Ideally, plants should obtain all their requirements from well-nourished soil fed with plant-based composts, turning in green manures, mulching and/or chipped branch wood. However, you may find that your greenhouse crops, vegetables and pot plants benefit from supplementary liquid feeding. We suggest various dosage rates but you may wish to experiment – remember that you can apply too much and this will damage plants, e.g. adding comfrey juice to seedlings can cause magnesium lock-up in older plants.

Seaweed feeds

It is possible to make a liquid feed using seaweed meal.

– Put three flat tablespoons of seaweed meal into two litres of water, preferably in a glass bottle.

– Leave to marinate for two to three weeks or more.

– Every month in the growing season feed plants with one mugful of this brew in two litres of water, shake the bottle well before use.

Seaweed meal contains all trace elements but some authorities believe the amount of available nitrogen and/or potash is low. A richer feed can be made using green leaves, but for indoor plants the problem is it stinks.

Plant tonics, e.g. comfrey liquid

– Take any size of container (e.g. a water butt with a tap) and fill with any or all of the following: grass cuttings, nettles, weed leaves or comfrey leaves.

– Nettles give the best multipurpose feed and comfrey alone will give a feed rich in potash, which will be excellent for tomatoes, cucumbers and courgettes.

– Cover with water, cover the container, and leave for two to four weeks. Try a dilution rate of about one part brew to three parts water and use it monthly on house and bedding plants.

– Strain through a sieve or old stocking to keep back weed seeds and bits of plant material, which will block your watering can.

– This feed, used fortnightly, is of course also excellent for vegetables but needs to be diluted more: about ten parts water to one part leaves. Liquid nettle feed brings on vegetables at least as well as chemical-based liquids when used at the same rate.

The pong problem is not too hard to work around, since once the brew is diluted and applied to the soil, the smell disappears after a few days. So it is possible to feed pot plants outdoors and bring them in the next day but ensure that they will not be harmed by any change in temperature.

The smell can be reduced very considerably if the tonic is made from freshly-picked dry leaves. The leaves are packed into a vertical length of say 6″ diameter plastic pipe and compressed by weighting them down with, e.g. a close fitting bag of sand. The bottom of the pipe is closed off and liquid accumulates there. A grill is fitted inside the pipe to prevent the bottom section clogging up, and below this is a tap to allow the resulting super-concentrated liquid to be drawn off as required, and then diluted in the usual way. This system works well and has been used by the stockfree organic farm Growing with Grace for many years – see www.youtube.com/watch?v=YDYpxiE933E starting at 15min 40sec. They have two separate ‘pipes’, one for nettles and one for comfrey. Growing with Grace recommend a dilution rate of one part liquid to 10 parts water.

Adding minerals

Mineral deficiencies can be confused with plant diseases. The use of mineral amendments is not sustainable, because mining and the transportation is fossil-fuel intensive and opencast mining destroys local environments. At the same time, occasionally growers have to make compromises to avoid crop failures, which may justify one-off applications. However, if other recommended practices are followed for improving soil fertility (plant-based composts, green manuring, mulching and chipped branch wood), stockfree systems should not need to rely on mineral amendments.

Deficiency Indicator Preferred remedy Last resort
Phosphorus Very dark green leaves with a tendency to develop purple colours, stunted Tunisian rock phosphate. Calcined aluminium phosphate rock (Redzlaag)
Potassium Yellow streaks in the leaves Wood ashes in the compost heap Sulphate of potash
Magnesium & Calcium Yellow drying and reddening of older leaves while veins remain green Limestone. Dolomite limestone. Gypsum /calcium sulphate. Foliar feed epsom salts (for acute magnesium deficiency).Magnesium rock (including Kierite)
Sulphur Younger leaves turn yellow and then all the leaves turn yellow Gypsum Sulphur
Boron Soil test – canker/curd browning More compost, less lime. Seaweed meal Direct application at 3g per m2 for extreme cases only
Copper Top leaves wilt and do not recover Seaweed meal Direct application for extreme cases only
Zinc Top leaves wilt and do not recover. May have a bitter after taste Seaweed meal Direct application for extreme cases only
Iron & Manganese Pale green leaves More compost, less lime. Seaweed meal Direct application for extreme cases only
Molybdenum, Cobalt & Selenium Soil test Seaweed meal Direct application for extreme cases only

Remember to use appropriate protective clothing. Lime and comfrey, for example, can irritate eyes and skin.

This article was originally the second half of VON information sheet Num 1. Last update: 2016.