Starter phosphorus (P) fertilizers supply small amounts of P (20 to 30 pounds of P₂O₅ per acre) at planting to supplement corn seedlings during the early stages of growth. Limited root growth associated with cool temperatures and saturated soils in the first weeks after emergence may contribute to reduced P uptake. This fertilizer is meant to supplement early plant growth before the root systems are large enough to obtain other P from the soil. Oftentimes, farmers use a starter fertilizer source that includes nitrogen (N). This widely adopted practice replaces the amount of N that winter rains may have leached out of the soil. If very little plant-available N is present at planting, an economic return is often realized by including N in the starter.

Previous studies, mainly conducted in other states, have shown that starter fertilizers (including both N and P) benefit yield when soils are wet and cold. Starter fertilizers are especially beneficial in minimum or no-till systems. Today, application of starter N and P is a common practice for many farmers. Looking at other studies, we learned that a greater response to starters occurred when soil test P was initially low. When soils had sufficient levels of P at planting, the addition of starter P fertilizer did not translate to higher yields. Starter P may promote faster early-season growth compared to plants not receiving it. However, plants without starter P catch up through the season and in the end, no yield difference was observed.

Many agricultural soils in North Carolina have significant accumulated P from prior fertilizer and manure applications, thus reducing the plant response to starter P. More than 84% of soils tested by the North Carolina Department of Agriculture and Consumer Services (NCDA&CS) had “high” or “very high” P index (P-I) levels, corresponding to 51 to 100 and > 100 P-I, respectively (Gatiboni et al. 2025). In such soils, no economically significant yield increase would be expected with the application of starter P.

The study described here was conducted to provide clarity about when a response to starter fertilizer might occur in North Carolina.

Eight trials were conducted in corn fields in North Carolina between 2022 and 2024. Fields with P-I (determined by NCDA&CS soil tests) varying between 24 (low P) and 56 (high P) were selected in Bertie, Beaufort, Tyrrell, and Washington counties. Three treatments were tested: (1) control—without application of starter fertilizer; (2) starter N—with application of 30 pounds per acre (lb/A) of N at planting; and (3) starter N+P—with application of 30 lb/A of N and 30 lb/A of P₂O₅ at planting. The starter fertilizers were dribbled over the row at planting. No other P fertilizer was used in the fields, and all other production practices followed the farmer’s management, including layby N fertilization. The data we collected included measured plant height between V2 and V4 growth stages, grain moisture content at harvesting, and grain yield.

Figure 1 summarizes the results of the multiyear study. Of the eight sites, two sites produced significantly higher yields with the addition of P and N. In the 2023 Tyrrell County trial, corn seedlings were substantially taller when P was included in the starter fertilizer, which translated into a 32% yield increase from 149 bushels per acre (bu/A) to 197 bu/A. This response was expected because that site had a low P index (24). Similarly, in the 2023 Beaufort County trial where the P-I was 56, both starter N and starter N+P fertilizer increased yields relative to the control; however, there was no difference between starter N alone and starter containing both N and P, indicating that the yield response was due to N rather than P.

For six sites, where the P-I ranged from 35 (medium) to 56 (high), no yield response to starter P was observed. Our results indicate that when existing soil P levels are medium (P index ≥ 25) or high (P index > 50), including starter P fertilizer usually does not provide a greater economic return.

Overall, our three-year, multisite study demonstrates that starter fertilizers containing P are warranted only at locations with very low or low soil P indexes. At higher soil test levels, a starter fertilizer containing N alone is usually sufficient.

Figure 1. Effect of starter fertilizers on corn seedling height and corn yield over eight site-years. Note that means with the same letter are not significantly different by the Tukey HSD test (P < 0.05). (Height data was unavailable for the Beaufort County site.)

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Figure 1. Effect of starter fertilizers on corn seedling height and corn yield over eight site-years. Note that means with the same letter are not significantly different by the Tukey HSD test (P < 0.05). (Height data was unavailable for the Beaufort County site.)

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Gatiboni, L., A. Lima, D. Osmond, and T. Tiecher. 2025. Overview of the Soil Fertility Status of Representative Row Crop Fields in North Carolina. NC State Extension. ↲

We gratefully acknowledge the Corn Growers Association of North Carolina for sponsoring this study.