The Soil Testing Laboratory operated by the North Carolina Department of Agriculture and Consumer Services (NCDA&CS) Agronomic Services Division analyzes about 300,000 soil samples per year submitted from across North Carolina. Previous evaluations of soil test results have provided insights into soil phosphorus content (Cahoon and Ensign 2004; Miller 2023), but studies had not provided an overall assessment of other factors affecting soil fertility throughout the state. Phosphorus is critical to evaluate because it is essential for plant growth and can be a potential environmental pollutant in excessive amounts. However, it is also important to assess other essential plant nutrients and soil parameters (including acidity, organic matter, and cation exchange capacity) to accurately gauge soil fertility. Furthermore, there have been a number of challenges with evaluating the soil test dataset due to spatial bias and variation in sampling methods. Growers in counties or regions in which precision agriculture techniques are heavily used tend to submit more samples for analysis, potentially skewing average results. In addition, variations in sampling depths and tillage practices can lead to inconsistencies that complicate the interpretation of the test results.

To provide a more accurate snapshot of the soil fertility status of row crop soils in North Carolina, we conducted a study in association with local Extension agents, who selected agricultural fields representing current crop management in each region. Soil samples were collected from two depths (0 to 4 inches deep and 4 to 8 inches deep), and information about soil type and crop management was recorded. The number of samples per county was proportional to the row crop acreage in each county, and 400 soil samples were collected and analyzed. The results were separated by region.

The number of soil samples collected per county was weighted based on the reported acreage of field crops in the 2017 Census of Agriculture (United States Department of Agriculture—National Agricultural Statistics Service 2019), with a target goal of collecting and analyzing 500 soil samples. Counties with fewer than 7,000 acres of field crops were excluded from the study, resulting in 83 counties initially selected for sampling. A 4-by-4-mile grid was overlaid onto each county to be surveyed, and an online tool was developed to randomly select one of the grid cells. Local Extension agents used the online tool to select a random grid cell for soil sampling. Based on their experience, the agents chose a field in the selected cell grid that represented the average soil and crop management for that county. Agents contacted the selected farmers for permission to sample their fields and collected information, including the types of tillage used, fertilizers used, and overall crop production history. The GPS coordinates of the field were sent to the sampling team. After soil sampling was completed, all data about the locations and identity of farmers were destroyed to maintain anonymity in the study.

Composite samples with 10 subsamples were collected with a push probe in each field at two depths: 0 to 4 inches and 4 to 8 inches. Due to logistical challenges, some target counties were not sampled, resulting in a total of 400 samples taken in 58 counties. The number of fields sampled per county, ranging from 4 to 20, was proportional to row crop acreage. Soil samples were then grouped by region: mountains, piedmont, coastal plain, and tidewater (a subregion of the coastal plain, following the organic soil types) (Figure 1). Fewer samples were collected in the mountains because the predominant agricultural systems there are pasture and hay.

After collection, samples were oven-dried, homogenized, ground to the standard particle size of 2 mm, and submitted to the NCDA&CS Soil Testing Laboratory for routine soil fertility analysis. North Carolina State University’s Crop and Soil Sciences Department conducted additional analyses for total carbon and texture. Organic matter was estimated by multiplying the content of carbon by 1.72.

Figure 1. North Carolina county map by region (tidewater, coastal plain, piedmont, and mountains) with the number of soil samples collected in each county. Counties without numbers were not sampled.

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Figure 1. North Carolina county map by region (tidewater, coastal plain, piedmont, and mountains) with the number of soil samples collected in each county. Counties without numbers were not sampled.

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The average clay content was 20% in coastal plain and tidewater soils and 30% in piedmont and mountains soils (Table 1). In general, tidewater soils are loams, coastal plain soils are sandy loams, and piedmont and mountain soils are clay loams. The different textures in North Carolina soils are related to the predominant geological parent material in each region.

The average soil organic matter increased from the coastal plain to the piedmont and mountain regions because soils with more clay generally protect organic matter from decomposition (Table 1). However, the tidewater region had the highest average organic matter (10.8%) due to the existing organic soils, reaching a maximum of 71% organic matter in one sample (Table 1). The content of humic matter did not follow the same pattern as organic matter among the different regions, indicating poor correlation between the content of soil organic matter and humic matter.

