NPK: What’s Missing, and Why?

John

13 hours ago

Understanding phosphorus, seasonal nutrition and the competition between new seedlings and established turf.

Walk into almost any garden centre and you’ll have shelves of fertilisers all promising better roots, greener leaves, quicker establishment and healthier plants. Each one has its own blend of NPK (nitrogen, phosphorus and potassium) and that’s usually quite prominently displayed on the packet.

It’s so easy to come to the conclusion that as grass needs all three nutrients we must be supplying all three. But there’s a very important difference between what a grass plant needs and what we need to put into the soil. That distinction is at the heart of the Bowls Central approach to fertilising bowling greens. Our aim is not just to get grass to grow. It’s to get the right grasses to grow in the right proportions at the right time and to develop a healthy, biologically active soil which will increasingly be able to support them. Sometimes the decision not to apply a particular nutrient is just as important as the decision to apply one.

The soil isn’t an empty container

One of the most pervasive myths in turf management is that grass is completely dependent on the fertiliser we apply for its nutrition. In reality a healthy rootzone is a very complex reservoir of nutrients, organic matter, microbes and various chemical compounds all of which are in a state of constant interaction. Some nutrients are readily available to the plant. Others are held temporarily by the soil minerals, some are incorporated into the organic matter and some are tied up in compounds which the roots cannot readily take up. But the nutrients aren’t necessarily lost. Through biological and chemical processes some of that reserve can be made available again. The roots take up the nutrients, the microbes immobilise and release them, the organic residues break down and the whole system is in a continual cycle.

Of course a bowling green isn’t a closed system at all. Nutrients get taken out in the grass cuttings, some are lost through leaching and so on and the soil reserves are not infinite. It makes absolutely no sense to apply large amounts of nutrients that the soil already has in sufficient quantity. This is why soil analysis is such an integral part of the Performance Greens Programme. We’re not just looking for things to fix. We’re looking to understand how the soil works and how best to maintain the grass in it.

Three essential nutrients, three different behaviours

Nitrogen, phosphorus and potassium are all vital for healthy grass growth but they work in the soil in very different ways.

Nitrogen (N) is vital for proteins, chlorophyll, enzymes and growth. Grass generally needs a fairly large annual supply and N availability can change quite rapidly through plant uptake, microbial activity, mineralisation, leaching and so on.

This makes nitrogen the obvious choice for regular, controlled feeding throughout the growing season.

Potassium (K) is involved in water regulation, enzyme activation and a whole host of other processes relating to plant health and stress tolerance. It can be lost from sandy rootzones quite easily, though the degree of retention will obviously be dependent on the soil texture, mineralogy and cation exchange capacity.

Maintaining adequate potassium availability throughout the year remains a key element of our programme, especially as we move into periods of environmental stress.

Phosphorus (P) is a different story.

It is absolutely fundamental to energy transfer, to nucleic acids, to cell membranes and to the formation of new plant tissue, roots in particular. Grass cannot survive without it.

Phosphorus is generally far less mobile in soil than nitrate nitrogen and its availability is very much a function of soil chemistry and soil biology. It can, on the other hand, build up over many years of fertiliser application. Significant reserves may already be present in the rootzone of an established bowling green even if only a proportion of that total is available to the grass roots at any one time. This goes some way to explaining the phenomenon we see so often in soil analysis where an established green will require a regular application of both nitrogen and potassium but will in fact require no further phosphorus.

What about phosphorus getting ‘locked up’?

There’s another fairly common Phosphorus assumption worth looking at. If P can become unavailable through chemical reactions in the soil then surely it makes sense to keep on adding more to ensure the grass gets enough? Not necessarily.

Phosphorus can react with iron and aluminium compounds (especially under acidic soil conditions) and with calcium compounds (under alkaline conditions). These reactions can reduce the concentration of available phosphate in the soil solution. Phosphorus availability isn’t an on/off thing. There are all sorts of different forms and degrees of availability and quite a bit of the P that’s in the soil can be made available again via chemical and biological processes. Plant roots and soil microbes have an effect on this cycling.

Mycorrhizae are of particular interest in that their hyphae can go into the soil well beyond the reach of the plant root. Some bacteria and fungi are also capable of releasing organic acids and enzymes which will help to mobilise the phosphorus from less available forms.

However, soil biology can’t manufacture phosphorus out of nothing, and it won’t automatically make every fixed reserve available. It simply means that nutrient availability is a function of more than just the amount of fertiliser you apply. A healthy root system, appropriate pH, good soil structure, adequate moisture and active biologicals all contribute to efficient nutrient uptake.

