If your soil is very fertile you won’t usually have to even think about additional fertilization of the growing crop. However there are situations when growing crops will benefit from a quick boost of a specific nutrient. In such cases you can use some form of post planting fertilizer. Reasons for fertilizing after planting include:
• It can help you get reasonable yields when you are first establishing a garden and the soil isn’t very fertile. Fast growing crops such as radish and spinach may be fed weekly and will greatly benefit from the steady and continuous supply of nutrients.
• It can help over-wintered crops get off to a fast start, by giving them a shot of nutrients (especially nitrogen) when the soil is cold and nutrients are least available.
• It can be used to give an easily absorbed boost to rapidly growing plants, or those that have been stressed by transplanting, harvesting or pests.
• It can be used to help maximize yields by giving plants the nutrients they need at critical times in their development (such as when food storage organs are enlarging).
• It can help long season crops get all the nutrients they need, even after they have exhausted the locally available nutrients.
• It can be used to save a nutrient deficient crop, by supplying the necessary nutrient in an easily absorbed form.
Foliar fertilization consists of spraying a dilute solution of liquid fertilizer onto the leaves of a plant. It works because plants are able to take up small quantities of nutrients directly through their leaves (apparently this is 8 times as efficient as absorbing them through the roots). Some organic purists frown upon the use of foliar fertilizers as “unnatural”, because it concentrates on feeding the plant rather than the soil. It is sometimes used as a way of growing plants without building up the soil and this certainly isn’t good practice. However it can be useful in some circumstances. It is most often used when raising transplants, to supply the seedlings with all the nutrients they need for good growth.
Commercial liquid fertilizers
The simplest way to foliar feed is with commercially prepared liquid fertilizer such as fish emulsion or seaweed. These are convenient and have minimal smell, but are quite expensive. A cheaper alternative is to make your own, from compost, manure, comfrey or whatever you have readily available.
It is important to read the label carefully before you buy any of these, to see what they actually contain. The labels on some of the cheaper products are deliberately deceptive, emphasizing their organic ingredients, while hoping you won’t notice that most of their nitrogen comes from synthetic sources. You would think fish guts would be cheap enough, but I guess not.
Old fashioned compost/manure tea
I like this for its simplicity. It is easily prepared from locally available materials (a renewable resource), doesn’t cost anything and supplies all of the most soluble nutrients found in those materials. The problem with this kind of tea is that it has a horrible sewage-like smell and may contain large quantities of pathogenic bacteria. The anaerobic decomposition is smelly enough to discourage many people from making it more than once. You wouldn’t want to use it in your house and in towns you could well get complaints from neighbors.
Prepare the tea by filling a plastic bucket one third full of composted or aged manure (avoid fresh manure as you don’t want to spread too many pathogens), or half full of compost and topping it up with water (larger quantities can be made with woven plastic sacks and 50 gallon oil drums). Cover with a tight fitting lid and leave to steep for 7 – 10 days (depending on how warm the weather). The tea is then strained into a bucket and diluted one part tea to 2 parts water (if you have two more buckets of equal size simply pour a third into each of the other buckets and then fill all three buckets with water). Some people add a ½ teaspoon of soap per gallon as a wetting agent, to help it stick to leaf surfaces (some leaf surfaces are hard to wet). You can also use molasses for this.
Modern actively aerated compost/manure tea
In recent years the making of compost tea has undergone a radical transformation. Those who were repulsed by the old stinky anaerobic compost tea now have an alternative in the form of actively aerated compost tea or AACT. This is made in a similar way to the old compost tea, with one big difference. Air is bubbled through the liquid for the whole time it is being made. This results in an aerobic tea with only a mild earthy smell.
To make this kind of tea you need to be able to bubble air through the liquid for the entire time it is brewing. You can buy commercially made aerating systems, but it is much cheaper to make your own. All you need is an aquarium aerator, a three gang valve, some plastic hose and a 5 gallon bucket. If you buy the right parts it probably doesn’t take you any longer to make your own than it does to assemble a commercial system.
