From Sarah’s garden to yours

Why do we need to fertilise? 

In the same way we feed our pets to ensure they are happy and healthy it is also important to feed our plants. Just as different animals require a different diet, plants also have varying needs, therefore we must offer them the kind of nutrition they require to not only survive but thrive.

As gardeners, we are always harvesting, trimming, deadheading, pruning, and taking material away from the garden. In nature, this plant debris falls to the ground where it breaks down and nourishes the soil. This is why we need to feed our gardens so that we can return nutrients to the soil to replace what has been taken away.

Each fertiliser, whether it be compost, manure, or a general-purpose synthetic fertiliser from a bag, can affect plants in different ways due to the needs of the plant or the structure of the soil it is growing in. A rockery plant like sedum growing in a light sandy soil will have needs completely different from a tomato plant in rich fertile loamy soil. A lawn with thousands of grass plants has completely different requirements from its fertiliser. There are many specific fertiliser options available for a wide range of different plants to keep them healthy. It may feel a bit like a sales gimmick; however, a lawn food used on a tomato or vice versa will result in less than healthy plants struggling to thrive due to the wrong diet.

What is fertiliser?

 What all plants have in common are the chemical elements that make up their essential diet. The most important key elements are Nitrogen (N) for leaf growth, Phosphorus (P) for the development of fruit, flowers and roots, and Potassium (K) for strength and the overall health and wellbeing of the plant. 

A NPK ratio showing the balance of these elements is often listed on the packaging of fertilisers to help gardeners decide which option is best for the needs of their plants.

Aside from NPK, all plants also need Calcium, Magnesium and Sulphur as minor elements and as trace elements Iron, Manganese, Copper, Zinc, Boron, and Molybdenum which all help keep their internal processes operating smoothly. Plants also require Carbon, Hydrogen and Oxygen but are able to access this readily from the atmosphere and don’t need any help gathering these resources. 

All plant food varieties, including organic and synthetic are based on these elements, although there can be a significant variation depending on the source.

It is possible to over fertilise, and it is a lot harder to rectify than an under fertilised garden. A general-purpose fertiliser may not necessarily be the best option and could result in adding too much of an unneeded nutrient or even not enough of another. Choose an option that matches the needs of your garden and always follow the application directions.

It is a good idea to test your soil every few years to better understand your soil to avoid problems.

Is organic fertiliser better than synthetic fertiliser?

The answer is: It depends! The plant itself doesn't care whether it receives its nutrients from an organic or artificial fertiliser. Nutrients are taken up through the roots as tiny molecules through a selective process based on what the plant needs and is dependent on a complex set of conditions in the soil, such as pH, temperature, moisture, and concentrations of other nutrients. There is complicated molecular chemistry occurring beneath the soil, but essentially the origin of nutrients for the plant does not affect its bioavailability.

The benefit of organic fertiliser is it supports the soil itself. It provides nutrients for micro communities which break them down to provide the essential elements for the plants, as well as improving soil structure and moisture retention. Adding just compost can go a long way to improving a poor soil. 

It takes at least three to four weeks - depending on the weather, for organic fertiliser to become incorporated into the soil and become bioavailable. To ensure a garden is ready to receive plants in the spring, it is best to apply the organic additions, such as well rotted manure, blood and bone or compost well beforehand for a healthy, flouring garden.

While it has its place providing precise nutrients, readily available to plants as a slow release or quick pick-me-up, synthetic fertilisers can become problematic when allowed to build up in the soil, altering the pH, creating a toxic environment for the micro communities or cause problems in nearby waterways with excess nutrients being washed away in heavy rain, becoming an environmental pollutant.

It is satisfying to have control over what happens in your garden; and as a home gardener there is a reassuring confidence in knowing exactly what has been added to the soil, giving surety that nothing harmful has come in contact with the backyard environment. Understanding not all fertilisers are the same, knowing how they work and becoming familiar with the needs of the plants you are growing will ensure the best results for your garden.