Adding more P to a soil which is already adequately supplied may simply result in an increase in the amount being held or fixed, with no beneficial effect on turf performance. That’s why we look at the plant available P in the soil analysis rather than simply assuming that if you’re putting in a low level of fertiliser annually you’re going to end up with a P deficiency. It’s also why we look at phosphorus in conjunction with the soil pH, the iron content, the level of organic matter and the overall physical condition of the soil.

Phosphorus and annual meadow grass: an old greenkeeping observation

There’s another very important reason for our cautious approach to phosphorus and that’s to do with the grasses we want growing on our bowling greens. For years greenkeepers have been talking about the relationship between phosphorus fertilisation and the spread of annual meadow grass (Poa annua).

The observation was that liberal applications of phosphorus appeared to promote annual meadow grass, both in new turf establishment and in the renovation of existing turf. For a long time the bulk of the discussion was based on practical experience and observations of how the various grasses behaved in response to management. More recent research has provided some useful evidence.

In 2022 researchers looked at annual meadow grass establishment in conjunction with creeping bentgrass and perennial ryegrass and found that P applications increased annual meadow grass establishment in low P sand based growing media. Creeping bentgrass did quite well with fairly low applications of P. You could go higher with P and it would improve the establishment of the annual meadow grass without necessarily resulting in any increase in bentgrass cover.

Another study (published in Crop Science in 2024) looked at the competitive interaction between annual meadow grass and creeping bentgrass under a range of P and pH conditions. The results indicated that under moderately acidic conditions relatively low P availability would favour creeping bentgrass over the annual meadow grass.

There was, however, an important qualification. Withholding P entirely reduced annual meadow grass growth and also resulted in very poor turf cover and quality overall. The best competitive result in that experiment was achieved with a very small application of P rather than complete omission. The aim isn’t to starve the annual meadow grass by starving out everything else. It’s to have sufficient phosphorus to maintain a healthy perennial turf without in any way encouraging a competitor that will respond very strongly to extra phosphorus.

These were experiments under very controlled conditions (mainly with creeping bentgrass) and they don’t establish a universal phosphorus threshold for every bowling green or for every bent/fescue mix. Nevertheless they do provide a sound scientific basis for a principle which experienced greenkeepers have been talking about for years. More phosphorus isn’t necessarily better and it can affect which grasses end up being the most competitive.

So why not use a high-phosphorus starter fertiliser in the autumn?

This brings us to one of the more interesting facets of our commonly applied autumn renovation programme. Traditionally grass seed and starter fertiliser have been used together. The idea is that the newly germinating grass needs the phosphorus to develop its root system so a high phosphorus fertiliser at the time of sowing will improve establishment. There’s a very good reason for that advice when soil phosphorus is actually deficient.

But overseeding of the bowling green is a very different proposition to establishing grass on bare, nutrient poor ground. We’re introducing new seedlings into an existing community of mature, highly competitive grass. And those plants already have an enormous head start.

Every seed comes with a packed lunch

A grass seed contains the reserves of carbohydrates, proteins, oils and mineral nutrients which will support germination and the initial stages of seedling development. When the moisture, temperature and so on are right the seed starts to draw on those reserves. The emerging root and shoot are initially supported by the resources already stored in the seed.

As photosynthesis gets going and the new root system establishes itself the seedling becomes increasingly reliant on light, water and nutrients drawn from the environment. Phosphorus is important during this transition, but that doesn’t necessarily mean that extra phosphorus fertiliser is required. If the soil already has sufficient available P the developing seedling will be able to take it from the rootzone.

And that brings us to what I think is the main problem with overseeding an established bowling green.

The problem isn’t just germination. It’s competition.

Think about the situation of a newly germinated bent or fescue seedling. It has a very small root system, very little leaf area and a poor capacity to take up light, water and nutrients. Surrounding it are the more mature grass plants with established crowns, extensive roots and a far greater capacity to capture those same resources. If we don’t intervene, the existing mature turf will have a massive advantage.

This is why we go to such lengths with intensive autumn scarification. The aim is not merely to strip out the dead stuff and get the surface looking completely renewed. We’re trying to alter the competitive balance between the existing turf and the young plants we’re putting in. By scarifying fairly intensively, we reduce the existing grass canopy and temporarily weaken the established plants; we create light, space, and opportunities for the new seedlings to get going. It’s a temporary, but important opening.

The mature turf will recover and as it does the competition will increase once again. Our job is to give the young seedlings enough time to get established prior to that. And that’s precisely why the fertiliser programme following renovation merits careful consideration.

Autumn feeding: support new seedling establishment without giving the mature grass too much of a boost.

