Bonsai Tree Fertilisers & Tonics

In order to keep a bonsai healthy, strong and resilient, good quality horticultural practice is vital and the use of a fertiliser is an important part of that practice. Here are the basics........

• Nutrient reserves are constantly depleted from the soil by absorption and leaching out through watering. These must be replaced by use of a fertiliser. For bonsai use a 'balanced' fertiliser is generally recommended, this means a fertiliser product that contains major, minor and macro nutrients in appropriate proportions as determined by the manufacturer. A balanced fertiliser is a one pack option and in general is the only product required to keep a bonsai tree healthy.
• Chemical bonsai fertilisers. Plants only absorb chemical compounds. Where a chemical fertiliser is used these are presented directly to the roots as soon as they are added to the soil. Within hours they are fully absorbed. Applying too often or in too strong a concentration can cause a chemical burn to the roots which in extreme circumstances can kill a plant. Chemical fertilisers need to be added very regularly as they are either entirely absorbed or leached out of the soil quickly.
• Organic fertilisers are natural compounds that contain nutrients in a complex form not instantly available to plants. Organic fertilisers are broken down by bacteria and fungi and as a result the nutrients are released slowly, often over several weeks or months. This means the plant has access to small amounts of nutrient all the time which in bonsai is very beneficial, it produces a stronger healthier and more resilient plant that grows in a more controlled manner. Organic fertilisers also generate lots of beneficial microbial activity within the soil. There is very little risk of fertiliser burn using organic fertilisers.
• Liquid fertilisers are to be diluted with water and applied directly to the soil in the same way as regular watering.
• Solid fertilisers are simply placed on the soil surface. As you water in the regular way these are slowly dissolved and broken down by microbial activity and disappear into the soil.
• NEVER fertilise a tree which is not actively growing. Only apply fertilisers when a plant is in active growth, if you don't see the plant growing don't apply fertiliser.
• Never fertilise a plant that is sickly, stressed or recovering from any form of trauma. Fertilisers are NOT plant food and you cannot 'feed up' a poorly plant, in fact applying fertiliser in this case will cause possibly irreversible damage.
• Unless the temperature is constantly above 12 degrees Celsius fertilisers cannot be absorbed by plants. Winter 'feeding' is a fallacy, don't bother.
• The golden rule is 'little and often', where fertilisers are concerned more is definitely not better. 
• Higher nitrogen fertilisers are used in spring time to support rapidly developing new growth.
• Higher potash fertilisers are typically used around the end of summer to harden off new growth before winter.
• If in doubt use a fertiliser at half the recommended strength but apply more often.
• Yellowing leaves rarely indicates a nutrient deficiency. In bonsai yellowing foliage is more often related to over or under watering or poor situation, giving too much or too little light or exposure to the elements or repotting may be long overdue. If you are regularly using a good balanced fertiliser product yellowing foliage will almost always be down the one of the above factors.