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Seasol’s Guide to Fertilisers

Have you ever wondered what a fertiliser is and how it works in your garden?

Follow our easy guide to “all you need to know about fertilisers” including:

What is a fertiliser?

How does a fertiliser work?

What types of fertilisers are there?

When should fertilisers be used?

What fertilise should be used on what plant and when?

What nutrients does a fertiliser contain?

How to select the best type of fertiliser for your specific plant?

What is fertiliser?

A fertiliser is any substance, either natural or chemical-based, that is applied to soil or plants to increase yield or performance and comprised of nutrients essential for plant growth.

To grow, plants need sunlight, water and carbon dioxide. To thrive they require essential nutrients referred to as macro (or major) nutrients and micro (or trace) nutrients. These essential nutrients are considered mineral nutrients and in an ideal world are absorbed by the plants through the soil.  There are many contributing factors that can affect the uptake of these essential nutrients and unfortunately, most soils across the planet are deficient in one or more elements.

The use of fertilisers dates back thousands of years, but it wasn’t until the early 20th century that agricultural and horticultural practices changed forever with the development of artificial fertilisers. Almost overnight there was a product available to farmers that would increase crop yields, was easy to transport and apply and allowed the world to feed a growing population with the same land resources.

Plant growth at different stages

How does fertiliser work?

Fertilisers supply essential nutrients to plants for balanced growth. Broadly speaking a fertiliser is any substance that is applied to the soil or foliage that will increase the health, disease resistance, productivity and yield of a plant. There are many factors that contribute to the effectiveness of nutrient uptake by plants. These include:

  • Soil acidity or alkalinity
  • Availability of moisture and rainfall
  • Organic content of the soil
  • Existing nutrient availability in the soil
  • Types of plants grown

Artificial fertilisers are specifically formulated to supply nutrients to a particular crop and usually are made up of macronutrients (Nitrogen, Phosphorous and Potassium – NPK) and micronutrients. When artificial fertilisers are manufactured the available nutrients are concentrated and converted into a compound that a plant can absorb (or take up) readily. If a plant is unable to take them up quickly many of these readily absorbed nutrients are water-soluble and will leach through the soil to waterways.


Organic fertilisers

Organic fertilisers are any substances that were once living such as manure, biosolids, leaf litter, crop residues and any by-products from meat production facilities such as blood and bone. These fertilisers have a lower NPK ratio than artificial fertilisers but also have soil conditioning properties.

Nutrients in organic fertilisers are not as readily available to plants as artificial fertilisers. Organic fertilisers require a conversion process so they can be absorbed by the plants;

This conversion process is carried out by;

  • Naturally occurring soil fungi
  • Beneficial bacteria
  • Animals such as earthworms, nematodes and arthropods
  • Microbes

The by-product of the conversion process is an improvement in soil conditions. These soil conditioning properties further improve the water holding capacity of the soil, increase soil structure and aeration. In addition, there is an increase in microbial activity, which in turn reduces the leaching of excess nutrients.


Artificial fertilisers – nutrient conversion for plant availability happens in a factory.

Organic fertilisers – nutrient conversion for plant availability happens naturally in the soil.

Garden and Seasol

What types of fertilisers are there?

There are many different types of fertiliser available on the market and to the new gardener selecting the right formulation for the garden can be overwhelming.

Types of fertiliser;

  • Complete inorganic fertilisers are often referred to as NPK formulations. They will give immediate results and deliver nutrients quickly to plants. They are available to buy as a granular or liquid formulation. Controlled release formulations such as PowerFeed Controlled Release All Purpose including Natives are also readily available and release nutrients through temperature and moisture. With controlled release formulations, there is almost no leaching and wasting of nutrients as plants will uptake nutrients as they are released.
  • Organic based fertilisers are not totally organic and have a mixture of organic and inorganic nutrients. Organic based fertilisers such as the PowerFeed range of products have been fortified with added minerals to increase the immediate effectiveness of the formulation. In addition to the readily available nutrients, these formulations have soil conditioning properties.
  • Organic fertilisers and plant tonics are those fertilisers and plant tonics that are made from a substance that was once living such as animal manures, seaweed, kelp such as Seasol, spent cereal crop waste or peat. These formulations are often fortified with naturally occurring rock minerals or nutrients.


