## 1 Introduction  
Olive leaves (OL) contain high levels of phenolic compounds, which are essential for human health and agriculture. These compounds, such as oleuropein and hydroxytyrosol, possess strong antioxidant properties that can help protect our bodies from oxidative stress and inflammation, thereby reducing the risk of chronic diseases like cancer and heart disease. In addition to their health benefits, phenolic compounds act as natural defence mechanisms against diseases and pests in olive trees, making them significant in promoting sustainable agriculture [1]. These dual benefits underscore the importance of olive leaves and their phenolic compounds, highlighting their potential to positively impact our health and the environment. Several research studies have revealed that the phenolic compounds found in olive leaves, such as oleupopein and hydroxytyrosol, have the potential to prevent and manage colon cancer [2]. These bioactive compounds have significant anti-cancer properties as they can inhibit the growth of colon cancer cells and promote programmed cell death or apoptosis [3; 4]. Moreover, their potent antioxidant [5] and anti-inflammatory effects [6] also make them effective in fighting cancer. Chronic inflammation and oxidative stress are known to be linked to the development of colon cancer [7]. Additionally, phenolic compounds from olive leaves are believed to help regulate gene expression related to tumour suppression [8]. Although more studies are needed to understand their full impact, these findings suggest that incorporating OL extracts or olive oil rich in these compounds into one's diet may be a valuable strategy in the fight against colon cancer [9]. Fertilisation is a process that can increase the phenolic content in various plants, including fruits, vegetables, and herbs [10]. This process, known as biofortification, involves enriching the soil with essential nutrients, which are then absorbed by the plants and utilised in synthesising phenolic compounds [11]. Specific fertilisation strategies, such as organic or mineral-based fertilisers, can be tailored to optimise the phenolic content in crops [12]. Adequate nutrient availability, particularly nitrogen and phosphorus, can boost plant metabolism and stimulate the production of phenolic compounds, essential for the plant's defence mechanisms [13]. By enhancing the phenolic content, we not only improve the nutritional quality of the plants but also their resistance to pests and diseases. This makes fertilisation important for promoting healthy plant growth [14; 15]. Fertilisation is essential for olive trees and can improve the phenolic content in their leaves by providing necessary nutrients that support the synthesis of these valuable compounds. Like other plants, olive trees also require essential nutrients for their growth and development. Nitrogen, phosphorus, and potassium are vital elements, but micronutrients like iron, manganese, and copper also play crucial roles in producing phenolic compounds [16; 17]. When olive trees receive appropriate fertilisation, they can maintain vigorous growth and overall health, enabling them to allocate more energy and resources toward the biosynthesis of phenolic compounds. Balanced and well-timed fertilisation practices not only improve nutrient availability to the trees but also help regulate the pH of the soil, which can influence the bioavailability of certain nutrients crucial for phenolic synthesis [17; 18]. Furthermore, organic fertilisers, like compost and organic matter, can introduce a diverse range of beneficial microorganisms to the soil, further supporting the production of phenolic compounds [19]. In this way, proper fertilisation can lead to an increase in phenolic content in olive leaves. Silicon foliar fertilisation has been found to positively impact the phenolic content in plant leaves [20]. Silicon is not an essential nutrient for most plants, including olive trees, but when applied as a foliar spray, it can boost the production of phenolic compounds.