Following phosphorus maps over fields

For farmers, phosphorus is both an important input and a potential pollutant. As an essential plant nutrient, farmers need to ensure that their crops receive sufficient phosphorus, which usually means adding it to their fields in the form of synthetic fertiliser or manure. Add more phosphorus than the plants need, however, and the excess can be washed off the field by rain to pollute nearby bodies of water. To make matter worse, the concentration of phosphorus in a field usually varies, meaning that different amounts should really be added to different parts of the field.

So farmers would benefit from being able to identify just how much phosphorus these different parts need, allowing them to apply enough to meet their plant’s nutrient requirements without causing any pollution. And it turns out that NIR spectroscopy can help them do just that.

This is a bit of a surprise, as phosphorus can’t be directly detected by NIR spectroscopy. However, when Abdul Mouazen and Boyan Kuang, two soil scientists from Cranfield University in the UK, tried building a model relating NIR spectra to phosphorus levels in soil they found it was reasonably accurate. This suggests that phosphorus levels are linked to some other soil property than can be detected by NIR spectroscopy.

Mouazen and Kuang next recorded NIR spectra from a 22 hectare field, by towing a portable vis-NIR spectrometer behind a tractor. This spectrometer is attached to a chisel that digs a small trench into the field, with the spectrometer collecting spectra from soil at the bottom of the trench. Applying the collected spectra to their model, they were able to produce a map showing how the phosphorus levels varied across the field.

Finally, they used this map to determine how much phosphorus they should apply to different regions of the field. As reported in Soil & Tillage Research, when they mapped phosphorus levels in the field the following year, they found they were less variable. They are now planning to repeat this work on other fields with different soil types.