What to do in the garden in September

  • Specialty fertilisers have been specifically formulated for different plant types. E.g. Roses have different nutrient needs to Australian plants or ferns. Specialty fertilisers take the guess work out of feeding and will deliver nutrients and quantities to maximise plant growth for a specific group of plants. This group of fertilisers are either available as a liquid, granular or controlled release formulations such as the PowerFeed Controlled Release Plant Food range and correct selection depends on what is easiest for a situation and type of plant.
  • Liquid fertilisers have nutrients suspended in solution. They are diluted with water and applied either via the soil or over the foliage of plants such as the PowerFeed All Purpose including Natives. Liquid formulations are absorbed readily and are most effective for specific nutrient deficiencies and individual plant groups. When applied over foliage there is almost no leaching as nutrients are suspended in a low volume solution and there is minimal run off. Liquid fertiliser applied via the soil is also an efficient way of feeding plants. Organic and organic based liquid solutions will also have soil conditioning properties, delivering soluble available nutrients and soil improving in one.
  • Slow release fertiliser is the name often given to organic formulations, such as manure or compost. Often confused with controlled release fertilisers their action is a little different. While controlled release formulations release nutrients through temperature and moisture in a controlled manner, slow release formulations such as PowerFeed with Trofore All Purpose including Natives deliver nutrients to plants over a period and the release of nutrients is not controlled. As the manures and compost breaks down, they have additional soil improving properties. Both controlled and slow release formulations are less likely to burn plants and it’s difficult to overfertilise.


When you should use fertiliser.

Fertilisers are an essential part of a plant health and maintenance program. Most naturally occurring soils on the planet are deficient in one or more of the essential nutrients that plants require to grow. Many of the plants we choose to grow in our gardens occur naturally in very different soils to what gardens we have in Australia.

Fertilisers will not revive dead plants overnight. What they will do is provide a continual supply of nutrients to ensure plants build up a natural resistance to disease, perform to their potential and thrive.

Even after improving soils with organic matter, compost and manures most garden plants will require regular feeding to stay healthy and flower and fruit. As gardeners, we expect a lot from our gardens. Gardens can be likened to an intensive horticultural set up on a mini scale. Every flower, leaf or piece of fruit we remove from the garden is removing essential nutrients from the soil. For garden plants to continually thrive these nutrients must be replenished, either by organic matter or a specially formulated fertiliser.

Fertiliser should be applied throughout the growing season and depending upon the type of fertiliser used, it’s best to use a small amount regular rather than a lot once a year. When plants are actively growing, flowering and fruiting their nutrient needs are at their peak.

Applying fertiliser when plants are dormant is a waste of resources, especially if using soluble formulations, as there is potential for leaching of nutrients past the plant’s root zone.

Fertilising lemons with PowerFeed

A general guide to plant fertilisers:

  • Citrus – throughout their growing season. Apply granular fertiliser around the root system every 4-6 weeks from August to May. Liquid formulations such as PowerFeed PRO SERIES Flowers, Fruit & Citrus can be applied fortnightly (especially if plants are in pots) around the soil.
  • Ferns – require mild doses of fertiliser as they are prone to burning and will benefit from applying organic or organic based solutions if growing in the ground. Apply at the start of every season PowerFeed Controlled Release All Purpose including Natives. Most ferns grow naturally in high organic content soils, so replicating these conditions will benefit the plants. Organically based liquid formulations such as PowerFeed All Purpose including Natives will feed the plant and improve soil conditions. Apply 10mL of concentrate per 9 litres of water (standard watering can or bucket) every 2 to 4 weeks.
  • Fruit Trees (deciduous) – require regular feeding throughout the growing and fruiting season from August to May. A speciality formulation for fruit trees such as PowerFeed with Troforte for Flowers Fruit & Citrus will contain all nutrients required for fruit development and flavour.
  • Garden Shrubs – depending upon the variety will respond to regular feeding at the beginning of spring, summer and autumn with a complete granular formulation. At the beginning of winter, an application of organic based fertiliser such as PowerFeed with Troforte All Purpose including Natives will benefit the health of the soil, which in the long run benefits the health of the plants.

