April 2024 Louisiana Soybean Update: Planting Progress and Replanting Considerations

David Moseley, LSU AgCenter Soybean Specialist

Louisiana's 2024 soybean planting season had a later start compared to last year but was ahead of the historical five-year average according to the latest USDA-NASS survey as of the week ending April 7th, 2024. Recent precipitation, particularly the heavy storms on April 10th, have likely slowed planting progress and may raise concerns about stand establishment due to factors such as soil crusting, hail damage, and flooding/saturated soils.

Planting and Replanting Decisions

The 2024 Soybean Variety Yields & Production Practices publication recommends planting approximately 130,000 soybean seeds per acre (adjusting for row spacing and planting date). The desired final stand is around 104,000 plants per acre. However, research suggests 95% of full yield potential can be achieved with final stands of 70,000-75,000 plants per acre, especially with even distribution. This information is supported by a national publication ("Soybean Plant Stands: Is Replanting Necessary?") from Science for Success (a national group of soybean specialists and agronomists) and LSU AgCenter research.

Data from a 2020 Dean Lee Research Station population trial showed no significant yield difference with final stands of 61,000 and 67,500 plants per acre for May and June plantings, respectively.

The Science for Success factsheet suggests repair planting might be a more economical option than a complete replant. Repair planting involves adding seeds in areas with low plant populations to achieve a final stand of at least 70,000-75,000 plants per acre.

Balancing Replant Timing

The timing of repair planting requires balancing several factors:

• Accurate Stand Assessment: Waiting allows for a more precise count of healthy plants per acre.
• Competition and Maturity: Planting additional seeds too late can create competition between plants with different growth stages, potentially delaying harvest and impacting yield.
• Later Planting Date: Additional plants may be needed to compensate for the later planting date of the repair seed.

Estimating Final Stand

To estimate the final stand, count the number of plants within a specific row length based on your row spacing (Table 1).

Table 1: Row Length for Plant Count by Row Spacing

Soybean Tolerance to Flooded/Saturated Conditions

While soybeans are generally sensitive to flooding and saturated soil conditions, some varieties offer better tolerance. The LSU AgCenter collaborates with the Universities of Missouri and Arkansas to screen varieties for flood tolerance. Data from the 2023 LSU AgCenter Red River Research Station flood screening trials can be found in the 2024 Soybean Variety Yields & Production Practices. Similar trials are planned for 2024 at the LSU AgCenter Red River and Northeast Research Stations.

Factors Need to be Considered Before Taking Tissue Sample for Diagnosing Nutrient Deficiencies

Md. Rasel Parvej, LSU AgCenter Assistant Professor and State Soil Fertility Specialist

Before taking a tissue sample to diagnose nutrient deficiencies in plants, several factors should be considered to ensure the sample is representative and meaningful. Here are some key factors to consider:

