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The Mechanics of Soil Fertility: Use of Sugar in Field Crops
Jolee Derrick, Precision Nutrient Management Ph. D. Student
Grace Williams, Soil Microbiology Ph. D. Candidate
Brian Arnall, Precision Nutrient Management Specialist
Recently, there has been increased interest in adding sugar to spray tank mixes, whether for post-emergence weed control or foliar nutrient applications. While there is limited work on impact of sugar inclusion in herbicide applications, some papers have posed potential enhancement (Devine and Hall, 1990). But since this is coming from a soil science group, we will only focus on soil impact. Following up the last blog, unlike humic substances, which represent more complex and relatively stable carbon forms, sugar is a highly labile carbon source. This rapid utilization of simple carbon sources is well documented to stimulate microbial activity and growth (Kuzyakov and Blagodatskaya, 2015). The general idea of utilizing sugar applications is that sugar has the capacity to improve spray performance, stimulate biological activity, increase organic matter mineralization, and ultimately result in improved yields.
Sugar additions can influence soil processes differently depending on system conditions. In systems with higher residual nitrogen and organic matter, responses may differ from those observed in Oklahoma production environments, where soils are typically lower in organic matter and microbial activity can occur for much of the year. Understanding how sugar functions in these systems requires a basic discussion of carbon dynamics. Sugar itself is almost entirely carbon and is readily consumed by microbes. It’s a simple molecule, which allows it to dissolve easily in water and be quickly utilized in the soil system. Crop residues, like wheat straw, are also carbon-rich but much more complex. They contain cellulose, hemicellulose, and lignin which are long carbon chains that take time to break down because microbes need specialized enzymes to access them.
For the sake of simplicity, we can group carbon into two key pools: labile carbon and particulate organic matter (POM). Labile carbon includes easily decomposed materials, which include the previously mentioned simple sugars that microbes can metabolize rapidly. These pools differ in turnover time and microbial accessibility, with labile carbon driving short-term microbial responses (Cotrufo et al. 2013). POM breaks down more slowly and serves as a longer-term nitrogen source through residue breakdown.
Soil microorganisms require both carbon and nitrogen to grow and maintain biomass, typically at a ratio of approximately 24 parts carbon to 1 part nitrogen. When readily available carbon is abundant, but nitrogen is limited, microbes increase their nitrogen demand and begin scavenging nitrogen from the surrounding soil. This process, better known as nitrogen immobilization, temporarily reduces nitrogen availability to crops. Additions of readily available carbon sources have consistently been shown to increase microbial nitrogen immobilization in soil systems (Recous et al. 1990).
In systems where sufficient nitrogen is present, microbial populations can expand rapidly. Fast-growing microbial species may dominate, continuing to immobilize nitrogen within their biomass. Eventually, when nitrogen becomes limiting, microbial populations decline to levels the system can support. This boom-and-bust cycle can disrupt nitrogen availability during critical stages of crop growth. These rapid shifts in microbial population and activity following carbon inputs are commonly observed in soil systems receiving easily decomposable substrates (Blagodatskaya and Kuzyakov, 2008).
This dynamic becomes especially relevant when considering residue management practices common in Oklahoma. Under no-till or limited-tillage systems, the crop residues have wide carbon-to-nitrogen (C:N) ratios, creating conditions where nitrogen immobilization can occur during the growing season.
Table 1 provides approximate C:N ratios for several crops commonly grown in Oklahoma. When additional carbon is introduced into these systems without accompanying nitrogen, the likelihood of microbial immobilization increases. While immobilization is not bad, it does create a question mark as Oklahoma’s variable climate means the following release of nutrients will be unpredictable.
Table 1. Table depicting the range of C:N ratios for residues of commonly utilized crops in Oklahoma. Ratios were obtained from Brady, N. C., & Weil, R. R. (2017). The Nature and Properties of Soils (15th ed.)
