Climate and seasonal impacts: Soil microbes drive many nitrogen processes, and their activity changes with weather and soil moisture.

• Wet conditions may speed up denitrification and increase the risk of nitrogen losses through leaching or gassing off.
• Dry conditions slow microbial activity and nitrogen cycling, reducing nitrogen availability to plants.

The role of climate is clear when comparing how the McKinleys and Michael Waring manage nitrogen. In their temperate and often dry climate in NSW, the McKinleys can bank on some applied nitrogen being available for the next crop rotation. In tropical North Queensland, Michael must carefully time cover crop termination and cane planting to prevent losing nitrogen gained from the cover crop to heavy rainfall. 

Soil texture: Soil’s mix of sand, silt, and clay affects how nutrients, water and air cycle through it. 

• Light soils (like sandy loams) drain quickly, hold fewer nutrients and may have slower nitrogen cycling due to lower microbial activity.
• Heavy soils (like clays) retain nutrients longer due to higher cation exchange capacity (CEC), but can compact and hold onto moisture, reducing oxygen for microbes and accelerating denitrification.

There’s no ‘right’ or ‘wrong’ soil texture – the key is knowing your soil and working with it. You can use simple in-paddock tests like ‘ribboning‘, soil testing and local soil maps to understand your soils. While texture is largely fixed, you can improve soil structure and organic matter to support better nitrogen cycling and nutrient retention. The McKinleys have seen that in extended wet periods, light soil textures can leach nitrogen deeper into the soil and out of reach of young crops. They also take into account the potential nitrogen loss from denitrification when areas of heavy clays get waterlogged.

Image 5. Michael Waring’s sugarcane farm near Ingham, QLD, where nitrogen management and efficiency is focused on careful timing between application of inputs and incorporating legumes to match crop nitrogen demand. Source: Michael Waring.

Monitoring plants and soil can give a real-time snapshot of your crop’s nitrogen demand and what’s already available in the soil. This helps avoid over-application, improve nitrogen efficiency and save money. Keep in mind that nitrogen availability will likely vary across the farm depending on soil types and paddock history.

• Pre-season soil testing: Pre-sowing soil tests show the pool of available nitrogen at sowing. As the season progresses, nitrogen will continue to cycle and mineralise, so pre-season tests are only a snapshot, not a guide, to a full-season nitrogen budget. Tests are usually taken from the top 10 cm of soil. Laboratories can measure mineral nitrogen (ammonium and nitrate) and total nitrogen (including organic forms). For accurate results, keep samples cool and ensure quick turnaround between sampling and testing at the lab.
• Deep nitrogen sampling: Testing deeper than 10 cm gives a better picture of nitrogen reserves at depth as crops extend their roots. Crops can access significant nitrogen deeper in the soil, especially if topsoils dry out. For winter crops, deep N testing is recommended before crop growth stage 31 (mid to late July).¹³ Grains Research & Development Corporation, Soil Testing Fact Sheet – Nitrogen soil testing for in-season fertiliser application [PDF], 2021, accessed 13 April 2025.
• Sap and leaf tissue testing: Sap testing can provide real-time insights into nitrogen uptake during the growing season.¹⁴MF Padilla, M Farneselli, G Gianquinto, F Tei and RB Thompson, ‘Monitoring nitrogen status of vegetable crops and soils for optimal nitrogen management’, Agricultural Water Management, 2020, 241:106356, doi:10.1016/j.agwat.2020.106356. Some broadacre growers now use sap tests to guide in-season foliar and soil nitrogen applications. Consistent sampling is critical, including leaf choice, timing, sample handling and lab processing. Avoid sampling during water stress, as it can throw off results.
• Harvest grain protein levels: Taking stock of grain protein levels post-harvest can be a useful way to gauge the success of the season’s nitrogen management. Industry standards aim for nitrogen and crop management that can produce 11.5–12.5% wheat protein.¹⁵Grains Research & Development Corporation, ‘GRDC GROWNOTES, WHEAT Southern Region – Nutrition and fertiliser’, 2016, accessed 7 April 2025.  

