From our perspective as geranium plants, nitrogen is fundamental for our vibrant green foliage and overall vigorous growth. When the soil lacks sufficient nitrogen, it directly impacts our ability to produce chlorophyll, the green pigment essential for photosynthesis. You will first notice a general paling or yellowing (chlorosis) of our oldest, lower leaves. This is because we strategically mobilize the remaining nitrogen from these mature leaves to support the growth of new, young leaves at our apical meristems. As the deficiency progresses, the entire plant may take on a light green or yellowish hue, and our growth will become noticeably stunted and spindly.
Phosphorus is crucial for our energy transfer systems, root development, and flowering. A shortage of this nutrient severely hampers our metabolic processes. Our growth becomes severely stunted, and our stems may turn a dull, reddish-purple or dark green colour, particularly on the leaf veins and petioles. This anthocyanin pigment (the red-purple colour) accumulates because without sufficient phosphorus, the sugars we produce through photosynthesis cannot be utilized properly for growth. Our older leaves will often turn dark bluish-green and may develop necrotic (dead) purple-brown patches before falling off prematurely.
Potassium acts as a regulator within our systems, governing water uptake, enzyme activation, and the movement of sugars. When potassium is deficient, the edges and tips of our oldest leaves begin to show scorching or browning (marginal chlorosis and necrosis). This "scorched" appearance often starts with a yellowing halo around the leaf margins before the tissue dies and turns brown and crispy. We become generally less robust, with weaker stems and reduced resistance to drought, temperature fluctuations, and diseases. Our flowering potential is also significantly reduced.
Magnesium is the central atom of the chlorophyll molecule, making it absolutely vital for our photosynthesis. A deficiency manifests as interveinal chlorosis on our older leaves. This means the tissue between the leaf veins turns yellow, while the veins themselves typically remain green, creating a distinct marbled or patterned effect. In severe cases, the yellow areas can turn brown and necrotic, and leaf drop can occur. Without enough magnesium, we cannot efficiently capture sunlight energy, leading to a overall decline in health and vigour.
Iron is another key player in chlorophyll synthesis, though its deficiency symptoms appear differently from magnesium. Iron is not a mobile nutrient within our plant bodies, meaning we cannot relocate it from older tissues to new ones. Therefore, the chlorosis appears on our youngest, newest leaves first. You will see interveinal chlorosis where the emerging leaves turn yellow but their veins remain sharply green. If uncorrected, the entire new leaf can turn pale yellow or almost white, and our growth will be severely stunted due to our inability to produce functional chlorophyll in new growth.
Calcium is a structural component of our cell walls and is critical for new cell development. Since calcium is immobile in our phloem, it cannot be moved from old growth to new growth. Therefore, deficiency symptoms appear in our youngest tissues: the emerging leaves and the root tips. New leaves may appear distorted, hooked, or crinkled, and growing points (apical meristems) may die back. Root development is also severely impaired, leading to a weak root system that cannot properly support the plant or uptake other nutrients and water, causing a cascade of further health issues.
