From our perspective as Grevillea plants, our vibrant, spider-like blooms and fine, often prickly foliage are a point of pride. However, to maintain this showy display and overall health, we require specific nutrients. When these are lacking, we communicate our distress through clear physical signs. Understanding these signs is key to helping us thrive.
It is critical to first address that what often appears as a nutrient deficiency in us is actually a toxicity. We are exceptionally sensitive to phosphorus. Many standard garden fertilizers contain high levels of phosphorus, which we cannot process. It binds with other elements in our roots, effectively locking them out and creating secondary deficiencies. The signs of phosphorus toxicity mirror a lack of other nutrients: stunted growth, yellowing (chlorosis) particularly of the older leaves, and tip die-back. The solution is not to add more fertilizer, but to immediately cease using any product containing phosphorus. Always seek out fertilizers labeled specifically for "Australian Native Plants" as these are formulated to be low in or free from phosphorus.
When our roots are compromised by high phosphorus or planted in alkaline (high pH) soils, we struggle to uptake iron. Iron is crucial for the production of chlorophyll, the green pigment that allows us to photosynthesize. Without it, our newest leaves will exhibit interveinal chlorosis—meaning the tissue between the veins turns yellow while the veins themselves remain green. In severe cases, the entire new growth can turn pale yellow or even white. To remedy this, apply a chelated iron supplement, which is a form of iron that we can easily absorb even in less-than-ideal soil conditions. This should be applied to the soil around our drip line according to the product's instructions.
Manganese deficiency presents very similarly to iron deficiency, making it tricky to diagnose. You will also see interveinal chlorosis on our newer leaves. However, a key distinguishing feature can be the development of small, dead, brown spots within the chlorotic areas. This deficiency is also often triggered by high soil pH or waterlogged conditions that damage our roots. The fix involves applying a manganese chelate to the soil. Improving drainage around our root zone is also a vital long-term solution to prevent this and other root-related issues.
Nitrogen is the engine for our overall growth and leaf development. A genuine nitrogen deficiency, while less common than phosphorus-induced problems, can occur in very sandy, infertile soils. We will signal a lack of nitrogen through an overall paling or yellowing of our oldest leaves, which may eventually drop. Our growth will become stunted, and we will produce fewer of the magnificent flowers we are known for. To address this, feed us with a low-phosphorus, slow-release native fertilizer or use an organic mulch like leaf litter, which breaks down slowly to release nitrogen in a form we can safely tolerate.
Zinc is important for our enzyme function and overall growth regulation. When zinc is lacking, our new leaves become unusually small and narrow, a condition known as "little leaf." The internodes (the spaces between leaves on a stem) will also shorten, giving our new growth a stunted, bunched-up appearance. Leaf margins may be distorted or wavy. This is also more prevalent in alkaline soils. A foliar spray containing zinc chelate can be an effective short-term solution, as we can absorb it directly through our leaves. For a long-term fix, correcting the soil pH to be slightly acidic is ideal.