Image 6. Michael Waring’s young sugarcane crop, which he monitors using leaf tissue testing to see if the crop has sufficient nitrogen. Source: Michael Waring. 

Foliar applications can be an effective way to supply nitrogen directly to the plant during key growth stages. This method helps improve grain quality and can support yield when soil nitrogen levels are insufficient or when environmental conditions limit root uptake. Timing and proper application rates are key to maximising benefits without overloading the plant with excess nitrogen.

Image 7. The spray equipment the McKinleys use to apply foliar applications of nitrogen in the second half of the crop’s growing season. Source: Matt and Belinda McKinley.

Adding a carbon source to nitrogen fertilisers can help improve nitrogen efficiency and reduce losses. Carbon inputs can help stabilise nitrogen and also act as biostimulants, stimulating soil microbes and promoting nutrient cycling.¹⁶ P Espie and H Ridgway H, ‘Bioactive carbon improves nitrogen fertiliser efficiency and ecological sustainability’, Scientific Reports, 2020, 10(1):3227, doi:10.1038/s41598-020-60024-3; Y Ma, X Cheng and Y Zhang, ‘The Impact of Humic Acid Fertilizers on Crop Yield and Nitrogen Use Efficiency: A Meta-Analysis’, Agronomy, 2024, 14(12):2763, doi:10.3390/agronomy14122763. A common approach is using products that are made from humic substances,¹⁷ Humic substances are organic compounds that are formed in the decomposition of plant, animal, and microbial residues. such as humate granules, with nitrogen fertilisers or humate coated urea products. Some farmers are also using composted manure pellets alongside nitrogen applications. 

Farms aren’t air and water-tight and nitrogen is lost through multiple pathways in soil, water and the air. These losses can be reduced through:   

• Reviewing your irrigation scheduling and paddock drainage to reduce waterlogging (where possible) to minimise the conversion of nitrates into nitrogen gases.
• Reducing soil erosion by maintaining ground cover and consider integrating permanent vegetation on farm (i.e. areas of diverse woody vegetation) to filter and slow overland flow and soil erosion,¹⁸PM Vitousek, N Rosamond, T Crews, MB David, LE Drinkwater, E Holland, PJ Johnes, J Katzenberger, LA Martinelli, PA Matson, G Nziguheba, D Ojima, CA Palm, GP Robertson, PA Sanchez, AR Townsend and FS Zhang, ’Nutrient imbalances in agricultural development.” Science, 2009, 324(5934):1519-1520. doi:10.1126/science.1170261. especially on slopes. 
• Enhancing farm dams, wet areas and wetland areas to actively manage paddock runoff before it enters a local waterway. This also helps reduce greenhouse gas emissions – wetlands support the denitrification of excess nitrogen from runoff water into harmless nitrogen gas rather than nitrous oxide, which is a more potent greenhouse gas.¹⁹JN Galloway, JD Aber, JW Erisman, SP Seitzinger, RW Howarth, EB Cowling and BJ Cosby, ‘The nitrogen cascade’, Bioscience, 2003, 53(4):341-356, doi:10.1641/0006-3568(2003)053[0341:TNC]2.0.CO;2; W Zhang, H Li and SG Pueppke, ‘Direct measurements of dissolved N2 and N2O highlight the strong nitrogen (N) removal potential of riverine wetlands in a headwater stream’, Science of The Total Environment, 2022, 848:157538, doi:10.1016/j.scitotenv.2022.157538. 

