Organic vs Inorganic Fertilisers

Good soil is a requirement for growing healthy plants. Soil testing can provide information about the soil pH (acidity-alkalinity), organic content, nutrients, and ability to hold nutrients. Armed with this information you will be able to amend the soil if required, turning poor soil into good soil.

Soil test kits can be obtained from good garden centres and we recommend testing your soil every 2-3 years.

You can learn about how to adjust the soil pH levels here.

But in this article, I will discuss adjusting soil organic matter and nutrient levels and the roles of organic and inorganic fertilisers.

You will need to add fertiliser and/or soil amendments to replenish nutrients depleted from your soil. For example, vegetables remove nitrogen (N), phosphorous (P), potassium (K), calcium (C) and magnesium (Mg) from soils in substantial quantities. You are likely to need to apply fertiliser that contains these five elements, and there may be other minor nutrients such as iron (Fe) and sulphur (S) that also need topped up.

These nutrients can be added to the soil in the form of organic or inorganic fertilisers.

Organic fertilisers include, composts, animal manures and other plant or animal products such as blood and bone meal, fish meal and wood ash. There are also green manure crops such as mustard and lupin that can be grown and then dug into the soil.

Inorganic fertilisers include various mineral salts, usually including nitrate, phosphate, and potash, which contribute nitrogen, potassium and phosphorous. They will also contain others which provide the minor nutrients.

Advantages of Organic Fertilisers

• Less caustic and less likely to cause burning to plants than inorganic fertilisers
• Nutrients are released more slowly and are thus available for longer
• Plant or animal material adds to the organic matter in the soil, and thus water retention, and encourages a healthy microbiome (life in the soil)
• May contain many micro-nutrients not in inorganic fertilisers
• Organic fertilisers are mostly sustainable and have a lower environmental impact

Disadvantages of Organic Fertilisers

• Often more expensive than inorganic fertilisers
• Level of the five major nutrients is generally low
• Organic material must be broken down (decayed) by soil organisms to a form available to the plants
• Fresh manures that have not been fully broken down may have high nitrogen levels that are harmful to plants
• Fresh manures may also contain harmful micro-organisms that could contaminate vegetables that would be consumed
• Overall, fertiliser requirement could not be met by supplies of organic material

Advantages of Inorganic Fertilisers

• As soluble salts the nutrients are more readily available for plants to take up
• Higher levels of nutrients compared to organic fertilisers
• Usually cheaper than organic fertilisers
• Can be made slow release, containing larger molecules that are coated, helping them to break down slowly in the soil

Disadvantages of Inorganic Fertilisers

• Can easily be overapplied and cause harm to plants
• Soluble salts can be more easily leached from the rhizosphere (plant root zone) and potentially harm other parts of the ecosystem
• Inorganic fertilisers do not add to the organic content of the soil
• Over time they are likely to make the soil more acidic and the soil will need pH amendment
• Inorganic fertilisers are typically made from petroleum products or mined and so have a negative environmental impact

As you can see there are advantages in both organic, and inorganic fertilisers, and it is likely that using a combination of them is going to give you the best results in growing your plants and crops.

David Brittain
Kiwicare