Is it necessary to use fertiliser to grow vegetables? Well, we all dream of perfect soil, bursting with life and humus, supplying all the nutrients the plants need. Our article Feeding, Fertilisers and Fertility last year made the point that if the soil is looked after, then crops grown in the open (as opposed to in containers) need little fertiliser. In fact, fascinatingly, recent research found, not only that the flavanoid level in organic tomatoes increased with an increased level of organic matter in the soil, but, once soil reached its ideal condition, required reduced applications of manure in order to maintain that increase.
However, not many of us can claim to have perfect soil and if it’s a difficult year weather-wise (any nitrogen available to plants in the soil is easily washed away in heavy rain), you have difficulty generating enough organic matter to spread on your beds, or you grow mostly heavy-feeding vegetables like brassicas, giving them extra nutrients along the way will keep them growing steadily through the season, give them strength against pests, and produce a better harvest. You just don’t want to overdo it.
The good thing is that fertiliser doesn’t have to come in expensive packets from the local garden centre, as we can grow our own. Comfrey and nettles are the most obvious home-grown fertilisers, but all organic matter feeds the soil. A mulch of grass clippings makes a supplemental fertiliser and weeds soaked in water will produce a liquid feed of some value, which is one way of getting your own back on them.
In fact, it’s recently dawned on me why straw is a traditional mulch for strawberries and potatoes. Straw, whether of wheat, oat, oilseed rape, rye etc., is high in potash which will be released as the straw rots down, and both these crops love their potash.
Fast and slow fertilisers
If you do decide to grow your own fertiliser, how is it best to use it? Well, fertilisers are generally described as fast- or slow-acting and those used by organic gardeners are generally slow-acting—broken down gradually by soil micro-organisms, providing food over a much longer period, often beyond the current growing season.
Liquid fertiliser is different. The nutrients have already been released into the water and are therefore readily available to plants. What liquid fertiliser shouldn’t be is a substitute for good soil. It is useful, though, while you build up the quality of your soil with organic matter.
How to use your home-grown fertiliser
The great thing about growing your own fertiliser is that you have a choice—you have a fertiliser that you can apply in different ways, depending on the need at the time.
Adding compost to the soil around, for example, your brassicas can’t be expected to deliver nutrients quickly to the hungry crop. In fact, it’s worth noting that composted manure gives more nitrogen to plants in the second and third year after application, because the micro-organisms take time to release it.
Nitrogen-rich plants such as nettles and comfrey will provide nutrients faster, over weeks, as they decompose quickly (which is why they are often used to fire up compost heaps), so add these to your beds from the start of the season, and as and when they need a top up. Just chop up the leaves and mulch around your plants (or lay whole leaves on the soil for slightly slower decomposition).
A liquid feed is the instant tonic of the plant world. Make compost tea by soaking a bag of well-rotted compost in a large bucket of water for about ten days, or nettle or comfrey tea by soaking leaves in a bucket for three weeks or so, and you’ll have a solution that, diluted with water to the colour of weak tea, will deliver a nutrient-rich tonic quickly to the vegetable roots.
Be careful with liquid fertiliser
One thing you don’t want to do is overfeed your plants. This is most likely to happen with too much liquid fertiliser. When I’ve just made a huge bucketful, I find it awfully tempting to keeping sprinkling it around.
Keep an eye on your vegetables and if something looks to be flagging, and doesn’t respond to a good water, then try a liquid feed. Vegetables need more nutrients when they’re using the most energy, so broccoli and cauliflowers, for example, will appreciate a bit extra as they begin to head up. Tomatoes, capsicum, aubergines, cucumbers—all these heavy bearers appreciate extra nutrients. Overwintered crops will benefit from a boost in early spring when they start to get going.
Root vegetables generally manage well on what’s in the soil and don’t waste the precious liquid on seedlings. They shouldn’t be encouraged to grow too fast, and might be affected by a fungal infection in the water.
Salads always look as if they’ll appreciated a lot of nitrogen (it’s all those green leaves), but they aren’t particularly greedy, so on the whole I don’t give them any tea. Cut-and-come-again crops do expend extra energy, though, and if they’re looking tired I feed them once a week with nettle or compost tea until they’re looking perkier. On the whole, I prefer to mulch salads with nettles or grass clippings, and build up the soil for the following years with compost, to give a more measured approach, as too much nitrogen given to leafy crops will make them grow soft and sappy and more vulnerable to pests.
It’s also worth remembering that comfrey tea majors on potash, which encourages fruiting and seeding, so if you apply it to saladings you might find them running to seed more quickly.
By Helen Gazeley