In a garden plants are in a situation where they are competing for nutrients. When plants receive all the nutrition they need they have lovely lush growth, plenty of flowers and strong resistance to pests and diseases. So it is a food idea for gardeners to add fertiliser to fortify the plants.
Go into any nursery and you will see a huge range of fertilisers all promising amazing results for your garden. Knowing a few tips could save you a lot of money, while ensuring your garden is the best it can be.
Spring is the ideal time to add fertiliser as plants are getting up and growing.
Different plants require different amounts of fertiliser. For the most part, garden plants will survive and thrive with one application annually in spring. Other plants such as herbaceous perennials or roses, which are highly productive may require a top up while they are flowering and fruiting.
The basic elements of a balanced fertiliser are Nitrogen, Phosphorous and Potassium. These are commonly known by their elemental reference of NPK. They are the nutrients that plants use the most of. You can purchase these elements individually to create your own unique fertiliser, but it is easiest to buy a ready blended option at your local nursery.
N – Nitrogen, used for foliage and plant growth.
P – Phosphorous, encourages root growth.
K – Potassium, boosts overall plant health (ripens wood) and encourages flowers and fruits.
The ratio of NPK is listed on the side of the fertiliser packaging as a percentage. In addition to these ‘macronutrients’, many fertilisers contain secondary and or trace or ‘micronutrients’.
S – Sulphur
Ca – Calcium
Mg – Magnesium
Fe – Iron
B – Boron
Zn – Zinc
Cu – Copper
In Australia most plants require fertiliser at the start of spring, when they are beginning to grow.
The best time to fertilise is before it rains, as the fertiliser will settle it into the soil without you having to water it in. The plants need to absorb the nutrients through the soil, not directly from the granules so need water to activate.
When you scatter fertiliser onto garden soil it is known as ‘topdressing’. Once you have put it on the soil, it should be scratched or lightly dug in to prevent it forming a crust on the top. Once this is done it is important you water the fertiliser in.
Fertiliser is best applied at the drip line. The drip line is where the feeder roots are that readily absorb the nutrients. The drip line is where the outer foliage of the plant is, it is also where the water runs to. Applying fertiliser too close to a stem, or directly to plant roots can burn the plant. If left on the leaves, the salts in granular fertiliser can also cause the foliage to burn.
If you think of a tree’s canopy as an umbrella, the drip line is the outside boundary where the water would run off.
‘Basedressing’ is when you dig the fertiliser through the soil prior to planting. This is good practice when creating new gardens. This method is especially suited to organic fertilisers.
As a general rule, you should avoid fertilising native plants, unless you are using a specific fertiliser as they don’t tolerate high levels of phosphorus.
Organic fertilisers are derived from animals or plants, they include manures, bonemeal, fish, blood and bone and seaweed extracts. This type of fertiliser is mild on the soil, but takes longer to become ‘active’ (be able to be absorbed by the root system), as the microorganisms in the soil first need to convert it into useful nutrients for the plant to take up.
Using this type of fertiliser may require you to add a combination of fertilisers to achieve an effective NPK ratio. For example Blood and Bone, while a good organic fertiliser does not contain potassium, so you need to add some sulphate of potash (high in potassium) to balance out the elements.
If you are planting a new garden bed we highly recommend adding organic fertilisers to the soil prior to planting to improve structure and add nutrients. They are useful in existing gardens as they are mostly environmentally friendly.
Also known as inorganic, controlled release or slow release fertilisers. Examples of these include Osmacote, Acticote or Miracle Gro, and some that are formulated for specific plants such as citrus or roses.
Artificial fertilisers are quickly absorbed into the soil and take all the hard work out of fertilising, even though they do cost a little more. They offer an easy to apply convenience, have consistent nutrient levels, come with instructions and even indicate how long they will last. The granules or prills just need water in order for them to release the nutrients into the soil so they can become accessible to the roots.
Artificial fertilisers are especially good for pots as they don’t smell and are easy to apply. In the garden it is better to apply small amounts often to avoid run off and damage to your soil from excess nutrients. You should work the granules through the top level of the soil with a garden tool to ensure they can be better absorbed.
Artificial fertilisers have a special coating which allows the granules or pills to emit more nutrients during warm, wet weather – which is when plants are growing the most – and less when it is cold, and or dry – which is when plants are growing the least. This makes them extra useful as they correlate their release with the natural growth cycle of the plant, and when it is cold they remain inactive so are not wasted or lost in run off.
Always follow the directions on the packet for correct dosage.
These fertilisers give plants a kick start as they are readily absorbed by the roots. It is a good idea to add this type of fertiliser when establishing new plants, or to promote fast growth for plants such as annuals. Liquid fertilisers are also useful for pots. It is a good idea to water before applying this type of fertiliser so it can be more easily absorbed. Liquid fertilisers usually need to be diluted and applied with a watering can so they are not practical for large gardens.