Now consider conventional tillage systems. In Oklahoma, no-till systems typically contain 2 to 3 percent organic matter, which is relatively high given our climate and extended periods of microbial activity. Conventional tillage systems often fall between 0.75 and 2.25 percent organic matter. Because soil organic matter is approximately 58 percent carbon, this represents a substantial difference in the soil carbon pool.
Tillage can temporarily enhance microbial access to both previously mentioned carbon pools. When tillage exposes previously protected carbon, microbial activity increases rapidly. This initial flush can temporarily increase nitrogen mineralization as organic nitrogen is converted to plant-available forms. However, this phase is short-lived. As microbial populations expand, nitrogen demand increases, leading to immobilization and reduced nitrogen availability.
Hypothetically, increased microbial growth and activity would rapidly mineralize organic matter, trigger a surge in NO₃⁻, deplete soil organic matter, and as resources become limiting and the environment can no longer sustain elevated microbial populations, this boom would be followed by a population crash. This relationship is ultimately driven by the soil C:N ratio, which introduces an interesting additional complexity of residue. Different residues bring very different carbon-to-nitrogen balances into the system, and microbes respond accordingly. High carbon residues give microbes plenty of energy but very little nitrogen, so they pull N out of the soil to meet their needs. Residues with lower C:N ratios (soybean, alfalfa, etc.) do opposite, releasing nitrogen as they break down. Now the real question becomes where the critical point sits, and when does management push the system from the threshold of immobilization and mineralization.
These hypotheses form the foundation for new research currently underway through the Precision Nutrient Management Program. Initial proof-of-concept work has already been completed, providing a necessary steppingstone to address these questions.
Figure 1. Graph depicting the different concentrations of nitrate leached corresponding to applied treatments in the proof-of-concept work
The preliminary work (Figure 1) evaluated different sugar sources applied alongside a high-nitrogen product to assess the extent of nitrogen immobilization. Although these studies were conducted using potting soils, clear trends were apparent. Treatments containing sugar consistently showed greater nitrogen immobilization compared to treatments without sugar. This response is consistent with studies showing that additions of simple carbon substrates stimulate microbial growth and increase nitrogen immobilization (Dendooven et al. 2006). Building on this work, an active field-based research project is underway to evaluate how sugar additions influence nitrogen availability and microbial dynamics under real-world Oklahoma production conditions.
From an agronomic standpoint, sugar functions primarily as a readily available carbon source that stimulates microbial growth. In nitrogen-limited systems, this response increases the likelihood that nitrogen will be incorporated into microbial biomass rather than remaining immediately available for crop uptake.
Finally, we conclude with a conceptual consideration. If increased OM mineralization leads to greater plant biomass, this process may partially offset losses of OM. Greater biomass production could return more residues to the soil, contributing to the OM pool in the upper soil profile. Therefore, the system may compensate for OM mineralization through the rebuilding of organic matter via plant inputs. However, the stabilization of this carbon depends on microbial processing and physical protection within the soil matrix (Cotrufo et al. 2015)
However, while the underlying logic is sound, this concept has not been extensively studied within Oklahoma cropping systems. This blog does not address the impact of sugar applications on residue breakdown, and the potential impact of such. Future research through the Precision Nutrient Management Program will further investigate the mineralization process to better understand carbon dynamics within these systems.
Take Home:
- Oklahoma production systems generally have lower residual N and high carbon residues, creating conditions conducive to N immobilization
- Adding sugar increases microbial growth, creating population booms that will momentarily increase mineralization, but then immediately immobilize residual nitrogen.
- Tillage can amplify the negative effects of sugar by exposing more carbon and reducing soil organic matter
- Proof-of-concept work shows sugar triggered a net nitrogen immobilization in a carbon heavy environment
- Proof-of-concept work also suggests that when additional nitrogen is present, sugar additions may shift the system toward net mineralization rather than immobilization.
Work Cited:
Blagodatskaya, E., & Kuzyakov, Y. (2008). Mechanisms of real and apparent priming effects. Biology and Fertility of Soils, 45, 115–131.
