Fall Phosphorus and Potassium Application Considerations

Rasel Parvej, David Moseley, Matthew Foster, LSU AgCenter Scientist and Jamil Uddin, LSU AgCenter Postdoctoral Researcher

Article Highlights:

• Spring applications of both TSP and MoP fertilizers are either equal to or better than fall applications; however, both fertilizers need to be incorporated by rehipping the bed (for 30-40-inch row spacing) when applied in the spring.
• Factors such as the soil-test P and K concentrations, soil pH, and soil texture should be considered in making decisions regarding the time of P and K fertilizer applications.

Louisiana producers mostly use triple superphosphate (TSP; 0-46-0) for Phosphorus (P) fertilization and muriate of potash (MoP; 0-0-60) for potassium (K) fertilization and apply both fertilizers mostly in the Fall rather than in the Spring. The main reasons for fall applications are due to wet soil conditions or limited application time in the Spring. A misconception about Spring applications of both P and K fertilizers is that they require a long time to dissolve and become available for plant uptake. Practically, both fertilizers are highly water soluble and can rapidly release nutrients, regardless of application time, when dissolved with adequate soil moisture and/or rainfall/irrigation water. Many studies including research conducted in Louisiana showed that spring applications of both TSP and MoP fertilizers is either equal to or better than fall applications in increasing crop yield especially in soils that are deficient and highly prone to nutrient losses via leaching, runoff, and/or erosion. However, both fertilizers when applied in the Spring need to be incorporated by rehipping the bed (for 30-40-inch row spacing) to bring most of the fertilizers, broadcasted in the furrow, back to the bed, to reduce fertilizer-P and K losses from furrow with rainfall and irrigation water over time, to increase fertilizer-P and K availability in the bed near plant roots, and to improve P and K uptake and crop yield. The following factors need to be considered in making decisions regarding P and K fertilizer application time.

• 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.
• 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.
• 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. However, applying P and K in fall in fine-textured soils with high CEC (>20) may not be a problem.
• 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.
• 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.

LSU AgCenter Conducts Soybean Variety Trials and On-farm Demonstrations

David Moseley, LSU AgCenter soybean specialist

Article Highlights:

• It is important for a producer to consider how varieties perform in an environment similar to their own and how varieties perform in multiple environments.
• The LSU AgCenter conducts an Official Variety Trial (OVT) and Core-block demonstration plots to provide unbiased data to assist in variety selection.
• The LSU AgCenter also conducted other on-farm demonstrations including: conventional and nematode resistant variety demonstrations, potassium demonstrations, and planting date by maturity group demonstrations.

One of the most important decisions a soybean producer makes every year is variety selection. The LSU AgCenter conducts an Official Variety Trial (OVT) and Core-block demonstration plots to provide unbiased data to assist in variety selection. The OVT and core-block demonstrations are planted throughout the state to collect performance data in different environments. It is important for a producer to consider how varieties perform in an environment similar to their own and how varieties perform in multiple environments. Varieties that perform consistently well across multiple environments and years could be considered to have more performance stability.

Official Variety Trial

The 2022 OVT included 118 varieties entered by 10 seed companies and one university soybean breeding program. The varieties consisted of several different herbicide technologies, and the maturity groups range from 3.8 to 5.6. The trial was replicated at seven research stations across the state in different soil types including fine sandy loam, silt loam, silty clay and clay. At each location, the varieties were replicated four times.

On-farm Demonstration Plots

In addition to the OVT, the LSU AgCenter collaborates with soybean producers to evaluate soybean varieties directly on farms. For the core-block demonstration program, the LSU AgCenter parish agents cooperate with producers to plant, maintain, and harvest strip trials submitted by seed companies and university soybean breeding programs. These demonstrations provide valuable yield data from local growing conditions and agronomic practices.

In 2022, seven seed companies and one university soybean breeding program submitted varieties to be evaluated in the core-block demonstrations. Thirty-five demonstrations were planted across 19 parishes. A total of 31 varieties were submitted to be evaluated. The demonstrations were divided by maturity group (MG). A demonstration consisted of varieties with a MG of 3.7 to 4.4; 4.5 to 4.9; or 5.0 to 5.6. The number of varieties submitted for each MG were ten (MG 3.7 to 4.4), fourteen (MG 4.5 to 4.9) and seven (MG 5.0 to 5.6).

Nematode resistance demonstrations were conducted again in 2022. Twelve varieties from five seed companies and one University soybean breeding program were entered into the nematode resistance screening trial. Among the varieties, eleven were entered as resistant varieties and one was entered as a susceptible variety. The nematode resistant trial was planted in four parishes (Pointe Coupee, Concordia, Avoyelles, and Tensas) in fields known to have nematode pressure. Along with yield, nematode assays were taken at planting, at the R6.5 growth stage, and at harvest.

Conventional soybean variety demonstrations were conducted in Beauregard, Acadia, and Tensas parishes. Eight varieties from the University of Missouri and a commercial check were included in the demonstrations. Data from these demonstrations will help farmers who produce soybean in herbicide restricted habitats and who are looking for a potential premium from conventional soybean varieties.

In addition to the variety trials, a potassium demonstration was conducted in Beauregard parish on a light soil with very low potassium soil levels. Granular and foliar applications of potassium was applied at different times and rates (seven different applications). This data will help farmers understand best management practices to economically correct low potassium levels.

A planting date by variety demonstration was conducted in Acadian parish. Unfortunately, the soybean plants were not harvested due to poor seed quality. We plan to repeat the demonstration in 2023.

The parishes in which the soybean demonstrations were located in 2022 are indicated in figure 1.

Variety Testing and On-farm Demonstration Results

The performance data from the soybean OVT and on-farm demonstrations will be published by the LSU AgCenter in the annual soybean variety testing summary. Maturity date, height, lodging and disease reaction information from the OVT will also be included. The 2022 OVT resultswill be published following harvest to assist with 2023 variety selections and planting decisions. The variety publication for the 2021 growing season can be found at 2021 Soybean Variety Yields and Production Practices.

More information on LSU AgCenter variety testing can be found in the Louisiana Agriculture Magazine Vol. 64, No. 1, Winter 2021.

Figure 1: Parishes in which the soybean on-farm demonstrations were located in 2022

LSU AgCenter Specialists