Measuring Nitrogen in the Field… From Soil to Seeds.

Measuring Nitrogen in the Field… From Soil to Seeds.

Take the guess work out of your nitrogen input decisions with the Cropscan. With nitrogen significantly influencing both yield and protein in a cropping system, the Cropscan shows you exactly where its needed most. Find fertiliser efficiency, reduce paddock variability, stream line your harvest and increase your bottom line. Keep reading for more information. 


Plants and animals are programmed to reproduce themselves. For plants, reproduction means that seeds are produced and then released back to the soil to propagate the species. Nitrogen is required by plants at all stages of the plant’s growth cycle in order for the production of seeds. Built into plant’s chemistry are checks and balances that the plant uses to ensure that seeds are produced and released. If there is sufficient water and Nitrogen as well as other micro nutrients then the plant will produce seeds to its full yield potential. In cereal crops, i.e., wheat and barley, if there is excess Nitrogen available at the Filling stage, then the plant will store the Nitrogen as protein. In cereal crops the yield potential is controlled by availability and uptake of Nitrogen throughout the plant’s growth cycle. If the Nitrogen is not sufficient in the soil or is not readily available then the plant will reduce the yield, i.e., the number of seeds.

Measuring Nitrogen in soil, the plant and the seeds are very difficult tasks. There are three opportunities to measure Nitrogen in the field, i.e., before sowing, during the growth stages and at the time of harvesting the seeds. This paper looks at the methods for measuring Nitrogen and how these measurements relate to the plant’s growth.


Fig 1: Nitrogen Cycle

What is Nitrogen?
Chemically speaking, Nitrogen is a gas that makes up 78% of the earth’s atmosphere. It is inert as a gas and plays only a minor role in transpiration in plants and nothing in respiration in animals. In soil, Nitrogen is found in the form of ions, Nitrates (NO3-), Nitrites (NO2-) and Ammonium (NH3+) or as organic matter including amines and amino acids. Ammonia is used by bacteria in the soil to produce Nitrite ions under the process called Nitrification. Nitrite ions are oxidised to form Nitrate ions which are water soluble and are easily transported through the soil both to the plant’s roots or leached out of the root zones by rainwater. Nitrate ions are absorbed by the roots and transported throughout the plant where they are used in Photosynthesis to produce sugars and amino acids. Sugars are used by the plant’s cells as energy and to make cellulose which is the main structural component of all plants, i.e. stems and leaves. The amino acids are used to produce proteins and nucleotides such as DNA and RNA.

N Fig2Fig 2: Amino Acids contain Nitrogen

Nitrogen is a key element in the formation of amino acids which make up proteins and DNA in plant cells. Nitrogen makes up approximately 17% of proteins by weight. Grains of wheat and barley are made up of between 6 and 18% protein. As such, every tonne of wheat stripped from the field removes between 10-32kg of Nitrogen from the soil.

N Fig3Fig 3: Measuring Soil Nitrogen during the Plant Growth Cycle

Comparing Nitrogen Measurements
Figure 3 shows a typical plant growth cycle from Emergence through to Filling. The Nitrogen measurement systems used at each stage of the growth cycle are shown in the schematic, i.e., Soil Testing, NDVI Imagery and On Combine NIR Analysis.

Soil Testing
Planted seeds have sufficient stored nutrients within the endosperm to initiate sprouting. Once the plant emerges from the soil and begins to grow, the Nitrogen availability is very important for development of Tillers. If the plant’s roots can access water and Nitrogen then the plant will develop a full complement of Tillers, i.e., 6-8 for wheat and barley. It there is insufficient Nitrogen available then the plant may abort several of the Tillers so that it can increase the likelihood of the plant producing seeds. The Yield Potential is set at this stage of the plant’s growth cycle by the number of Tillers that go on to produce stems and hence heads. If the number of Tillers has been reduced, then the Yield Potential cannot be recovered later in the plant’s growth cycle by the addition of more nutrients. As such it is critical that there be enough Nitrogen available and accessible by the roots at the Emergence and Tillering stage.

Water is extremely important throughout the entire growth cycle of a plant; however, the timing of rain events can have a big impact of the availability of Nitrogen during the early and later stages of growth. Since Nitrate ions are water soluble and therefore mobile, a rain event immediately after application of Nitrogen fertilizer may leach the Nitrogen through the soil and beyond the roots of the emerging seed. This could result in insufficient Nitrogen being available to the plant and hence limiting the number of tillers produced by the plant.

