Phosphorus

Dr Ross Monaghan from AgResearch at Invermay in Otago explains the importance of phosphorus and then talks us through phosphorus cycle.

Jargon alert

Ross mentions that phosphorous is adsorbed to soil particles. Adsorption refers to atoms or molecules adhering (or sticking) to the surface of something. It is different to absorption, where atoms or molecules pass through or enter a material.

Transcript

DR ROSS MONAGHAN

Phosphorous is another nutrient that is critically important for legumes and the clover plants that are fixing nitrogen, but all plants need phosphorous as they do nitrogen. Legumes need a higher level of P fertility than the neighbouring grass plants. So it’s really driving that biological N fixation process that gives us nitrogen otherwise for free.

The phosphorous cycle is this process of inputs to the cycle from fertilisers that are ultimately derived from rock – geological formations. Those fertilisers provide dissolved phosphorous that the plants will uptake and convert to an organic form as the plant grows. That plant organic phosphorous is then ingested by the animals, often mineralised as part of that process, then they excrete it in dung, and will then be exposed to a number of competing processes – could be taken up by the plants again or it could be retained by the soil, either gently adsorbed to soil particles, which act as a bit of a reservoir for future desorption and reuse by the plants, or it could be a more strongly adsorbed by minerals and effectively made unavailable for the next short to medium term.

Or that phosphorous could be actually lost to the environment, and the main way that occurs really is via losses in surface run-off. So that is water running across the surface of the soil either mobilising phosphorous that’s attached to soil particles, soil sediments, or just removing the dissolved phosphorous in the surface layer. And what we find is that the surface layers of soil are very enriched in phosphorous simply because that’s where all the fertilisers land, where a lot of the root material is.

Acknowledgements:
Dr Ross Monaghan, AgResearch, Invermay
Ravensdown
Tamara Douglas