• Plant Growth Stage: Most agronomic crops exhibit different nutrient needs at different growth stages. It’s crucial to sample plants at a consistent growth stage to make accurate comparisons. For example, for potassium (K) deficiency diagnosis, tissue needs to be sampled at the R2 (full-flower) stage for soybean and the V10-12 collar leaf stage for corn.
• Sampling Time: Proper sampling timing is essential when taking tissue samples. For the most accurate results of comparing good vs bad areas, samples should be taken when nutrient deficiencies are first observed but before severe damage occurs. Avoid sampling during extreme weather conditions or stress periods.
• Sampling Location: Selecting the right location within the field is crucial. Samples should be taken from areas representative of the entire field to avoid bias. Consider factors such as soil type, topography, and historical management practices when choosing sampling locations.
• Plant Part: Determine which plant part is most appropriate for sampling based on the nutrient deficiency symptoms, the plant species, and most importantly correlation between tissue nutrient concentration and yield. The most commonly sampled plant part is the leaves. For example, soybean leaves need to be sampled from the 3^rd node from the top and corn leaves from the uppermost matured leaves with collar.
• Sample Size: Collect an adequate number of samples to ensure statistical reliability. The number of samples needed depends on the agronomic crop type, the variability within the field, and the desired level of accuracy. For one composite sample, it is recommended to sample at least 15-20 leaves of soybean and 12-15 leaves of corn.
• Sampling Method: Use your hand to collect tissue samples, ensuring they are clean and free from contaminants. Wash the samples with deionized water if they are dusty before sending to the lab. Maintain consistency and reliability across samples for accurate results.
• Nutrient Interactions: Consider potential interactions between nutrients when interpreting tissue analysis results. Nutrient deficiencies or excesses in one nutrient can affect the uptake and availability of other nutrients.
• Diagnostic Tools: Supplement tissue analysis with other diagnostic tools such as soil testing, visual observation, and plant tissue culture if necessary. Integrating multiple diagnostic approaches can provide a more comprehensive understanding of nutrient status and deficiencies.
• Laboratory Analysis: Choose a reputable laboratory with experience in plant tissue analysis. Ensure that the laboratory uses reliable testing methods and provides accurate interpretation of results. If possible, send both soil and tissue samples to the LSU AgCenter Soil Test and Plant Analysis Laboratory (STPAL) for nutrient analysis.
• Record Keeping: Keep detailed records of sampling locations, dates, plant growth stages, and laboratory results. This information is valuable for tracking nutrient trends over time and making informed management decisions.

By considering these factors before taking tissue samples, you can optimize the accuracy and usefulness of the diagnostic process for identifying and addressing nutrient deficiencies in plants.

Importance of Cation Exchange Capacity in Crop Production

Md. Rasel Parvej, LSU AgCenter Assistant Professor and State Soil Fertility Specialist

Cation exchange capacity (CEC) is a crucial parameter in soil fertility management and crop production, assessing the soil’s capability to retain and interchange positively charged ions, known as cations such as calcium (Ca²⁺), magnesium (Mg²⁺), potassium (K⁺), ammonium (NH₄⁺), sodium (Na⁺), etc. The importance of CEC in crop production stems from several key factors:

• Nutrient Retention: Soil with a higher CEC can hold more nutrients, reducing the likelihood of nutrient leaching and ensuring essential nutrients remain accessible to plants over an extended duration. This is particularly important for slowly available nutrients like potassium (K), which can be retained by soil colloids and released as needed by plants. Potassium is very prone to leach through soil profile in low CEC soils with high rainfall and therefore it is recommended to apply K in the Spring near planting rather than in the Fall. In addition, low CEC soils have a high potential for nutrient deficiency, especially K, and may require K fertilization every year.
• Nutrient Availability: The exchangeable cations held by the soil colloids, which contribute to CEC, can be readily exchanged with plant roots, influencing nutrient availability. Cations like Ca, Mg, and K are vital for plant growth and are often provided through cation exchange processes. High CEC soils are usually loaded with these cations and from a fertilization standpoint may not require any fertilizer-K addition for maximizing crop yield.
• pH Buffering: Soils with higher CEC tend to have greater buffering capacity against changes in pH. This is because the exchangeable cations can neutralize excess acidity or alkalinity in the soil solution, helping to maintain a stable pH level conducive to optimal plant growth. However, in low CEC soils, soil pH can easily be changed through soil amendments and irrigation (if irrigation water contains acidic or alkaline cations).
• Fertilizer Efficiency: Understanding the CEC of soils is crucial for efficient fertilizer application. Soils with higher CEC require larger amounts of fertilizer to compensate for nutrient retention, while soils with lower CEC may require less fertilizer but may also be more susceptible to nutrient leaching. For example, corn requires 1 lb nitrogen (N) per bushel grain yield in silt loam soils but 1.25 lb N per bushel yield in clay soils.
• Soil Health and Structure: CEC is closely related to soil organic matter content and soil structure. Soils with higher organic matter content typically have higher CEC due to the presence of humic substances, which can hold and exchange cations effectively. Improving soil organic matter through practices such as organic amendments and cover cropping can enhance CEC and overall soil health.
• Crop Yield and Quality: Maintaining optimal soil CEC levels is essential for maximizing crop yield and quality. Adequate nutrient availability, pH balance, and soil structure contribute to healthy plant growth, increased resistance to pests and diseases, and improved crop productivity.
• Environmental Sustainability: Proper management of soil CEC can contribute to sustainable agriculture by minimizing nutrient runoff and leaching, reducing the likelihood of groundwater contamination, and promoting long-term soil fertility and productivity.

