The best heirloom tomato fertilizer has less nitrogen and more phosphorus for your plants. All three nutrients matter but the mix should change as the season goes on. Too much nitrogen early leads to big green plants with very few fruits.
I learned this lesson when I first started growing heirlooms about ten years ago. My plants grew six feet tall with gorgeous leaves but gave me almost no tomatoes. A friend pointed out I was using too much lawn fertilizer. The nitrogen made leaves but starved my flowers.
The next season I tried an organic tomato fertilizer with a lower first number. My Brandywine plants went wild with blooms and I picked my biggest harvest ever. Now I tell everyone to check those three numbers before they buy fertilizer.
The tomato nutrient requirements change as your plants grow through the season. Early on they need some nitrogen to build strong stems and leaves. Once flowers start forming, you want to cut back on nitrogen and boost phosphorus. Potassium helps your fruits ripen with better flavor.
For feeding heirloom tomatoes the organic way, you have many good options to pick from. Fish emulsion works great early in the season to get plants growing strong. Bone meal adds the phosphorus your flowers need to set fruit. Kelp meal gives you potassium plus trace minerals.
Compost tea makes an excellent all-around feed that won't burn your plants. Brew it by soaking finished compost in water for 24 to 48 hours. Strain out the solids and pour the liquid around your tomato roots. Your plants get a gentle nutrient boost plus helpful soil microbes.
I like to side-dress my heirlooms with granular organic fertilizer every three to four weeks. Look for products with numbers like 3-4-6 or 4-6-8 on the label. These give your plants more phosphorus and potassium than nitrogen. Scratch it into the top inch of soil around each plant.
Watch your plants for signs they need more food or less. Yellow lower leaves often mean nitrogen shortage. Purple stems can point to low phosphorus levels. Slow growth and pale leaves tell you the plant is hungry. Brown leaf edges sometimes mean too much fertilizer salt in your soil.
Stop all fertilizer about four weeks before your first frost date. Your plants need time to ripen fruit without putting energy into new growth. Let them focus on finishing what they started and you'll enjoy sweeter tomatoes at the end of your season.
Read the full article: Best Heirloom Tomato Varieties to Grow
