When plants are grown in the soil they can send out roots, make friends with fungi, and source their own nutrients from their surroundings. In gardens we help them do this by improving and feeding the soil, a topic I will be returning to in chapter four. But when they’re confined in containers plants have a limited volume of soil and therefore a limited amount of nutrients to tap into.
Commercial potting composts generally contain enough nutrients for around six weeks of plant growth, and then you’ll need to supplement that with regular feeding. We covered the basics of plant nutrients and nutrient deficiencies in chapter two, so here I’m going to concentrate on choosing suitable fertilizers for your container-grown plants.
Fertilizers can either be solid or liquid. Solid fertilizers may be slow-release or fast-acting, and can be applied in several different ways. A base dressing is included in the growing medium – fine for potting mixes, but not ideal for seed composts as high levels of nutrients can hinder germination and harm seedlings. Top dressings are layers of fertilizer applied to the soil or compost surface, which can either be left on top or incorporated into the top layer of the soil. A surface broadcast is a scattering of solid fertilizer over an area of soil (it’s not applicable to containers) which is then left on the surface.
Liquid fertilizers are all fast-acting, as the nutrients are in a solution and readily available to plant roots. When suitably diluted, liquid feeds can be applied as a foliar feed – a spray applied directly to leaves. Foliar feeds are most commonly used to apply micronutrients (e.g. a seaweed feed), and to help ailing plants, rather than as a general application.
Commercially available fertilizers will display their nutrient ratio on the packaging, in the order N:P:K. A balanced fertilizer is the best choice for general potting mixes, as growing mediums that do not contain any soil (loam) need additional phosphorus (P). If you want to encourage rapid, leafy growth (e.g. for salad crops) then a high nitrogen (N) feed is the way to go, but don’t apply it late in the year to plants that you’re hoping to over-winter, as sappy growth is not very hardy. Fresh growth is also more vulnerable to pests and diseases.
If you want to encourage the production of flowers and fruits, or winter hardiness, then a high potash (K) feed is what you need – this is exactly what you get when you buy tomato feed, which is used from the moment that tomato plants start to flower to encourage a bountiful crop.
Always follow the instructions on the packaging, as it is important not to over-feed your plants. High levels of fertilizer can cause the build-up of salts (particularly in containers) that harm roots. Excess fertilizer won’t be used, and will wash (leach) away in the rain – a waste of money and a potential environmental contaminant as excess plant nutrients cause problems in wild environments (particularly rivers and lakes).
Solid fertilizers are often based on animal products. Common ones include chicken manure pellets, ‘hoof and horn’, ‘blood, fish and bone’ and fish meal. Vegan options are available, with fertilizers based on plant products such as seaweed or alfalfa meal. Rock phosphate can be used in place of animal bones, and synthetic nutrients are available. Bear in mind that mined sources of nutrients are non-renewable, and the manufacture of chemical fertilizers uses a lot of energy.
In a garden setting, home-made compost is a very good solid fertilizer. You can add it to your potting mixes, or use it as a top dressing. Worm compost is particularly rich in nutrients as it is mainly made from food waste. Compost is free, easy to make, and creates a healthy environment in the container while providing nutrients in a form that plants can use. Animal manures need thorough composting before they can be used as fertilizers for containers.
Bunny poo comes in handy pellet form, and makes a good slow-release fertilizer
If you are growing legumes (peas and beans) in containers then you may prefer to give them a bean booster rather than to use a fertilizer. Legumes form a beneficial relationship with soil bacteria (rhizobium) that allows them to capture nitrogen from the air and use it as food. Rhizobia bacteria are naturally present in soils (although not always the strains that you need – different legumes make friends with different strains of bacteria), but won’t be included in potting mixes. Adding the appropriate bean booster (also called an inoculant) can help your plants produce a good crop without supplementary feeding from you.
