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Plants, like all other living creatures, have nutrient requirements necessary for growth. Their roots extract nutrients from the soil to fuel processes within their cells, continuously taking in nutrients that have attached to soil particles or organic matter. Over time, this continuous feeding can deplete even the most fertile soil, making it necessary for gardeners and growers to replenish the nutrient reserves. Fertilisers are applied to the soil to add nutrients and encourage strong, vigorous plant growth.

Plant Essential Nutrients

The theory of plant essential nutrients is fundamental in plant nutrition. Essential nutrients for all plant growth include:

  • Major Nutrients: Nitrogen (N), Phosphorus (P), Potassium (K)
  • Secondary Nutrients: Calcium (Ca), Magnesium (Mg), Sulfur (S)
  • Micronutrients: Boron (B), Chlorine (Cl), Copper (Cu), Iron (Fe), Manganese (Mn), Molybdenum (Mo), Zinc (Zn)

These nutrients each have specific roles within the plant. When any of these nutrients are deficient, plant growth will be affected.

Nutrient Depletion and Replenishment

Over time, the levels of plant essential nutrients found in the soil profile deplete due to various reasons, such as plant uptake, water runoff, and soil erosion. As these levels reduce, plants will show deficiency symptoms, highlighting the importance of reintroducing nutrients to depleted soils through fertilisers and other plant foods like manure or compost.

Fertiliser Basics

Fertilisers come in many forms, but in simple terms, they are chemicals or natural substances added to soil or land to increase fertility. They add nutrients back to the soil, providing food for plants.

N-P-K Ratio

Understanding the N-P-K ratio is crucial. Fertilisers are labeled based on the three main elements they contain: nitrogen (N), phosphorus (P), and potassium (K). This universal labeling method makes it easier for consumers to compare products.

  • N-P-K Ratio Example: In a 10-10-10 formulation, a 10-kilogram bag of fertiliser would contain 1 kilogram each of nitrogen, phosphorus (as P2O5), and potassium (as K2O).

Types of Fertiliser

Since it was first understood that plants need nutrients to grow, many products have been developed for agricultural and horticultural use. Here are the main types of fertilisers:

Conventional (Inorganic) Fertiliser

  • Characteristics: Made from synthetic materials; nutrients are quickly available to plants.
  • Advantages: Quickly fixes plant deficiencies; generally less expensive and readily available.
  • Disadvantages: Nutrients can leach out of the soil easier, causing possible environmental problems.

Organic Fertiliser

  • Characteristics: Derived from all-natural ingredients; nutrients are not always readily available for plant uptake.
  • Advantages: Adds organic matter to the soil, improving water retention and soil structure.
  • Disadvantages: Often appears to have lower nutrient contents due to labeling laws; nutrients take longer to be released.

Common Fertilisers and Plant Foods

There are many options available for feeding your garden plants. You can choose commercial products or apply natural ingredients to enrich the soil.

Blood Meal

  • Source: Dried blood of slaughtered animals.
  • Nutrient Ratio: Typically around 12-0-0.
  • Benefits: Excellent source of organic nitrogen; increases soil acidity and deters pests.

Bone Meal

  • Source: Finely ground animal bones.
  • Nutrient Ratio: Typically around 3-15-0.
  • Benefits: Provides slow-release phosphorus without adding too much nitrogen or potassium.

Compost Tea

  • Source: Liquid produced by “brewing” compost.
  • Application: Apply directly to leaves or soil once a month during the growing season.
  • Benefits: Adds beneficial microorganisms and soluble nutrients.

Fish Meal

  • Source: Ground, dried fish.
  • Benefits: High in micronutrients, macronutrients, amino acids, and vitamins.


  • Types: Chicken, cow, horse, rabbit.
  • Benefits: Provides nutrients and organic matter; improves water retention and soil structure.
  • Considerations: Ensure manure is composted to prevent nitrogen burn and reduce weed seeds.

Understanding the Role of Key Nutrients

Nitrogen (N)

  • Role: Regulates vegetative growth, builds proteins, supports chlorophyll production, and drives enzymatic reactions.
  • Labeling: Listed first on fertiliser labels due to its high demand in plants.

Phosphorus (P)

  • Role: Supports root growth, flowering, and is a structural component of genetic material in plant cells.
  • Labeling: Listed second on fertiliser labels.

Potassium (K)

  • Role: Activates vital enzymatic reactions critical for overall plant health.
  • Labeling: Listed third on fertiliser labels.

DIY Fertilisers and Plant Foods

Gardeners increasingly use DIY fertilisers to reduce their environmental footprint. Common DIY options include:

  • Aquarium Water: Nourishes plants with fish waste nutrients.
  • Coffee Grounds: Provides nitrogen and acidifies the soil.
  • Eggshells: Increases calcium levels in the soil.
  • Epsom Salts: Supplies magnesium and sulfur.

As plants grow, they constantly consume essential nutrients from the soil, making it necessary to replenish soil nutrient reserves. Commercially available fertilisers allow gardeners to add “food” back to the soil while tailoring a fertiliser program to fit plant needs and growing philosophies. Knowing the basics of fertilisers and plant foods will help ensure you are feeding your plants correctly, encouraging strong, vigorous growth.

Quick Reference Tables

Essential Nutrients and Their Roles

Nutrient Type Role
Nitrogen Major Vegetative growth, protein synthesis, chlorophyll production
Phosphorus Major Root growth, flowering, genetic material structure
Potassium Major Enzymatic reactions, overall plant health
Calcium Secondary Cell wall structure, transport of nutrients
Magnesium Secondary Chlorophyll production, enzyme activation
Sulfur Secondary Protein synthesis, enzyme function
Boron Micronutrient Cell wall formation, reproductive growth
Chlorine Micronutrient Photosynthesis, osmotic regulation
Copper Micronutrient Enzyme activation, photosynthesis
Iron Micronutrient Chlorophyll synthesis, enzyme function
Manganese Micronutrient Enzyme activation, photosynthesis
Molybdenum Micronutrient Nitrogen fixation, enzyme function
Zinc Micronutrient Enzyme activation, protein synthesis

Types of Fertilisers

Type Source Nutrient Availability Pros Cons
Inorganic Synthetic Quick Rapid deficiency correction Potential environmental impact
Organic Natural (plants, animals) Slow Adds organic matter, eco-friendly Lower immediate nutrient availability

Common Organic Fertilisers

Fertiliser Source Nutrient Ratio Benefits
Blood Meal Animal Blood 12-0-0 High nitrogen, deters pests
Bone Meal Animal Bones 3-15-0 High phosphorus, slow-release
Compost Tea Compost Variable Adds microorganisms and soluble nutrients
Fish Meal Fish Variable High in nutrients, amino acids, and vitamins
Manure Animal Waste Variable Adds nutrients and organic matter

By understanding the role of nutrients and the types of fertilisers available, you can make informed decisions to support the health and growth of your garden plants. This knowledge helps ensure that you are feeding your plants correctly and sustainably, promoting vigorous growth and bountiful harvests.

For further information and specific product recommendations, visit Growseed's Fertiliser Section.