Brady, N. C., and R. R. Weil. “The Nature and Properties of Soils, 15th Edn (eBook).” (2017).
Cotrufo, M. F., Wallenstein, M. D., Boot, C. M., Denef, K., & Paul, E. (2013). The Microbial Efficiency-Matrix Stabilization (MEMS) framework. Global Change Biology, 19, 988–995.
Cotrufo, M. F., Soong, J. L., Horton, A. J., Campbell, E. E., Haddix, M. L., Wall, D. H., & Parton, W. J. (2015). Formation of soil organic matter via biochemical and physical pathways of litter mass loss. Nature Geoscience, 8(10), 776–779.
Dendooven, L., Verhulst, N., Luna-Guido, M., & Ceballos-Ramírez, J. M. (2006). Dynamics of inorganic nitrogen in nitrate- and glucose-amended alkaline–saline soil. Plant and Soil, 283(1–2), 321–333.
Devine, M. D., & Hall, L. M. (1990). Implications of sucrose transport mechanisms for the translocation of herbicides. Weed Science, 38(3), 299–304.
Kuzyakov, Y., & Blagodatskaya, E. (2015). Microbial hotspots and hot moments in soil: Concept & review. Soil Biology and Biochemistry, 83, 184–199.
Recous, S., Mary, B., & Faurie, G. (1990). Microbial immobilization of ammonium and nitrate in cultivated soils. Soil Biology and Biochemistry, 22, 913–922.
Mechanics of Soil Fertility: Understanding Humic and Fulvic Acids
Brian Arnall, Oklahoma State University, Precision Nutrient Management Extension Specialist
Oliver Li, Oklahoma State University, Soil Chemistry
Interest in humic and fulvic acid products has increased substantially in agricultural production systems during the past two decades. These materials are frequently promoted as tools for improving soil biology, increasing nutrient availability, enhancing fertilizer efficiency, and stimulating plant growth. Because humic substances are known to be important components of soil organic matter, it is reasonable to ask whether adding humic or fulvic products to soil can meaningfully influence soil fertility.
As with many soil fertility questions, the answer depends on understanding two key factors: the mechanism involved and the magnitude of that mechanism relative to the soil system. Soil processes operate within large natural pools of organic matter, nutrients, and microbial activity. Therefore, evaluating the potential effects of humic products requires examining both how these compounds function chemically and biologically and how their application rates compare with the soils organic matter.
What Are Humic and Fulvic Acids?
Humic substances are heterogeneous organic compounds formed during the decomposition and transformation of plant and microbial residues. Historically, soil scientists have divided these materials into three operational fractions based on their solubility behavior: humic acid, fulvic acid, and humin (Stevenson, 1994; Tan, 2014). Humic acids are relatively large molecules that are insoluble under acidic conditions but dissolve in alkaline solutions. Fulvic acids are smaller molecules that remain soluble across the entire pH range, which allows them to move more freely in soil solution.
Both humic and fulvic acids contain numerous functional groups, particularly carboxyl and phenolic groups, which carry negative charge. These functional groups allow humic substances to interact with metal ions and nutrient cations and contribute to several important soil properties, including cation exchange capacity, buffering capacity, and metal complexation (Stevenson, 1994; Lehmann and Kleber, 2015). Because these materials originate from decomposed organic residues, they represent one portion of the complex mixture that collectively makes up soil organic matter. The distribution of the soil organic matter fractions varies among soil types and land uses, but fulvic acids and humic acids are each typically estimated to comprise approximately 10–35% of total soil organic matter (Guimarães et al., 2013).
Nutrient Retention and the Role of Cation Exchange
One of the most commonly cited mechanisms associated with humic substances is their ability to retain nutrients through cation exchange. The negatively charged functional groups present on humic molecules attract positively charged ions in soil solution. Through this electrostatic attraction, humic materials can retain several plant nutrients, including ammonium, potassium, calcium, magnesium, and certain micronutrients such as zinc and copper (Stevenson, 1994; Tan, 2014). This mechanism functions in the same manner as cation exchange on clay minerals. Of course, negatively charged surfaces do not retain negatively charged ions. As a result, nutrients such as nitrate are not held by humic substances and remain mobile in soil solution.
