Fertilizing trees - a comprehensive guide
Urban trees have a very different surroundings to natural forests. There's a lack of naturally available nutrient and organic matter. Trees are unable to just up and leave for a better place to have a better life, so we have to create a better environment for them and fertilizing is another method. Many times you will see trees competing with lawn and other plants. Hard surfaces such as concrete paths and bitumen roads alter the soil, change pH, drainage and prevent root growth. Compaction is a common problem, lack of mycorrhizal fungi and lack of root grafting with more trees of the same species to share resources.
There's a myriad of different fertilizers and different tolerance to fertilizers by various types of trees. So what we will do here is break it down to the different categories of fertilizer. Then we will look at the effects of what the nutrients are, what they do etc. But remember, a tree can only take up nutrients when it's in it's preferred pH range so always check that first.
All nutrients - elements are taken up by the tree as soluble ions through the root system and transported via the vascular xylem. Remember this when fertilizing, have you ever seen avid gardeners fertilizing in the rain with their coats on? Smart people.
Organic vs Inorganic Fertilizers, the difference.
Organic fertilizers are derived from plant and animal products (carbon based). They can be processed such as say Blood and Bone vs spreading chook poo (manure) around your trees.
Inorganic fertilizers are processed or refined from natural sources, such as limestone and rock phosphate. They can also be chemically engineered.
In the various forms that fertilizers come in, they can be organic, inorganic or a combination of both. So for example you can apply an inorganic liquid fertilizer such as Miracle Grow or an organic liquid fertilizer such as Seasol.
Advantages of organic fertilizers are:- Environmentally friendly, less chance of overdose, slow release, improves soil structure and encourages microbes/humus.
Disadvantages of organic fertilizers are:- Can be expensive, unknown analysis, bulky as a lot more needs to be applied due the nutrient content being lower concentration, can smell and slow release if immediate results are required.
Advantages of inorganic fertilizers:- Wide range of choices, known analysis, easy to handle and relatively cheap.
Disadvantages of inorganic fertilizers:- May harm the environment, easily overdosed, no improvement to soil structure, can lead to pH changes and salinity.
Liquid and Soluble Fertilizers
The advantage of this fertilizer is it is ready for take up by the tree. Also on smaller plants and shrubs the foliage may absorb the nutrients. You will generally see a quick response however it will not be a lasting response and repeated applications at regular intervals will be required.
The downside of these fertilizers is mixing, watering in and perhaps hosing which may be prohibited if in a water restriction area. Temperature at the time of application needs to generally below 30C as scorching to foliage may occur. They are quickly dispersed and leached through the soil.
Granular Fertilizer
Is easily applied by broad casting either by hand or a spreader. These fertilizers require watering or rain to both dissolve and disperse them into the soil. A variety is available, some use osmosis to release the fertilizer, a special coating that doesn't allow release of the fertilizer until a required level of moisture is available. Be aware that you may see these granuals and be fooled into thinking that there's still plenty of fertilizer lying around when in actual fact the interior of those small granuals is gone. This fertilizer is usually slow release.
16 Essential Nutrients - Elements For Healthy Trees
Often when you pick up a bag of fertilizer you will see things like this 18-5-12 or NPK with a bunch of numbers, sometimes you'll also see "includes trace elements" or "with micro nutrients" etc. Now this is all important stuff and it's telling you what's inside. There are macro and micro nutrients.
The numbers refer to the percentage of N (nitrogen), P (phosphorous), K (potassium) in the bag. So imagine a 100kg bag with 18-5-12 on it. It has 18kg of N, 5kg of P and 12kg of K. Now you say that adds up to 35kg so what's the other 65kg? With inorganic fertilizers it's called a filler, could be clay, sawdust, sand, soil etc. With organic fertilizers it's pretty much just the organic matter as the nutrient concentrations are a lot lower than manufactured inorganic fertilizers.
Macro nutrients
C (carbon), readily available from atmosphere.
H (hydrogen), readily available from atmosphere and water.
O (oxygen), readily available from atmosphere and water.
N (nitrogen), from soil, N in atmosphere is unavailable for plant take up!
P (phosphorous), from soil (and same for rest below).
K (potassium)
Ca (calcium)
Mg (magnesium)
S (sulphur)
Micro nutrients
B (Boron)
Cl (chlorine)
Cu (copper)
Fe (iron)
Ma (manganese)
Mo (molbdenum)
Zn (zinc)
So what do you do now? Why fertilize? What about natives?
Well, decisions decisions hey. Lets get one thing very clear, over doing things can be harmful too, even toxic. Some natives will burn if phosphorous is applied. So you really need to read the labels. Australian soils are amongst the oldest and most nutrient depleted in the world. Many of the tree species have evolved with modified roots systems such as casuarinas, banksias, hakeas etc have adjusted to the conditions, they have what's called a proteoid root system. So you will often see special fertilizers for Australian natives. Most if not all organic fertilizers are safe for natives, it's those inorganic ones you need to watch, too much P and it may burn the tree, so look for a low "P" on the NPK numbers.
You fertilize to keep trees healthy and add nutrients to their environment. They need a balance of nutrients (just like us) to grow and defend themselves from a jungle of attackers (borers, wasps, scale, thrips, mites, caterpillars etc etc).
Fertilizing stressed or wounded trees
A tree will compromise self defence for growth. This is extremely important to know because fertilizing a stressed tree with the wrong NPK ratio could induce growth, the tree then looks good to the layman but then becomes infested and has little resources left for defense. The secondary predators attack and the tree can even die.
The main nutrient you need to avoid is N (nitrogen). Nitrogen promotes growth which is not what you want from a stressed tree. Buy organic fertilizer with trace elements, most are low in N, or look for a low N inorganic. The objective here is to have a sound nutrient base, moisture and mulch so the tree can have reserves to grow callus wood and sap to flood out borers.
What time of year to fertilize
This also depends on the climate you have. Here in Brisbane it is likely to have a year around growing season depending on moisture levels. In southern states where there's distinct seasons and dormancy it's important to get it right.
A tree has to have leaves and be photosynthesizing to make food. So fertilizing in winter (cooler climates) or when trees have dropped their leaves is bit of a waste. But it could be the opportune time to get the soil conditioning right for the next growing season. In the beginning of spring trees use their stored food reserves to grow leaves, branches, buds etc. It is wise to lightly fertilize as the tree develops it's new crown ... however if you have been a green thumb you would have already prepared this with the winter soil conditioning. Fertilize regularly from late spring thru to autumn. In autumn and as the climate cools the trees begin to store food reserves. This is the crucial time to assist the tree in having plenty of storage for the next spring burst after winter.