Feed lawns with PowerFeed

  • Indoor Plants – due to the artificial conditions they are placed in and the climate-controlled conditions indoors there is not a defined dormant season for these plants. Applying a slow release granular fertiliser or a controlled release formulation such as PowerFeed Controlled Release Pots & Planters Indoor & Outdoor at the beginning of each season will deliver a supply of nutrients gradually over time. This will also minimise salts building up in pots. Indoor plants tend to be in small pots with minimal soil around roots. Indoors benefit from regular (fortnightly applications) foliar feeding with a soluble formulation such as PowerFeed PRO SERIES Pots & Plants Indoor & Outdoor.
  • Lawn – requires regular fertilising with a quality granular fertiliser throughout the growing season. Try PowerFeed LawnFeed or more specific for Buffalo lawns PowerFeed Buffalo LawnFeed.
  • Native – it is very easy to over fertilise native plants with fertilisers that contain readily available nutrients. It is the reason to use specific native fertilisers or low phosphorous formulations at the beginning of spring and autumn. Australian plants have evolved over thousands of years and developed specialised roots to extract required nutrients out of deficient soils. A sudden application of extra nutrients will result in the death of certain plants. NB – Many South African species, such as Leucadendron and Protea will benefit from using native fertilisers such as PowerFeed with Torforte All Purpose including Natives at the beginning of spring, summer and autumn.


  • Potted Plant – will require regular fertilising with a liquid and a granular or controlled release formulation. This two-pronged approach will deliver nutrients required to support continual growth in a restricted environment and the liquid formulation will be absorbed quickly through the foliage. Apply granular at the beginning of each season and liquid fertiliser every fortnight from August to May. Check out the range of PowerFeed liquid and controlled release plant foods for potted plants.
  • Roses – are heavy feeders. We expect a lot from our roses, including almost continual flowering throughout the warmer months. Monthly applications of granular rose specific fertilisers such as PowerFeed with Troforte Rose Food & Soil Improver which will result in strong, healthy plants and many flowers. It’s important to select a rose fertiliser that will also improve the root system and build the plant’s natural resistance to disease.  If you want to apply a fertiliser to your roses more often, try PowerFeed PRO SERIES for Roses & Flowers ever 2 to 4 weeks.
  • Shade Plants – plants growing in shade are in a unique situation. If the shade is cast by a fence, building or structure the fertiliser application times are the same as general garden shrubs. If gardens are growing under the shade of large trees organic fertilisers are preferred for soil application. Large trees will quickly develop fine feeder roots that will take the nutrients before the understory plants have the full benefit of nutrients and organic fertilisers will condition the soil benefiting and strengthening the plant’s roots. Liquid foliar fertilisers such as PowerFeed PRO SERIES All Purpose including Natives come into their own when plants have root competition because nutrients can be targeted to species that require feeding. Low volume, high pressure (through a small pressurised sprayer) is the most effective way of applying foliar fertilisers on shade plants.
  • Trees – will benefit from a granular fertiliser at the beginning of each season such as Seasol Plant + Soil Booster.  The soil conditioner properties of organic compost, natural composts and humic acids will benefit the tree in the long term.


What is NPK for plant life

What does each nutrient do?

Different plant groups have varying requirements for nutrients.

The correct balance of nutrients is important for optimum plant growth. It’s a case of more isn’t better as excessive amounts will have detrimental effects on plants and often result in death.

Macro (or major) nutrients are those nutrients required by plants in greater quantities than micro (or trace) nutrients.

Macro nutrients:

N         Nitrogen is required for leaf growth and protein formation. Too much nitrogen results in soft weak growth and susceptibility to pests and diseases. Plants over fertilised with nitrogen will not survive drought conditions and are often referred to as ‘thirsty’, at the first sign of drying conditions they will collapse.

