The significance of nitrogen to agricultural science is a recurring theme for the Legume Laboratory, having recently been the focus of articles about the possibility of artificial nitrogen fixation and estimating nitrogen mineralisation rates from legume crops.
A review piece was recently published in the journal Agronomy about nitrogen use in cereal crops, focusing on our current knowledge of how to increase efficiency in its uptake and identifying areas where developing technology may result in further efficiency improvements.
Wheat crops (and plants generally) require nitrogen to:
- establish and maintain photosynthetic capacity and activity;
- maximise the number and size of seeds; and
- increase the quality of crop products.
Nitrogen is highly mobile in soil and the efficiency of plant uptake depends on many variables including the type of crop and environmental conditions such as soil type and rainfall. For example, rice is known to have the lowest rate of uptake amongst cereal crops while barley has the highest.
The literature review doesn’t delve into the research behind many of its statements but it is a great starting point for some basic knowledge and further research into different sources and uses of nitrogen fertilisers, different application methods and current and developing technologies to diagnose nitrogen status in crops.
Mind-map figure. Left side – inorganic nitrogen fertiliser sources split based on nitrogen content. Right side – current and potential technologies for nitrogen sources and in-crop monitoring. Figure reproduced from article.
The article describes the pros and cons of different inorganic nitrogen sources besides their nitrogen content. For example:
- Anhydrous ammonia, which contains the highest nitrogen content of inorganic fertilisers, is a gas requiring pressurised storage, specialised equipment for storage, handling and application and can lower soil pH undesirably;
- Aqua ammonia has high ammonia content but is volatile at temperatures above 10ºC and needs to be injected into soil;
- Ammonium nitrate combines two different forms of nitrate, reportedly improves baking quality of wheat, but has low nitrogen content compared to other sources.
- Ammonium sulfate contains both nitrogen (but in a low amount compared to other sources) and sulfate and is useful for acid-requiring crops and high pH soils;
- Ammonium chloride contains a low concentration of nitrogen but one that is suitable for chloride responsive crops such as cereals and coconut but not for crops that cannot tolerate the chloride. It also lowers soil pH.
- Urea, the most widely used nitrogen fertiliser source due to ease of manufacture, transport, and low cost, is also volatile, phytotoxic to susceptible crops and can be toxic to germinating seedlings when emerging.
The article discusses the advances in matching as closely as possible in time the release of nitrogen from fertiliser to when the plant requires it. A method such as urease inhibition controls nitrogen release and has been found in studies to increase potato tuber yield and nitrogen use efficiency while also decreasing nitrification. However, as with many fertilisers, the positive effects are affected by the status of the soil prior to treatment, it is difficult to predict the time-frame and release rate of nitrogen, and yield increase doesn’t consistently match the use of nitrogen inhibitors.
The use of microogranisms is a potential nitrogen source which could increase crop yields. Plant growth-promoting bacteria have been shown to promote growth in row or horticultural crops, effecting the production of growth regulators that stimulate the plant uptake of nutrients.
New sources of nitrogen such as nanofertilisers have the potential to deliver more nitrogen from lower dose amounts but require more research on possible adverse effects. Coating seeds in nutrients appears to have some potential to improve nitrogen uptake, and recapturing nitrogen lost from agricultural fields or after food consumption could be the basis of new fertiliser production.
Nitrogen fertiliser application and management
Different fertiliser sources, different crops and different application times require different methods of application. The tables below set out the different nitrogen sources and applications methods for different application times to match nitrogen availability with crop demand (top) and the effects of management practices on nitrogen nutrition.
Tables sourced from article.
Diagnosing crop nitrogen needs
Matching nitrogen supply to crop requirements is an area where significant improvements in efficiency can be found. Diagnosing when crops require nitrogen application during growth is limited to a few methods such as sap nitrate tests (high accuracy but labour intensive and destructive to the plant) and optical sensors (non-destructive and reliable but cannot detect over-fertilisation). The development of satellite and drone technology has opened up the possibility of remote sensing nitrogen status. Developing accurate methods of optically determining whether, and how much, nitrogen is needed by a particular crop is both a matter of technology development and increased knowledge of nitrogen uptake pathways and efficiencies.
Crop nitrogen assessment techniques. Figure from article.
The article provides some useful knowledge that can be used now to help match nitrogen source, application method and application time. But more importantly it shows where the limits of our knowledge are and what technological improvements could assist nitrogen supply and crop production and its a pretty safe bet that the authors have written the review as a springboard for further research.
Incorporating suitable fertilisers for particular crops, soil types or environmental conditions at suitable times will improve crop production, particularly throughout the developing world. Research into the matrix of factors that effect nitrogen uptake in crops to elucidate the conditions that enhance efficiency would build on our current knowledge, and the ability to deliver nitrogen with maximum efficiency at times predicated on real-time in-crop knowledge of nitrogen content will significantly increase food production, reduce the costs and side-effects of fertiliser production and mitigate the environmental effects caused by the nitrification of over-supplied fertiliser.