Laboratory measurements indicate that humic materials may possess relatively high cation exchange capacity on a mass basis. Reported values commonly range from approximately 300 to 600 cmolc kg⁻¹ depending on the source material and extraction method (Stevenson, 1994; Tan, 2014). These values demonstrate that humic substances can retain a large amount of cationic nutrients. A question that can be posed, however, is how this capacity compares with the nutrient retention already provided by soil organic matter.
Understanding the magnitude of humic additions requires comparing product application rates with the organic matter already present in soil. Calculations based on typical cation exchange values suggest that one pound of humic material with a CEC of 300–600 cmolc kg⁻¹ could theoretically retain approximately 0.04 to 0.08 pounds of ammonium-nitrogen. When viewed in isolation this number may appear meaningful. However, agricultural soils already contain large quantities of organic matter. An acre furrow slice, representing approximately the upper six inches of soil, weighs roughly two million pounds. Soil containing one percent organic matter therefore contains about 20,000 pounds of organic material per acre (Brady and Weil, 2016). Humified organic matter typically has cation exchange capacities ranging between 150 and 300 cmolc kg⁻¹ (Stevenson, 1994), meaning that the exchange capacity associated with native soil organic matter is already substantial. To put this into perspective, one pound of humic material can retain roughly 0.04 to 0.08 pounds of cation charge. Ammonium and potassium carry a single positive charge, while calcium carries two, meaning two ammoniums can be held for every two calcium. To provide contrast to the application of a humic substance, increasing soil organic matter by just 0.1% equivalent to about 2,000 pounds of additional organic material per acre can provide the capacity to retain approximately 40 to 80 pounds of cation charge or 40 to 80 pounds of ammonium.
The key point is not that humic materials cannot retain nutrients. They clearly can. Rather, the scale of material already present in soil is extremely large compared with the few ounces or pounds of humic products typically applied in agricultural systems. Consequently, the nutrient retention capacity associated with soil organic matter overwhelmingly dominates the soil system.
Micronutrient Complexation
Humic and fulvic substances are also known to interact with micronutrients through metal complexation reactions (also known as ‘chelation’). Carboxyl and phenolic functional groups can coordinate with metal ions such as iron, zinc, copper, and manganese to form organic complexes (Stevenson, 1994; Tan, 2014). These complexes can influence micronutrient mobility and availability in soils.
Fulvic acids are particularly effective at forming soluble complexes because they remain dissolved across the full range of soil pH. In some cases, these complexes may increase micronutrient mobility and transport within the soil solution. This mechanism has been well documented in soil chemistry research and may explain some responses observed in systems where micronutrient availability is limited.
Effects on Plant Physiology
In addition to soil chemical interactions, humic substances may influence plant growth through physiological mechanisms occurring in the rhizosphere. Several studies have shown that humic substances can stimulate root development, including increases in root elongation, lateral root formation, and root hair production (Nardi et al., 2002; Canellas and Olivares, 2014).
Research suggests that these responses may involve interactions with plant hormonal pathways and membrane transport processes. Humic substances have been shown to activate plasma membrane H⁺-ATPase enzymes, which are involved in proton pumping and nutrient uptake across root membranes (Canellas et al., 2002; Trevisan et al., 2010). Activation of these transport systems can enhance nutrient absorption and influence root architecture.
These physiological effects appear to occur primarily at the root–soil interface, where dissolved organic molecules interact directly with plant tissues. As a result, the responses observed in plant growth experiments are often attributed to rhizosphere signaling processes rather than large changes in bulk soil fertility.
