograms of nitrogen per hectare (kg N/ha). The formula is:
Soil test value x soil bulk density x sample depth (cm) / 10Yield
(t/ha) Grain protein (%) 7
|Soil type||Soil depth (cm)|
|Western Downsand and Maranoa|
|Cracking Clay (Jambin)||1.22||1.28||1.28||1.4|
|Black Vertisol (Capella)||1.04||1.22||1.21||1.29|
|Rolleston Scrub (Banana)||1.13||1.32||1.32||1.32|
A more comprehensive database of soils characteristics is available at the Farmscape website.
N fertiliser application - If paddock records or soil tests indicate that nitrogen fertiliser is required, the calculation below can be used to obtain the quantity of actual fertiliser product required. For example if 40 kg N/ha is required, this rate of nitrogen can be supplied by applying 87 kg/ha of urea.
Fertiliser product required (kg/ha) = Rate of nitrogen required kg N/ha x 100 / % nitrogen in fertiliser product
Applying nitrogen fertilisers High rates of nitrogen fertiliser applied at planting in contact with or close to the seed will severely damage seedling emergence. If high rates of nitrogen are required then it should be applied pre-planting or applied at planting but not in contact with the seed (i.e. banded between sowing rows). Table 5 (below) indicates the maximum rates of fertilisers containing nitrogen that may be applied with the seed at planting using conventional planting equipment. Rates should be reduced by 50 % for very sandy soil and may be increased by 30 % for heavy textured soils or where soil moisture conditions at planting are excellent. Rates should be reduced by 50 % when planting equipment with narrow disc or tine openers are used as the fertiliser concentration is increased around the seed.
|N kg/ha||Urea kg/ha||DAP kg/ha||MAP Starterfos|
Phosphorus deficiency is one of the most widespread nutrient deficiencies. Phosphorus forms part of the proteins in plant cells, so it is important in growing tissue where cells are actively dividing, (e.g. development of seedling roots, flowering and the formation of seed). Phosphorus deficient plants are stunted, dark green plants with short, erect leaves and stout stems that often develop orange, red or purplish discolouration. Many soils in wheat growing areas will respond to the application of phosphate fertilisers.
Approximately 3.2kg of phosphorus is removed in every tonne of wheat harvested. Phosphorus deficiency is more likely to occur after a long fallow due to low numbers of arbuscular mycorrhiza fungi (AMF, also known as VAM) in the soil. AMF are the beneficial soil fungi that help plant roots take up both phosphorus and zinc.
Crop demand for phosphorus can be considered in two distinct phases - during the early development (from emergence to the end of tillering, but before stem elongation), and then during the subsequent growth and grain-filling period. During early development, the requirement for P is small (perhaps 1=kg of P), but the root system is small and inefficient so the crop responds to a concentrated phosphorous source close to the seed and developing roots. Ensuring these young plants have adequate phosphorous during this phase is essential to determine grain number (ie. yield potential) and ensure vigorous seedling development.
Subsequent P requirement is much larger, and largely mirrors the accumulation of crop biomass. As a general rule, crops require approximately 5kg phosphorous to be accumulated by the plant to produce each 1=t of grain yield, so a 3t crop will typically take up 15kg/ha of phosphorus. Only 1-2kg will be taken up from the banded phosphorous fertiliser applied at planting (either in or below and beside the seeding row). The rest comes from the soil profile, with approximately half coming from the top 10-15cm and the rest coming from the next 15-30cm. These proportions will change somewhat with seasonal conditions, as root activity in surface layers will be minimal in dry periods. Having plant available phosphorous in the immediate subsoil (i.e. the 10-30 cm layer preferably) becomes a critical success factor for crop performance.
The need for phosphorus fertiliser can be determined through using soil tests (0-10 and 10-30 cm) and/or test strips of fertiliser. When interpreting Darling Downs soil tests, the best predictions of P response are obtained by using both the BSES and Bicarbonate (or Colwell) P test results as shown in the table below. These guidelines are currently based only on results from the 0-10cm layer of the soil profile, but current research is developing guidelines to determine the soil phosphorus status in the 10-30cm layer, and any resulting fertiliser requirement.
|Bicarb P test (mg/kg)||Acid P test (BSES) (mg/kg)||Black clay||Red brown earth|
|Recommended kg P||% probability of response||Recommended kg P||% probability of response|
|Bicarb P test (mg/kg)||Acid P test (BSES) (mg/kg)||Brigalow soils|
|Recommended P kg/ha||% probability of response|
|> 20||Any level||Nil||Nil|
|Bicarb P (mg/kg)||Recommendations (P/ha)|
|0-10||Response most likely - Apply 8 kg P|
|11-15||Response likely - Apply 6 kg P|
|16-20||Response infrequent - Test strip of 6 kg P|
When fallow length exceeds 12 months, moderate to high rates of P fertiliser are often required to prevent long fallow disorder affecting the crop. A suggested rate is 10kg P/ha.
