Reading a Soil Test: A Farmer's Guide to NPK, pH and Micronutrients

Published 2026-02-14 · Updated 2026-05-01 · For Farmers, agronomy students

A soil test is the cheapest agronomy decision you will make all year. For the cost of a bag of urea you get a season's fertilizer plan, calibrated to your field's actual chemistry.

The three numbers to read first

Soil pH controls whether nutrients are available at all. Below 5.5 phosphorus locks up in aluminum; above 7.5 iron and zinc become unavailable. Target 6.0–6.8 for most vegetables and grains.

Organic matter above 3% indicates a resilient soil that mineralizes 30–60 kg N/ha per season for free. Below 1.5% you are farming sand — build organic matter with cover crops and compost before spending on fertilizer.

CEC (cation exchange capacity) tells you how much fertilizer the soil can hold. Sandy soils (CEC <10) need split applications; clay soils (CEC >20) hold nutrients between rains.

Turning ppm into kg/ha

Multiply soil-test ppm by 2 to approximate kg/ha in the top 15 cm. A phosphorus reading of 15 ppm is roughly 30 kg P/ha available — below the 25–40 ppm target for most crops, so plan a starter dose of DAP or triple superphosphate at planting.

Visual clues when the test hasn't come back

Older leaves yellowing uniformly → nitrogen. Purpling of stems and leaf undersides → phosphorus. Marginal leaf scorch on older leaves → potassium. Interveinal chlorosis on new growth → iron or zinc, usually pH-driven. Correct pH first; deficiencies often disappear.