Getting to the root of the shortage

Phosphorus is needed for crops to grow, so could we breed plants that don't need it? Or use bactieria to unlock more from the soil? Dr John Hammond talked through the possibilities with Graihagh Jackson...

Graihagh - Surely the first plan of action should be to reduce the amount of phosphorus we use in the first place - right?

John - Yes, that's right. Phosphorus is essential for growing our food so can't do away with it completely. We have to have some inputs into the system and, as we've already heard to the second world war, the amount of phosphorus we were using has increased probably more than it should have done but we are on the case now and looking at that. And certainly in the UK over the last 20 or 30 years, the amount of phosphorus going into our fields has roughly halved.

Graihagh - But that's not enough then? I mean what else should we be doing or can we be doing?

John - Well, there are a number of things that we can look at. There are land management issues looking at creating a healthy soil so the phosphorus in there is turned over.  Looking at alternative fertilizers and sources of phosphorus from things like sewage or we can look at breeding crops which are better at capturing the phosphorus in the soil and using it more efficiently.

Graihagh - Because I suppose there are places in the world like Australia, I'm thinking, where it's really poor in phosphorus. So there are plants that grow there so can we just steal some of their tricks of the trade and use them in our crops in the UK.

John - Yes, that's one of the types of things we're looking at doing. So, these plants in Australia live on very little phosphorus and yet they're able to produce large amounts of biomass. They have a number of attributes in their root systems and in their leaves that allow them to capture phosphorus more efficiently by exuding acids and enzymes into the soil to extract the phosphorus. Making very good, close relationships with thing like bacteria and fungi in the soil to help them capture the phosphorus and very extensive root systems that can mine the phosphorus from the soil.

Graihagh - And these bits of research you were mentioning there. Have they been successful? Have we been able to breed them into other crop varieties?

John - There's been a number of attempts to do these things. There's a couple of examples. For example, in China, there's some soya beans that are now available commercially where they've over-expressed in the roots some of these enzymes which are released from the root into the soil, and these enzymes can break down some of the phosphorus locked away in organic compounds. They are more efficient at capturing that phosphorus and producing higher yields for less phosphorus input.

And another example is in beans, where we've got this phenotype or characteristic of the root system where they are able to put a lot of their roots in the topsoil. Because phosphorus is very reactive, it doesn't move very far in the soil, it tends to stay in the top 30cms. So, if you can get a plant to put a lot of it's roots into that area of the soil, it can capture more phosphorus.

Graihagh - Is that not a bit of a risky thing to do though? I'm thinking if there's a drought or anything you want deep roots so, I mean, you can't really put all your eggs in one basket, surely?

John - No, no, that's it.  So that is one of the downsides to that approach is if you put all your roots in the topsoil, you're at risk of maybe not being able to capture maybe water and nitrogen which are further down in the soil profile. So there has to be some aspect of plasticity in the root system that allows the plant so respond to an appropriate stress,.

Graihagh - And you mentioned this sort of inaccessible phosphorus in the soil before and how these root system produce enzymes to get this phosphorus out. I wondered if you could just sort of unpick that a little bit for me?

John - Well, a plant can only take up phosphate which is in the soil solution and the rest of the phosphorus in the soil is either bound to the soil particles through very strong bonds or in organic compounds, so dead plant material or dead microbes. But, in the soil, there are fungi and bacteria which have to scavenge that phosphorus as well from those sources, and they have these enzymes and ability to release acids into the soil to release that phosphorus from it. So plants, historically, are very good at making mycorrhizal symbioses, which help the mycorrhizal fungi catch the phosphorus for the plant and, in exchange, the plant gives the fungi some carbon so the plant can encourage those microbes to grow and help scavenge the phosphorus for them.

Graihagh - I imagine though trying to unpick what microbes are doing that job is next to impossible. There are thousands, if not millions of species of microbes in soil so, how do you go about finding the right one?

John - It is very tricky and up until probably the last five or ten years, it's been nye on impossible because a lot of the microbes in the soil won't grow very well in the lab. So it's very difficult to take them out of the soil and find out what's there.

With the advent of new sequencing technology, we've been able to take the DNA out of the soil and sequence that DNA, and start looking and identifying all the microbes that are there. We are actually part of a new BBSRC funded project where we're looking at, not extracting the DNA, but actually extracting the protein and sequencing the proteins that are in the soil around the roots, so that we can identify the types of enzymes that are active in the phosphorus cycling process, but also the microbes that those enzymes came from in the first place.

Graihagh - So you can sort of match them together and then, hopefully, design some microbial soil magic dust that you can sprinkle on and get your plants to extract phosphorous magically?

John - That's the idea.

Graihagh - I just have a bone to pick with you because, surely, you're kicking the can further down the road for future generations to pick up though because we're still reliant on this phosphorus despite reducing it?

John - Certainly. There's no way we can produce food without that so it's still a question of minimizing the amount we actually use, and using it as efficiently as possible in the farming system. But we do need to look at a whole host of mechanisms to close that phosphorus loop so we're not losing it to the water where it's causing eutrophication, and bring it back onto the farm so we're not washing it all away.