Fertilizing: Test First, Then Feed

Why it matters

Big harvests need the right nutrients at the right time. Too little starves plants, and too much can burn or kill them. A soil test tells you what your ground actually needs, so you buy only what helps.

Step by step

1. Take a soil sample Scrape off surface debris, then dig 8 to 10 cores about 6 inches deep across the garden. Mix them in a clean bucket, pull out roots and rocks, and air-dry the soil if it's wet.
2. Bag and label it Put about a pint of the mixed soil in a 1-quart zip-top freezer bag. Write a sample name like 'Back Garden' on it with a permanent marker, and use the same name on the form.
3. Fill out the urban form Home gardeners use the Texas A&M Soil, Water and Forage Testing Lab's urban/small-area form, either the PDF or the online form at soiltesting.tamu.edu/soil-testing/.
4. Pay and send it A routine test costs $12 per sample (prices valid through 12-31-2026). Pay by check or money order to 'Soil Testing Laboratory' or online. Mail it to Soil, Water and Forage Testing Laboratory, 2478 TAMU, College Station, TX 77843-2478, or drop it off at the lab lobby on weekdays from 8 a.m. to 5 p.m.
5. Read the bag numbers The three numbers are the percent of nitrogen (N), phosphorus (P) and potassium (K). A 100-lb bag of 10-20-10 holds 10 lb N, 20 lb P and 10 lb K. The rest is filler.
6. Feed before planting Without a soil test, AgriLife suggests 2 to 3 lb of 10-20-10 per 100 sq ft. Use 2 lb on sandy soil and 3 lb on clay. Spread it evenly and mix it 3 to 4 inches into the soil before making rows.
7. Side-dress growing plants Sprinkle about 1/2 cup of garden fertilizer per 10 feet of row along the sides of the row, then water it in. This helps most on sandy soil or after heavy rain.
8. Give transplants a starter drink Mix 2 tablespoons of fertilizer in 1 gallon of water and pour 1 cup into each hole when you set out tomatoes, peppers, eggplant or cabbage.

Key facts

• Soil test about every 2 years (AgriLife EHT-069).
• Routine soil test: $12. Routine plus micronutrients: $20. Routine plus organic matter: $32. Emailed results are free. Mailed paper results cost $3 per shipment.
• The routine test covers pH, nitrate-N, conductivity, P, K, Ca, Mg, Na, S and boron.
• Pre-plant rate on untested soil: 2-3 lb of 10-20-10 per 100 sq ft. A 10x10 or 5x20 ft plot is 100 sq ft.
• Two cups of most fertilizers weigh about 1 lb.
• 2 lb of 5-10-5 gives the same nitrogen as 1 lb of 10-20-10.
• Side-dress: about 1/2 cup per 10 ft of row.
• Fall garden after a well-fed spring garden: use about half the spring rate, 1-2 lb per 100 sq ft.
• Organic option: 20-30 lb of aged manure per 100 sq ft, worked into the soil.
• Green beans: 1/2 cup of 15-5-10, or 1 cup of organic fertilizer, per 10 ft of row after flowering and bean set begin (AgriLife EHT-132).
• Squash: 2-3 lb of 10-10-10 per 100 sq ft before planting, then about 2 tablespoons per hill at first bloom (AgriLife EHT-041).
• AgriLife's fall guide alternative: 1 lb ammonium sulfate per 100 sq ft before planting, then 1 tablespoon per plant every 3 weeks.
• Lime is needed only when soil pH falls below 5.7.

Texas notes

• The state soil lab is the Texas A&M AgriLife Soil, Water and Forage Testing Laboratory in College Station: soiltesting.tamu.edu.
• Your county AgriLife Extension office can hand out sample bags and explain how to submit.
• Sample in midwinter so results are back in time for spring planting, when soil is moist but not wet.
• Sandy East Texas and Gulf Coast soils lose nutrients faster after heavy rain, so side-dressing pays off most there.
• Use the urban form for home gardens and the ag form for large acreage.