At the most basic level, a plant needs four things to grow: sunlight, carbon dioxide, water, and nutrients. In nature, the nutrients are provided by the soil, and their levels are maintained over time when plant and animal waste rots down as part of the natural ecosystem cycle.
However, in gardens the situation can be more complicated. Removing flowers, crops, and other plant material from the garden depletes the soil's fertility over time, by breaking the cycle of rotting down and re-growing.
What's more, many plants aren't native or perfectly adapted to the local conditions, and the soil's natural nutrient balance doesn't quite meet their particular needs.
And lastly, many cultivated flowers, fruits, and vegetables are much hungrier than their wild counterparts, and will not achieve the expected growth without an especially rich, fertile soil.
All of this means a typical garden's soil can sometimes use a little help in the nutrient stakes, and that often means applying a fertiliser.
In essence, a fertiliser is a concentrated source of nutrients, applied to the soil or an individual plant to promote stronger, healthier growth, producing higher harvest yields or better flower displays.
However, behind that simple concept lies quite a lot of complication. Fertiliser isn't a one-size-fits-all solution which you can apply routinely and casually. Using the wrong type of fertiliser can actively harm your plants, or encourage them to grow in a way you didn't intend.
It's also important to note that a fertiliser won't magically transform your plant-growing success. In most cases, a fertiliser will do nothing for soil structure, drainage, pH levels, pest control, or any of the other factors essential to healthy growth.
However, when used in the right way for the right reasons, fertiliser is a powerful technique in a gardener's repertoire.
There are two basic types of fertiliser. Throughout history, using natural organic fertilisers was the only option, typically by spreading manure, applying compost, growing green manures to fix nitrogen, and so on. More recent industrial techniques have refined these base organic materials into convenient products which are easier to apply and store.
In modern times, there are is also the option of synthetic fertilisers, sometimes known as mineral fertilisers. These are are produced under industrial conditions and almost always from inorganic sources.
Both types have advantages and disadvantages.
- Organic Fertilisers
Organic fertilisers are those which are derived from plant or animal sources, and include bone meal, pelleted chicken manure, seaweed, and more. They may be processed to increase their concentration and to make them easier to apply, but the nutrients they contain are generally bound up in complex organic molecules rather than appearing as pure elements.
The effect of this is that the nutrients need to be broken down in the soil before they're available to plants, and this takes time. This slow release means organic fertilisers are a great choice for providing steady nutrient levels over weeks or months, giving the plant the minerals it needs without overwhelming its metabolism.
For example, adding organic fertiliser at the start of the growing season (or even after the end of one) provides small amounts of nutrition to a seedling in spring. As the organic molecules start to break down more quickly, the nutrient availability increases in step as the plant grows and requires more feeding.
A second advantage of organic fertilisers is that many of them improve the soil structure at the same time as feeding your plants. This is especially the case with powdered or pellet fertilisers, which often contain large-molecule organic materials which help improve the texture and water retention of the soil as they break down.
However, it's important to note that in a fertiliser context, organic refers only to the plant or animal origins of the product. It doesn't imply any particular eco-friendly benefits, nor that the fertiliser has received any organic certification.
For example, there's no intrinsic reason why a manure-based fertiliser couldn't contain traces of hormones, antibiotics, and so on, depending on the rearing conditions of the source animals. However, products with formal organic certification are also widely available to avoid these issues.
- Synthetic Fertilisers
Synthetic fertilisers are produced in a variety of ways, but are always formed by chemistry rather than by re-using organic materials.
Many synthetic fertilisers are produced from mineral mining, as byproducts from the chemical industry, by processing inorganic wastes, or even by taking nitrogen and methane from the air and combining them to produce ammonia.
The key difference compared to organic fertilisers is that the nutrients are present in a purer, more readily absorbed form, and are more immediately available to the plants.
This makes synthetic fertilisers ideal for giving plants a quick boost to rectify any deficiencies, or to simply promote faster growth and higher yields by greatly increasing nutrient concentration in the soil.
However, few mineral or artificial fertilisers provide any benefits for the soil itself, and their quick release can also mean the nutrients are quickly leeched away in wet conditions.
Of course, many commercial products blend fertilisers from different sources, giving the resulting feed a mix of characteristics to suit different situations.
There are three primary elements to most fertilisers: nitrogen, phosphorous, and potassium. These provide the basic nutrients that a plant needs to grow, and fulfil three different broad functions.
i) Nitrogen (N) - Promotes green foliage and fast, leafy growth. As nitrogen is essential for creating proteins, it's the nutrient a plant needs in the largest amounts.
ii) Phosphorous (P) - Helps the development of roots and shoots, while increasing the health of fruit and seeds. Also plays a key role in photosynthesis, where the plant turns sunlight into usable chemical energy.
iii) Potassium (K) - Increases flowering and fruiting, adds strength to the plant's stems and other structures, and increases hardiness to severe weather.
Although fertilisers are available containing just one of these basic elements, the vast majority are made up of a blend of all three. Considering the different effects the elements have, it's important to choose the right formulation to achieve the results you're looking for.