For more information, see ´Improving phosphorus fertiliser management on southern Queensland grains farms
Yield responses to zinc from research and grower experience are common in many areas of Queensland, including the Darling Downs, Western Downs and Maranoa, and through Central Queensland. Zinc plays a vital role in a plant's ability to use nitrogen and transform it into yield and protein, and should not be overlooked in a balanced crop nutrition program.
The availability of zinc to many crops is increased by the presence of arbuscular mycorrhizal fungi in the soil. Crops grown after long fallows or other events that deplete soil mycorrhiza population will be at most risk of suffering zinc deficiency.
Deficiency symptoms include pale, stunted plants, with brown lesions developing on severely deficient plants. Zinc deficiency is not easy to identify but responses to zinc fertilisers occur frequently in old cultivation with heavy clay soils and high soil pH levels (>8) or low organic carbon (<0.7%). Soil erosion, soil structural problems (e.g. hard pans), root diseases, and Group B herbicide residues can all increase the likelihood of zinc deficiency. Soil zinc status should be checked by plant tissue testing, through soil tests (0-10 cm) and/or the establishment of fertiliser test strips.
Critical levels for zinc are: Soil pH <7.0 - 0.4 mg/kg; Soil pH >7.0 - 0.8 mg/kg.
On the Western Downs, the deficiency is usually associated with low soil zinc test (<0.3mg/kg), high soil pH (>8) and low organic carbon (<0.7%).
Zinc can be applied directly to the soil (zinc sulfate monohydrate), as a component of a starter fertiliser, as a foliar spray (zinc sulfate heptahydrate) or as a seed dressing.
- Soil application (pre-plant) - Applying 15-20kg/ha of zinc sulphate monohydrate 3-4 months before planting should meet the total plant requirement for 5-8 years. Zinc is not mobile in the soil and needs to be evenly distributed over the soil surface, and then thoroughly cultivated into the topsoil. In the first year, a foliar zinc spray may also be required.
- Soil application (at planting) - Banding zinc with phosphorus at planting is an efficient means of delivering zinc to the plants roots. Water injection with either zinc monohydrate or zinc heptahydrate may also be an option for some growers. If applied as a starter fertiliser component, the amount should be at least 1 kg of zinc/ha (about 40 kg/ha of product) which will only provide enough zinc for that crop.
- Seed dressing may provide sufficient Zn to meet crop requirement, but will not build up residual zinc to the soil. May be a cost-effective option where soil P levels are adequate.
- Foliar sprays - Two applications are necessary. Apply at three weeks and again at five weeks after emergence. It is important both sprays be applied before the crop is six weeks old. The most economical form of foliar zinc is a tank-mix of 1 kg zinc sulphate heptahydrate/ha plus 1 kg urea/ha in 100L water/ha plus surfactant at 100mL of 1000g/L product/100L of spray mixture (or 160mL of 600g/L product/100L of spray mixture).
Water high in carbonate will produce insoluble zinc carbonate. Consider having the water analysed for suitability in crop protection programs and using a commercial buffering agent.
Zinc heptahydrate sprays may not be compatible with herbicides. Several chelate forms of zinc are available which are generally more compatible with herbicides. Always read the label to determine compatibility.
Due to the gradual decline in soil potassium levels with crop removal and historically low fertiliser application rates, some situations (particularly red soils) require K fertiliser applications. However, crops also vary in their response to improved soil K levels. Generally winter cereal responses are low to moderate unless gross deficiencies occur.
Crops generally take up as much potassium as they do nitrogen although this may not be reflected in crop removal. In particular irrigated cotton, grain legumes and hay baling/silage can affect the levels of K reserve in the heavier soil types.
- Young plants grow very slowly and are often stunted.
- In older plants, the lower leaves exhibit a marginal ´scorch´, with yellow to brow margins towards the leaf tips.
- Potassium deficient plants may also lodge more readily.