Cation exchange capacity is also very important for N management in corn and cotton. Low CEC soils require split application of N due to the less retention capacity. High CEC soils can hold NH₄⁺ for a longer period of time for crop uptake and in most years with adequate rainfall, a single application may be sufficient for maximum yield.

In summary, understanding and managing CEC in agricultural soils is critical for optimizing nutrient availability, pH balance, soil health, and crop productivity. Incorporating practices that enhance CEC, such as organic matter management, judicious fertilizer application based on soil testing, and soil conservation techniques, can enhance soil fertility and promote sustainable crop production systems.

Questions Need to be Asked to Diagnose Nutrient Deficiencies

Md. Rasel Parvej, LSU AgCenter Assistant Professor and State Soil Fertility Specialist

When diagnosing a nutrient deficiency in plants, it's important to gather detailed information from the producers to accurately identify the issue. Here are some questions you might ask:

• Plant Symptoms:
  • What are the observable symptoms on the plants? (e.g., yellowing of leaves, stunted growth, leaf curling, necrosis)
  • Where on the plant are these symptoms occurring? (e.g., lower leaves, upper leaves, entire plant)
  • Are there any patterns to the symptoms? (e.g., localized to certain areas of the field, affecting specific crop varieties)
• Crop Information:
  • What type of crop variety is affected?
  • What growth stage is the crop currently in?
  • Has this issue been observed in previous seasons, or is it new?
• Field Conditions:
  • What are the soil type and pH of the affected area?
  • Has soil testing been conducted recently? If so, what were the results?
  • Are there any known issues with soil drainage or compaction in the field?
  • Has there been any recent fertilizer applications? If so, what type and amount?
• Environmental Factors:
  • What has the weather been like recently? (e.g., temperature, rainfall, humidity)
  • Are there any environmental stressors present? (e.g., drought, flooding, extreme temperatures)
  • Is the crop exposed to any pollutants or contaminants?
• Management Practices:
  • What crop rotation or tillage practices are used in the field?
  • Have any pesticides or herbicides been applied recently?
  • How frequently is irrigation applied, and what method is used?
• Pest and Disease Pressure:
  • Have any pests or diseases been observed on the plants?
  • Are there any signs of insect damage, fungal infections, or other diseases?
• Historical Data:
  • Have nutrient deficiencies been a problem in previous crops?
  • What is the history of nutrient management in this field?
• Observations from Surrounding Areas:
  • Are neighboring fields experiencing similar issues?
  • Have other growers in the region reported similar problems?

By gathering detailed information through these questions, you can better understand the context of the issue and narrow down potential causes of nutrient deficiency, helping to develop an effective solution.

Crazy Top in Corn

Boyd Padgett, LSU AgCenter Pathologist

Article Highlights:

• Recent rains and flooding are conditions favorable for crazy top in corn.
• Symptoms will not be evident until later in the growing season, usually during the reproductive stages.
• Usually not a yield limiting disease.

