Most plants grow best in soil with a pH of about 6.5. To lower your soil's pH, use a soil acidifier, such as soil sulfur or ammonium sulfate. Follow the directions on the package. Then amend your soil with organic matter such as compost, well-rotted manure, or peat moss. Soil nitrogen levels change from week to week because the nutrient quickly moves through the soil. Adding organic matter to your soil will increase nitrogen levels. Another way to increase them is by adding ammonium sulfate or any granular fertilizer containing nitrogen in the form of nitrate or ammonium. Phosphorus is more stable in the soil. To increase the phosphorus level, add superphosphate, rock phosphate, or bonemeal, all of which should be available at your local garden center. Because phosphorus moves slowly through the soil, work these products into the root zone.