Pros and cons of inorganic and organic fertilisers

This article will discuss the roles of inorganic and organic fertilisers in adjusting soil nutrient levels and organic matter.

Good soil is necessary for growing healthy garden plants. Soil testing can provide information about the soil pH (acidity-alkalinity), organic content, nutrients, and ability of the soil to hold nutrients.

Soil test kits can be obtained from good garden centres and we recommend testing your soil every 2-3 years. Armed with this information you will be able to amend the soil if required, turning poor soil into good soil.

It is likely that you will need to add fertiliser and/or soil amendments to replenish nutrients depleted from your soil. E.g., 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 inorganic or organic fertilisers.

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.

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.

Pros of Inorganic Fertilisers

• Higher levels of nutrients compared to organic fertilisers
• As soluble salts the nutrients are more readily available for plants to take up
• Usually cheaper than organic fertilisers
• Can be made to break down slowly in the soil by containing larger molecules and/or are coated granules

Cons of Inorganic Fertilisers

• Inorganic fertilisers do not add to the organic content of the soil
• 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
• 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

Pros of Organic Fertilisers

• 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)
• Organic fertilisers are mostly sustainable and have a lower environmental impact
• Less caustic and less likely to cause burning to plants than inorganic fertilisers
• Nutrients are released more slowly and are thus available for longer
• May contain many micro-nutrients not in inorganic fertilisers

Cons of Organic Fertilisers

• Organic fertilisers are often more expensive than inorganic fertilisers
• The level of the five major nutrients is generally low
• Organic material must be broken down 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

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.