Compost vs fertiliser: Which is better for your garden?

A lush garden with vibrant blooms and bountiful harvests doesn’t grow overnight. You need to work for it by watering the plants every day, watching out for pests, and taking out the weeds. More importantly, you must add some compost and use fertilisers to ensure the plants get the proper nutrients to thrive. Knowing how and when to use compost vs fertiliser is equally essential because, as they say, anything in excess can be harmful.

What is compost?

Organic kitchen waste being converted into compost. (Source: iStock)

Compost is a soil booster composed of organic matter like garden waste, grass clippings, fallen leaves, plant debris and other vegetative waste. Over time, these organic materials break down into a dark, soil-like substance called a humus. So, what is compost used for? It works to improve soil structure because the humus creates a crumbly texture that allows oxygen and water to circulate and reach the plant roots.

Compost also provides plants with an added source of nutrients. As organic matter decomposes, it slowly releases nutrients over time, supplying both plants and soil with a steady source and ensuring the soil remains fertile.

One of the benefits of composting is that it introduces beneficial microbes and bacteria into the soil. These include Thermophilic bacteria, which increase soil temperature to kill weed seeds; Azospirillum, a nitrogen-fixing bacterium that aids in root growth; and Bacillus subtilis, which releases antifungal molecules to keep plants free from diseases.

Compost comes in many types, including:

• Green compost - nitrogen-rich compost made from grass clippings, dead green plants, and fruit and vegetable waste; boosts beneficial microbes growth.

• Brown compost - carbon-rich material composed of paper, cardboard, and wood pruning; provides beneficial microbes energy, bolstering the composting process.

• Fresh compost - partially decomposed organic material high in nutrients.

• Mature or finished compost - fully decomposed organic matter with a dark, crumbly texture.

• Recycled compost - processed compost made out of green and organic waste.

What is fertiliser?

Granular fertiliser being added to garden soil. (Source: iStock)

A fertiliser is a substance added into the soil to provide plants with essential nutrients like nitrogen (N) for leaf and stem growth, phosphorus (P) for root development, flowering, and fruiting, and potassium (K) for plant disease resistance. It can be classified into two categories:

• Organic fertilisers are derived from natural sources like plant residues, animal matter or manure, bone meal or dried animal bones, and blood meal or dried, ground-up blood from cows and pigs.

• Synthetic fertilisers are chemically processed and formulated to meet the precise nutritional needs of plants. They typically have higher nutrient content than organic fertilisers.

Fertiliser vs compost: Which is the better choice?

If you’re new to gardening, learning about plant growth and maintenance can be overwhelming. Take it slow and start with the plant food basics, such as fertiliser vs compost, since these are essential for growing a garden.

In terms of use and purpose

Enriching garden soil with homemade organic compost. (Source: iStock)

Both fertilisers and compost help boost plant health, but they do it differently. Fertilisers correct and address specific nutrient deficiencies in plants, while compost creates an ideal soil environment for plants to grow.

Here’s a specific scenario for using fertilisers: when the leaves appear very yellow, it indicates a lack of nitrogen. You can use an organic fertiliser such as blood meal for this, as it’s high in nitrogen. Another scenario where fertilisers can be helpful is when plants experience slow and stunted growth. You could add phosphorus-rich fertiliser to encourage root development and boost plant growth.

Meanwhile, compost is best used when plants can’t absorb the nutrients from soil that’s dense and compacted. This can loosen up the compacted soil particles because compost comprises organic matter that decomposes over time. The decomposition process helps create spaces in the soil since beneficial organisms burrow and tunnel through the soil.

You can also use compost to improve soil water retention. The organic materials will serve as sponges, helping to hold onto the water molecules and preventing the plants from getting overwatered or underwatered.

In terms of nutrient content

Compost and fertilisers contain similar nutrients, as both are rich in nitrogen for leaf and stem growth, phosphorus for root development, potassium for plant disease resistance, and micronutrients like zinc, iron, and calcium. However, fertilisers have a higher nutrient content than compost because, unlike compost, which is used as a soil amendment, fertilisers are plant foods formulated to address nutrient deficiencies.