These are usually applied through a spray. They are quick to activate and useful if the soil conditions will not allow for nutrient absorption. The plants soak up the nutrients through their stomata or leaf pores. Foliar fertilisers are especially useful if plants are suffering from micronutrient deficiencies. The drawbacks are it can be expensive and it is impractical for large gardens.
In addition to fertilisers, there are other elements which can be added to your garden soil. These elements include secondary and micronutrients that work to improve soil and plant health, these include:
Lime – commonly known as a soil sweetener, lime is a natural soil conditioner that will increase the PH level of soil. When a soil is fertilised, it becomes more acid, so over time this can build up and affect your plants ability to absorb nutrients, especially micronutrients. The ideal soil PH is between 5.5-6.5 as this is when the microorganisms best function to convert nutrients into a usable form for plants to absorb. If they PH is higher or lower the nutrients are tied up or insoluble.
Adding lime improves the PH levels which encourages the microorganisms and increases the availability of nutrients.
Plants such as Peonies love lime as they originate in areas with alkaline soils. Conversely, ericaceous (acid loving) plants such as Rhododendrons will suffer with too much lime in the soil. If you use lime in combination with a high nitrogen fertiliser such as blood and bone, it will react to form a gas and will be ineffectual. You can use a PH tester kit to see if your soil requires lime.
Gypsum Lime – this is a form of lime used to aerate heavy clay soils. It is physical rather than chemical in nature so will not affect PH levels, but it does add calcium. It is useful to improve soil structure and drainage.
Dolomite and Garden Lime – these are the soil conditioning limes that raise PH. Garden Lime (Calcium carbonate) adds calcium to the soil, it is fine so easily absorbed. Dolomite is coarser so takes longer to absorb, and it has trace elements of magnesium.
Magnesium Sulphate – this is usually applied as Epsom Salts. It is used to add magnesium to deficient soils. A magnesium deficiency is usually indicated in a plant by yellow leaves.
Sulphate of Potash – This is an easily absorbed form of potassium for plants. It is used to encourage flowers, fruit and all round health.
Iron – is used by plants to produce chlorophyll and carry elements throughout the plants circulatory system. If your plants are deficient in iron, new growth may be white or yellow – you should test your soils PH if this is the case, because it may have ample iron, but the PH may be too high to make it readily available to your plants. Encouraging a more acidic soil by adding lots of well rotted organic matter will make the iron more available. If iron levels are low, you can add rusty nails or iron chelates to the soil.
Seaweed – this is more of a tonic for plants as it doesn’t contain a significant amount of the essential NPK elements to qualify as a fertiliser. It is useful for helping plants settle in and to fortify them as they grow. It contains trace elements and bioactive compounds. Seaweed tonics also encourage microorganisms in the soil.
Calcium – helps to strengthen the cell walls of plants and to carry nutrients. It can be added with Garden Lime.
Compost – this is one of the most important elements of great soil. While the nutrients in compost are not high enough to change soil composition, they do help the soil to retain existing nutrients and enable them to be better available to plants.
Copper – facilities plant growth, without it plants may become sterile, fail to thrive and produce flowers. It can be added with copper sulphate and copper oxide.
Sulphur – this is used to acidify soil that has become too alkaline.
Boron – this micronutrient is essential for plant growth and development. A soil is deficient when it has less than 0.14 mg/kg. If it is deficient you can use borax to add it to the soil.
Zinc – A micronutrient that is in plant enzymes and proteins. A deficiency can affect growth, hormone production and cell formation. It can be added through Zinc chelates or Zinc sulphate.
Fertiliser is not always the answer, if your plant is not thriving, it may be due to too much or too little water, light levels, or you are trying to grow the wrong plant for your climate. Make sure this isn’t the case before you load on the fertiliser.
• If you over fertilise your plants it can lead to straggly growth that is easily toppled.
• Having excess fertiliser in the soil can cause new growth at the wrong time which may be susceptible to frost damage.
• The verdant foliage from increased growth can be a magnet for slugs and snails.
• Too many nutrients can kill the fragile microorganisms living in your soil. The salts from the minerals can also flow on to waterways and damage ecosystems.
• Using too much of an organic manure fertiliser can be hazardous, especially if it is applied when it is too fresh as the ammonia it produces may be toxic to plants.
• If you apply granular fertiliser in dry conditions it can damage surface roots.
• Too much fertiliser may cause the total dissolved salts (salinity) to increase in the soil which will cause plants to be stunted and to wilt.
If you are unsure how much fertiliser to apply, a bit less is always better than a bit more.
As a general rule fertiliser should be applied in spring as the weather is warming and plants are beginning to grow. Always check your growing guide to see if exceptions apply to this rule. We also recommend reading the directions on your fertiliser for best results.
Generally new plant s require more fertiliser than established plants, which is another good reason to dig through new beds with some organic matter.
While establishing trees and shrubs need a light dressing of a fertiliser high in phosphorous for root growth and potassium for overall health. This is most beneficial in autumn for deciduous trees as they don’t have any vegetative growth.
We hope this helps keep your garden happy and healthy.