Fertilizers and Fertility: How to Feed Plants Effectively

, written by Bob Flowerdew gb flag

A range of organic fertilizers are available

A trip to some garden centres can leave you feeling that nothing will grow without copious quantities of fertilizers, specialized 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 fertilizers. 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 fertilizer

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 fertilizer as well. But be careful never to over-feed as this makes plants more prone to pests and diseases, and in great excess, most fertilizers kill plants along with much of the soil micro-life. Organic and green gardeners avoid soluble ‘chemical’ fertilizers simply because of detrimental effects and instead prefer slower acting, usually more natural, fertilizers 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 fertilizer
Adding liquid seaweed fertilizer to a watering can

Because of this slower incorporation, the action of many fertilizers (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 fertilizers 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 fertilizers 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 flavor) 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, beets, 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 fertilizer, 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.

Plants in pots
Plants grown in pots need regular feeding to ensure continued good growth

There is also the option of foliar feeding. This is no substitute for fertilizers, 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.

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Show Comments


"I put Fish Fert. in my hose end sprayer and just sprayed everything in my garden and then watered with the overhead sprinklers to wash of fert off and into the ground. How long should I wait before I can pick my strawberries and lettuce. I picked everything I could before putting the Fish Fert. on."
Faith on Friday 15 July 2011
"This is my 1st year using large raised beds, I have filled them with top soil and compost for the most part and some composted manure for certain veg. The reason for the raised beds is heavy clay soil and rock. I live in Portugal. Next year, should I just add compost or mix it with more composted manure?"
Judy on Friday 15 July 2011
"Faith - I would wait a week after spraying before picking the harvest and make sure that you wash the produce well before consuming them."
Jeremy Dore on Saturday 16 July 2011
"Judy - personally I'd just add a layer of compost but some gardeners swear by adding manure every year. With raised beds you usually don't need to dig them over every year since the soil is never compacted by walking on them so adding compost on top a month or so before planting is adequate and worms will mix it into the soil for you."
Jeremy Dore on Saturday 16 July 2011
"Regarding feeding - some of my opalka tomatoes are experiencing blossom end rot. I've read lots on what may cause it, but the treatments are very confusing. I'm most interested in feeding the plant calcium in some form, but can't find two people that agree on how best to do it. Fortunately it's not happening on all my plants, but maybe 20% or so. I've planted 25 opalkas. Any help as to how best to deal with it now, would be greatly appreciated. All of the hybrids I've planted are doing fine."
Nick Manley on Monday 18 July 2011
"Nick- I'm not sure that feeding calcium is the best course of action to prevent blossom end rot. Barbara Pleasant gives a good concise summary of the causes and options for treatment in the following article and this may also explain why it's just your opalka tomatoes that are experiencing the problem as it mainly affects larger-fruiting varieties: http://www.motherearthnews.com/ask-our-experts/bottom-end-rot-on-tomatoes-zb0z08zblon.aspx"
Jeremy Dore on Monday 18 July 2011
"Thanks much, Bob! Speaking of compost, I'm finding I have a ton of green stuff to cut up (fresh tomato stems that overgrew the garden, zucchini leaves, etc.) and it takes an enormous amount of time that I don't have. Is there a great shredder you can recommend that works on soft green material? Prefer an electric one."
Jan on Tuesday 19 July 2011
"Thanks, Jeremy. I hate the thought of losing those tomatoes that are affected. But it make sense to pick them off. I guess they are no good anyway. The soil I garden in is mostly clay, so this year I decided to do raised (more like mounded) beds. I covered the beds with landscape fabric and straw mulch in an effort to keep the soil moist. It has mostly worked. I'll have to add in the dolomitic lime next year. I also used MiricleGro on them twice early on, which likely contributed to this with lots of nitrogen."
Nick Manley on Tuesday 19 July 2011
"Nick, I came across this yesterday: someone reporting the same problem with the opalka variety: http://ktownhomestead.blogspot.com/2010/07/blossom-end-rot.html Good luck!"
Jeremy Dore on Tuesday 19 July 2011
"Jan, in my experience it's not worth buying a cheap rotary shredder as they tend to jam when you have lots of green stuff to feed into them. Better to go for a good quality one which crushes or has a corkscrew action but you'll probably still need to mix in some dryer 'brown' material too, particularly if you want to make good compost with it."
Jeremy Dore on Tuesday 19 July 2011
"Very interesting, Jeremy. Thanks for the link."
Nick Manley on Tuesday 19 July 2011
"Hello: Thanks for the article on Container Plants. I am growing plants in containers on my balcony, usually with great success. I've grown herbs inside and outside for years, but this year I cannot keep my Terragon growing. Each leaf, one by one turns brown almost over night. The soil looks healthy, and I use Fish Fertilizer on all my plants. Also, I am trying to grow Marjoram from seed, having tried 2 different packages of seeds, with nothing growing. I love both these herbs,I am disappointed with such results... so, what do you think is happening? I love this site, thanks for all the info. and help you send us. "
Gaia on Tuesday 26 July 2011
"Gaia, it's difficult to say what's going wrong with the tarragon, unless the container is waterlogged at the bottom which would rot the roots. Are the container's drainage holes blocked? Marjoram is slow to germinate (up to 14 days) and often needs to be started in a warm place indoors, covered with a plastic bag to keep the moisture in until the seedlings emerge."
Jeremy Dore on Wednesday 27 July 2011
"Thanks for your prompt reply. It's possible the drainage is poor for the Terragon because of the type pot I am using...(one with the wick) I certainly will check that out. ...and the Marjoram... I did not use the method you suggest, I will start over and plant the seeds again. I was not fertilizing often enough either...but I did after reading your article. Thanks for all the good info....I really enjoy this site. "
Gaia on Wednesday 27 July 2011
""I find that, in open ground, feeding plants is far less important than most people imagine...." But if my cheap, take-home soil test says a nutrient is lacking, then I should listen to it, right? Because the tests show me how much of a nutrient is actively available to the veggies, and it does not show me how much nutrient is or might be in the future available? Right? Have I been assuming wrong? Or do the roots have access to a lot more nutrients than my tests show? Here's an example of one of the soil test kids I've used before: http://www.ferry-morse.com/product_detail.aspx?id=980"
Sylvia B. in Dallas, TX on Monday 8 August 2011
"Is fish emulsion effective against onion fly ?"
Dat on Monday 28 December 2015

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