No-Till Corn Response to Phosphorus and Sulfur Placement Under High Soil Fertility

• Starter fertilizer provides value in high-yield, no-till corn, even when soil test phosphorus (P) and potassium (K) are high.
• Reducing P and K rates should be done cautiously on soils with low nutrient holding capacity based on this trial.
• Banded placement of sulfur (S) may provide additional benefits over broadcast S in fields with low organic matter levels.

Introduction

Vertical stratification (high nutrient concentration at the surface with decreases at lower depths) of phosphorus (P) and potassium (K) due to repeated surface-applied fertilizers is a great challenge in no-till systems because of the low mobility of these nutrients. While subsurface banding of nutrients can address stratification concerns, research has shown that the advantage of banding over broadcast is minimal when soil P is sufficient.^1,2,3

Sulfur (S) deficiencies in corn continue to become more common in high-yield situations due to the shift to low-S fuels (as atmospheric deposition had served as an important source) and utilization of fertilizer blends with low S composition. With increased reliance on S mineralized from organic matter, demand can often exceed supply on coarse textured or low organic matter (OM) soils.^4,5 Since mineralization of S is driven by soil temperature, its availability can be delayed or reduced by the cooler soils attributed with no-till. Unlike P and K, S is a highly mobile nutrient. However, can its placement in a no-till setting provide any additional value?

Agronomy In Action Trial Details

A trial was established in an irrigated, no-till corn-soybean rotation field at Clay Center, KS, to answer the following questions:

1. Is there value to starter fertilizer when soil P and K are above sufficient levels?
2. Does reallocating a portion of the amount of nitrogen (N) and P applied preseason to a starter application provide positive benefits?
3. Can broadcast P be reduced when soil test P is high and starter fertilizer is used?
4. Does the S application method matter under no-till conditions?

Soil tests P and K (0-6-inch depth) were very high at 89 and 305 parts per million (ppm), respectively. OM was 1.6% and cation exchange capacity (CEC) was 9.7.

Fertilizer treatments were applied either by broadcast in the spring or in 2×2 placement with the planter. Nutrient amounts were balanced for total N, P, K and S in all treatments (21, 55, 55 and 10 lbs/acre, respectively) except in the reduced P broadcast treatments (21, 45, 55 and 10 lbs/acre, respectively). Fertilizer sources for the broadcast treatments were monoammonium phosphate (MAP, 11-52-0), potash (0-0-60), urea (46-0-0), and ammonium sulfate (AMS, 21-0-0-24). In the reduced P treatment, applied P was reduced 20% from the broadcast combined with 10-34-0 treatment by only reducing the MAP amount.

Starter fertilizer consisted of ammonium polyphosphate (10-34-0) at a rate of 7.5 gal/acre for all treatments except the broadcast-only and check treatments. Broadcast fertilizer amounts were reduced accordingly based on starter fertilizer rates being added to maintain equivalent rates to broadcast-only treatments. Potassium thiosulfate (KTS, 0-0-25-17) at a rate of 5 gal/acre was added to 10-34-0 as an alternative S source for some treatments. When this occurred, ammonium sulfate was removed from the broadcast application to keep rates balanced across treatments. No broadcast or starter fertilizer was applied to the check plots. Amounts of total nutrients of each fertilizer source applied within each treatment are provided in Table 1. All plots were side-dressed at the V3 growth stage with 150 lbs N/acre of NH₃.

Response To P Fertilizer Allocation

Numerical responses to the fertilizer treatments were apparent in the data, but individual plot variability limited most responses from being statistically significant. Fertilizer application either through preseason broadcast or starter significantly increased grain yield by 27.9 bu/acre (11% increase) (Graph 1). This response was likely attributed to N provided in the fertilizer, as pre-plant soil tests only indicated 5 ppm NO₃-N (~9 lbs N/acre) in the seedling zone and the base field N application was delayed until V3 crop stage. In a study geographically adjacent from this one, greater early growth was observed in plots that received 60 lbs N/acre in addition to starter fertilizer than those that did not (Figure 1). Reallocation of P from broadcast to starter did not affect yield, which is consistent with academic research when soil test P is sufficient.^1,2,3

Although soil P levels were more than sufficient, numerical reductions in yield were observed when total applied P was reduced (Graph 1). It is important to remember that sufficiency levels on soil test reports indicate the probability of response to additional nutrient applied. Although the very high sufficiency level observed at this site suggests that probability of response is very low, it is not zero. Since CEC at this site is low, opportunity for tie-up of fertilizer P by free cations is reduced, which may explain the response to fertilizer P when soil P was high. Soil K was also very high at this site (305 ppm). It is unlikely that any responses observed in the trial were associated with or confounded by K, but it cannot be completely ruled out.

Response To S Placement

Banded placement of S typically does not provide any additional advantage over broadcast S, except when soil OM is low.⁵ This site with 1.6% OM exhibited positive, though not significant, responses of ≥5.2 bu/acre to banded application compared to broadcast (Table 1, Graph 1). The limited S supply by organic matter paired with a higher S concentration placed in the root zone with banding compared to broadcast may explain the response. The response to banded S was even more apparent when P and K were not limiting, as yield with banded S increased 13.0 bu/acre as compared to 5.2 bu/acre within reduced P treatments.

Summary

Starter fertilizer is valuable in high-yielding, no-till fields, even when soil P and K are sufficient, due to N responses. This trial did suggest that P fertilization may be advantageous in fields with low nutrient holding capacity, as opportunity for nutrient tie-up is lower. Banding S in fields with low OM also seems to provide additional value over broadcast, especially when other macronutrients are nonlimiting.