Soil Organic Matter: The Key to Enhanced Soil Chemical Properties

Leandro O. Vieira II and Saulo A.Q. de Castro, LSU AgCenter Scientists

Soil organic matter improves soil chemical properties, which includes the increase of nutrient status, cation exchange capacity (CEC), and anion exchange capacity. Soil organic matter is also known for the slow release of nutrients to the plants, protects nutrients in available forms to the plants, and reduces nutrient leaching.

Starting with the improvement of nutrient status, soil organic matter is a source of plant nutrients such as nitrogen (N), phosphorus (P), sulfur (S), and micronutrients. Nitrogen is the primary nutrient in soil organic matter, presenting about 1,000 lbs per 1% of soil organic matter in one acre, although most of it (~95%) is not immediately available to plants without microbial activity. Through mineralization, 2 to 3.5% of this N becomes available annually, providing 20 to 35 lbs of N per acre each year (Figure 1). Furthermore, soil organic matter boosts microbial activity, enhancing biological N fixation by free-living bacteria. Organic compounds within soil organic matter can increase phosphorus availability through mechanisms such as forming more soluble organophosphate complexes, displacing P retention sites with organic anions, and enhancing the mineralization of organic P into plant-available forms. Additionally, increased soil organic matter positively impacts S status by retaining it in a form that minimizes leaching while keeping it accessible for plant uptake.

Figure 1. Each 1% of soil organic matter can release approximately 20 to 35 lbs of plant-available nitrogen per acre per year.

Another critical aspect is the increase in cation and anion exchange capacity. A significant portion of the soil's CEC is attributed to soil organic matter, which can hold positively charged elements (cations) and reduce their leaching. The relatively weak interactions between soil organic matter and cations ensure that these nutrients remain available to plants. Examples of such cations include nitrogen (in ammonium form, NH₄⁺), potassium (K⁺), calcium (Ca²⁺), magnesium (Mg²⁺), zinc (Zn²⁺), and manganese (Mn²⁺). Despite soil organic matter being predominantly negatively charged, it can also retain negatively charged elements (anions). Anion exchange capacity is not measured in regular soil testing, but it refers to the ability of a soil to retain anions, reducing their leaching while keeping them accessible to plants. This is crucial because not all plant nutrients are positively charged, and soils typically have limited anion exchange capacity. Examples of anions include nitrogen (in nitrate form, NO₃^-) and sulfur (in sulfate form, SO₄^2-).

While most nutrients in soil organic matter are not immediately accessible to plants, soil organisms decompose organic matter, breaking down organic nutrient forms into simpler inorganic forms that plants can absorb. This mineralization process supplies a considerable amount of the N, P, S, and micronutrients required by plants. Moreover, some micronutrients, such as iron, zinc, and manganese, are often present in unavailable forms in certain soils. However, through chelation, these micronutrients remain in forms that can be absorbed by roots. Chelates are byproducts of organic matter decomposition or root exudates, which bind nutrients to multiple parts of their organic molecules.

Last but not least, cation and anion exchange capacity of soil organic matter, along with the presence of chelates, reduces nutrient leaching. This is particularly important for enhancing fertilizer use efficiency and preventing water body contamination (eutrophication). By retaining nutrients in the soil, soil organic matter helps ensure that plants have a steady supply of essential elements, promoting healthier and more productive crops.

Nickel: a micronutrient with macro functions

Saulo Augusto Quassi de Castro, LSU AgCenter, Sugarcane & Soybean Agronomist

To be considered a plant nutrient, the element must fulfill both essentiality criteria, that is, the plant cannot complete its life cycle without this element and no other element can substitute the target element. Nickel (Ni) essentiality to plants was demonstrated in 1987; it is the most recent plant nutrient.

