How to Adjust NPK in Soil

Testing

• 1

  Determine whether you want to use the soil site to plant a lawn, to grow small fruits, flowers or vegetables, or to grow trees or shrubs. Dig up 10 cross-sections of soil that measure 1/2 to 1 inch wide and 3 to 4 inches tall by using a clean spade or trowel if you want to plant a lawn. Dig up 10 cross-sections of soil that measure 1/2 to 1 inch wide and 6 to 8 inches tall if you want to plant small fruits, flowers or vegetables. Dig up three or four cross-sections of soil that measure 1/2 to 1 inch wide and 10 to 12 inches tall if you want to plant trees or shrubs. Place the soil cross-sections in a bushel basket.

• 2

  Remove debris and organic matter from the collected soil using latex-gloved hands. Don’t use bare hands because your skin’s oils affect the soil's pH level. Organic matter, such as twigs and leaves, and debris, such as rocks, also influence soil test results.

• 3

  Spread the soil in an even layer on a sheet pan or on a table covered with newspapers. Let the soil air-dry for a few days, until it is friable or crumbly.

• 4

  Mix the soil with your gloved hands, pulverizing it as you proceed. Place the pulverized soil to a home soil pH and nutrient test's four vials, putting enough soil in each vial to reach the vial's soil fill line. Use a plastic spoon for the task.

• 5

  Twist open one of the test's supplied capsules, and drop its contents into one vial containing soil. Repeat the task, using one capsule per vial. One capsule will test for nitrogen, one for phosphorous, one for potassium and one for pH level; the capsules and test vials usually have an “N,” “P,” “K,” or “pH” on them to identify them.

• 6

  Drop distilled water in the vials to the vials' water fill lines, using the test’s supplied dropper. Cap each vial with its matching, supplied lid. The vial lids usually are in four colors: green for pH, purple for nitrogen, blue for phosphorous and red for potassium.

• 7

  Shake the vials vigorously until the soil and powder from the capsules dissolve, which may take about 30 seconds. Hold the vials up to light, and compare their colors to the test's colorimetric chart. The chart lists each nutrient – nitrogen, phosphorous and potassium – on a color scale that corresponds with text, usually “High,” “Medium” and “Low” or “Poor,” “Moderate” and “Good.” Record the results of the home soil test for reference.

Adjusting

• 8

  Pull the weeds and remove debris from the planting area. Cultivate the soil to a depth of 12 inches using a fork, spade or, if fertilizing a large area, mechanical tiller.

• 9

  Spread a 2-inch layer of compost over the soil by hand. All soil benefits from compost, which improves aeration and permeability, and adding it poses no risk of burning your plants. Plant-based compost contains about 2-0.3-1.5 NPK, respectively; cow-manure based compost contains about 1.5-1-1.5 NPK; and chicken-based compost has about 3-2-2 NPK. Cultivate the soil 12-inches deep a second time.

• 10

  Distribute the appropriate soluble, continuous-release NPK fertilizer, as determined by the soil test, over the soil, using a drop spreader for the task.

  Use 2/3 to 3/4 pound of 24-0-0-24 NPK fertilizer for every 100 square feet if the soil pH measured above 5.0 and you need to feed shrubs that require acidic soil, such as azaleas (Rhododendron spp.), which are hardy in U.S. Department of Agriculture plant hardiness zones 4 through 9.

  Use 2 pounds of 15-15-15 NPK fertilizer for every 100 square feet if your soil’s potassium and phosphorous measured low. Use 2 pounds of 16-20-0 NPK fertilizer per 100 square feet if the phosphorous measured low but the potassium measured high.

  Use 1 1/2 pounds of 15-15-15 NPK fertilizer for every 100 square feet if the phosphorous and potassium measured moderate. Use 1 1/2 pounds of 16-20-0 NPK fertilizer for every 100 square feet if the phosphorous measured moderate and the potassium measured high.

  Use 1 1/2 pounds of 21-0-0 NPK fertilizer for every 100 square feet if the phosphorous and potassium both measured high.

• 11

  Work the fertilizer into the top 4 to 6 inches of soil with the fork, spade or mechanical tiller. Water the fertilizer into the soil to a depth of 4 to 6 inches if you don’t expect rainfall for one or two days, and allow rainfall to work the fertilizer into the soil if your area receives at least an average of 1 inch of rainfall per week.