Soil testing has been a powerful tool for farmers and their agronomists for more than 40 years. Elemental Soil testing provides all the key nutrient concentrations in the soil including, Nitrogen, Sulphur, Potassium, Phosphorus, Calcium, Magnesium, Aluminium, Copper etc. However, the costs of sampling and analysis are high, and most farmers only get soil samples from the extremities and the middle the field or at best a 50-meter grid. Measuring Nitrogen in soil is mostly used to assess the availability of Nitrogen prior to sowing. Based on the soil Nitrogen tests, agronomists recommend the rate of Nitrogen fertilizer application. Variable Rate Nitrogen Fertilization can also be used to apply Nitrogen into zones where the Nitrogen levels are higher or lower than the agronomists recommend.

N Fig4Fig 4: EM-38 Soil Scan

Soil scans such as Electromagnetic Induction/Electrical Conductivity and Gamma Radiation provide maps of the elemental composition of the soil, soil texture, soil boundaries, soil types, water content and the soil depth. EM-38 (Electromagnetic Induction) scanners measure the ionic strength of the minerals in the soil however this technology does not measure Nitrogen selectively. All mineral salts in the soil contribute to the electrical conductivity in the soil and the EM-38 maps provide a high-density image of the relative distribution of elements across the field. Figure 4 shows an EM-38 map of a field in South Australia.

Gamma Radiation sensors measure the natural gamma ray radiation from elements in the soil. This technology is used to develop zones across the field that may reflect the soil texture, compaction, moisture saturation and micro nutrient content. The zone maps are then used to collect soil samples in areas where the greatest variation in soil parameters exists. The farmer and their agronomist can then make decision on fertilization strategies based on the soil sample testing.

NDVI using Satellite Imagery, Drone or Tractor Mounted N Sensors
70% of the Nitrogen required by a plant during the growth cycle is consumed during the Stem Elongation, Leaf Development and Head Development stages. The Nitrogen needs to be accessible to the roots. If the Nitrogen has leached down to the bed rock, then the roots may not be able to access sufficient Nitrogen which is required by the leaves for Photosynthesis to grow the plant.

N Fig5Figure 5. An example of a satellite image of a field in North America.

NDVI measurements from satellite imagery, drones or tractor mounted N sensors detects green, yellow and brown colours of the crop canopy. A healthy crop will exhibit a green canopy. Yellowing leaves can indicate that the plants do not have sufficient nutrients to maximize growth. Nitrogen is a nutrient in plant growth, but other parameters can also show similar symptoms in the leaf canopy including water stress, disease and other nutrient deficiencies. Imagery can help farmer to identify zones where there are problems within the crop and thereby take some corrective action. For example, crop dusting may be useful to apply more fertilizer, insecticide or fungicide. The limitation of imagery is that it does not measure Nitrogen or other micro nutrients directly but detects the effects of nutrient deficiencies. As well the data from imagery is a relative measure and can only be quantified by ground truthing the images.

On Combine NIR Analysis
Near Infrared Spectroscopy actually measures Nitrogen in seeds as Protein. An On Combine NIR Analyser measures the Protein in the seeds by trapping the sample in a Sample Head mounted to the clean grain elevator. Data is collected every 5-10 seconds, 1 sample reading very 15-17meters down the row or around 15 data points per hectare. This is much lower than satellite imagery, drones or crop scanners, however it is much higher density than soil sampling. The Protein, Yield and GPS data collected off the combine are used to generate maps in real time and post-harvest. Real-time Protein maps are extremely useful for farmers in order to blend and segregate grain in the field. Figure 6 shows an example of a real time Protein map generated on the PC screen inside the combine’s cabin.

N Fig6

Figure 6. Real Time Protein Maps generated as the grains are harvested.

Post-harvest the Protein, Moisture and Yield data can be combined to generate a range of maps including:
• Nitrogen and Sulphur Removal
• Protein/Yield Correlation
• Nitrogen Use Efficiency
• Water Use Efficiency

The Nitrogen and Sulphur removal maps based on both Protein and Yield provide a complete picture of the availability and uptake of these nutrients during the plant’s growth cycle. The Protein/Yield Correlation Quadrant maps provide a powerful diagnostic tool that allows the farmer and their agronomist to access the crop’s performance over the growing cycle.

N Fig7Figure 7. Protein/Yield Correlation Map

N Fig8Figure 8. 4 Performance Zones

Figure 7 shows a Protein/Yield Correlation Quadrant map for a wheat field in central New South Wales. Four Performance Zones, figure 8, can be identified using this map.

