Can You Put Cooking Oil in the Garden?

Cooking oil has many uses. From hair conditioners to biofuels, leather protection or deep fat frying, oil and its high-fat content allows it to cover surfaces in a thin film and soak into almost anything it touches. Oil has been used throughout history for these properties in the garden, however with a modern understanding of water treatment, is this safe? 

You should never dispose of used cooking oil in the garden because it’s bad for wildlife and can cause pollution. However, you can use small quantities for gardening purposes. For example, you can put oil in your garden as its ability to cover surfaces can inhibit air and water from getting to the leaves of weeds, killing them. It’s also this ability to cover and saturate leaves, however, that makes it harmful to the environment and is advised against by modern gardeners and their understanding of groundwater. 

Let’s review oil’s historic use as a weedkiller, its effect on the environment and ways in which oil can be used for environmentally friendly purposes. 

How Does Disposing Of Cooking Oil In The Garden Affect The Environment?

As oils are less dense than water, they float on the surfaces of puddles, lakes and oceans where they are spilt. I’m sure you have seen an unfortunate oil spill where oil doesn’t mix with water but instead floats out over the surface causing something called an ‘oil slick’. 

These slicks are caused by oil spreading out over the surface of bodies of water, making the slick grow thinner and thinner over time. Sometimes the layer of oil on the water’s surface can be as thin as 1 molecule deep, which is through a process known as a Langmuir–Blodgett film effect. 

This film, considering it can be 1 molecule thick, has the massive potential to cause harm to plant life in a large area of land as it has the potential to spread. But by how much?

One teaspoon of waste oil in your garden can spread out to over half an acre of surfaces in the wrong conditions. 

It is strongly advised against pouring used cooking oil into your garden, as this has future implications for the environment which are hard to manage and remove. 

So why is cooking oil used in gardening, and why is it known as a weedkiller?

How Is Cooking Oil Used In Gardening?

There are several ways in which used oil has been used in gardening. Let’s review them, in turn, to see how oil, even used oil, can benefit gardeners if used correctly.

Does Cooking Oil Kill Weeds?

Used oil has been used throughout history for its ability to cover the leaves and roots of weeds in microfilm preventing light, air and water from getting to the weed. Many other weedkillers are made from petroleum products and can add harmful chemicals to the biodiversity and chemical balances found in soil. 

It is debated as to whether cooking oil is safer for groundwater than other, modern weed killers. It has been found that ‘atomising’ waste cooking oil and applying it in small amounts can be an eco-friendly alternative to other modern weedkillers. It is still up for debate as to whether using oil in this way is more environmentally friendly than using other homemade weed-killer methods. These can include using:

  • boiling water,
  • lemon juice or vinegar,
  • baking soda.

These methods don’t cause lasting effects on groundwater and would be much more beneficial to your garden than the use of cooking oil as a weedkiller. 

Used Oil For Water Retention

Oil’s property of sticking to the surface of water is linked to its ability to retain moisture in soil. If you live in dry climates, incorporating a tiny amount of oil into ground soil has been shown to contribute to water retention. This works via the process of evapotranspiration. This can reduce water loss in soil by up to 50%. However, this oil will stay in the soil until it is broken down by natural, organic processes. 

Once added to soils in small amounts, bacteria and fungi will feed on waste oil, excelled by captured particles of foods such as starch which are suspended in the oil. This will allow the oil to break down over time. 

What are The Environmental Effects of Disposing of Oil in a Compost Pile?

Composting is important for the diversity of soils in your garden, but it also allows cities to create a place for many people to dispose of their garden waste in an environmentally friendly way. Not everything that is compostable is obvious and some household items like ‘eco-friendly coffee pods’ boast compostability without actually being compostable (at least, not in the compost pile in your back garden). 

Oil is similar as it can be composted to a degree. Composting oil from animal fats can attract wildlife and pose a significant risk as an environmental hazard. Some reports claim that a small amount of oil may benefit compost, however, this amount is negligible and would still contaminate groundwater. 

The only appropriate way of disposing of oil at home is to see if you have disposal provided by your local authority.

Used Oil and The Environment

Used oil has both beneficial and detrimental effects on the environment when added to plants or the soil containing them. It can add properties to the soil, such as water retention and the addition of nutrients stored within the used oil itself. 

However, the detrimental effects on groundwater once oil-polluted water runs off into nearby streams, lakes and rivers should be taken seriously. Modern understandings of the harmful effects of oil have caused minds to change around the use of oil as a ‘natural weedkiller’. This causes water to not oxygenate properly as the thin layer of oil on the surface creates a barrier. 

Furthermore, underground water conduits such as sewer systems can become clogged with ‘fatbergs’ which create nightmares for council workers and water companies alike. So what is a positive way to dispose of oil in 2023?

Turning Used Oil into BioDiesel with BioUKFuels

One huge step forward in recent years is the ability to turn used cooking oil into biodiesel, forgoing the process of disposal entirely. Biodiesel is a type of fuel suitable for existing diesel engines. Biodiesel is made from used cooking oil via a process called ‘transesterification’. This completely removes the issue of waste oil becoming an environmental hazard in its own right. 

For large industrial premises such as restaurants or fast food outlets, the problem of what to do with waste oil has caused concern for years. The Food Standards Agency has attempted to manage the disposal of waste cooking oil for years, with some businesses catching heavy fines if the duty of care measures are not met. 

Businesses, therefore, pay for companies to collect their waste oil and dispose of it properly. However, with the advent of biodiesel technologies, companies can now sell their waste oil to businesses like BioUKFuels and make a profit. 

Find out more about our process of waste oil disposal with our blog How Do You Dispose of Used Cooking Oil?

Contact BioUKFuels today to see how we can help your business contribute towards a positive environment, reducing oil exposure to groundwater by turning it into Biodiesel. 

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