How to Increase the Acid in My Potted Tomato's Soil

Soil test kits are inexpensive, easy to use and available at retail plant nurseries. Follow the directions on the kit to test the pH level of the soil you plan to use in the pot. The kits are accurate enough to identify pH and give you a rough idea of the soil's nitrogen, phosphorus and potassium content. Many university agricultural extension services offer more accurate soil testing for a small fee. Always use potting soil, not garden soil, to pot a tomato, since garden soil harbors pests and diseases.

Choose a commercial potting soil with sphagnum peat moss listed near the beginning of the ingredients list. Peat moss is acidic, so the potting mix will be on the acidic side. Or, mix acidic organic matter such as sawdust, peat moss or acidic planting compost (usually labeled for blueberries and rhododendrons) into any commercial potting soil at a 1/3 amendment to 2/3 soil ratio. Mulch the top of the pot with pine needles, wood chips or shredded oak leaves both to keep moisture levels even and add acidity.

Feed the plant with a fertilizer with a higher middle number in its NPK. The N stands for nitrogen, the P for phosphorus and the K for potassium. Tomatoes need more phosphorus than the other two elements, and adding it in the form of fertilizer will help maintain the acidity of the soil.