• High Yield/High Protein – Blue
• High Yield/Low Protein – Green
• Low Yield/High Protein – Red
• Low Yield/Low Protein – Yellow

These Performance Zones can be used to create Variable Rate Fertilization prescriptions that take into account the amount of Nitrogen that has been removed from the soil by the plant. At least this amount must be returned to the soil, however a better strategy is to apply more Nitrogen into the zones identified as Low Yield/Low Protein (Red) and High Yield/Low Protein (Yellow). These are the zones that will respond best to more Nitrogen fertilizer. The fertilizer rate in the High Protein/High Yield (Green) zones should not be increased since these zones performed very well. The Low Yield/High Protein (Blue) zones are the most difficult to diagnose and may reflect problems due to moisture, soil or other nutrient deficiencies.

Studies on Nitrogen fertilization from around the world recognise that Protein and Yield in cereal crops are related. Brill et al, showed that the Yield and Protein increases with the application of additional Nitrogen fertilizer of 30, 60, 90 kg/ha. However above 90kg/ha the Yield flattened out and only the Protein continued to increase.

N Fig9

Fig 9: Grain yield (t/ha) and protein concentration (%) from 10 wheat varieties with 0, 30, 60, 90 and 120 kg/ha applied nitrogen in a trial at Parkes in 2011.Brill et al, 2012.

Figure 9 shows the plot of Yield, Protein and Nitrogen fertilizer rate. The explanation is perfectly consistent with the understanding of how plants take up and use Nitrogen throughout the growth phases. Once the plant has reached its Full Yield Potential, i.e., a complete set of heads filled with grain, then any excess Nitrogen is stored inside the seeds in the form of Protein. If the crop has not had sufficient water, Nitrogen or other nutrients, then it will not have reached it Full Yield Potential. Late application of Nitrogen fertilizer as a result of collecting NDVI data, can drive the Yield, however if the plants have been forced to abort tillers or reduce the number of seeds per head, then the Yield cannot be recaptured by applying more Nitrogen.

It is therefore critical to understand where and how much Nitrogen has been used by the plants across the field. The Protein/Yield Correlation Quadrant maps provide his understanding. The Green zones are the Sweet Spots where the Full Yield Potential and optimum Protein were achieved. For the next crop, the farmer can either maintain the existing rate of Nitrogen application in the Green Zones or when the Protein is higher than what the market requires, then it may be possible to reduce the rate of Nitrogen fertilization. The target value for Protein should be approximately 11.5% according to research conducted by Brill et al and support by research in the US, UK and Canada.

The Yellow and Red zones are where the Nitrogen rate was too low, and the net results are lost Yield and low Protein. Applying more Nitrogen in these two zones in the next harvest will produce a Positive Yield Response.

The Blue zones are where there was sufficient Nitrogen to produce high Protein content, but some other factor still limited the Yield. It could be Moisture, pH or other minerals. Or it could have been a combination of soil type and rain events that leached out the Nitrogen from the root zones early in the growth cycle thus causing Tillers to be aborted and Heads to be reduced. The Blue zones may require investigation by soil scientists. Maybe deep N soil analysis, soil breakup using lime or other remediation processes are required.

If the purpose of measuring Nitrogen is to optimize the yield and better manage fertilizer inputs, then the best tool available today for the farmer is the Protein/Yield Correlation Quadrant map. Protein data provides the missing piece of information that when combined with Yield data, provides the understanding of where in the field you can get a Positive Yield Response to application of additional Nitrogen fertilizer. The only sensor that measures actual Nitrogen Availability and Uptake is an On Combine NIR analyser.

A food manufacturing plant measures the incoming raw materials, i.e., inputs, the materials in process and then the final product, i.e., outputs. Grain growers have been measuring the inputs for 40 years in the form of soil testing. Over the last 20 years growers have had access to satellite imagery that helps them to monitor the crops as they are growing. Now there is technology that allows farmers to measure the outputs, i.e., the seeds.

Michael Eyres, Field Systems Australia, commented. “The Yield map correlates directly to soil performance and the Protein map is a very good proxy for plant performance. The Nitrogen data is what makes everything else fit together, i.e., productivity and performance. The On Combine Grain Analyser is a tool of exceptional value whose true value is only just starting to be well enough understood.

This article was republished with permission from Next Instruments, Sydney, Australia. For more information, visit or contact AGree precision agronomist on 03 5382 9546 or