- Tasmanian farmers grow many different crops with various nutrient requirements within a rotation, leading to large variation in residual soil N post-harvest.
- Soil testing prior to sowing and a deep nitrogen (N) test in early spring will give a greater indication of available and required N.
- Early N applications create large biomass with greater disease pressure, and if not appropriately managed may result in yield loss.
- For cereals, 40kg N/ha is required for every tonne of grain yield produced and 80kg N/ha is required for every tonne of canola grain yield.
Retaining stubble can have a significantly positive benefit to soil health and nutrient cycling. Retaining stubble contributes to the soil organic matter which can be mineralised by microbes to plant available forms of nitrogen (N). Many farmers in Tasmania’s high rainfall zone (HRZ) are growing cereals under irrigation and are producing high grain yields and stubble loads, sometimes exceeding 16t/ha. These heavy stubble loads have their challenges for the following crop including sowing and trash flow implications, reduced establishment and increased pest and disease pressure.
Residual N available in the soil can be variable in Tasmanian cropping systems depending on soil type and previous crop history. Considering the complexity of the local farming systems, careful nutrient budgeting is required. Following are some key steps for nitrogen management when approaching the new season.
The importance of potential yield in relation to total soil nitrogen
Wheat, on average, requires 40kg of N per tonne of grain yield (assuming 50% efficiency), so a crop with a targeted yield of 8t/ha would need approximately 320kg N/ha. This does not include crop biomass development.
Stubble causes initial N tie up through mineralisation but N becomes available for plant use later in the growing season. At sowing, N needs to be available for initial crop growth. At least 40 N kg/ha is required at sowing for cereal crops. It is important to have established nutrient levels in the paddock prior to sowing through soil testing.
In 2015 a trial was sown in Cressy, western Victoria into an 8t/ha poppy stubble that had been disced prior to sowing. Results indicated that in retaining stubble Victorian trials, legume based pasture residues that were incorporated by disc had increased microbial activity and conversion of organic N into plant available forms (Angus and Peoples 2012).
Table 1. Deep N results from the Cressy trial site 2015 before and after harvest. Sown 28th April 2017. Soil type is a sandy loam soil over clay.
|Pre-sowing||Post-harvest nil N plots|
|(8t/ha poppy stubble)||(10.4t/ha Trojan yield)|
|Sample depth (cm)||0-30||30-60||60-90||0-30||30-60||60-90|
|Nitrate nitrogen (NO3) mg/kg||22||40||9||8.5||6.4||5.9|
|Ammonium nitrogen mg/kg||2||2||5||4.9||1.3||1.3|
|Bulk density (g/cc)||1.2||1.4||1.4||1.2||1.4||1.4|
|Total N available (kg/ha)||86.4||176.4||58.8||48.2||64.7||90.7|
|Total N (kg/ha)||Before sowing: 321.6||After harvest: 110.8|
|Soil organic carbon %||4.2||From topsoil 0-10 cm|
|50% potentially mineralisable N (kg/ha)||142.1||If soil conditions allow for mineralisation of nitrogen from organic matter in the soil|
|Total potential available N (kg/ha)||463.7|
|Yield potential, nil additional N (t/ha)||11.5 to 16.6|
Under Victorian and NSW loam soils, the rate of mineralisation is 1kg/ha/day with 1 per cent of organic carbon (Angus J, 2015). As shown in Table 1, the organic carbon level is 4.21 per cent meaning the peak mineralisation can be four times greater. The rate of mineralisation can vary with soil temperature, soil moisture and amount of oxygen in the soil. Mineralisation occurs quickest in warm, well aerated moist soils, and in Tasmania this is usually during spring (winter soil temperatures are below 10oC). Spring is also when cereals commence stem elongation (GS30) and most of the crop growth occurs, slowing by harvest in late December. During this spring growth period, a fast-growing crop will require up to 12-14kg N/ha/day.
Available nitrogen and potentially mineralisable nitrogen (PMN)
Soil tests taken prior to sowing showed that this site had high levels of available soil N to a depth of 90cm, with more than 320kg N/ha available, see Table 1 above. 86kg N/ha of this was available within the early root zone of the plant (top 30cm).
The target trial grain yield of 8t/ha required 320kg N/ha to be available. This amount was available in the soil before any mineralisation during crop growth has been included. When PMN is included, total potential available N increases to over 460kg N/ha. This level of N has a potential yield range of 11.5t/ha to 16.6t/ha, yield difference due to efficiency of the mineralisation process.
The trial plots that received no additional N yielded 10.4t/ha. This yield was less than the predicted potential yield. Leaching of nitrate was not a factor as to why the actual yield was less than potential yield due to the underlying clay soils. Following denitrification and harvest, additional soil samples were measured in nil N plots and there was still 111kg N/ha not used by the crop (Table 1). The 2015 season had a hot dry finish and plants, although site was irrigated, would have suffered heat stress during flowering and grain fill contributing to the loss in potential yield.
What strategies are required to maximise retained stubble crop nutrition in Tasmania?
