Botanical Name: Raphanus sativus
Radishes need little in the way of fertilizer added to soil. In fact, a common problem with growing radishes is having too much of certain elements, usually nitrogen. The results may include spindly radish roots, overly thick greens on top, inferior flavor, and low levels of antioxidants in the mature radishes.
Well-balanced soil in the vegetable garden will contain a mix of several important nutrients: nitrogen, phosphorus, and potassium. Let’s look at how they work together.
Nitrogen nurtures plant leaf development, so leafy crops like kale and spinach need a lot of it. Phosphorus helps several aspects of plant development, including flowering, root growth, and the transfer of water content and energy through the plant structure. Potassium nurtures root growth, which is crucial for healthy plant growth.
When there’s too much nitrogen – it’s easy to over-fertilize – a root crop like radishes will suffer because the plant is trying to make the leaves as big and profuse as possible. What happens to the roots? They simply can’t form correctly or grow big and plump.
The root – the part of the radish we eat most commonly – is starved for nutrition and may appear long and skinny rather than round and full. In addition, the special flavor of a radish may not develop properly, either, and even the normally high level of antioxidants in the radish may be suppressed.
Radish fertilizer requirements, then, are minimal. If you apply commercial fertilizer to the garden be sure to look for one with a low nitrogen content. Standard garden fertilizers are marketed witha series of three numbers, for example 5-20-10; the numbers indicate the percentages of nitrogen, phosphorus, and potassium – in that order. If that first number is low, that’s good because it indicates a low nitrogen fertilizer. If the number were 20-10-5 it would indicate a much higher percentage of nitrogen than is recommended.
Now, gardeners who make their own soil amendments from ingredients at hand can add certain components with lower nitrogen content. Examples are sawdust and straw, both of which require the slow decomposition that keeps active nitrogen levels fairly low.
Nurture your radishes by providing a low-nitrogen soil that still contains plenty of organic materials mixed in. Leaf mold is a good mixer. It’s important to give radishes a fairly loose, open soil so that the roots can develop easily.
A heavy clay soil is difficult for growing radishes, so it may be necessary to help clay soil develop a better texture. One way to do that is to work some powdered gypsum into the soil. That helps the clay particles clump together and leave more room between the particles; water and nutrients can move better.
Finally, soil that’s too low in pot ash may not enable radishes for form round bulbs. In that case try adding commercial potash or some hardwood ashes.
The best radish fertilizer may be no fertilizer at all!