The compost (or vermicompost or aged manure) is put in a bag (a pair of old panty hose works great) to make a kind of tea bag. This prevents the manure from clogging the hose. This is then left in the bucket for 24 – 48 hours, with air bubbling through it. It should be kept at a temperature of 70 – 80°F and it should be in the dark. The end product should not be offensive in any way. If it is then it didn’t brew properly and you have made old fashioned manure tea.
The finished product is diluted and used like traditional manure tea.
By using your kitchen waste to feed the worms you can produce vermicompost, which can then be used to make tea to feed your plants and grow vegetables which then go to the kitchen. This creates a nice closed loop that doesn’t require you to buy anything.
Green plants can also be made into foliar fertilizer. If a specific nutrient is in short supply you might want to use a plant that accumulates that nutrient. You can also mix different plants of course, or add liquid seaweed.
The usual method of preparation is pretty much the same as for old fashioned compost tea. You fill the plastic bucket with as much foliage as it will hold (best for comfrey, seaweeds, nettles), top it up with water, then cover and leave for 2 – 5 days. Unfortunately these smell just as bad as manure tea.
You may also want to experiment with making aerated teas as described above. These are much less offensive and so more pleasant to work with.
Comfrey: This is best known s a source of potassium, but is also rich in nitrogen, phosphorus, calcium and magnesium. It is especially valued for feeding young plants.
Stinging nettle: A quick source of nitrogen,phosphorus and iron.
Alfalfa: A source of phosphorus.
Horsetail: Biodynamic gardeners believe Horsetail tea increases plants resistance to disease.
Oak bark: Rich in calcium.
Seaweeds: Rich in nitrogen, potassium and many trace elements (wash thoroughly before use to remove salt)
Yarrow: A source of potassium and silica. Some people believe it also increases a plants resistance to disease.
Grass: Lawn clippings are an easily available source of nitrogen.
Applying foliar fertilizers
Plants only absorb nutrients through their leaves as they need them, so it’s more effective to use frequent applications of very dilute fertilizers. If they are applied in too high a concentration they may burn the plants and you will waste nutrients. Don’t apply foliar fertilizers to water stressed plants, as it may do them more harm than good. In such cases remedy the water stress first and then feed.
The best way to apply foliar fertilizers is with a sprayer, as this enables you to wet both sides of the leaves for maximum nutrient absorption. To cover larger areas you can get an injector system that attaches to a hose and watering wand. You could also use a hand sprayer or backpack unit normally used for spraying pesticides. If you have nothing else you can use a watering can with a fine rose (though it’s harder to cover the undersides of the leaves).
The best time for applying a foliar feed is in early evening, because the nutrients are most easily absorbed at night when leaf stomata are open (it is up to 10 times more effective at this time). It also helps if the air is still and humidity is high. Early morning is also good. Ideally the plants shouldn’t get wet again until they have had 24 hours to absorb the nutrients through their leaves (wetting will wash off the nutrients). Generally it is better to give several light applications, rather than one heavy one.
In less than ideal soils it may be necessary to fertilize several times during the growing season. Perhaps begin with a feed as soon as the plants have recovered from transplanting, to get them up and going. Feed them again just before they start developing the food producing part and once again as this part is sizing up. Of course, rather than keeping track of every crop and stage of growth, you could just feed everything in the garden every couple of weeks
Don’t harvest and eat green leaf crops within 2 weeks of fertilization, as nitrogen rich feeds can temporarily increase the level of toxic nitrates in the plant. Of course you wouldn’t want to eat salad greens that have just be sprayed with diluted manure anyway.
Applying liquid fertilizers
The difference between liquid fertilizers and foliar ones is in the manner in which they are absorbed. The same materials are used and for the same purposes. Rather than being absorbed through the leaves, liquid fertilizers are absorbed through the roots in the usual way.
Liquid fertilizers are usually applied with a watering can. You merely pour water on to the soil around the plants individually, as if you were watering them in after transplanting. After applying the fertilizer you then irrigate the whole bed to soak the nutrients down to the root zone
Top dressing is another method of applying fertilizers to growing plants. It usually means sprinkling small quantities of soluble fertilizers onto the soil around the plants to give them a quick boost. They will be washed down to the root zone by rain or irrigation water.