Microbial Responses to Humic and Fulvic Compounds
Soil microorganisms respond strongly to carbon availability, and different carbon sources can produce very different microbial responses. Simple carbohydrates such as glucose and sucrose are readily metabolized by soil microbes and therefore produce rapid increases in microbial respiration and biomass. Humic substances, in contrast, consist of chemically complex and partially oxidized organic compounds that decompose much more slowly (Lehmann and Kleber, 2015).
Experimental studies comparing carbon sources consistently show that microbial respiration increases dramatically when simple sugars are added to soil, whereas humic substances produce smaller responses (Blagodatskaya and Kuzyakov, 2008). This difference reflects the relative degradability of these compounds as microbial energy sources.
Carbon Inputs from Humic Products Compared with Natural Soil Carbon
Soil microbial activity is largely driven by carbon supplied from plants through root exudation, residue decomposition, and organic matter turnover. The carbon pools already present in soil are therefore important for understanding the potential influence of humic product additions. A soil containing one percent organic matter holds approximately 11,600 pounds of carbon per acre (Brady and Weil, 2016).
Research on plant–soil carbon cycling indicates that living roots release significant quantities of organic carbon into soil each growing season through root exudation and rhizodeposition (Kuzyakov and Domanski, 2000). These plant-derived carbon inputs commonly amount to hundreds of pounds of carbon per acre and serve as a major energy source for soil microbial communities. Viewed in this context, humic product applications represent extremely small additions to the soil carbon pool. Consequently, microbial stimulation in agricultural soils is dominated by carbon inputs from plant residues and root exudates rather than by small additions of humic materials.
Building Organic Matter in the Central Plains
Increasing soil OM in the central Great Plains is achievable, but the magnitude of change is governed primarily by carbon inputs and water availability rather than any single management practice. Systems that combine no-till, increased residue return, diversified crop rotations, and where feasible cover crops or manure inputs are the most effective because they simultaneously increase carbon inputs and reduce decomposition losses (Lyon et al., 2007; Mikha et al., 2013; Nielsen et al., 2016). In semi-arid systems, realistic rates of OM increase are modest: over a 5-year period, changes are often small, approximately +0.05 to 0.1% OM, but significant in relation to the system which is often at total OM levels between 0.7 and 1.25 prior to establishment of conservation practices. The increase is confined to the top inch of the soil surface (Mikha et al., 2013; Saha et al., 2024). Mechanistically, these gains occur through greater residue and root-derived carbon inputs, reduced soil disturbance which slows microbial oxidation, and improved aggregation that physically protects organic matter from decomposition (Six et al., 2002; Lehmann and Kleber, 2015). However, as emphasized throughout this discussion, the scale of change is small relative to the large existing organic matter pool, and meaningful increases require long-term, system-level management focused on maximizing biomass production rather than relying on small external carbon additions such as commercial products.
Take-Home Points
- Humic and fulvic acids can retain cations, chelate micronutrients, and influence plant and microbial processes.
- Typical application rates are small relative to existing soil organic matter, so whole-soil impacts are limited.
- Most observed effects are localized in the rhizosphere, not broad changes in soil fertility.
- Evaluating both mechanism and scale is key to understanding their role in nutrient management.
References
Blagodatskaya, E., & Kuzyakov, Y. (2008). Mechanisms of real and apparent priming effects and their dependence on soil microbial biomass and community structure. Biology and Fertility of Soils, 45(2), 115–131.
Brady, N. C., & Weil, R. R. (2016). The nature and properties of soils (15th ed.). Pearson.
Canellas, L. P., Olivares, F. L., Okorokova-Façanha, A. L., & Façanha, A. R. (2002). Humic acids isolated from earthworm compost enhance root elongation and lateral root emergence in maize. Plant Physiology, 130(4), 1951–1957.
Canellas, L. P., & Olivares, F. L. (2014). Physiological responses to humic substances as plant growth promoters. Chemical and Biological Technologies in Agriculture, 1, 3.
Guimarães, D. V., Gonzaga, M. I. S., Silva, T. O., Silva, T. L., Dias, N. S., & Matias, M. I. S. (2013). Soil organic matter pools and carbon fractions in soil under different land uses. Soil and Tillage Research, 126, 177–182.