Crazy top is caused by the soilborne pathogen Sclerophthora macrospora. Disease development is favored when young corn (most susceptible from seedling to V5) is subjected to flooding and saturated soils for 24 to 48 hours. The pathogen infects the corn through the corn growing point. Pooling water in the whorl provides optimum conditions for infection. Symptoms will be twisted and deformed leaves, deformed leaves where the tassel should be, and usually no ear develops (Figures 1 and 2). Resistance has not been identified in hybrids and seed treatments are not effective for managing this disease. Fortunately, this is usually not a yield limiting disease.

Figure 1. Crazy top.

Figure 2. Crazy top.

Early-Season Insect Pest Control for Soybean and Cotton

James Villegas, LSU AgCenter Entomologist

Soybean:

The optimal planting window for soybeans in Louisiana typically falls between mid-April and mid-May. During this period, soybeans strike a balance between yield potential and environmental conditions. However, planting dates can vary based on soybean maturity groups. The wide range of planting dates—from early March to late June—exposes soybean plants to various insect pests. These pests, including bean leaf beetles, three-cornered alfalfa hoppers, wireworms, grape colaspis, and thrips, can harm both above-ground and below-ground parts of the plant. To mitigate risks associated with insect damage, consider insecticidal seed treatments. These treatments are particularly important when planting occurs in suboptimal conditions.

Some examples where insecticidal seed treatment use may be warranted:

• Early-planted soybeans: When environmental fluctuations stress seedlings, making them more susceptible to insect injury.
• Late-planted soybeans: High insect pest pressures may affect seedlings.
• Soybeans planted into wheat stubble: This environment favors three-cornered alfalfa hopper development.
• Weedy fields: Weeds can harbor insect pests.
• Conservation tillage systems: Reduced soil disturbance may impact pest dynamics.
• Historically problematic areas: Regions with a history of insect issues.
• Direct planting into cover crops: Cover crops can influence insect pest populations.

When soybeans are planted within the recommended window, under optimal soil conditions, and with historically low insect pest densities, the risks associated with insect damage decrease. In contrast, when planting occurs in suboptimal conditions, insecticidal seed treatments become significantly more crucial. It’s worth noting that these seed treatments typically remain effective in soybeans for approximately 30 days.

Cotton:

Thrip is a common early-season insect pest of cotton in Louisiana. Management of this pest is critical for successful cotton production, and it requires proactive measures from the outset. Thrips have a unique biology that complicates the precise timing of insecticide applications without seed treatments. Their eggs are typically deposited in the cotyledons, and as the first true leaf emerges, immature thrips hatch and prefer to feed within the furl stage. Unfortunately, they are well-protected within the furl, making it extremely challenging to reach them with insecticides. Consequently, the first true leaf often sustains damage before it fully expands. Effective thrips management necessitates realistic expectations for sprays and consideration of the thrips’ life cycle.

Key Strategies for Thrips Control:

• Insecticide seed treatments (ISTs): ISTs are the primary approach for controlling thrips. Their effectiveness varies based on prevailing weather conditions and thrips infestation levels. In low to moderate-pressure situations, ISTs can effectively manage thrips. However, under high pressure or unfavorable growing conditions, supplementary foliar treatments may be necessary.

• In-furrow (IF) applications: In-furrow treatments provide another effective method for thrips management. Options include acephate (note that acephate-resistant thrips are present in the state), imidacloprid, and aldicarb (AgLogic). When cotton is treated with aldicarb, additional foliar applications for thrips control are typically unnecessary.

• ThryvOn cotton: ThryvOn, a new Bt cotton trait, offers an at-plant option for thrips management. ThryvOn demonstrates high efficacy against thrips and eliminates the need for supplementary foliar treatments. Additionally, ThryvOn provides some control against tarnished plant bugs, although to a lesser extent than thrips
• Foliar rescue treatments: When seed treatments are no longer effective, foliar treatments become necessary. These should be applied when immature thrips are present or when significant adult populations cause damage before the fourth true leaf. Refer to the “Louisiana Insect Pest Management Guide (https://www.lsuagcenter.com/portals/communications/publications/management_guides/insect_guide)” available at the LSU AgCenter website for foliar options to manage thrips.

