Potassium decides whether nitrogen works

“You don’t always notice potassium when it’s right, but you see it very quickly when it isn’t… usually about two weeks after you thought everything looked perfect.”

Part 1: Potassium, the nutrient everyone measures but few really understand.

Potassium doesn’t get talked about in the same way as nitrogen, but it has just as much influence on how a crop actually performs.

You can push a crop with nitrogen and get a big canopy, plenty of colour, and everything looking right early on. But potassium decides whether that growth turns into yield, quality, and a crop that actually finishes.

Nitrogen drives growth. Potassium keeps that growth working.

Potassium (K) doesn’t build the plant in the same way nitrogen does. Instead, it moves through it, regulating processes rather than forming structure. It controls water movement, drives sugar transport, and keeps the plant functioning as conditions change. That’s why it’s sometimes described as the “spark plug” of the plant, and once you start looking at it that way, a lot of what you see in the field begins to make more sense.

Or at least explains a few of those “that looked a good crop in April” conversations.

At its simplest level, potassium is about moving sugars. That is what fills grain, sizes fruit, and ultimately drives yield. You can grow plenty of biomass with nitrogen, but if sugars are not being moved properly, you do not convert that into output. You just end up with a lot of crop… doing a very good impression of yield.

This is also where the link with nitrogen becomes clear. A crop short of potassium cannot use nitrogen properly. You see crops that look strong early on, then lose momentum. They go soft, disease creeps in, and they fail to finish as expected. They look like they should yield, right up until they don’t.

Potassium keeps nitrogen moving through the plant and helps convert it into protein rather than excess leaf tissue. It balances growth so the crop continues to build rather than simply expand. When potassium is right, nitrogen behaves as it should. When it is not, nitrogen becomes more expensive than it needs to be.

Where things start to shift is when you look at soil potassium levels.

Most soils are not actually short of potassium. In fact, many contain large reserves, sometimes 20 to 40 tonnes per acre in the topsoil.

Which sounds great, until you realise most of it is about as accessible as money in someone else’s bank account.

The problem is that almost all of it is locked up, with only a very small fraction, often less than 2%, available to the plant at any one time.

So the real question is not how much potassium is there, but how much is available, and how consistently that supply is being delivered.

This is why soil tests do not always tell the full story. You can have a soil test that looks fine for potassium and still have a crop that behaves as if it is short. Soil tests measure quantity, not accessibility. Or to put it another way, they tell you what’s there, not what the crop can actually get hold of on a Tuesday afternoon in June. Potassium exists in different pools within the soil. Some is immediately available, some is held on clay particles, and some is locked into mineral structure and only released slowly over time.

Clay soils in particular can hold large reserves, but that does not mean the plant can access them easily. So you end up chasing numbers rather than fixing the function.

This is where things get more interesting. There is evidence showing that applying potassium, particularly in chloride form, can actually reduce availability over time by tightening clay structures and fixing potassium into less available forms. So you can be applying potassium, increasing soil levels, and still not improving plant uptake.

That does not mean fertiliser does not work. It means it does not always work in the way people expect.

It also highlights that not all potassium sources are equal. Most conventional programmes rely on muriate of potash, KCl. It is cheap and widely used, and in some situations it does a job, but it comes with trade-offs.

It’s cheap for a reason.

And like most cheap things, it usually costs you somewhere else.

It has a high salt index, which can stress roots, and the chloride component can negatively affect soil biology.

That is why many growers move towards sulphate forms, which are generally more stable and kinder to the system. But even then, the focus is still largely on supply, and that brings you back to the same question: Is the plant actually accessing it?

The more you look at potassium, the more it becomes clear that it is not just about what you apply, but how the system is functioning.

Compaction reduces availability. Cold soils reduce availability. Dry soils reduce availability. Poor rooting reduces availability. Low biological activity … you get the idea.

The list is long, and none of them is fixed by another tonne of fertiliser. All of these factors limit access to potassium, regardless of the amount present.

That is why K problems often show up under stress. The supply is there, but the system cannot deliver it.

This is where I think the approach starts to change. If you focus purely on applying potassium, you are always chasing. And chasing nutrients is a tiring way to farm. If you focus on availability, things start to become more consistent.

Why not start using approaches that support both supply and availability at the same time? That might mean choosing better potassium sources, but more importantly, it means supporting the biology and the rhizosphere so more of what is already there becomes usable.

That is where products that combine potassium with biology and carbon start to make sense, not as a replacement for everything else, but as a way of making the whole system work better.

So what have we learned?

Potassium is not just another nutrient to tick off. It is one of the main reasons nitrogen either delivers or disappoints. And in many cases, the issue is not how much potassium is in the soil, but whether the plant can access it when it needs it.

If you can improve that, everything else starts to work better.

If this is something you are looking at on farm this season, it is worth having a conversation around how potassium is being supplied and, just as importantly, how available it actually is. There are options that can support both sides of that equation.

Contact us info@soilfertilityservices.co.uk

Steve Holloway

Technical Manager.

Next time in Part 2, I will look at how to improve potassium availability in practice, how biology fits into that, and how to time potassium properly through the season so it delivers when it matters.

Bio-K

Vitaplex Foli-K