The Phosphorus Paradox – Part 2

Phosphorus has a slightly awkward habit of causing two problems at once.

Crops often struggle to access it, while waterways struggle to get rid of it.

In the previous article I looked at why phosphorus often sits in the soil, yet crops still struggle to access it. Rather than revisiting the chemistry again, the more useful question is what actually improves that access.

In many cases, the answer lies in how efficiently the soil system functions.

When biological activity around the root is strong, nutrients tend to move through the soil–plant system far more effectively. As that activity slows, the phosphorus supply to the crop slows as well.

Early root systems are small, and phosphorus movement through soil is extremely limited. The crop therefore depends heavily on the surrounding biology to continually release small amounts of phosphorus into circulation as the roots absorb it. If that biological engine is slow, the crop can experience a shortage even though the soil may contain plenty.

This is one of the reasons Mega-Fos still remains one of my favourite products.

Rather than simply adding more phosphorus fertiliser, the idea is to help mobilise the phosphorus already present in the soil and return it to circulation so plant roots can actually reach it.

Trials help illustrate the point quite nicely. In winter oilseed rape work carried out by NIABTAG, the use of Mega-Fos increased yield from 4.48 t/ha to 4.88 t/ha and delivered the highest profit margin among the treatments tested.

What is worth noting is that the soil itself did not suddenly gain more phosphorus. The crop simply accessed what was already there more effectively.

In practice, that is often where the margin improvement comes from, not adding more nutrient, but helping the crop reach the nutrients that were already paid for.

Another aspect of this biological activity is often overlooked.

The rhizosphere (the narrow band of soil surrounding plant roots) is also where many soil-borne diseases try to establish themselves. When beneficial microbes colonise that space early, they compete directly with pathogens for both space and resources.

Some produce enzymes or compounds that suppress harmful organisms, while others simply occupy the root zone so thoroughly that opportunistic pathogens struggle to gain a foothold. In practical terms, a diverse microbial population acts almost like a protective buffer around the root system.

This can be particularly useful against common soil-borne pathogens such as Pythium, Rhizoctonia, and certain Fusarium species, all of which can damage emerging roots or slow early establishment. Where beneficial microbes dominate the rhizosphere, these organisms often find it much harder to establish themselves.

When it comes to establishing a new crop, the bacteria in ActiV8-Bio contribute to nutrient cycling, but they also colonise the root zone early and compete with some soil-borne pathogens. In practice, the benefit is often seen in stronger early root development and more resilient crop establishment.

One of the comments on the previous article also raised an interesting point about recycled phosphorus.

Andrew mentioned sewage sludge as a useful phosphorus source, which is entirely fair. Materials such as sewage sludge, compost, and manure can certainly return valuable nutrients to the soil system.

The difficulty is that much of that phosphorus still ends up joining the soil’s wider reserves rather than immediately entering circulation where crops can use it. As Andrew also pointed out, supply-chain restrictions are increasingly limiting where materials such as sewage sludge can be applied depending on the crop being grown.

So recycled phosphorus certainly has a place, but it does not remove the underlying challenge.

The soil may already contain the phosphorus.

The real question is whether the soil system is working well enough for the crop to reach it.

Improving that efficiency carries benefits beyond crop nutrition alone. When crops can access nutrients already present in the soil, reliance on additional fertiliser inputs begins to decline. That improves nutrient efficiency, reduces dependence on mined phosphate, and lowers the risk of phosphorus leaving fields through soil erosion or runoff into waterways.

In other words, a soil system that functions well tends to benefit both the crop and the environment.

The most productive soils are rarely the ones with the highest phosphorus numbers on a laboratory report.

They are the soils where nutrients are able to move.

Typical applications

Mega-Fos

Applied at 10 L/ha (approx. £20/ha)

Typical timings include:

Winter wheat – GS30

Winter barley – GS25

Winter OSR – Green bud / early stem extension

Peas / Beans – Pre-flowering

ActiV8-Bio

Applied at 20 L/ha (approx. £40/ha)

Suitable for most crops – apply at drilling or pre-emergence.

In seasons where fertiliser prices remain high and grain prices are not exactly helping margins, improving nutrient efficiency is becoming just as important as deciding how much fertiliser to apply.

If phosphorus availability is something you are questioning on your own soils, feel free to get in touch. I’m always happy to talk through what we’re seeing on other farms and where approaches such as Mega-Fos or ActiV8-Bio might fit.

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