Remember that successful cotton planting depends not only on timing but also on choosing the right thrips control strategy to ensure a healthy crop yield.

Importance of Soil Organic Matter in Soil Fertility, Health, and Nutrient Management

Md. Rasel Parvej, LSU AgCenter Assistant Professor and State Soil Fertility Specialist

Soil organic matter (OM) refers to the organic materials present in the soil that originate from the decomposition of plant and animal residues, as well as from the activities of soil organisms such as microbes, insects, and earthworms. Soil OM consists of a diverse array of organic compounds derived from plant and animal residues (leaves, roots, stems, etc.), humus (chemically complex, stable OM), microbial biomass (bacteria, fungi), and soil fauna (earthworms, insects). In contrast, soil organic materials encompass a broader category of substances derived from living organisms or their byproducts, such as compost, manure, plant residues, and other organic inputs used for soil improvement and fertilization. While soil OM is a component of organic materials, not all organic materials are incorporated into the soil as OM; they may serve as inputs to enhance soil fertility and structure without becoming part of the soil’s OM pool.

Soil OM is formed through dynamic processes influenced by biological, chemical, and physical factors. Key processes include: i) decomposition of plant and animal residues by soil organisms (microbial activity), ii) humification: the transformation of OM into humus through microbial and chemical processes, and iii) stabilization: the incorporation of OM into soil aggregates and protection from decomposition by physical and chemical mechanisms.

Soil OM plays a vital role in crop production and agricultural sustainability due to its numerous benefits:

• Nutrient Supply: Organic matter plays a crucial role in cation exchange capacity (CEC) by serving as a site for the exchange of positively charged ions (cations) in the soil. Specifically, OM contains negatively charged sites known as exchange sites or functional groups. These sites attract and hold cations such as calcium (Ca²⁺), magnesium (Mg²⁺), potassium (K⁺), ammonium (NH₄⁺), etc., allowing them to be exchanged with soil solution cations. Soil OM also functions as a storage unit for sulfur (S), zinc (Zn), etc. These nutrients are released slowly as OM decomposes, providing a steady supply of nutrients to crops throughout the growing season.
• Soil Fertility: Organic matter enriches soil fertility by enhancing soil structure, augmenting water retention, and improving nutrient availability. It creates a granular soil structure that promotes root growth, aeration, and drainage, which are essential for healthy plant development.
• Water Retention and Drought Resistance: Soil OM increases soil water-holding capacity, reducing water runoff, and enhancing drought resistance. Soils rich in OM prove invaluable during prolonged dry periods or in areas experiencing unpredictable rainfall patterns.
• Biological Activity: Organic matter serves as a source of energy and habitat for soil microorganisms, encompassing bacteria, fungi, and earthworms. These organisms are pivotal in processes such as nutrient cycling, decomposition, and the maintenance of soil health. Healthy populations of soil organisms contribute to improved soil structure, disease suppression, and nutrient availability.
• Carbon Sequestration and Climate Regulation: Soil OM represents a substantial carbon reservoir in terrestrial ecosystems. Techniques aimed at boosting organic matter levels, like conservation tillage, cover cropping, and organic amendments, have the potential to augment carbon sequestration in soils. This mitigates climate change by curbing atmospheric carbon dioxide levels.
• Erosion Control: Organic matter helps stabilize soil aggregates, reducing soil erosion by water and wind. It acts as a natural barrier, protecting the soil surface from erosion and maintaining soil fertility and productivity.
• pH Buffering: Organic matter acts as a buffer, helping to maintain soil pH within the optimal range for crop growth. It mitigates the effects of soil acidification or alkalization resulting from fertilizer applications or other soil management practices.