P          Phosphorous is essential for strong root development and fruit and seed production. Phosphorous deficiency in plants manifests itself as a red or purple tinge on foliage and stunted root and tops growth.

K         Potassium contributes to a plant’s resistance to pests and diseases. This nutrient plays an important role in the vascular system and the translocation of vital plant sugars. Potassium thickens cell walls and is responsible for strong stems in all plants. It will also intensify flower colour and perfume, an essential element for roses.

Potassium deficiency will be identified by scorching of leaf edges and tips, reduction in flower size and early leaf shedding.

What is Magnesium, Calcium and Sulphur for plant life

Secondary nutrients:

Calcium, Sulphur and Magnesium are known are secondary nutrients and are important elements required by plants:

Ca        Calcium is essential for building cell structure and cell walls in a plant and plays a vital role in the uptake of many nutrients.

Calcium deficiency results in stunted roots, distorted growth and stress symptoms on new foliage.

Mg       Magnesium plays an important role in the photosynthesis process and is an important part of chlorophyll, the compound that makes leaves green. Magnesium is movable within the plant, so deficiency shows up in lower and older leaves.

Magnesium deficiency shows up first as pale leaves and often leads to chlorosis on foliage that takes on a patchy appearance with yellowing between veins.

S          Sulphur is now thought of as the 4th macro nutrient, even though it is only required in small amounts it is essential for plant growth. Sulphur is found in organic matter however isn’t available to plants until it has been converted by microbial activity in the soil. It is a building block of protein chains and essential for chlorophyll formation.

Sulphur deficiency is first noticed in plants as pale leaves. It is not mobile around the plant, so deficiency will show up in new leaves. Sulphur deficient plants are small and stunted.



Fe        Iron is essential for plant enzyme systems and chlorophyll production.

Mn      Manganese is essential for photosynthesis.

Zn        Zinc is needed for the plant hormone that is responsible for stem and leaf expansion.

         Boron assists in the formation of cell walls in the rapidly growing tissue.

Mo       Molybdenum helps bacteria and soil microbes convert nitrogen from the air to a soluble form available to plants.

TIP: Avoid fertilising plants when they are water stressed, have experienced transplant shock or have sustained damage in some way. Roots are susceptible to burning.

Choosing the right fertiliser for your plant

How to select the right fertiliser.

Selecting the right fertiliser can be a little daunting as there are so many choices on the market.

There are a few questions to ask:

  • What type of plants do I want to fertilise?
  • How much time do I have to apply fertiliser?
  • Am I looking for soil conditioning properties in addition to plant nutrients?
  • Do I want to just apply and forget?
  • What type of soil do I have?

Answering these questions will make fertiliser selection easier. It will also ensure your fertiliser choice will live up to the expectations and will do what it was purchased to do.


How to apply Fertilisers to your plants

Spreading Fertiliser.

Achieving even coverage is important when applying fertiliser. There is less risk of over feeding and causing damage to plant’s roots. It is important to follow the directions on the container as over fertilising can cause long term damage and even death of plants in extreme cases.

When fertilising trees and shrubs apply granular formulations around the drip line of the tree, this is the area directly under the circumference of the branches and leaves. Do not apply granular fertilisers to close to the trunk or stem or the plant as this may burn it. If applying over the top of mulch apply before a shower of rain or water, it in so the granules are in contact with the soil surface.

When fertilising pots sprinkle evenly around the soil surface and water in well. Always wash any fertiliser off the foliage after applying to reduce the potential of burning delicate leaves.

Liquid fertilisers can be applied via watering can or a pressurised garden sprayer. Watering cans apply high volume quickly and are the ideal choice for watering pots in or feeding and watering at the same time.

Pressurised sprayers are lightweight and easy to cart around. They are the ideal choice for applying soluble nutrients that will be absorbed by foliage only.

TIP: When applying granular fertiliser, a handful is roughly 30-50grams.

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