1. Soil test– soil testing is essential. identify the nutrient and organic carbon content of the soil to assist in tailoring fertiliser programs. Sampling soil to 100cm is also beneficial to quantify nutrition available through the soil profile. Due to the cold soil moisture conditions, not a lot of N is mineralised during the high rainfall period of winter. Deep N testing in spring when soil temp is above 10oC is essential for an accurate figure of mineralisable N.
2. Estimate yield potential. The management tool Yield Prophet® can predict yield potential with the use of historical yield and climate data. In-crop tools such as moisture probes provide accurate levels of plant available water. Targeted soil and plant tissue testing can also be used to increase the accuracy of crop nutrient requirements.
3. Calculate N requirements for the crop. Use soil test data to establish how much N is available and what nutrients are lacking. For wheat, 40kg N/ha is recommended per tonne of estimated yield and for barley 35kg N/ha. For canola, 80kg N/ha is recommended per tonne of estimated yield.
N top dress requirement= Estimated yield x 40, 35 or 80 dependent on crop type (as above)– (N from soil test + N applied at sowing).
4. Timing of N top dressing. As previous studies show, top dressing crops at stem elongation (GS30) is recommended. Applying N too early will increase the biomass of the crop. This additional early growth of biomass can reduce available soil moisture that would be used later in crop growth and may also increase the risk of foliar diseases. Both these effects can reduce potential yield. N applied later than GS30 is likely to increase protein levels rather than the crop yield. Local agronomists and advisors work on the rule that only 10. When calculating a nitrogen budget it is important to allow for mineralisation that will occur in season.
5. Monitor crop response. Visually inspecting crops and using other tools such as tissue tests and dry matter production will help indicate the N response of a crop. The use of N rich strips or N zero strips in conjunction with NDVI can assist greatly in estimating N response in crop.
6. Disease. To ensure a crop can reach its potential yield any crop diseases in particular foliar diseases must be identified and controlled.
7. Weather Forecast: In the high rainfall zone (HRZ), due to increased rain activity ideal weather conditions are more frequent to apply fertilisers however, can make paddock access limited. Using weather forecasts allows the grower to plan ahead and apply products confidently knowing it is the most effective application window. It is important to understand where nitrogen is being added to the system and removed and the impact of rainfall events. While spreading nitrogen will add to soil N so too will in season mineralisation whereas excessive rainfall can see leaching of N through the profile along with denitrification. It is important to understand the consequences of the timing of spreading in regards to rainfall events.
Retaining stubble can have a significantly positive benefit to soil health and nutrient cycling. In areas such as the Mallee in Victoria, nitrogen levels in the soil and stubble can be low compared to Tasmania. Tasmania has many high value crops ie. poppies in the rotation demanding high N inputs which can leave high levels of residual N in soil for use by the following crop.
In Tasmania, low soil temperatures and high rainfall events restrict plant growth during winter and also reduce the rate at which N is mineralised. Spring deep N testing when crops have reached GS30 will give growers an accurate measure of N available to the crop in various forms and can be used to assist in ensuring nitrogen budgets reflect crop potential. Timing of N applications in regards to crop growth stage should be considered. Application of N following GS30 will have a greater impact on grain protein whereas applications prior to this stage will contribute more to yield gains. Using all these strategies including weather forecasts is very important to minimise N wastage and maximise profitability.
Angus JF. Plant-Available Nitrogen Fact Sheet. (2015) Accessed at: https://grdc.com.au/news-and-media/news-and-media-releases/south/2015/01/understand-the-factors-that-affect-mineralisation
Angus JF and Peoples MB (2012) Nitrogen from Australian dryland pastures, Crop and Pasture Science 63: 746–758. Accessed at: www.publish.csiro.au/paper/CP12161.htm
Carson J & Phillips L. Soil Nitrogen Supply Fact Sheet. Soil Biology Initiative, 2014. Accessed at: http://soilquality.org.au/factsheets/soil-nitrogen-supply
Cosgriff H. Can we reduce nitrogen inputs for wheat sown onto high-input crop stubbles and still achieve high yields? (2015) Accessed at: https://thestubbleproject.wordpress.com/2016/04/22/can-we-reduce-nitrogen-inputs-for-wheat-sown-onto-high-input-crop-stubbles-and-still-achieve-high-yields/
Growing eight tonnes a hectare of irrigated wheat in Southern NSW, NSW DPI Primefact No. 197, 1996. Accessed at: www.dpi.nsw.gov.au/__data/assets/pdf_file/0010/79075/irrig-wheat.pdf
Nutrient Management in Broadacre Cropping, SARDI/CropCo. Accessed at: http://www.croppro.com.au/resources/SARDIsustain_nutrient_broadacre.pdf
By: Georgina Moloney Research and extension officer – (SFS) Tasmania firstname.lastname@example.org
This research is being conducted by SFS as part of the GRDC Maintaining profitable farming systems with retained stubble initiative (project BWD00024 Maintaining profitable farming systems with retained stubble in Victoria and Tasmania).