This is made easier by a standardised system known as the N:P:K ratio, with the three letters representing the chemical symbols of nitrogen, phosphorous, and potassium respectively. All commercially available fertilisers should give an accurate ratio on their packaging.
The numbers in the ratio show the percentage of each nutrient in the mix, making it easy to see both the balance of each and the actual concentration. In nearly all cases, the numbers won't add up to a full 100%, and the rest of the fertiliser's bulk will be formed by an inert substance such as sand or perlite.
For example, a ratio of 10:10:10 means the fertiliser is equally balanced with the three major elements each contributing 10% of the final volume, making it a good choice for all-round nutrition and healthy growth,
In contrast, a ratio of 5:10:15 would mean the fertiliser is weighted towards a higher potassium content, with lower nitrogen levels. This is a common kind of ratio for tomato feeds as it promotes flowering and fruiting, without encouraging excessive, energy-draining foliage.
On the other hand, a fertiliser with a ratio of 15:10:5 or similar is a good choice for feeding grass and other greenery, with its high levels of foliage-friendly nitrogen. It's particularly useful when applied before the first growth spurt of spring.
- Secondary Nutrients
Most fertilisers also contain at least some secondary nutrients alongside the main N:P:K elements. These are needed for all-round plant health, with different species requiring different combinations.
The three most common secondary nutrients are:
i) Calcium (Ca) - Helps the plant to absorb other nutrients, and increases disease resistance.
ii) Magnesium (Mg) - Necessary for photosynthesis, and also for making absorbed phosphorous available to the plant's metabolism.
iii) Sulphur (S) - Used to build amino acids from nitrogen and other nutrients.
Unless you're growing a plant with a specific need for extra secondary nutrients, it's not usually necessary to worry about them. Commercial fertilisers normally have a good balance that's suitable for most uses, and in any case ordinary gardening techniques such as composting and mulching will likely provide the necessary amounts.
- Trace Elements
Lastly, all but the most synthetic, highly processed fertilisers will also contain trace amounts of other minerals, including zinc, iron, copper, boron, nickel, and manganese.
Again, while these elements are essential for plant health, they're effective in tiny amounts, and so a broad mix is usually completely acceptable unless you have a known deficiency in your soil.
Fertiliser can come in several physical forms. The most natural form of all is so-called green manure. These are fast-growing plants such as cress or alfalfa, which are sown after the main crop and help to fix nitrogen and other nutrients back into the soil.
Other all-natural fertilisers include homemade compost or rotted manure, used as a mulch and allowed to slowly break down and fertilise the soil naturally.
However, most commercial fertilisers can be divided into four main categories.
i) Powders and Granular
As the name suggests, these fertilisers come in a coarse or fine powder form. They should be spread over the soil surface or gently worked into the soil, and then watered in. Typically, they'll need to be reapplied every month or two depending on the amount of rain and how hungry the plants are.
Pelleted fertilisers are powdered or granular ones which have been processed into larger sizes. This means they take longer to degrade, spreading their nutrient release over a longer period, and will need less frequent reapplication. To use, simply sprinkle them around the base of each plant, and leave soil biology and the weather to start breaking them down.
Soluble fertilisers are mixed with water before use, and can then be applied with a watering can or through an irrigation system. They provide quick nutrition, but need to be used more frequently as the dissolved nutrients are easily washed from the soil by rain or further watering.
Liquid fertilisers are highly concentrated and should be diluted according to the instructions on the label. They can then be used in the same way as soluble fertilisers, being quickly absorbed but also providing short-lived nutrient availability.
Warning: Synthetic fertilisers of all kinds are often highly concentrated, and can be irritating to skin. It's advisable to wear gloves as a precaution when using them, especially when scattering powders or pellets.
Different kinds of fertiliser release their nutrients at different speeds, depending on both their source and their physical form. There are three main types of release speed to choose.
i) Fast Acting
Fast-acting fertilisers are most often artificial types, where the nutrients are available in a chemically straightforward, pure form. Either liquid or easily soluble in water, they can be applied to the soil around a plant, or occasionally the plant itself, to give a quick nutrient boost. This makes fast-acting fertilisers ideal for correcting deficiencies, or giving emergency feeding to a plant that's struggling.
ii) Slow Release
Most organic fertilisers are the slow-release type. The nutrients are present in chemically complex molecules, and these need to be broken down in the soil before they become available to the plant's roots.
This process relies on micro-organisms in the soil, and so using these fertilisers requires a relatively healthy soil to start with.
The breakdown process also requires a reasonably warm soil to kick into action, and this makes slow-release fertilisers a good choice for spring application. As the weather begins to warm, the nutrients will be released in ever greater amounts, matching the growth of the plant to provide weeks or months of steady feeding.
Depending on the form of the fertiliser and the bulking ingredients it uses, slow-release types can also improve the soil structure as the organic material decays.
iii) Controlled Release
Controlled-release fertilisers are another slow-acting type, but rely on a different process to spread their effectiveness over weeks or months. These fertilisers are generally inorganic granules coated with a semi-permeable outer layer that lets the nutrients slowly leech into the surrounding soil.