Legumes form relationships with microbes in the soil to convert nitrogen from the air into plant-available nitrogen (like ammonium, nitrate and some forms of organic nitrogen). Including legumes in your cropping rotation, multispecies cover crop, or break crops²³In cereal cropping, break crops are non-cereal crops used strategically in crop rotation to reduce the buildup of weeds, diseases and pests that can affect cereal production. is a key strategy for supplying organic nitrogen to a cropping system. Bear in mind that it is possible to have too much of a good thing – for example, repetitive monocultures of legume rotations can also create excess nitrogen and may contribute to soil acidification and loss of soil structure.²⁴ RJ Haynes, ‘Soil acidification induced by leguminous crops’, Grass and Forage Science, 1983, 38(1):1–11, doi:10.1111/j.1365-2494.1983.tb01614.x.  

Biological nitrogen fixation relies on plant structures and enzymes made from a variety of other nutrients. In low input systems that rely on nitrogen via biological fixation, it’s important to ensure these other nutrients are available, specifically molybdenum, iron, phosphorous and nickel. Calcium, boron, copper, cobalt are also important for nitrogen fixation by legumes.²⁵GW O’hara, N Boonkerd and MJ Dilworth, ‘Mineral constraints to nitrogen fixation’, Plant and Soil, 1988, 108(1)93–110, doi.org/10.1007/BF02370104; W Weisany, Y Raei and KH Allahverdipoor, ‘Role of Some of Mineral Nutrients in Biological Nitrogen Fixation’, Bulletin of Environment, Pharmacology and Life Sciences, 2013, 2(4):77-84. This varies depending on soil type and existing nutrient stocks, so it is important to keep monitoring soil over time and to use inputs strategically to remedy deficiencies that could impact nitrogen cycling.²⁶M Reimer, TE Hartmann, M Oelofse, J Magid, EK Bünemann and K Möller, ‘Reliance on biological nitrogen fixation depletes soil phosphorus and potassium reserves’, Nutrient Cycling in Agroecosystems, 2020, 118(3):273-291, doi:10.1007/s10705-020-10101-w.

Image 8. Nodulation on a legume plant at the McKinleys’ farm. Nodules on legume roots are where the plant hosts nitrogen fixing bacteria to biologically fix nitrogen from the air Source: Matt and Belinda McKinley.

In addition to including legumes in crop rotations, the design of overall crop rotations can also support improved nitrogen efficiency and cycling. For example, canola can leave more mineral nitrogen in the soil than a wheat crop, which is then available for the next crop rotation.²⁷MH Ryan, JA Kirkegaard and JF Angus, ‘Brassica crops stimulate soil mineral N accumulation’, Soil Research, 2006, 44(4):367–377, doi:10.1071/sr05143. Pasture phases, especially those with clovers and perennial grasses, can also boost soil nitrogen supply for subsequent crops.²⁸ JF Angus, TP Bolger, JA Kirkegaard and MB Peoples, ’Nitrogen mineralisation in relation to previous crops and pastures’, Soil Research, 2006, 44(4):355–365, doi:10.1071/sr05138.

Biological or recycled sources of nitrogen can replace some or all of your existing synthetic nitrogen inputs, depending on your goals. The amount of nitrogen in biological inputs like composts can be measured with lab tests, though many biological inputs won’t have the same concentration of nitrogen as synthetic alternatives. Other non-synthetic products are being developed such as fertiliser made from microbial biomass, which tends to have a higher nitrogen content than other organically sourced alternative fertilisers. Biological inputs can have the added benefit of providing a longer term source of nutrients and often other soil health and biostimulant effects on crops.

After plants absorb nitrogen, they rely on internal processes and enzymes to move and utilise it to build plant proteins. Many of these processes depend on specific nutrients. Making sure nutrients are available in the soil or, in the case of micronutrients, topping them up with foliar applications can help fine-tune nitrogen efficiency even further. Soil and plant educator, Joel Williams from Integrated Soils, recommends ensuring sufficient levels of molybdenum, sulphur, phosphorous, potassium, manganese, magnesium, iron, boron, zinc and nickel as these are important for a plant’s ability to use nitrogen.