Fertilisers and Fertility: How to Feed Plants Effectively

15 July 2011, written by Bob Flowerdew

A trip to some garden centres can leave you feeling that nothing will grow without copious quantities of fertilisers, specialised compost and a variety of plant tonics for every imaginable problem. Feeding your plants is big business and while some gardeners swear by these products others believe nature supplies everything necessary. So, what are the best guidelines to follow for growing edible crops?

What to Feed Plants With

I find that, in open ground, feeding plants is far less important than most people imagine. Indeed, if anything must be neglected then not feeding does little harm. With most soils, simply giving your plants sufficient space and watering more liberally will be more successful in improving your results than applying all manner of fertilisers. In practice, for almost all plants (edible or ornamental) a modest dressing of compost or well-rotted manure applied annually is quite sufficient fertility.

Lettuce bed — Steady growth of crops can often be enhanced with careful supplementation of compost or organic fertiliser

However, a few vegetable crops are more demanding because they respond with far bigger yields. So, it’s worth ensuring these get extra compost, well-rotted manure, or some other more concentrated fertiliser as well. But be careful never to over-feed as this makes plants more prone to pests and diseases, and in great excess, most fertilisers kill plants along with much of the soil micro-life. Organic and green gardeners avoid soluble ‘chemical’ fertilisers simply because of detrimental effects and instead prefer slower acting, usually more natural, fertilisers such as seaweed meal. These feed our plants a full diet but do not overwhelm them with too much all in a rush.

Surprisingly, even apparently insoluble substances such as rock dusts soon start to dissolve in the soil thorough the action of microorganisms. All dust has a huge surface area and this encourages soil life to digest and turn the minerals into their waste products and dead bodies. These, in turn, feed another tier of micro-organisms and so on, as the new supply is passed around. All of these populations multiply then decay, continuously feeding our plants with a steady stream of nutrients thus encouraging strong healthy and unchecked growth.

~Diluting liquid seaweed fertiliser — Adding liquid seaweed fertiliser to a watering can

Because of this slower incorporation, the action of many fertilisers (ground rock phosphate, ground rock potash, ground rock calcium or garden lime, dolomitic lime, calcified seaweed, seaweed meal) continues over a long period, so to provide adequate minerals, we only need apply a light dressing of these fertilisers every so many years. However, two important elements, nitrogen and potassium, often become in short supply. Thus, to provide more nitrogen (which stimulates growth) and to have it balanced with other nutrients, we can spread and rake in top dressings of compounded fertilisers such as blood, fish, and bone meal or composted manures such as chicken dropping pellets when plants need it most – from spring through early summer. For potassium, (essential for disease resistance and flavour) it’s simplest to rake or water wood ashes into the soil anytime from the start of the growing season. Potash is most important for tomatoes, potatoes, beetroot, onions and fruit such as cooking apples, gooseberries and grapes. And as vegetable plots need a liberal supply of calcium (to keep them slightly alkaline), one quarter of the area (usually that part about to grow brassicas) should be dressed with garden lime the start of each year.

We can also help our most demanding plants with faster acting liquid feeds (fish emulsion, seaweed solution, comfrey liquid, borage, compost, or manure tea) added to their water. Though this is best restricted to the known hungry feeders- sweet corn, the marrow and pumpkin family, the cabbage tribe, potatoes and tomatoes. But remember; too much at any time is worse than too little, so feed lightly but often.

Feeding Container Plants

It’s completely different of course with plants in pots and containers. These can never access more soil which means it is up to us to provide their nutrients when the initial supply from their potting compost is used up. So for these, liquid feeds are almost essential. True, you can sometimes pot up into a larger size or top dress with a compounded fertiliser, but most plants grown in pots need heavier and more continuous feeding than this can provide, especially heavy croppers such as tomatoes. So, small amounts of liquid feed can be added every watering or at least once or twice weekly during the growing season. And as with your own diet, it’s better to have a spread of nutrients. So rather than always adding the same type of feed, it’s more effective to use a range working the changes.

There is also the option of foliar feeding. This is no substitute for fertilisers, but is a useful additional treatment. Plants can absorb nutrients through their leaves, so very diluted solutions, usually of seaweed or compost tea, can be sprayed on. These then act quickly and more like tonics or vitamins making our plants grow robustly. I spray everything with dilute seaweed monthly throughout the growing season, and fortnightly or weekly for those known prone to pests or disease as this makes them more resistant. (Spraying anything is usually best on cloudy days as it may scorch in bright sun.)

By Bob Flowerdew.