Soil Testing Guide: How to Use Soil Analysis to Plan Your Fertilizer Program

Without soil testing, fertilizer recommendations are essentially guesswork. A farmer who applies 200 kg N/ha of nitrogen fertilizer without knowing their soil's nitrogen supply might be applying twice as much as needed — wasting money and risking environmental damage — or half as much as needed — limiting yield and leaving money on the table. Soil testing removes this uncertainty by providing actual data on what nutrients are present in the soil and in what quantities.

The economic case for soil testing is compelling. Research consistently shows that soil-test-based fertilizer programs reduce fertilizer costs by 15-25% while maintaining or improving yields, compared to programs based on blanket recommendations. The cost of soil testing ($20-50 per sample) is typically recovered many times over in the first season through fertilizer savings and yield improvements.

The quality of a soil test is only as good as the sample collected. A poorly collected sample that does not represent the field will give misleading results that lead to incorrect fertilizer recommendations. The key principles of good soil sampling are: collect from multiple locations across the field (20-30 sub-samples per field), sample to the correct depth (0-20 cm for annual crops, 0-40 cm for perennial crops), avoid sampling from unusual areas (wet spots, headlands, areas near old fence lines), and collect samples at the same time of year for consistent comparison.

For large fields with visible variation in soil color, texture, or crop performance, divide the field into management zones and collect separate samples from each zone. This allows zone-specific fertilizer recommendations that can significantly improve fertilizer efficiency compared to applying a single rate across the entire field.

A standard soil test report includes: pH, organic matter content, and available levels of phosphorus, potassium, calcium, magnesium, and sulfur. Some tests also include micronutrients (zinc, manganese, iron, copper, boron) and cation exchange capacity (CEC). Each nutrient is typically reported in mg/kg (parts per million) or kg/ha, along with a rating (low, medium, high, very high) based on calibration research for your region and crops.

pH is the most important single measurement in a soil test — it affects the availability of virtually all other nutrients. Organic matter is important because it is the primary source of nitrogen, phosphorus, and sulfur through mineralization. Phosphorus and potassium levels determine how much of these nutrients need to be added as fertilizer. Calcium and magnesium levels indicate whether lime or gypsum applications are needed.

Phosphorus and potassium soil test results are used to determine the 'build-up and maintenance' strategy for these nutrients. In soils with low phosphorus (below the critical level for your crop), apply phosphorus at rates above crop removal to build up soil levels over time. In soils with adequate phosphorus, apply only enough to replace what the crop removes. In soils with very high phosphorus, reduce or eliminate phosphorus applications until levels decline.

The critical phosphorus level varies by crop and soil type — typically 15-25 mg/kg (Olsen method) for most crops in mineral soils. Potassium critical levels are typically 100-150 mg/kg for most crops. GreenGrow's agronomists can help you interpret soil test results for your specific crops and soils, and develop a fertilizer program that builds soil fertility over time while optimizing current-season yields.

Once you have soil test results, the process of building a fertilizer program involves four steps: (1) identify nutrient deficiencies (nutrients below the critical level for your crop); (2) calculate the fertilizer rates needed to correct deficiencies and meet crop requirements; (3) select fertilizer products that supply the required nutrients in the most efficient form; and (4) schedule applications to match crop demand timing.

GreenGrow provides a free soil test interpretation and fertilizer recommendation service for our customers. Send us your soil test results along with your crop type, yield target, and irrigation system, and our agronomists will develop a complete fertilizer program recommendation. This service is available to all customers and distributor partners, and is one of the key ways we support our customers in achieving maximum return on their fertilizer investment.