Nitrogen is the classic example of how fertiliser timing can affect competition. Apply a good quantity of readily available nitrogen straight after scarification and the mature turf will respond quite vigorously. The existing plants have established roots, crowns and growing points so are in a position to exploit the favourable conditions and readily available nutrients.

Rapid recovery may look impressive but could just as easily be the canopy closing before the seedlings have had a chance to develop properly. That’s absolutely not the outcome we’re after. Our preference for the controlled and slow release autumn nutrition is a reflection of that concern. We need sufficient nitrogen to support healthy growth and recovery (including the development of the new seedlings) without necessarily forcing a rapid flush of the existing turf.

Potassium is also kept at appropriate levels to support normal plant function and to enable the plant to get ready for the more difficult conditions that lie ahead.

But where the soil analysis shows P is already adequate there’s very little basis for applying more P simply because you’ve sown new seed.

The seedlings do need access to adequate phosphorus though, they don’t necessarily need a fresh application from a fertiliser bag. Additional phosphate could also be of benefit to the annual meadow grass seedlings establishing on that same disturbed surface.

In other words we’re trying to avoid turning the renovation into an opportunity for the very species we’re trying to reduce.

What happens when the seed’s reserves are exhausted?

The autumn grass seedlings cannot live off the reserves in their seeds forever. As those reserves are exhausted, successful seedlings become established, photosynthesising plants which get their water and their nutrients via their developing root systems. By the time winter comes the majority of the autumn sown seedlings that have established will have long since moved on from being dependent on their seed reserves. But they are still young plants.

Their root systems are typically less extensive, their crowns less developed and they may have fewer tillers and less capacity to tolerate stress than the adjacent mature turf. Autumn nutrition is all about aiding that transition and subsequent establishment, with the attendant competition from the existing vegetation.

The slow release characteristics of the nitrogen fertilisers we’ve selected will help to moderate the supply of nitrogen over that period. Slow release fertiliser isn’t a magic bullet which feeds the seedlings and doesn’t feed the established turf. Both can utilise the nutrients that are released. The advantage we’re after is a more steady supply of nutrition rather than big bursts of growth.

Combined with intensive scarification, appropriate moisture management and sufficient existing soil P this forms the basis of our autumn feeding programme for the majority of greens.

Why supply a little P in the spring?

So far we’ve explained why P isn’t automatically added at the autumn renovation. By spring the seedlings from the previous autumn should have survived the winter and be ready for another period of active growth. Their root systems are still quite young in comparison to those of more mature plants and spring growth does provide an opportunity for further rooting, tillering and consolidation.

Phosphorus is vital to these processes as it is to the growth of mature turf. A fairly modest spring application therefore provides a handy opportunity to provide a little extra, available Phosphorus just as the young plants start to grow again. Of course we can’t say that the spring applied P is going to preferentially feed the younger plants and give them a competitive edge over the established grasses. Both young and mature plants can benefit and in soils where P status is already good an extra application may well produce little if any response.

Spring isn’t necessarily the best time of year to apply phosphorus where there is a deficiency. Where soil testing reveals a real lack of P, getting that corrected prior to or at the time of autumn establishment may be far more important than waiting until spring. However, my preferred approach is based on the more normal scenario we come across, i.e. an established bowling green with sufficient existing phosphorus reserves. A fairly modest spring input of manure is a sensible way of providing some top up without necessarily enriching the soil at the more sensitive autumn establishment period. Phosphorus is relatively immobile in soil so its benefits and interactions go well beyond the date of application. The intention is to maintain a sufficient level of phosphorus over time rather than the idea that some particular calendar date has the ability to ‘direct’ the phosphorus to the grasses we want.

Why only N in summer?

During the main growing season we’ll be relying on regular applications of liquid slow-release nitrogen, plus other inputs as dictated by soil analysis and the prevailing turf conditions. These N applications are intended to provide a measured and controllable level of nutrition during the periods of active turf growth.

They don’t routinely contain phosphorus as in many of the established greens there is no requirement for additional phosphorus. There’s very little point in just reapplying a nutrient simply because it’s one of the three figures on the fertiliser bag. If soil P is adequate then we can focus on N management, maintaining K sufficiency where required, promoting good rooting and helping the soil to cycle nutrients.

Over time this approach can also help in avoiding the need to unnecessarily build up soil phosphorus. The amount of P removed in clippings and in other losses is still an issue. Soil reserves are finite and a permanently P-free fertiliser programme wouldn’t be suitable for every green. But whether or not a particular nutrient needs replenishing and by how much is best worked out by the evidence rather than by habit.

What about the soil organisms?