Nickel is important in nitrogen metabolism. It is a cofactor in the urease enzyme, which is responsible for breakdown urea [CO(NH₂)₂] into carbon dioxide (CO₂) and ammonia (NH₃). This is an essential transformation since urea can be absorbed by the roots but can’t be assimilated by the plants. Moreover, plants naturally produce urea during plant tissue senescence, specifically, by the catabolism of the amino acid arginine. Therefore, in the absence of Ni, urea accumulates in the plant, mainly on leaf tip, promoting toxicity (leaf scorch). Urea toxicity due to Ni deficiency was reported in controlled conditions for some crops and first reported in field conditions on Pecan in 2004; on soybeans, Ni deficiency in field was proved in 2018. In leguminous crops, such as soybeans, urea is also produced during the ureide degradation. Ureides allantoin and allantoic acid are the main nitrogen forms exported out of the nodules to the plants (~80%).

Leguminous crops are known to require no or a minimum amount of nitrogen-fertilizer due to biological nitrogen fixation (BNF). BNF is a process in which atmospheric nitrogen (in the form of N₂) is converted to ammonium (NH₄⁺), a form of nitrogen that can be assimilated by plants. Nickel is also a key-factor for BNF, it is a component of hydrogenase, an enzyme that recycles the H₂ produced in the BNF keeping the process energetically efficient and improving the efficiency of BNF. Hidden Ni deficiency was already reported for soybeans in other countries, and the addition of Ni has shown improvements in nodule activity and BNF efficiency, showing benefits to plant growth and yield.

Plants absorb Ni as a cation (Ni²⁺). Nickel is found in soil minerals, such as serpentine, and the cation exchange capacity of the soil. Soil pH down-regulates Ni availability to the plants. Moreover, glyphosate has been shown to reduce the nodulation and activity of enzymes related to BNF as well as the Ni available to the plants. Glyphosate is an herbicide commonly used in soybeans, for weed control, and in sugarcane, to increase the sucrose content in the stalk. Nickel is not determined in routine soil analysis and accurate methods to determine Ni available to plants were recently developed. Therefore, there is scarce data on the Ni dose, source, and application method. Results have shown differences in the function of plant genotype and edaphoclimatic conditions, and since Ni has a narrow dynamic range among the minimal requirement by the plant and toxicity, it is important to investigate Ni deficiency and application in Louisiana. Needing Ni-fertilizer, improvements in nitrogen reassimilation by the plant and efficiency of BNF in leguminous crops must be achieved. Improving BNF will not only bring benefits for soybeans, but can also increase the nitrogen content in the soil, benefiting the next crop, a good strategy in a soybean-sugarcane double-crop system.

Ag Water Management Field Day at the Red River Research Station

Freeze Injury on Wheat

Boyd Padgett, Steve Harrison, and Noah DeWitt, LSU AgCenter Scientists

Freezing temperatures can be detrimental to wheat. The impact of freezing temperatures (two-hour duration) on wheat at specific growth stages is listed in Table 1.

Most oat varieties are somewhat less tolerant of cold weather than wheat and there is substantial variation among oat varieties for degree of cold tolerance. Oat varieties sustained more leaf injury than wheat in general. However, the oat varieties in juvenile growth stages with growing points below the soil surface are protected. Where leaves were significantly damaged by cold the plants should survive and regrow with no significant impact on yield.

Individuals should assess injury 7-10 days after the freeze event.

For more information on freezing temperatures on wheat and oats contact your local county agent or specialist.

Table 1 is taken from Kansas State publication C-646 http://fieldcrop.msu.edu/uploads/files/Knsas%20freeze.pdf

Table 1. Injury Symptoms of Wheat Resulting from Freezing Temperatures

Selecting Soybean Varieties Video

David Moseley, LSU AgCenter Soybean Specialist

The Science for Success team is a national group of soybean agronomists. One activity the team completed was recording informative videos for soybean clientele. The following link is for a video on selecting soybean varieties.

Optimum Soybean Planting Dates for the Louisiana Northeast, Central, and Southwest Regions

David Moseley, LSU AgCenter Soybean Specialist

Article Highlights:

• Included are optimum planting timings for soybean in the Northeast, Central, and Southwest Louisiana regions based on a recent article from the Agronomy Journal.
• It is possible to achieve near full yield potential and reduce the risk of a severe weather event by planting soybean within a certain time frame.