Here’s a table of the nutrient breakdown of compost vs fertilisers, focusing only on the primary macronutrients:

Even though compost doesn’t provide immediate nutritional value to plants, it can help boost plant and soil health. The beneficial microorganisms in compost help release the nutrients slowly, providing plants with a consistent supply.

In terms of application

Hands spreading fertiliser around a seedling. (Source: iStock)

Fertilisers have a more complex application process than compost because you usually have to measure the amount you apply to avoid harming the plants. Remember that fertilisers are high in nutrient content, so overapplication can cause the roots to shrink and make the plants more susceptible to diseases. Here are some of the ways to properly apply fertiliser:

• Broadcasting is ideal for lawns and large gardens, where the fertiliser is spread evenly to distribute nutrient content uniformly to the soil.

• Side-dressing is ideal for vegetable gardens and row crops. It involves sprinkling the fertiliser beside the plants to deliver nutrients directly to the roots.

• Foliar feeding is ideal for plants with yellowing leaves. It involves directly spraying the liquid fertiliser onto the leaves for immediate nutrient absorption.

• Incorporation is ideal for preparing new planting areas. It involves mixing fertilisers into the soil to provide nutrients as the plant grows.

Compost also comes with different application methods. Here’s how to use compost:

• Top dressing involves adding compost to the upper soil layers, ideal for lawn care and established gardens.

• Mulching involves spreading compost around the soil to keep it moist and prevent weed growth. It’s ideal for trees, shrubs, and garden beds.

• Compost tea involves brewing the compost and directly applying it to the leaves. It’s ideal for plants with nutrient deficiencies, like yellowing leaves.

• Incorporation involves mixing the compost into the topmost layer of the soil to keep it moist, suppress weeds, and gradually add nutrients.

How frequently you should apply them depends on the plant’s needs. But to give you an idea, fertilisers are typically applied monthly, while compost is generally applied once or twice a year.

In terms of environmental impact

Compost has less environmental impact than fertilisers because compost is made from repurposing food scraps and yard waste. This helps decrease landfill waste and reduces methane emissions, one of the largest contributors to greenhouse gases. Plus, compost helps trap carbon in the soil.

Conversely, fertilisers can be detrimental to the environment because their manufacturing process involves the use of fossil fuels. Chemical fertilisers, specifically synthetic nitrogen-based fertilisers, release nitrous oxide, a potent greenhouse gas.

Even though some fertilisers are made from organic materials, they can still lead to nutrient runoff if used excessively. In other words, the excess nutrients from fertilisers can be washed away to nearby rivers, lakes, or oceans, especially when it rains heavily in your area. This can cause harmful algal blooms to grow, polluting the water.

In terms of cost and availability

Garden supplies being loaded for transport at a store. (Source: iStock)

Fertilisers are more affordable upfront because they’re available in small packs, unlike compost. Their price ranges from as low as £1 to £380. Their price and availability depend on the market status of natural gas because, as mentioned earlier, their manufacturing process involves the use of fossil fuels. If the prices of natural gas change, this can also affect the cost and availability of certain chemical fertilisers.

Meanwhile, compost ranges from £4 to £1,000 because it’s usually bought in large sacks of bags. Naturally, commercial compost is more expensive because the production process involves rigorous and extensive testing and processing to ensure the compost is free from contaminants.

Despite a slightly higher initial cost, compost is more cost-effective because you can purchase it in bulk, significantly reducing costs if you have an extensive garden. But the good news is that good quality compost is readily available at various retail and bulk outlets.

In terms of long-term impact

Compost provides better long-term effects because its organic materials help bolster the growth of beneficial bacteria and fungi. These microbial communities allow the soil nutrients to cycle continuously, increasing plants’ disease resistance. Moreover, since compost mixtures release nutrients slowly, they help prevent soil degradation and nutrient overload.

On the other hand, continuous use of fertilisers can cause plants to depend on them instead of naturally producing nutrients through photosynthesis. Another negative long-term effect of fertilisers is that the chemicals can cause the soil to become too acidic, inhibiting the growth of beneficial microbes.

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