Rasel Parvej, Brenda Tubana, Syam Dodla, David Moseley, and Matthew Foster

For corn and soybean production in Louisiana, producers mainly supply macronutrients such as nitrogen (N; for corn only), phosphorus (P), potassium (K), and sometimes sulfur (S). To meet these nutrient demands, organic and/or inorganic fertilizers are often applied onto the soil surface at or before planting. Nitrogen is mainly applied for corn and the recommendations depend on yield goals and soil types. For P, K, and S fertilizations, soil-test-based fertilizer recommendations are used. Fertilization is recommended if soil-test nutrient concentrations fall below the critical level. The critical nutrient level is defined as the range of soil-test nutrient concentration below which crop response to added fertilizer is expected, within which is uncertain, and above which is unlikely. In general, the critical soil-P and K concentrations for both soybean and corn production in Louisiana range from 21 to 35 ppm P (42 to 70 lb P/acre) for all soil types and 98 to 141 ppm K (196 to 282 lb K/acre) for silt loam, 142 to 264 ppm K (284 to 528 lb K/acre) for clay loam, and 142 to 317 ppm K (284 to 634 lb K/acre) for silty clay soils. Detailed soil-test-based fertilizer recommendation for row crop production in Louisiana can be found at the LSU AgCenter (Variety Trials and Production) website.

For P and K fertilization, Louisiana producers mostly use triple superphosphate (TSP; 0-46-0) for P and muriate of potash (MoP; 0-0-60) for K and apply both fertilizers mostly in the fall rather than in the spring. One of the main reasons for fall application is due to wet soil conditions or limited application time in the spring. However, lots of producers believe that they must apply both TSP and MoP in the fall since both fertilizers are rocky materials (TSP is originated from phosphate rock and MoP is from potash ore) and require long time to dissolve and become available for plant uptake. Practically, both fertilizers are water soluble and can rapidly release nutrients, regardless of application time, when dissolve with adequate soil moisture and/or rainfall/irrigation water. Many studies showed that spring application of both TSP and MoP fertilizers is either equal to or better than fall application in increasing crop yield especially in soils that are highly prone to nutrient losses via leaching, runoff, and/or erosion.

In 2019-2020, we evaluated the effect of P and K fertilizer application timings on soybean and corn yields at the Macon Ridge Research Station (MRRS) in Winnsboro and Northeast Research Station (NERS) in St. Joseph, LA (Fig. 1-2). The trials were conducted in silt loam soils at both locations. The soil-test P and K concentrations across all sites were either below or within the critical level except for the corn trial at the NERS where soil-test P and K concentrations were above the critical level. As expected, both corn and soybean yields responded positively to both fall and spring fertilization at sites with soil P and K concentrations below or within the critical level and did not responded at site with soil P and K concentration above the critical level (corn at NERS; Fig. 1). Although both corn and soybean yields were not significantly different between fall and spring P and K application, our one-year preliminary results showed that there was a trend of getting numerically higher yield (~3 bu/acre soybean and ~8 bu/acre corn) from spring P and K application compared to fall application especially in silt loam soils with P and K concentrations below or within the critical level.

Overall, both fall and spring fertilization have advantages and disadvantages. Fall application may help save critical time in the planting season but reduces available quantity of applied nutrients due to losses through leaching, runoff, erosion, or soil fixation. Although our one-year study showed no significant bump in yield between fall and spring P and K application, the following factors need to be considered in making decision regarding fertilizer application time.

1. The rapidity of P and K fixation to unavailable forms usually increases with the decrease of soil-test P and K concentrations. For example, soils with deficient P and K will fix applied P and K more rapidly than soils with sufficient P and K. Therefore, fertilizers should be applied in the spring at or near planting in P and K deficient soils to ensure maximum nutrient availabilities during the time of rapid plant uptake.
2. Soil P availability is maximum between soil pH 6.0 and 7.5. Fertilizer-P is fixed to unavailable forms as aluminum phosphate when soil pH falls below 5.5 and as calcium phosphate when soil pH exceeds 7.5. Therefore, fertilizer-P should be applied in the spring as close to planting as possible for fields with low (pH <5.5) or high (pH >7.5) soil pH to ensure maximum fertilizer-P availability for plant uptake.
3. Spring application of fertilizer-P and K should be considered for coarse-textured soils with very low cation exchange capacity (CEC <10) such as loamy sand to sandy loam (sometimes silty loam) soils where nutrient leaching and soil erosion are common, and nutrient deficiencies are often observed.
4. For soils that are very prone to waterlogged/flooded conditions, fertilizer-P and K should be applied in the spring at or near planting since the alternating flooding (anaerobic) and non-flooding (aerobic) conditions decreases soil nutrient availability and increases losses.
5. Fall application of P and K should be considered for soils with nutrient concentration within (medium) or above (sufficient) the critical level, where fertilizers are mainly applied to replace soil nutrients that are removed by harvested grains to maintain the same level of soil nutrient level. In addition, fertilizer-K should be applied in the fall in fields that have long history of chloride (Cl) toxicity problems and are poorly drained. Since K fertilizer (MoP) mainly consists of KCI, fall application will allow enough time to decrease Cl toxicity by reducing Cl accumulation from fertilizer KCl through winter and early spring rainfall.