Managing soil OM is crucial for maintaining soil health, fertility, and productivity. Here are some strategies for managing soil OM effectively:

• Reduced Tillage: No-tillage or minimum tillage, which reduces soil disturbance, helps preserve soil structure, minimizes erosion, and retains OM within the soil profile.
• Cover Cropping: Introducing cover crops like legumes, grasses, or brassicas during winter can elevate soil OM levels by shielding the soil surface, mitigating erosion, and augmenting organic matter via root biomass and residue breakdown.
• Crop Rotation: Rotating crops with different root structures, residue quality, and nutrient demands helps sustain soil OM levels and promotes soil health.
• Organic Amendments: Integrate organic amendments like compost, manure, litter, or crop residues into the soil to boost soil OM content. These amendments offer accessible nutrients, enhance soil structure, activate microbial activity, and facilitate soil OM decomposition and nutrient cycling.

Soil OM is a cornerstone of soil fertility, structure, and health, with far-reaching implications for agricultural productivity, environmental sustainability, and ecosystem resilience. Understanding the composition, formation, functions, and importance of soil OM is essential for effective soil management and sustainable land use practices. Through sound management practices that promote soil OM accumulation and maintenance, we can enhance soil quality, productivity, and resilience for current and future generations.

Is Ammonium Sulfate a Good Sulfur Source for Soybean Production in Louisiana?

Md. Rasel Parvej, Md. Enamul Haque Moni, Abrar Bin Wahid, Md. Moklasur Rahman, Jiad-Ur Rahaman, and Iftekhar Alam, LSU AgCenter

Sulfur (S) is an essential nutrient for soybean growth and development, playing critical roles in amino acid synthesis, chlorophyll formation, enzyme activation, and nitrogen (N) fixation. Crucially, three of the nine essential amino acids in soybean protein contain S, including cystine, cysteine, and methionine. Soybean plants obtain S from various sources such as soil organic matter, atmospheric deposition, and the mineralization of S-containing compounds. However, S deficiency has emerged as a significant concern for crop production, exacerbated by a 90% reduction in atmospheric S deposition via rainfall since the “Clean Air Act” was adopted.

Sulfur deficiencies can occur in soils with low organic matter content, acidic pH, or very low S content and availability. Remarkably, around 95% of total soil S is bound within organic matter, with even a mere 1% of organic matter housing over 100 lb S/acre, releasing only 3-5 lb S/acre depending on environmental factors such as temperature, rainfall patterns, and soil management practices. Additionally, trace amounts of S may also be found in pesticides and irrigation water.

The response of soybean to S fertilization depends on a multitude of factors, including soil-test S levels, organic matter content, prevailing environmental conditions, crop management techniques, and the type of S-containing fertilizer utilized. Here are several critical points regarding soybean response to S fertilization:

• Symptoms of Sulfur Deficiency: Soybean exhibits S deficiency as chlorosis (yellowing) of younger leaves, stunted growth, delayed maturity, and reduced seed yield. Notably, S deficiency symptoms tend to manifest primarily on the upper, younger leaves of soybean plants, often resembling symptoms of N deficiency. However, a distinguishing feature is that while N deficiency primarily affects lower, older leaves due to N’s mobility within plants, S deficiency affects upper, younger leaves owing to S’s immobility within plants.
• Soil Test S Level: Soil testing plays a vital role in evaluating S availability within soybean production systems. Soil test, sulfate-S (SO₄-S) test, offers insights into soil S levels and helps determine the need for S fertilization. According to the current recommendations from LSU AgCenter, soybean requires S fertilization in soils containing less than 10 ppm (20 lb S/acre) of Mehlich-3 S within the 0- to 6-inch soil depth.
• Sulfur Fertilization: Soybean requires 20 lb S/acre in soils with S concentration ≤ 10 ppm. There are several dry granular S fertilizers available in the market including gypsum (~23% Ca and 16-19% S, depending on purity), sul4R-Plus (23% Ca and 17% S), tiger 90CR (90% S), K-mag (22% K₂O, 11% Mg, and 21-22% S), poly4 (14% K₂O and 19% S), ammonium sulfate (AMS, 21% N and 24% S), etc. Additionally, some liquid S products are also found in the market including ammonium thiosulfate (ATS, 12% N and 26% S), potassium thiosulfate (KTS, 25% K₂O and 17% S), etc.