Currently, potassium soil tests are reported as exchangeable potassium (meq/100g or cmol/kg) or, in the case of a Colwell K test, as mg/kg available K. Research is currently underway to better define critical soil test potassium levels but, in the interim, exchangeable K < 0.3-0.4 meq/100g or 130-160mg/kg Colwell P would be considered low-marginal, and test strips worth a try.
Remember that, like P, K is effectively immobile in the soil, so profiles are tending to stratify (much higher levels in the top 10cm, with significant depletion in the 10-30cm layer below). Testing for soil K in both the 0-10cm and 10-30cm layers is advisable, with the deeper K essential when the topsoil is dry.
Potassium fertilisers can be side-banded at planting, drilled in pre-plant, or broadcast and cultivated in fallow or even prior to preceding crop. The residual value of K fertilisers is excellent, so sporadic applications at higher rates can be an effective alternative to lower rates with each crop. However, potassium banded in the seed row can affect germination. Use the safe rates of nitrogen table as an application guide.
Nutrient levels must remain adequate throughout the top 25-30cm, as K is not highly mobile. Surface applications of K without thorough mixing into the soil profile can lead to problems if deeper layers are also low in K and the surface soil dries out. Similarly, relying solely on deep banding of K (e.g. at 20-30cm) when surface layers are also K deficient will not allow the crop to obtain enough K to meet plant demands.
Once K fertility of the surface layers has been restored, deep K application is the best way to apply K fertilisers to maintain soil productivity. This deep K helps maintain crop K status when the surface layers are dry. At the same time, a proportion of the deep K taken up by the crop gets returned to the soil surface in the litter and crop stubble, which replenishes the K fertility of these layers.
While soil K reserves are greatest in the heavier alluvial and cracking clay soils, it is important to maintain adequate potassium soil levels by replacing K removed in harvested product as often as possible. Once soil K levels have been depleted in these soils, very heavy fertiliser K
Because sulfur forms part of many proteins, sulfur deficiency may depress protein formation and prevent nitrate being converted into protein. This may result in low protein grain with high total nitrogen content and thus flour of poor bread-making quality.
Sulfur responses are widespread on the eastern and southern Darling Downs, more so on the Anchorfield and Haselmere soil types, and in areas of the Jimbour plain. It is prevalent on basaltic black earth soils that have been intensively farmed for more than 20 years, particularly if they have been eroded, waterlogged or irrigated, and especially where double cropping is practiced or fertilisers containing gypsum and/or sulfur have not been used regularly.
- Young plants are pale green or yellow with only limited stunting.
- Mature plants' upper leaves are pale green to yellow, with the lower leaves remaining green (unlike nitrogen).
- Tiller production is not affected, though mature tillers will produce small heads.
- In severe deficiencies, the upper leaves are yellow to white in colour with the lower leaves turning pale green. Tiller number will also be reduced.
Critical soil test levels for sulfur have not yet been devised. As a guide, testing to 60cm is suggested with soil test levels <4 mg/kg sulfur indicative of areas to undertake test strips of S application for growth responses. Consider applying either ammonium sulfate into the soil at 75-100kg/ha (15-24kg S/ha) or broadcasting gypsum (calcium sulphate) at the rate of 500kg/ha as test strips.
Copper deficiency occurs in a narrow band stretching from Taroom to Wyaga in the Goondiwindi district, and in a belt of scattered ironbark country stretching from Cecil Plains through to Inglewood. Soils are generally brigalow/belah grey or grey-brown clays adjacent to ridges where the natural vegetation is ironbark, molly box and wattles. Plant tissue testing is most likely the best indicator of copper adequacy in plants.
Deficiency symptoms in wheat include:
- wilting despite ample water supply
- leaf tip dieback of youngest leaves with leaves becoming tightly twisted
- ear tip dieback - where the top of the head turns white or yellow. The remainder of the head stays green, but may not set grain
- white heads and delayed maturity, and melanism (blackening) of stems
- ear branching
- upper node tillering.
Copper deficiency can be corrected by applying copper sulphate either to the soil as a fertiliser or to the foliage as a spray. Copper sulphate applied to the soil at 10-20 kg/ha prior to or at planting will last a number of years. One foliar spraying at booting may still be necessary in dry years.
Alternatively, apply two foliar sprays of 1 per cent copper sulphate per hectare (1kg copper sulphate to 100L water/ha). Apply the first spray 3-5 weeks post-emergence, and the second spray any time from when the ear begins to swell the stem of the plant to when the plant is in the boot stage. Foliar sprays have little residual value and must be applied every year.