Again, the warmer the soil, the quicker this happens, and so the season needs to be taken into account when judging how often to apply these fertilisers.
A particular advantage of controlled-release fertilisers is that they can be designed to closely match a plant's life cycle, with nutrient release timed to give a particular species of plant the most benefit over a set period.
There are four main ways of applying fertiliser, depending on the problem you need to solve.
i) Top Dressing
Most often used with fast-acting fertilisers, top dressing means to spread a solid or powdered fertiliser over the soil surface, around the plants to be treated. This method stimulates growth, and is useful early in the season to give young plants a solid start.
Top dressing is also useful for feeding tomatoes, which have an upper layer of roots close to the soil surface specifically for absorbing nutrition, with deeper roots responsible for taking in water.
However, it's important to follow the instructions carefully when using this method. If you over-apply, or if a mixture is too concentrated, it can raise mineral proportions to toxic levels and even pollute underlying ground water.
Also, avoid contact with the plant itself, as fast-acting fertilisers can scorch organic material if not sufficiently diluted.
ii) Base Dressing
Base dressing involves mixing fertiliser down into the soil before sowing seeds or transplanting young seedlings. It's most often used with slow-release fertilisers, so that the plant's roots have a steady supply of nutrients throughout the season.
Base dressing is most helpful when the soil is relatively poor. However, a nitrogen-rich base dressing is also useful for fast-growing plants such as peas and beans, promoting vigorous early foliage growth before a later top dressing of potassium to encourage flowering.
iii) Watering On
With watering on, liquid fertilisers or dissolved powders are watered around the base of the plant. This provides a quick nutrient boost, and is especially good for containers or indoor plants where nutrients in the soil are quickly used up.
Again, take care not to over-concentrate the mixture or to splash the plant too heavily.
iv) Foliar Feeding
Lastly, fertiliser can be heavily diluted and applied directly to the plant for extremely fast absorption. This is the ideal method for rescuing plants suffering from a particular deficiency, and is known as foliar feeding.
For the best results, apply the fertiliser to younger, more tender foliage which will absorb it most easily.
Diluting the fertiliser by the correct amount is essential for this technique, and even with mild solutions try not to use foliar feeding on a sunny day, or the leaves can quickly scorch.
Fertiliser shouldn't ordinarily be a routine, knee-jerk treatment, unless your garden features particularly poor soil. Applying fertiliser when it's not needed is at best a waste of time and money, and at worst it can actively harm your plants.
Bearing that in mind, there are three main ways of deciding when to apply fertiliser to any particular plant.
i) Time of Year
Most plants have different nutritional requirements over the course of a their life cycle. Planning your fertiliser application to match this can increase yields for vegetables, encourage a more impressive flower display, or promote better overall health.
For example, apply a general or nitrogen-rich fertiliser in spring to produce robust early growth, using a slow release type to provide a steady benefit.
Follow this with a potassium-rich formula in summer to encourage flowering, and finish with a phosphorous-based fertiliser as autumn approaches to discourage tender foliage while promoting healthy roots for overwintering.
ii) Stage of Growth
A variation on this schedule is to add specific fertilisers only when the plant has reached the growth stage where it will most benefit. The most common example of this is for tomatoes, chillies, and their relatives, which will almost always benefit from a potassium-rich feed every couple of weeks from when they begin to flower. Even in generally fertile soils, this will tend to produce a higher fruit yield.
iii) Emergency Measures
Fertiliser can also be used when the situation immediately demands it, such as if your plants are showing signs of a particular mineral deficiency. For example, if a plant's leaves are a pale, sickly greenish-yellow rather than a deep vibrant green, this is often a sign of a nitrogen deficiency, and foliar feeding with an appropriate formula can be a quick and effective remedy.
Similarly, translucent and weak leaves can point to an iron or magnesium deficiency, and a good all-round fertiliser will often cure the problem.
Lastly, regular fertiliser application is useful for container gardening, where the rule of not fertilising as a matter of course can be broken. Container plants are often under crowding stress to begin with, and watering can easily leech out the nutrients already present in the soil, leaving it depleted.
In these cases, use either a general purpose fertiliser or one formulated for your specific plants, and apply it relatively frequently but with a light touch.
Although fertiliser is extremely useful in many situations, it's far from a panacea. Most synthetic products do nothing for improving soil quality, and with their quick leeching times you'll need to keep applying them to see results.
Many organic fertilisers will improve the soil to some extent, but that doesn't remove the need for good soil management tactics. To see the best results from fertilisers, build your soil up with plenty of home-made compost, add green manures to your rotation schemes, and use organic mulches when you can.
When your soil is in great fundamental condition, any fertilisers you use will have the precise effect you're looking for, rather than making up for a poor growing medium depleted of nutrients.
Nonetheless, soil improvement is a long-term project, while fertiliser is a quick and easy fix. When used with care and planning, fertilisers will help you get much improved results from your garden, whichever kinds of plants you're growing.