Lime is sometimes considered to be a fertilizer, but what it actually does is raise the pH of the soil or growing medium (making it more alkaline). You need to test the pH of your soil or potting compost, and know the pH requirements of your plants, rather than apply it indiscriminately. The right pH makes nutrients available to your plants, but the wrong pH can lead to a nutrient deficiency. Similarly, sulphur can be used to lower the pH of soil or potting mixes for acid-loving plants.
Solid fertilizers are great for slow-growing plants and for adding to growing mediums, but hungry plants grow in containers need nutrients they can suck up quickly.
Liquid fertilizers contain soluble plant nutrients that are readily available to roots and can be quickly taken up and used by plants. They need to be correctly diluted so that they are not too strong, particularly when applied as a foliar feed, and any excess nutrients are easily washed away by rain. Although the instructions on commercial liquid fertilizers may recommend using them once or twice a week, it is possible to dilute them further and use your solution whenever you water your plants, which can give them a more regular supply of nutrients. Don’t apply liquid feeds to dry soil – you need to rehydrate the soil and the plant before feeding.
Common liquid feeds include the high potash tomato feeds (which are also used for other fruiting vegetables) that can be synthetic or organic. Seaweed is often used to provide a balanced diet of micronutrients. Other synthetic, organic and vegan choices are available. Again, each commercial feed should give the NPK ratio on the packaging, to help you make the most suitable choice for your plants.
It’s also common for people to make their own liquid feeds. Comfrey (Symphytum species) is a hardy perennial, leafy plant that is easy to grow. Its long roots dig down into the subsoil (under the nice topsoil layer where most plant roots hang out) and bring up nutrients that are accumulated in its leaves (it’s called a dynamic accumulator). And comfrey leaves have an unusual composition that means they rapidly rot down and leave very little fibrous matter. They are ideal for adding to the compost heap, for using as a mulch or for making a nutritious plant feed that is high in potash (K) (and so ideal for tomatoes and other fruiting vegetables) but also contains other nutrients. The usual method of making comfrey feed involves drowning the leaves in a bucket. Be warned – the liquid is ready after a couple of weeks but it stinks like sewage. In an urban area it won’t make you popular with your neighbours. I use a different method; stuff comfrey leaves into a lidded bucket and leave them there until they rot down on their own, with no water. It doesn’t take much longer, and there’s almost no smell.
Nettles make a liquid feed which is high in nitrogen, but you have to use the water method so there will be a pong. In fact you can make a liquid feed out of any plant matter in this way (and it’s a nice way to deal with horrible weeds), but what you use affects the nutrient content of the feed.
You can dissolve animal manure in water to use as a liquid feed, and the run-off from worm composters is also good. Compost tea is made via a ‘brewing’ process that keeps a mixture of compost and water aerated to allow the growth of beneficial bacteria. A similar idea is to use EM (Effective Microorganisms). If you use a Bokashi bin to dispose of your food waste then you can use the run-off as a plant feed; you can also buy special garden ‘yoghurt’ makers that brew up EM fertilizers.
However you make your liquid feeds, be sure to dilute them before use. A good general rule is that they should have the colour of weak tea or coffee before use; err on the side of caution for younger plants.
Comfrey liquid (or liquor) before dilution
Your own urine is a good source of nitrogen, best used fresh (when there are less problems with odour and the nitrogen content is highest – the ammonia that comes off as it ages is made from nitrogen). There is an entire book devoted to the use of urine in the garden (Liquid Gold), and I did a podcast on the subject, but the highlights are that you have to dilute urine before use and shouldn’t apply too much in one place (particularly for containers) due to the potential build-up of harmful salts. Urine is often referred to euphemistically as recycled beer or recycled tea!
Now that your plants aren’t hungry, it’s time to make sure they’re not thirsty either – watering is coming up in the next installment 🙂
This blog post was written by Emma Cooper and was published on The Unconventional Gardener website. If you're reading it elsewhere you may want to navigate away from plagiarised content.