Figure 1. Case IH and John Deere tractors equipped with tanks for liquid starter application at planting.
Starter fertilizer placed in contact with the seed ("pop-up" fertilizer) is another option, but its use requires a great deal of caution to avoid possible germination and seedling injury. The amount of pop-up that can safely be applied is limited, and depends on the fertilizer used and soil properties. For example, starter fertilizer containing ammonium thiosulfate should not be placed in contact with the seed (Hergert and Wortmann, 2006).
A starter fertilizer is usually composed of two or more nutrients. Under most situations, a combination of nitrogen and phosphorus constitutes an effective starter material. Liquid 10-34-0 and dry 18-46-0 are common starter fertilizer materials. Liquid 7-21-7 and dry 8-32-16 are also commonly used. Other fertilizer formulations may be used as starters; for example, the addition of zinc and/or sulfur may be warranted in sandy, low organic matter soils.
To fully understand the role of starter fertilizer in providing basic nutrition to corn seedlings, it is useful to review the physiology of early corn root development.
Soon after emergence (VE) the young corn seedling will begin to develop its nodal root system, the primary roots for water and nutrient uptake of the plant (Figure 2). If normal development of the nodal roots is impeded and the endosperm reserves of the seed become depleted, the plant will lack the basic nutrients needed for optimum growth. When nodal roots are impeded, above-ground symptoms may appear as less vigorous or less green plants.
By the V2 stage (2 leaf collars visible) it is important for corn plants to be actively taking up nutrients such as N (Figure 2). Although these nutrients are needed in very limited quantities, they are none the less essential in the plant's ability to run its newly developing photosynthetic machinery.
Starter fertilizer applications to corn have been well researched and documented. The scientific literature shows numerous cases where starter has produced positive, meager and no corn grain yield increases. This array of results means that positive grain yield responses are likely related to both environmental and cultural interactions. Several important studies are discussed below.
Starter Research Results by Geography and Type of Nutrients in Starter
Because starter fertilizer experiments are often conducted using N, P, and K fertilizers it is not always exactly clear which nutrient provided the yield increase; however:
Conclusions: It can further be concluded that if growers in the central Corn Belt states are using high rates of broadcast fertilizer (P and K) in a build-and-maintain fertilizer program, starter P and K yield responses would be even less consistent and unlikely, especially if conventional tillage practices are being utilized (Kaiser et al., 2005).
Research Results by Soil Type
Consistent grain yield responses to starter fertilizers may also be expected on soils that have low soil organic matter or soils that have coarse (sandy) soil surface textures. Many soils formed from Mississippi River alluvium that stretch from portions of central Minnesota to the Gulf of Mexico fit this description. Several studies report results by soil type:
- These researchers found that starter N and P applications significantly increased corn grain yields in five out of 15 environments (and numerically increased yield in 12 of 15 environments) even when extractable soil P levels were high on these soils (Figure 4).
- Moreover, the average grain yield response was 12.5 bushels an acre when averaged across all years and locations, although yields were not always significant at the 0.05 probability level.
- In addition, it was concluded that the observed grain yield responses were more likely from the P in the starter fertilizer and the largest, most consistent grain yield responses were on the coarsest textured (sandiest) soils (Mascagni et al., 2007).
Conclusions: It seems apparent from the reviewed literature that starter fertilizers of N, P and K in sandy loam and coarser-textured soils might be warranted even if P and K levels are high. However, it is impossible to tell if N, P and K are needed or just N, due to conflicting findings in these environments.
Hybrid Responses to Starter Fertilizer
Kansas State researchers studied five hybrids ranging in maturity from 103 to 113 RM, with and without starter fertilizer (N and P) in a no-tillage production system (Gordon, 1997). This Kansas study found that starter fertilizer significantly increased early season growth, N and P uptake at V6, and N and P concentration in the ear leaf. However, no hybrid by starter interaction was found (i.e., all hybrids responded similarly to starter application). The study also measured growing degree units (GDUs) to pollination. All hybrids required less heat units to begin pollination when starter fertilizer was used (Table 1). This is a key finding for dryland corn production in Kansas, where yield is often limited by late season drought stress. Furthermore, earlier pollination under these conditions can lengthen the grain filling period and increase corn grain yield potential.