When applying organic material to your garden beds, should you top-dress or dig it in deep?
Over time some gardening advice has changed. In the past, digging compost, rotted manure and seaweed deep into garden beds was the recommendation. The theory behind this was simple; get the natural nutrients down deep into the soil where the roots of the plants are. However, thinking has changed over time. As we have gained a greater understanding of the role of invertebrates and micro-organisms in soils and the layering of complex soil ecosystems, the advice is now to top dress, i.e. lay the organic material on the top of the soil or only mix it with the top layer. The theory with this approach is to allow the good organisms (earthworms, insects, bacteria, fungi, etc.) in the soil to break the organic material down and naturally transport the nutrients and organic material down into the soil. It is thought that deep digging disturbs the layers in the soil and unbalances the ecosystems, reducing the efficiency of the breakdown of the organic material and the health of the soil.
So, top dress garden beds with compost or well-rotted manure or seaweed in preparation for planting. Resist the urge to dig the organic matter in deep; the complex soil ecosystem of established beds is best left undisturbed. Nutrients added to the top will move their way down into the soil where they will be available to the plant roots. As a top dressing, the organic matter will also act as a mulch and help suppress weeds.
However, if the bed you're working on is new and has not been cultivated recently, it is recommended to dig in the organic matter on the first application. Then when the soil has settled down, change to top dressing.02 March 2021