This is where the wider philosophy of natural greenkeeping comes into its own. Nutrients don’t function in isolation from the soil environment. A rootzone with the right structure, the right moisture, the right amount of air in the pore space and with active biological processes will give the roots far more scope to explore and take up nutrients.

Mycorrhizae (if they get established) can in fact help to extend the effective area of root absorption. Other microbes are involved in the breakdown and recycling of organic matter, releasing the nutrients which can then be made available to the plants. Root exudates also have an effect on microbial activity and the chemistry right around the root. These relationships are complicated and the mere presence of microbes doesn’t necessarily equate to P being available.

Nevertheless root health and the promotion of biological activity are valid objectives in a programme aimed at reducing the reliance on unnecessary fertiliser inputs. The other way around — i.e. just keep on adding more fertiliser whenever it looks like the plants aren’t responding well — runs the risk of missing the real reason why the plants aren’t making effective use of the nutrients that are already there. The answer may well be in poor rooting, poor aeration, water stress, unfavourable soil chemistry or simply excessive competition. More fertiliser won’t reliably fix those underlying problems.

The annual programme is a sequence, not a shopping list

It might be helpful to think of the fertilising programme as part of an annual cycle rather than a series of individual products.

Autumn: Create the opportunity to add new plants.

Intensive renovation will of course temporarily reduce the competitive advantage of the mature turf. Overseeding will introduce the desirable perennials and careful nutrition will be used to support establishment without the need for any excessive phosphorus.

Winter: Protect the investment

The work doesn’t stop once the seed has germinated. Autumn and winter management can have a considerable influence on how successfully the new seedlings become part of the established sward. Maintain appropriate nutrition and rootzone conditions, encourage resilience and avoid operations that unnecessarily weaken the developing turf during periods of restricted growth.

But importantly, keep the green mown as fine turf. There’s a persistent belief that allowing a newly seeded green to grow long through autumn and winter will help the young plants establish. Some clubs even allow the grass to reach two or three inches in height, believing they’re giving the seedlings a better chance. Unfortunately, this can be counterproductive. An excessively long, dense canopy restricts light reaching the smaller seedlings, increases competition and can create humid, poorly ventilated conditions that favour fungal disease. It can also encourage softer, more elongated growth rather than the compact, well-tillered plants we’re trying to establish.

Regular, careful mowing, using a sharp, correctly adjusted mower at a sensible seasonal cutting height ( I recommend 8mm), encourages tillering, maintains light penetration and helps the developing plants adapt to life as closely maintained fine turf. Of course, mowing frequency and height need to reflect growth rates, weather, ground conditions and the maturity of the seedlings. We aren’t suggesting aggressive close mowing of newly germinated grass, or taking heavy machinery onto a wet, vulnerable green.

The principle is simple: protect the seedlings without abandoning the management practices that help create a dense, healthy, fine-turf sward. After all, we’re not trying to grow a field of long grass that we’ll eventually turn into a bowling green. We’re establishing bowling green grasses, and the sooner they can develop under appropriate fine-turf management, the better their prospects of becoming part of a smooth, dense and resilient playing surface.

Spring: Consolidate the gains.

As temps and growth go up then support rooting, tillering and recovery via controlled nutrition. A small amount of P may be incorporated into the programme provided the existing soil P status warrants it.

Summer: Keep the performance levels where they are without forcing any growth.

Use a measured application of nitrogen and other appropriate inputs to maintain healthy functional turf and avoid the soft, excessive growth which can have a detrimental effect on playing quality and increase the turf’s susceptibility to stress.

Throughout the year, soil analysis informs the decisions. Not every green receives precisely the same programme, because not every green has the same nutrient reserves, soil texture, organic matter, pH or biological condition. The calendar provides the framework. The soil provides the evidence.

Less fertiliser isn’t necessarily less nutrition

Perhaps the most important message is that reducing unnecessary fertiliser inputs doesn’t mean depriving grass of the nutrients it requires. A healthy grass plant needs adequate access to all its essential nutrients. But there is no requirement for every one of those nutrients to arrive in a fertiliser bag, or to be applied repeatedly throughout the growing season.

In some soils, phosphorus genuinely is deficient and needs to be supplied. In others, decades of fertiliser use have left sufficient reserves for healthy turf growth, and further applications may offer little benefit. In some situations, excessive nutrient availability may even change the competitive balance between desirable perennial grasses and opportunistic species such as annual meadow grass.

That’s why we test the soil, pay attention to the grasses growing on the green, use the soil sampler frequently to see what’s going on beneath the turf and it’s why the timing and composition of our fertiliser applications are deliberately different throughout the year.

It’s a rather different way of looking at greenkeeping, which begins by asking a simple question:

What does the soil already provide before we decide what to add?