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.
Key takeaways
- Always read pH, organic matter and CEC first — they drive everything else.
- Target soil pH 6.0–6.8 for most crops.
- Multiply ppm × 2 to estimate kg/ha in the top 15 cm.
- Fix pH before chasing micronutrients.
Related in the pathology library
Products you can buy for this problem
Typical registered products for the diseases covered in this article, with their active ingredient, mixing rate, spray interval and pre-harvest interval. Rotate FRAC/IRAC groups to avoid resistance.
Urea 46-0-0
Active ingredient: Urea nitrogen 46%
NPK 15-15-15 compound
Active ingredient: Balanced N-P-K
Calcium ammonium nitrate (CAN 27%)
Active ingredient: Ammonium nitrate + calcium
Well-rotted farmyard manure / compost
Active ingredient: Organic nitrogen
Doses shown are typical registered field rates. Always read the product label, confirm the product is registered for your crop and country, respect the pre-harvest interval (PHI), and wear the protective equipment the label specifies. PlantWatch AI provides agronomic guidance, not a legal spray recommendation.
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