You needn't be a boffin to understand basic plant nutrition. Plants use a process known as photosynthesis, taking in light from the sun, water from the soil and carbon dioxide from the air to manufacture carbohydrates. But these are the ways the plant stores energy; it's a bit like having batteries but not the torch/radio/clock that they power. To make structures such as tree trunks, woody stems, petals, seeds and the hormones that dictate the way they grow, and when they flower or shed their leaves, plants need a large number of different minerals in tiny quantities. These are taken up from the soil or, in the case of container-grown plants, the compost in which we grow them.
It's common to find that underfed plants are short of several nutrients all at once, so symptoms don't look like the pictures in books; the plants just look starved, stunted and sickly. Instead of trying to identify what's missing and how best to replace it, simply start each new growing season with an all-round dose of general-purpose feed, as it contains the 'big three' nutrients (nitrogen N, phosphorus P and potassium K) that plants need most of. That, plus a helping of well-rotted organic matter, gets most borders - and your edible crops - off to a flying start.
Nitrogen promotes leaves, is fast acting and encourages lush green growth, but too much will inhibit flowering. Deficiency produces small, pale leaves and weak, spindly shoots.
Use nitrogen in spring (the start of the growing season) and regular summer top-ups for lawns and leafy veg. Apply as liquid feed or easily soluble powder/granules - for example, dried blood.
Phosphorus assists the development of roots and tubers, but is also beneficial for general plant health. Deficiency is rare as phosphorus doesn't wash out of soil, even during very wet weather.
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Use phosphorus at the end of the growing season to promote strong roots underground over winter. This will increase the root area, helping uptake of other nutrients and water.
Potassium helps flowers and fruit to form, but is also crucial for toughening growth tor resisting pests and diseases. It also helps increase resistance to drought or extreme cold. Deficiency is more common on light, sandy soils. Signs include brown scorching and curling of leaf tips.
Apply potassium (also known as potash) little and often during the growing season, so plants can take it up when they need it.
Calcium, usually in the form of lime, neutralises acid soils and makes micronutrients available that would otherwise be chemically 'locked up'. It's also essential for prevention of disorders such as blossom-end-rot (tomatoes) and bitter pit (apples).
Use calcium in early spring, if needed (do a soil pH test), but not at the same time as organic matter, with which it can react adversely.
These vital nutrients, including iron and magnesium, are needed regularly in far smaller quantities, for plant health. Deficiency of certain nutrients can produce disorders in particular plants.
Use micronutrients in the growing season if a deficiency exists. Give magnesium to tomatoes using Epsom salts, or sulphate of iron to rhododendrons with yellowing leaves.
A wide range of minerals (selenium, molybdenum and even gold) are needed in minute quantities for optimal growth, reputed health benefits and flavour in fruit and vegetables.
Use trace elements at any time of the year. Mulches of home-made garden compost usually provide all that's needed. Some brands of organic fertiliser contain trace elements.