Plants need six macronutrients. The three most important are nitrogen (N), phosphorus (P) and potassium (K, presumably because when the periodic table was put together, P was already taken, so they used its Latin name: kalium). Generally speaking, nitrogen promotes the growth of stem and leaves, and helps a plant resist disease. Phosphorus promotes root growth, which is why it is used (often in the form of bonemeal) when planting. And potassium aids the development of fruits and flowers, which is why it is the dominant element in fertilisers for tomatoes.
In theory, a well-maintained soil will provide enough of these nutrients for most plants. However, they are washed out by heavy rain, and can be locked up in forms that are inaccessible. Also, of course, many of the fruits and vegetables we grow are bred for very specific purposes, and require artificial support. For some people, this will come from a box or a bottle; organicists will prefer to make an evil-smelling soup from comfrey or nettles. Seaweed extracts are also used, and very good they are, too, if used with care. (If you are wondering about the other three macronutrients, they are magnesium, calcium and sulphur…and then we get into the micronutrients. But shall we leave that for now?).
It follows that if your plants have a specific problem, it can often be cured by a careful application of the right chemical. If your cut-and-come-again has been cut, and gone away – and this was, in fact, the original question – it may well be a good idea to give it a quick feed of nitrogen. Not too much, though, or the leaves will go all floppy. And I am afraid that salad plants will not survive if cut below their growing point. Ideally, snip out the big leaves, and leave the little ones to grow. OK?
Jon Gunson, Transition Town Lewes.