Detecting soil nitrogen supplies by canopy sensing, proof of concept

Abstract

This research tested whether sensing the crop can indicate soil nitrogen supplies.

Nitrogen fertiliser experiments on cereals were established at four sites in 2005-6. In the following year, commercial cereal crops were grown. In 2006-7 at each site, the plot positions as used in the previous year were marked out once again for crop testing using a sensor.

Reflectance was measured monthly, December to March, during tillering using a Crop Circle instrument (provided by Soil Essentials Ltd) in the 2006-7 plots relating to the 2005-6 experiments. This comprised two sensors to measure reflectance at 880 nm (near-infrared, NIR) and 590 nm (orange). A Normalised Difference Vegetation Index (NDVI) was calculated to give a measure of vegetation cover. Soil mineral N data were obtained from another study at the same sites. Yields of grain and straw and their N concentrations were measured to give total N uptake. Data were interpreted for relationships between canopy reflectance and soil N.

Establishment and tillering were early and rapid at Boxworth and Rosemaund, but later and slower at Terrington and High Mowthorpe. The highest level of N applied in 2006 had a small effect on soil mineral N at Terrington and Rosemaund, but a large effect at Boxworth. Using total N uptake at harvest as an estimate of soil N supply, the largest response to N supply was also at Boxworth.

Over-fertilisation only led to high SMN residues at Boxworth, which had the lowest optimum N requirement in 2005-6 : 117 kg/ha, compared to, 220 to 237 kg/ha at other sites). Use of the sensor successfully detected the high SMN residue at Boxworth using the relationship between NDVI and soil N supply, especially at soil mineral N values below 150 kg/ha. This relationship was less variable and more useful (shallower slope) when N uptake (rather than soil mineral N) was used to estimate soil N supply. The relationships improved with later assessment of NDVI. Change in NDVI between assessment dates showed that canopies sometimes declined and sometimes grew during winter. However, change in NDVI was less useful for predicting soil N supply than absolute values of NDVI.

In conclusion at Boxworth, but not at three other sites, NDVI differentiated plots in need of fertiliser (soil mineral N up to around 150 kg/ha N) from those with ample soil N supplies.