After what was, if I’m honest, a pretty brutal winter – to have felt the sun start to make some progress into properly warming our days made me a happy man. Happier still when I finally started to catch wafts of the scent of spring (always as a precursor to the actual thing), drifting past from time to time. This winter, for the first time in years, temperatures at the nursery dipped below -5°C for extended periods and keeping the glasshouse warm, the outdoor water taps working and starting the vehicles each morning was becoming a bit of a grind. In a show of defiance, the large diesel powered air-heater that warms the glasshouse decided that the time was ripe to demonstrate that it felt unappreciated, and in a show of solidarity with French airport workers, it proceeded to stage a series of unannounced shutdowns (without the mandatory 48 hour warning that we are supposed to receive with “essential services”) – resulting in the poor plants near the doors and walls getting completely hammered by the cold. Quite inconsiderate in our opinion as those plants had nothing to do with the working relationship we have with our heater.
Mysterious numbers: they know we know
Every year around this time I usually feel the urge to stand and spread the word about fertiliser: that bit of Billy Graham in me itches to jump up and reach out with the importance of correctly feeding your plants (I’ll spare you the donation requests), and this year’s no exception. Leaving the dormant season and starting their spring growth, plants and lawns alike are in desperate need of a nutrient boost to ensure healthy growth and disease resistance.
This is best applied as a good dose of fertiliser, and those that follow my column will know that I’m a convert and now avid advocate of quality organic fertilisers (such as those fabricated by DCM), and have discussed this extensively in the past. One area however I realise continues to confuse many people is the numbering system on the fertiliser packs. If these mystical three-number combinations seem nonsensical to you, take comfort in the fact that you aren’t alone. Moreover, fertiliser manufacturers actually capitalise on this, selling plant feeds of every imaginable kind with beautiful pictures of each type of plant on the front of the boxes, allowing customers that aren’t already making their selections by way of tarot cards to be guided by the photos (rather than the actual nutrient content), and purchase more little boxes than they actually need. At this point I am breaking an industry taboo: What “they” don’t want you to know. The reality is that with a basic understanding of the numbering system and plant requirements, you’ll see that the vast majority of fertilisation needs can be suitably addressed with only a few nutrient compositions. Implicit proof of this is, rather ironically, provided by the fertiliser manufacturers themselves: I recently looked through a well-known fertiliser supplier brochure with over twelve pages of small dose compositions for the retail market – little boxes with pictures of everything from hedges to citrus trees. The very same manufacturer also provided a professional range in the same brochure, covering only two pages. Could it be that we professionals aren’t as conscientious as the amateur gardener? That we don’t really care what we put on our plants? Or course not! What they know is that we understand the meaning of the three little numbers on the packs and consequently that many of the “specific” fertilisers are all very similar in content.
So what is N-P-K?
To clarify the situation, I’ll explain. Nearly all that counts is the ratio of the three main elements: Nitrogen (N), Phosphorus (P) and Potassium (K). (The addition of one or two important microelements such as Magnesium (Mg) and Iron (Fe) tacked on the end of these compositions is one of the only areas individual fertilisers can truly differentiate, but these are not really “game changers”.) I won’t discuss what the individual elements do as I have addressed this before and a little reading on the internet will provide all you need to know.
Nitrogen focuses on growth and is most important early season.
Phosphorus is key for roots and flowers.
Potassium plays a vital role in cold weather tolerance, the formation of sugars and oils and fruit quality.
High N-P-K numbersdo not mean a “better product”, but you can put less on for the same effect: so the price and the weight of the bag should be considered.
A typical fertiliser for a lawn would be 14-5-8 (lawns need more nitrate than other nutrients to support their excessive growth) and the numbers represent the ratio of the N-P-K levels to one another. The attentive reader will note that these do not ever sum to 100. This is because they are not a total percentage: rather they are a percentage by weight of the respective elements* with the remainder being filler. One key advantage with organic feeds is that the filler material acts as a soil improver as it adds organic content, while with simple “chemical” feeds this remaining bulk has little added value.
Some real world examples
I like to use three main feeds on grass, although you could skip the first and use just two. At the start of a season I recommend a slightly higher nitrate level to boost the lawn into growth (18-4- 3), moving onto a normal nitrate level (14-5-8) through all the main season and then reduce this at the end of the season and increase Potassium levels to keep the lawn green over winter without growth (6-5-20).
For the beds, the principle is similar, using lower nitrogen content for “off” season than “on” season. I only use two feeds for my entire garden: 7-6-12 for the winter season and 10-4-8 for summer – just making sure these are applied regularly. I avoid high nitrate chemical compositions on beds as, although these produce spectacular growth, the resulting plants are not as disease and drought resistant.
The secret is not to be obsessed by the actual numbers – they don’t really matter to the average gardener: instead look for a pattern in the weighting between the nutrients and you’ll start to understand on a more general level what nutrient is important for what function. Make a point of looking up a few well-known feeds and checking their ratios to improve your understanding of this system. In no time at all you’ll be impressing your friends by advising them they need a high Potassium feed on their lawn in the autumn to keep it green.
*For accuracy, and for the chemists among you who will probably point out that I have made an error here, the P and K numbers do not represent the elemental amount in the fertiliser, but instead the amount of the associated oxide (P2O5 and K2O) that would theoretically be present if all the elemental P and K were oxidised. The elemental amount of P and K (unlike the N which is given as a direct elemental amount) present in a fertiliser composition are therefore slightly lower than the given ratio. (The elemental content of P in P2O5 is 43%; K in K2O is 83%.)