Gardening

Fertilizer

Trees and plants require macronutrients and micronutrients for healthy growth, fruit production, and resilience. These can be supplied naturally through compost and other organic materials, with adjustments for tree type and growth stage. The most important ones are Nitrogen, phosphorus and potassium, the three primary macronutrients required in the largest quantities by plants for growth, development, and reproduction. They are often the most limiting nutrients in soils, meaning plants deplete them faster than others, necessitating supplementation. For this reason, fertilizer are labelled through their NPK content, for instance fish blood and bones have a NPK of 5-5-6 [1].

1. Nitrogen (N)

Role: Promotes leaf and shoot growth, essential for young trees establishing foliage. Excessive nitrogen can lead to lush, weak growth and reduced fruiting (e.g., concern about fast growth in the lemon tree).
Sources: Homemade compost (from fruit peels, vegetable scraps, coffee grounds), grass clippings (dried, herbicide-free). Compost is nitrogen-rich, supporting new leaves on the lemon tree.
Application: Apply 2–3 inch layer of compost around the drip line (outer canopy edge), 2–3 inches from the trunk, in early spring (plum/cherry) or year-round for lemon (warm climates). Use 1–2 cups for young trees. Avoid overuse to prevent excessive foliage.

2. Phosphorus (P)

Role: Supports root development, flowering, and fruit ripening (e.g., lemon tree’s green lemon). Balances nitrogen to prevent overly leafy growth.
Sources: Bone meal (0-10-0 to 3-15-0 NPK), banana peels, potato peels, melon rinds, legume scraps (all vegetarian-friendly for compost). Bone meal is high in phosphorus; banana peels (0.5–1% P) are ideal for no-meat/fish compost.
Application: For young trees, sprinkle 1–2 tablespoons bone meal or bury 2–3 dried banana peels/potato peels around the drip line in early spring (plum/cherry) or now (lemon, late August 2025). Mix lightly into soil and water well. Add phosphorus-rich scraps to compost for long-term use.

3. Potassium (K)

Role: Enhances fruit quality (size, flavor), disease resistance, and winter hardiness (critical for deciduous plum/cherry trees).
Sources: Banana peels (10–12% K), wood ash (0-0-5 to 0-0-10 NPK), kelp meal. Compost includes banana peels, which provide potassium.
Application: Add 1–2 teaspoons wood ash or 2–3 banana peels around the drip line in spring. Incorporate banana peels into compost (20–30% of pile). Water well. Avoid excessive wood ash to prevent raising soil pH.

4. Calcium

Role: Strengthens cell walls, supports root and leaf health, and prevents fruit disorders (e.g., cracking in plums/cherries, blossom-end rot in lemons).
Sources: Eggshells (already in compost, 35–40% calcium), bone meal.
Application: Continue adding crushed eggshells to compost. For direct use, sprinkle 1–2 tablespoons crushed eggshells around the drip line, mix into soil, and water. Apply in spring or as needed for young trees.

5. Magnesium

Role: Essential for photosynthesis (part of chlorophyll) and cell structure. Prevents yellowing between leaf veins (common in citrus).
Sources: Epsom salt (magnesium sulfate, 10% Mg), kelp meal. Coffee grounds (in compost) provide only trace magnesium, not a primary source.
Application: Dissolve 1 tablespoon Epsom salt in 1 gallon of water and apply as a soil drench around the drip line every 2–3 months if yellowing occurs. Add kelp meal (1–2 tablespoons) to compost or soil for trace magnesium.

6. Micronutrients (Iron, Zinc, Manganese, Boron, etc.)

Role: Support chlorophyll production, enzyme function, and fruit development. Prevent chlorosis (yellowing leaves) in lemon, plum, and cherry trees.
Sources: Compost tea (from homemade compost), kelp meal, coffee grounds (trace amounts). Compost provides some micronutrients, but diversity enhances availability.
Application: Steep a handful of compost in water for 24–48 hours, strain, and apply as a soil drench around the drip line every 2–3 months. Sprinkle 1–2 tablespoons kelp meal in spring for young trees.