Data from soybean trials conducted between the 2013 – 2020 seasons in the Northeast, Central, and Southwest regions of Louisiana were used to publish an article in Agronomy Journal called “Soybean planting dates and maturity groups: Maximizing yield potential and decreasing risk in Louisiana" (Moseley et al., 2024). The data in this article suggest there are planting windows in each region that 99 - 100% of the max yield potential can be achieved.

Northeast Region:

Even though there were varieties with different maturity groups in the Northeast Louisiana region, there was not a significant difference in yield according to maturity group. This does not mean there is not a most optimum maturity group in the Northeast Louisiana region, but there were not enough data points to separate the maturity groups using statistics.

Table 1. Most optimum planting date in the Northeast Louisiana region and the planting window to achieve 99% of yield from the most optimum planting date.

Central Region:

There were enough data points in the Central Louisiana region to suggest planting windows for 99 - 100% yield of the optimum planting date for maturity group sections of 3.0-4.4, 4.5-4.7, 4.8-4.9, and 5.4+.

Table 2. Most optimum planting date by maturity group section in the Central Louisiana region and the planting window to achieve 99% of yield from the most optimum planting date. This table is from Moseley et al. (2024).

Southwest Region:

There were enough data points in the Southwest Louisiana region to suggest planting windows for 99% yield of the optimum planting date for maturity group sections of 3.0-4.4, 4.5-4.7, 4.8-4.9, 5.0-5.3, and 5.4+.

Table 3. Most optimum planting date by maturity group section in the Southwest Louisiana region and the planting window to achieve 99% of yield from the most optimum planting date. This table is from Moseley et al. (2024).

The amount of yield loss when planting earlier or later than the optimum timing will depend on the environment of the growing season.

Last year, there was an estimated economic damage of $23.7 million to the Louisiana soybean crop due to a prolong period of excessive rain including hurricane Francine between August 28 to September 12 (Guidry, 2024). Fields planted by April 16 at the Dean Lee Research and Extension Center were already harvested and fields planted on May 6 or later were not damaged by the extended rainy period. However, soybean plants that were planted at the Dean Lee Research and Extension center between April 17 – May 5 suffered extensive yield and quality loss.

Decreasing the risk of severe weather events by spreading out the planting dates may be one strategy to consider. Based on this data, the planting date for each region can be spread between the dates shown in tables 1, 2, and 3 and still achieve 99 - 100% of the yield of the optimum planting date.

Planting in March does come with the threat of freezing temperatures that can kill plants. The National Centers for Environmental Information shows a map of the expected last freeze across the United States. The map shows some areas in north Louisiana can have a last freeze between March 16 – March 30.

Additional information can be found in the Moseley et al. (2024) article and in a Science For Success factsheet called The Best Soybean Planting Date.

References:

Conley, S., Holshouser, D., Inman, M., Lee, C., Lindsey, L., Licht, M., Kandel, H., Kleinjan, J., Knott, C., Naeve, S., Nafziger, E., Ross, J., Singh, M., Specht, J., Vann, R. The best soybean planting date. United Soybean Board. The Best Soybean Planting Date

Guidry, K. (2024). Estimates of the economic impacts on the Louisiana Agricultural Industry from Hurricane Francine. LSU AgCenter. Staff Report No: 2024 - 74

Moseley, D., Reis, A., Gentimis, T., Campos, P., Copes, J., Netterville, M., Egbedi, P., Harrell, D., Kongchum, M., Levy, R., Padgett, B., Soignier, S., Scroggs, D., Sanders, J., Pankey, J., & Fic, K. (2024). Soybean planting dates and maturity groups: Maximizing yield potential and decreasing risk in Louisiana. Agronomy Journal, 1–12. https://doi.org/10.1002/agj2.21626

LSU AgCenter Specialists