Tips for Selecting Fertilizers Based on North Carolina Soil Tests

Understanding how to choose the right fertilizer begins with a good soil test. In North Carolina, soil test reports provide specific, actionable information about pH, nutrient levels, lime requirements, and often crop-specific recommendations. This article explains how to read those reports, match fertilizer types to the measured needs of your soil, and apply practical strategies to improve yield, plant health, and environmental stewardship across lawns, gardens, and agricultural fields in North Carolina.

How North Carolina soil tests are structured

Soil tests from North Carolina Cooperative Extension or state-supported labs typically report a similar set of parameters: soil pH, buffer pH (for lime requirement), extractable phosphorus and potassium (often by Mehlich-3), calcium, magnesium, zinc and other micronutrients, cation exchange capacity (CEC) or a proxy, and an estimate of organic matter. Reports often include explicit fertilizer and lime recommendations, stated per acre and sometimes per 1,000 square feet, and tailored to a specific crop you indicate when submitting the sample.

Key values to focus on in the report

• pH and buffer pH (if provided): pH affects nutrient availability and guides lime recommendations.
• Phosphorus (P) and Potassium (K): given as extractable P and K; recommendations specify if added P or K is required.
• Calcium (Ca) and Magnesium (Mg): help determine if dolomitic lime (adds Mg) or calcitic lime (adds Ca) is needed.
• Micronutrients (Zn, Mn, Cu, B, Fe): deficiency warnings may appear; many are recommended as foliar or banded applications.
• CEC and soil texture: indicate nutrient-holding capacity and affect choice of fertilizer form and application frequency.
• Lime requirement: often given as tons per acre, calculated from buffer pH and soil texture to raise the pH to a crop-specific target.

Interpreting the numbers: practical decoding

Soil test reports can look technical. Here are concrete translations that matter when you select a fertilizer.

pH and lime first

If the test recommends lime, address pH before heavy fertilization. Many nutrients (especially P, Fe, Mn, Zn) are less available at pH extremes. For most North Carolina crops, target pH ranges are: 6.0 to 6.8 for most vegetables and turf; 6.0 to 6.5 for legumes and soybeans; 5.5 to 6.5 for acid-tolerant crops. Use the lime amount on the report; it is calculated to reach the target pH for the soil texture and buffer pH provided.

Phosphorus and potassium: check the sufficiency level

Soil tests typically classify P and K as low, medium, or high. If P is “high” or “sufficient,” do not apply P unless a starter band is needed for seedlings; excess P increases runoff risk and is wasteful. For K, note crop sensitivity to chloride: standard potash (muriate of potash, KCl) contains chloride, which can harm chloride-sensitive crops (some vegetables, tobacco). For those crops use sulfate of potash (K2SO4) or potassium-magnesium sulfate (K-Mag) instead.

Understanding fertilizer label numbers

Fertilizer labels are in N-P-K form and list percentages as N, P2O5, and K2O. Many growers and lab recommendations are expressed as elemental P and K or as P2O5 and K2O; conversions are important:

• Elemental phosphorus (P) = P2O5 x 0.44.
• Elemental potassium (K) = K2O x 0.83.

When a soil test recommends “X lb P2O5 per acre,” use the label percentage of P2O5 to calculate how much product to apply. For example, a 10-10-10 fertilizer contains 10% P2O5; to supply 20 lb P2O5 per acre you would need 200 lb of that product per acre (20 / 0.10 = 200).

Nitrogen recommendations

Nitrogen (N) recommendations are usually crop- and yield-goal based and may not come directly from a basic soil test (because soil N is dynamic). Use the extension recommendation on the report and split applications where advised to reduce leaching: an initial small starter, then sidedress applications during active growth. For sandy soils with low CEC common in parts of eastern North Carolina, apply N in smaller, more frequent doses or use stabilized/slow-release N sources to minimize leaching.

Choosing fertilizer types by nutrient and situation

Knowing which nutrients are needed is step one. Step two is choosing appropriate sources and formulations for your soil, crop, and schedule.

Nitrogen sources

• Urea (46-0-0): high N content, inexpensive, but can volatilize (lose ammonia) if left on the surface without rain or incorporation. Use with urease inhibitors or incorporate soon after application.
• Ammonium nitrate (34-0-0): quick-release, lower volatilization than urea but more tightly regulated in some areas.
• Ammonium sulfate (21-0-0-24S): provides sulfur in addition to N; acidifying over time, so useful on high-pH soils or when sulfur is needed.
• Stabilized/slow-release N (urea formaldehyde, sulfur-coated urea, polymer-coated urea): reduce leaching and provide season-long N; best for lawns, high-value crops, and sandy soils.

Choose a source based on timing, risk of volatilization, soil type, and whether sulfur is also required.

