Hi, welcome to Taiga Bonzai in this post we discuss the purpose of fertilizer, often misunderstood and misused.
Introduction – the ideology that fertilizer is a source of food is a misnomer, because plants produce their own food in the form of sugars via photosynthesis and moisture from the soil. The minerals in fertilizer provide the ingredients needed for photosynthesis and growth, when minerals are deficient or absent in the soil, fertilizer is added to maintain an adequate supply.
Plants – in their natural settings are able to survive quite well mainly due to their root systems, which have the ability to spread and travel great distances in search of moisture and nutrients. But bonsai are confined to relatively small containers and thus are restricted from this practice therefore, they have to be fertilized. The questions of which fertilizer to use, solid or liquid, what is the dosage rate and how often to use it, one might assume it would be relatively straightforward and this is where mistakes are made, which in many cases cannot be undone; hence a little more thought on the subject is required.
The pH factor – the first step is understanding from where the plant originates and the soil type in which it is grown be it ericaceous (coniferous) or organic (deciduous) and the relevant pH factor. Detailed descriptive articles on these topics can be found on this site, ‘Bonsai Soils’ March 27th 2016, ‘The pH factor (part I)’ April 22nd 2017 and the ‘The pH factor (part II)’ May 6th 2017, these are important steps in the learning curve of knowing, which directive to adopt; especially for those new to bonsai horticulture.
Soils – of course many horticulturists make there own soil compositions depending on the species and their specific needs, some use Akadama (akadamatsuchi, red ball earth) a naturally occurring, granular clay mineral used as soil for both deciduous and coniferous species, Seramis, Turface and Oil-Dri that are fired clays whilst others will use soil from the same location of where the plant originated; if at all possible.
However, there are other factors to consider because soil contains a multitude of living organisms that consume, digest, and cycle nutrients. These include archaea, bacteria, actinomycetes, fungi, (mycorrhiza) algae, protozoa, and a wide range of insects; mites, nematodes, earthworms and ants all of which are important to the vitality of a soil composition. Such organisms are classed as either acidophiles, (that thrive under acidic conditions) Neutrophiles (that exist in a neutral pH environment) and Alkaliphiles (a class of extremophilic microbes capable of survival in alkaline environments).
Another factor is the balance or imbalance of a soil’s chemical structure and the three primary nutrients are nitrogen (N), phosphorus (P) and potassium (K). Secondary nutrients include sulphur, calcium and magnesium, minor nutrients consist of iron, manganese, copper, zinc, boron, molybdenum and chlorine. Soils with high acidity may have toxic amounts of aluminium and manganese.
Nitrogen (N) is essential for growth and a necessary part of chlorophyll helping plants photosynthesise, phosphorus (P) is needed for the development of flowers, fruits, and root systems. Potassium (K) keeps roots healthy, aids in the flowering/fruiting process and assists in aiding plants tolerate stress to some degree through periods of drought therefore, a soil test kit is advisable to check the balance between them. (See article the ‘The pH factor (part II)’ section Soil testing applications).
Having conducted a soil test and determined the balance or imbalance one can select a fertilizer that will give you the correct amount of nutrients required. On any package of fertilizer be it powder, granulated or liquid there are three numbers that correspond to the amounts of nutrients in the product for example. 5-5-5 is referred to as a balanced fertilizer due to (N) (P) and (K) having equal quantities. Other fertilizers may show a different numbering for example, 4-10-6 which indicates that (N) is low (P) is high and (K) is medium. This numbering system is the same for all manufactured fertilizer products regardless of their form.
However, different species require fertilizers suitable to their needs for example, conifers are not considered to be heavy feeders hence one annual application of a complete garden fertilizer such as 10-10-10 or 16-8-8 will suffice, which can be applied in early spring before the plants break dormancy, or in late autumn. Deciduous trees and shrubs require a more well-balanced fertilizer, in which the three main nutrients are closer in proportion, such as 10-10-10, that provides nitrogen for green healthy foliage, phosphorus and potassium for flowering, fruiting and root development.
Applying fertilizer – how often are bonsai trees fertilized due to their confinement? There are a lot of arguments on this topic because all species are individuals each having their own requirements. Some advocate a weekly basis whilst others state fortnightly or monthly is adequate in addition, fertilizer should be given sparingly after watering has taken place. The general consensus that bonsai can or should be fertilized during the entire growing season from early spring to mid-autumn has logic however, older mature trees are often fertilized less frequently. Much depends on the species, time of year, stage of development and health; indoor trees can be fertilized all year round. The problem with over fertilization boosting the (N)-(P)-(K) levels can weaken and stress out a plant often causing its demise.
Which form of fertilizer to use, powdered, granulated or liquid? – There are many different opinions from all quarters on this subject, some argue that liquid is better because it is instantaneous, but it usually drains out of the bottom of the container although much depends on the quantity given. Some maintain that a top dressing of powdered fertilizer is better as it penetrates into the soil after watering, although the majority of the particles will remain in the pot there will be a loss due to drainage. Others plumb for granules tiny pellets that are mixed in the soil when the container is being prepared, these are slow release and are preferred by many.
Another factor to consider is the cost of fertilizer, many fertilizers if in liquid form normally come in small bottles and if purchasing online the cost of shipment is added increasing the overall price. A 250ml bottle diluted 4 times (1.5 litres) will not last long during a season although much depends how many trees are in the collection and how often they are fertilized. Whereas half a teaspoon of granulated (slow release) added to the soil medium at repotting time will last at least two seasons. If one is unsure of what fertiliser to use the well known horticulturist and TV presenter Alan Titchmarsh gives a good presentation on this subject. (link to his presentation is given below)
As stated fertilizer is ‘not food’ nor is it a ‘one-size-fits-all’ it is a way of replenishing the nutrients within the soil medium which the plant needs for healthy growth and each species will require a specific level of nitrogen (N), phosphorus (P) and potassium (K) to sustain this. Hence it is advisable to do a soil test before purchasing fertilizer, because there has to be an accepted balance between (N), (P) and (K) for each species, this does not mean to say that you need an assortment of fertilizers far from it; one for conifers and one for deciduous will suffice. Until next time, BW, Nik.