Soil Testing in Late Fall for a Better Spring

By James Wilson · October 8, 2026

Why Test Soil in October

Fall is the best time to test garden soil. Amendments like lime and sulfur take 3-6 months to alter pH, so a fall test gives you results before spring planting. Spring tests often come back too late to act on.

What a Soil Test Tells You

A standard soil test measures pH, phosphorus, potassium, calcium, magnesium, and organic matter percentage. Some labs also report micronutrients like iron, manganese, and zinc. Nitrogen is rarely tested because it fluctuates too rapidly.

How to Collect Samples

Use a clean trowel to collect 6-inch deep cores from 10-15 spots across your garden. Mix them in a plastic bucket — never metal — and send about one cup to your lab. Avoid sampling right after rain or fertilizer application.

Interpreting pH Results

Most vegetables prefer pH 6.0-7.0. Blueberries and potatoes thrive at 4.5-5.5. If your pH is below 6.0, apply garden lime at 5-10 lbs per 100 square feet. If above 7.5, elemental sulfur at 1-2 lbs per 100 square feet will lower it gradually.

Reading NPK Recommendations

Lab reports recommend fertilizer amounts based on your target crop. A phosphorus reading of 30-50 ppm is adequate for most vegetables. Potassium at 150-250 ppm is ideal. Follow the lab's specific recommendations rather than generic fertilizer schedules.

Organic Matter and Soil Biology

Aim for 3-5% organic matter. Add 2-3 inches of compost each fall. Cover crops like crimson clover and winter rye add organic matter when tilled under in spring. Mycorrhizal fungi populations recover over winter when soil is undisturbed.