Because the cation exchange capacity (CEC) is dependent on the clay and organic matter content, it increased from 5 meq/100 cc in the coastal plain to 7 meq/100 cc in the piedmont, and 10 meq/100 cc in the mountain and tidewater regions. In all regions, the average phosphorus (P) index was greater than 50, which is considered the critical soil test value for North Carolina soils (Hardy et al. 2014). The average potassium (K) index varied from 49 to 69 across regions, which is similar to the critical soil test value (K index = 50) established for North Carolina. In North Carolina, sulfur (S) fertilization is recommended for soils with an S index lower than 25; all regions presented average values above 25. The average contents of the cationic micronutrients copper (Cu), zinc (Zn), and manganese (Mn) were above their respective critical soil test values (index = 25) recommended for North Carolina.

Among all the samples, 66% had a pH between 5.5 and 6.5, which is the recommended range for most crops (Table 2). Tidewater soils had the highest percentage of samples with a pH below 5.5 due to the occurrence of organic soils in that region.

For the macronutrients P, K, and S, the percentage of soils with values considered “high” or “very high” were 77%, 48%, and 89%, respectively (Table 3), indicating that among these nutrients, K would be the most likely to be deficient in row crop fields. Regarding the cationic micronutrients Cu, Zn, and Mn, we found that 74%, 78%, and 94% of the soil samples, respectively, were above the critical values for those nutrients.

Buffer acidity and pH were not different between the two soil layers (0 to 4 inches and 4 to 8 inches) (Table 4). Organic matter was lower in the deeper soil layer. In the tidewater and coastal plain, the levels of P, K, Ca, Mg, Mn, Cu, and Zn were an average of 26% lower in the deeper layer. In the piedmont and mountains, P, K, Ca, Cu, and Zn were 40% lower in the deeper layer; this pattern is expected in clayey soils, as clays tend to retain more nutrients from fertilizers in the top layers. Furthermore, no-till practices are more common in the piedmont, which promotes accumulation of nutrients in the top layer. In this study, however, we also compared the nutrient stratification in fields under conventional tillage and no-till systems, but the only significant difference occurred for P in the piedmont and mountains (data not shown).

Notes: The percent values are presented for soil parameters different by Mann-Whitney U Test (p < 0.05).

ns = not statistically significant. ↲

By selecting soils randomly and in proportion to the amount of row crop agriculture, we captured a representative snapshot of the soil fertility and texture of North Carolina soils. Soils were sampled across four geographical regions (tidewater, coastal plain, piedmont, and mountains) to represent the diversity of parent material, soil characteristics, and fertility in the state. Statewide, soil fertility was good, although excess P levels suggest that farmers could reduce applications of that macronutrient to save money. The micronutrients Cu, Zn, and Mn also were above the critical values. We observed some nutrient stratification with respect to soil depth, which is to be expected for organic matter and for plant nutrients; however, the tillage system had little effect on stratification overall.

The Corn Growers Association of North Carolina provided grant funds for this research. The authors are also grateful to Chandler Fulmer, who helped collect soil samples, and the Extension agents who helped select fields and collect soil samples.

Cahoon, L., and S. Ensign. 2004. “Spatial and Temporal Variability in Excessive Soil Phosphorus Levels in Eastern North Carolina.” Nutrient Cycling in Agroecosystems 69: 111–125. ↲

Hardy, D. H., M. R. Tucker, and C. E. Stokes. 2014. Crop Fertilization Based on North Carolina Soil Tests. North Carolina Department of Agriculture and Consumer Services Agronomic Division. ↲

Miller, S. 2023. A Manureshed Analysis and Assessment of Two Alternative Manure Products in North Carolina. Master’s Thesis, North Carolina State University. ↲

United States Department of Agriculture—National Agricultural Statistics Service. 2019. 2017 Census of Agriculture. U.S. Summary and State Data. USDA. ↲