Alan's success with… fertiliser
Confused by all the different fertilisers on offer? Alan explains which plants you should feed, when to do it and how often When we grow plants in pots, tubs and troughs, we really do need to ‘play God’
Would you ever re-use a tea bag? No, I thought not. So why do we often assume that the helping of fertiliser we gave our plants last season is still working? It’s easy to forget that cultivated plants can’t survive on rain alone – and in fact rain only makes matters worse by washing or ‘leaching’ valuable nutrients out of the soil. As a result, many garden plants are far from displaying their ful l potential, with weak growth, poor colour, and a lack of f lowers and fruit . Kept wel l supplied with the three main nutrients that are present in what we call a ‘general’ fertiliser – nitrogen (N), phosphorus (P) and potassium (K) – they are given every chance not only to f lourish, but also to better withstand attacks by pests and diseases. There are some circumstances in which feeding plants is even more vital than when they’re growing in the garden with a free and unrestricted root run. When we grow plants in containers – pots, tubs and troughs – we really do need to ‘play God’, as we’re then totally in charge of the amount of food and water made available to them. What’s more, moisture and food are likely to run out more rapidly than in open ground, as the plants can’t send their roots further in search of extra sustenance. You’ll know when your plant is hungry, as opposed to thirsty: a lack of water results in the foliage wilting and soft, sappy stems. Lack of food isn’t quite as obvious at first. It manifests itself in a gradual diminution of growth, a lack of lustre in the foliage, coupled with pallor. The plant will become stunted – it may have one final go at producing a few tiny f lowers, but this doesn’t mean it’s happy; it means it’s making one brave, last- ditch attempt to set seed and produce progeny to carry on its good name. In short, a starving plant wi l l look ‘of f colour’, all on account of nutrient starvation. The three main plant foods each have their own part to play in plant health and well-being. Nitrogen promotes healthy leaf and shoot growth, phosphorus helps root development, and potassium encourages flower and fruit production. A general fertiliser offers a balance of all three major nutrients, plus lots of minor ones and trace elements too – everything from magnesium and iron to manganese and molybdenum. The availability of some of these nutrients is governed by the very nature of the soil in which the roots grow. We know that rhododendrons and azaleas, camellias and pieris, for example, don’t grow well on chalky or limestone soils. This is because the alkaline nature of the
Slow-release or controlled-release fertilisers are handy for busy gardeners who are often absent, as they supply continuous nutrition over a period of time
soil ‘locks up’ iron and magnesium, making it difficult for these ‘lime-haters’ to extract what they need. As a result, their leaves turn yellow and their growth becomes stunted. Grown in ericaceous ( lime-free) compost, they are perfectly happy. The nutrients that we give plants can be supplied in organic or inorganic forms. Organic fertilisers are plant- and animalderived products, such as hoof and horn, bonemeal, seaweed extract, and blood, fish and bone. These usually need to be broken down in the soil before the elements they contain can be absorbed in solution by the plants’ roots. This process makes soil bacteria ‘work’ to break down the organic material into simpler constituents, to which the plants have direct access. Inorganic fertilisers, on the other hand, are derived from minerals or are synthetic products, which, once dissolved in water, are immediately avai lable to plants. Examples include sulphate of ammonia, sulphate of potash, superphosphate, Growmore, Miracle-Gro and Phostrogen. Their advantage is their relatively rapid action (although it can be short-lived), as they can be absorbed the moment they’re dissolved in water. However, because they don’t need soil bacteria to break them down into a readily absorbable form, such bacteria are made redundant. Inorganic fertilisers, while providing instant nutrition, do nothing for the long-term health of the soil. Some inorganic fertilisers are marketed as slow release or controlled release, Osmocote being an example of the latter. Here, the ef fects of the fertiliser are delivered over a longer period. These products are handy for busy gardeners who are often absent, as they supply continuous nutrition over a period of time. Applied in spring, they can last well into summer. Remember that soil bacteria aside, even organic fertilisers do little to improve soil structure – only bulky organic matter (garden compost and manure) can do that, and both are vital ingredients of good earth.
When and how
The next issue is timing. Plants slip into a semi-dormant or completely dormant state in winter, so it’s a waste to offer them any kind of food during this period. They’re most in need of nutrition just as growth is about to start, so I usually give plants a good helping of general fertiliser in March, so it can be readily absorbed by the time the strong growth spurt begins in April or May. They then get another dose in June or July to see them through the summer. Don’t feed plants in cold, frosty weather, or in hot, dry summers. The compost or soil must always be moist at the time of feeding, whether with a liquid or solid fertiliser. Powdered or granular fertilisers are best sprinkled on to the surface of the soil and gently hoed in, to allow rain, or watering,
to dissolve them and take them down to the roots. A mulch laid over the soil after application will seal in moisture and help to keep the fertiliser working. Liquid feeds (in which the fertiliser is dissolved and applied in solution) are watered on and therefore act more quickly, but they – and foliar feeds, which are dissolved and sprayed on to leaves – provide benefits for a shorter period. Liquid feeds can be given weekly or fortnightly during the growing season and are especially useful for plants in containers. It is possible to overfeed plants, so always follow the instructions on the packet. Overfeeding can lead to foliage scorch and a ‘burnt’ appearance. In severe cases, the plant may even collapse, due to a process called exosmosis – when the surrounding soil water is richer than the sap, which then flows outwards from the plant. Moderation in all things, then.
Potting and seed composts already contain a certain amount of fertiliser to ‘see the plant on its way’. This is known as a base dressing, but will run out after a month or six weeks, so you need to provide additional fertiliser from then onwards. Perennial plants that spend all their lives in a container need to be ‘potted on’ (given a larger container and fresh compost) in spring, either every year or when their size demands it. After a few years though, such a move becomes impractical and then the top few centimetres of compost can be removed in spring and replaced with fresh – a process known as top dressing. A modest amount of fertiliser can be mixed in with the new compost to offer a greater boost. Most plants will enjoy an annual dose of general fertiliser, while leafy vegetables will appreciate a boost of nitrogen-rich feed. Lawns are generally fed in spring with a high-nitrogen fertiliser and in autumn with a more balanced dressing that won’t encourage lush, frost-tender top growth, but wi l l promote root growth. These products always give clear instructions on their time of use on the packet. Applied correctly, fertilisers can make a big difference to your plants’ performance. To get started, just use my quick guide on p88 and you’re sure to reap the benefits.
Dilute liquid fertiliser in a watering can before applying Add slow-release pellets when planting up containers Scatter rose feed on the soil around your shrubs to improve flowering
gardenersworld.com Check the instructions carefully and don’t be tempted to overfeed – more isn’t always better! April 2018