What could overall yellowing of the plant starting from the bottom indicate?

Overall yellowing of a plant, beginning from the bottom leaves and progressing upward, is commonly indicative of nitrogen deficiency. Nitrogen is a key nutrient essential for healthy plant growth, particularly because it is a vital component of chlorophyll, the molecule responsible for photosynthesis, which enables plants to convert sunlight into energy.

When a plant lacks nitrogen, it cannot produce enough chlorophyll, leading to a general yellowing of the leaves. The yellowing starts at the bottom because older leaves are more likely to show deficiency symptoms first as the plant reallocates available nitrogen to support new growth and younger leaves. This symptom is often referred to as "bottom-up yellowing."

While other nutrient deficiencies can also cause yellowing, each has specific characteristics and patterns of leaf discolouration. For example, potassium deficiency tends to show yellowing at the margins of leaves or tip burn. Phosphorus deficiency may cause a darker green coloration in older leaves with stunted growth rather than a general yellowing. Calcium deficiency often manifests in symptoms such as blossom end rot or tip burn, rather than a generalized yellowing.

Thus, recognizing the pattern of yellowing and knowing nutrient functions helps in diagnosing nutrient deficiencies accurately in plants.