Phosphorus sources

• Triple superphosphate (0-46-0), monoammonium phosphate (MAP, 11-52-0), diammonium phosphate (DAP, 18-46-0): high P2O5 content for banding or starter applications. MAP and DAP are common starter fertilizers because they supply both N and P.
• For most soils with adequate P, avoid broadcast P applications and consider banding small starter amounts near roots instead.

Potassium sources

• Muriate of potash (KCl, 0-0-60): common and economical, contains chloride.
• Sulfate of potash (K2SO4, 0-0-50 or similar): lower salt index and supplies sulfur; preferred for chloride-sensitive crops.
• K-Mag (sulfate of potash-magnesium): supplies K, Mg, and S where Mg is deficient.

Sulfur, calcium, magnesium, and micronutrients

• Sulfate forms (ammonium sulfate, gypsum, potassium sulfate) supply plant-available sulfur.
• Gypsum (calcium sulfate) adds Ca and S without changing pH; used where Ca is low but pH is acceptable.
• Dolomitic lime supplies Mg and Ca while raising pH; use when soil test shows low Mg and lime is required.
• Micronutrients are often best applied as foliar sprays for quick correction or as chelated soil-applied products if soil tests show chronic deficiency. Follow the lab recommended rates carefully because micronutrient overdosing can cause toxicity.

Application methods and timing for North Carolina conditions

Correct product selection combined with proper timing and placement maximizes efficiency and reduces environmental risk.

• Band and starter placement: placing P and starter fertilizers in a band near the seedling root zone increases early availability and reduces total P needed.
• Split N applications: apply part of the N at planting and sidedress the rest according to crop growth stage and phenology.
• Incorporation vs surface application: incorporate granular fertilizers when possible to reduce volatilization of urea and to protect phosphorus from surface runoff.
• Use slow-release forms on sandy Coastal Plain soils to reduce leaching and on home lawns to reduce burn and extend feeding.
• Time applications to avoid heavy rain forecasts in eastern North Carolina; heavy rainfall soon after broadcast application increases runoff and leaching risk.

Practical calculations and calibration

Accurate application requires converting lab recommendations to product rates and calibrating spreaders or sprayers.

• Convert lb/acre to lb per 1,000 sq ft: divide lb/acre by 43.56. For example, 50 lb/acre is about 1.15 lb per 1,000 sq ft (50 / 43.56 = 1.147).
• To calculate product needed from an N recommendation: if you need 40 lb N/acre and the fertilizer is 12-12-12 (12% N), you need 333 lb of that product per acre (40 / 0.12 = 333).
• Calibrate spreaders annually: apply a known amount to a measured area and adjust settings so the actual matches the target.
• Read labels for safe maximum rates and re-application intervals; many crop labels and extension bulletins set annual maximums for certain nutrients and micronutrients.

Crop-specific tips for North Carolina growers

• Turfgrass: focus on regular soil testing, maintain pH near 6.0-6.5, use slow-release N for steady growth, and avoid P unless test indicates deficiency. Split N into fall and spring depending on grass species.
• Vegetables: use small starter P bands for transplants, sidedress N during fruiting, and choose K sources that avoid chloride on sensitive crops. Correct pH for optimum micronutrient availability.
• Row crops (corn, soybeans, cotton): follow extension yield-goal based N recommendations, band P at planting if needed, adjust K according to test status, and use conservation practices to minimize runoff.
• Specialty crops and orchards: rely on tree and tissue tests for micronutrient timing, use band applications and fertigation (when available), and emphasize soil organic matter management.

Environmental and safety considerations

• Follow soil test recommendations rather than applying blanket rates; over-application of P and N is wasteful and contributes to water quality problems.
• Store fertilizers safely and away from waterways. Mix and apply away from wells, streams, and storm drains.
• Consider buffer strips, cover crops, and reduced tillage to decrease erosion and nutrient loss in fields and gardens.

Bottom-line takeaways

• Start with the soil test: adjust pH and follow the lab’s lime recommendation before implementing major fertilization changes.
• Match the fertilizer form to the soil and crop: choose slow-release N on sandy soils, use sulfate or K-Mag for chloride-sensitive crops, and prefer banded P when soil P is low.
• Convert lab recommendations to product rates using label percentages and simple math, then calibrate application equipment.
• Apply N in split doses, minimize broadcast P unless necessary, and use micronutrients only when indicated by the test.
• Prioritize environmental stewardship: avoid over-application, time applications to avoid heavy rains, and use conservation practices to reduce runoff.

By reading North Carolina soil tests carefully and matching fertilizer choices to the soil chemistry, texture, and crop needs, you can improve nutrient efficiency, crop performance, and environmental outcomes. Use the concrete calculations, product guidelines, and application strategies above to make informed fertilizer decisions on your property.