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.