In recent evaluations of soybean response to S rates and sources in Louisiana, a 1-year study across 21 sites revealed a positive yield response to S fertilization in two sites where Mehlich-3 soil-test S concentrations were < 7 ppm and soil organic matter content was 1.2% at the 0- to 6-inch soil depth. However, no significant yield increment was observed with S fertilization in sites where soil S concentrations were near 10 ppm. Moreover, no yield response was measured in soils with S concentrations exceeding 10 ppm. These preliminary results indicate the necessity of S fertilization in soils with S concentrations less than 10 ppm, while no S is needed in soils with S concentrations exceeding 10 ppm.

In a 2-year study evaluating S sources (gypsum, sul4r-plus, tiger 90CR, ammonium sulfate, K-mag, and poly4), no significant yield response to S fertilization was observed across four sites where soil S concentrations were near or greater than 10 ppm. However, in three out of four sites, a numerically negative yield response (2-5 bu/acre) to S fertilization was observed when using ammonium sulfate (AMS) compared to the no-S control. This negative response may be attributed to the presence of 21% N in AMS, which can adversely affect soybean nodulation. Examination of soybean roots revealed minimal to zero nodules in plants that received AMS compared to gypsum and the no-S control (Figure 1).

Numerous research suggests that AMS has the potential to negatively impact nodulation in soybean under certain conditions. Here's how:

1. Nitrification Effect: Ammonium ions (NH₄⁺) released from AMS can accumulate in the soil solution and eventually undergo microbial conversion to nitrate (NO₃^-), the process called nitrification. High concentrations of nitrate ions (NO₃^-) can be toxic to N-fixing bacteria (rhizobia) present in the soil, inhibiting their growth and activity. Consequently, this can interfere with the establishment of beneficial rhizobial populations and the formation of effective nodules on soybean roots.

Some studies conducted in the mid-west have observed a positive yield response in soybean to AMS, possibly attributed to the slower nitrification process. Nitrification, influenced by factors like temperature, soil moisture, pH, and microbial activity, tends to proceed more rapidly in warmer and moister conditions, compared to cooler and drier environments. Therefore, the conversion from ammonium to nitrate may occur at a faster rate in the southern USA than in the northern USA. However, the nitrification process exhibits variability across different years and fields, and its impact on soybean nodulation may not be negative in every instance.

1. Acidifying Effect: When ammonium sulfate undergoes nitrification, it induces soil acidification, thereby lowering soil pH by providing more hydrogen (H⁺) ions. This decrease in pH creates unfavorable conditions for the survival and nodulation activity of rhizobia. Ideally, rhizobia thrive in soil with neutral to slightly acidic pH levels (pH ~6.5). Excessive soil acidity (pH <6.0) can impede the optimal function of rhizobia.

1. Nitrogen Competition: Ammonium sulfate offers soybean a readily accessible N source. However, excessive N availability from AMS can diminish nodulation and N fixation in soybean plants. In situations where soil contains abundant N, soybean tends to prioritize N uptake from the soil over forming nodules and fostering symbiotic relationships with rhizobia. This phenomenon is particularly concerning in soils with high residual N levels.

To sum up, S fertilization proves to be a valuable approach for addressing S deficiencies and enhancing both soybean yield and protein quality. Nonetheless, it's crucial to consider soil S levels, environmental conditions, and the choice of S fertilizer source when determining S fertilization in soybean production. For soybean producers in Louisiana, opting for S-containing fertilizers with minimal or no ammonium or nitrate content is advisable to fulfill soybean S requirements without adversely affecting nodulation.

LSU AgCenter Specialists