Kuzyakov, Y., & Domanski, G. (2000). Carbon input by plants into the soil: Review. Journal of Plant Nutrition and Soil Science, 163(4), 421–431.
Lehmann, J., & Kleber, M. (2015). The contentious nature of soil organic matter. Nature, 528(7580), 60–68.
Lovley, D. R., Coates, J. D., Blunt-Harris, E. L., Phillips, E. J. P., & Woodward, J. C. (1996). Humic substances as electron acceptors for microbial respiration. Nature, 382, 445–448.
Lyon, D. J., Stroup, W. W., & Brown, R. E. (2007). Crop production and soil water storage in long-term winter wheat–fallow tillage experiments. Soil and Tillage Research, 94(2), 387–397.
Mikha, M. M., Vigil, M. F., Benjamin, J. G., & Sauer, T. J. (2013). Cropping system influences on soil carbon and nitrogen stocks in the Central Great Plains. Soil Science Society of America Journal, 77(2), 702–710.
Nardi, S., Pizzeghello, D., Muscolo, A., & Vianello, A. (2002). Physiological effects of humic substances on higher plants. Soil Biology and Biochemistry, 34(11), 1527–1536.
Nielsen, D. C., Lyon, D. J., Hergert, G. W., Higgins, R. K., Calderón, F. J., & Vigil, M. F. (2016). Cover crop mixtures do not use water differently than single-species plantings. Agronomy Journal, 108(3), 1025–1038.
Saha, D., Kukal, S. S., & Bawa, S. S. (2024). Long-term impacts of conservation agriculture practices on soil organic carbon and aggregation. Soil Science Society of America Journal.
Six, J., Conant, R. T., Paul, E. A., & Paustian, K. (2002). Stabilization mechanisms of soil organic matter: Implications for C saturation of soils. Plant and Soil, 241(2), 155–176.
Stevenson, F. J. (1994). Humus chemistry: Genesis, composition, reactions (2nd ed.). Wiley.
Tan, K. H. (2014). Humic matter in soil and the environment. CRC Press.
Trevisan, S., Francioso, O., Quaggiotti, S., & Nardi, S. (2010). Humic substances biological activity at the plant–soil interface. Plant Signaling & Behavior, 5(6), 635–643.
For any questions or commments please feel free to reach out to Brian Anrall, b.arnall@okstate.edu
Corn Hybrids’ Yield Response to Limited Well Capacities in the Central High Plains
Macie McPeak: M.S in Irrigation and Water Management
Sumit Sharma : Extension Specialist for High Plains Irrigation and Water Management
Background
The Central High Plains, which include the Oklahoma Panhandle, Southwest Kansas, Southeast Colorado, and Northern Texas Panhandle, is a heavily farmed semi-arid region that depends on the Ogallala Aquifer for irrigation to ensure stable crop yields. However, the continuous decline of the Ogallala Aquifer has resulted in increased need for irrigation strategies that conserve water while maintaining crop profitability. Corn remains the most water consuming crop with highest productivity per unit of irrigation applied, and strong economic returns in the Central High Plains region. However, corn is also the most sensitive to water stress among all the existing cropping systems (including sorghum, cotton, and sunflower, soybeans and wheat). Declining water table has reduced the well capacities in many areas in the region, which cannot meet crop water demand, making it a growing challenge for corn production. Therefore, there is a need for research in irrigation strategies and agronomic choices such as drought tolerant hybrids, seeding rate, planting date, and hybrid maturity for sustainable and profitable corn production with reduced well capacities in the region. This blog discusses the yield response of different corn hybrids to limited well capacities in the Oklahoma Panhandle area of the Central High Plains.
Limited well capacities only meet partial crop water demand, which in general leads to yield declines especially in high water demanding crops such as corn. Several previous studies suggest that crop productivity does not significantly decrease as long as irrigation is maintained at approximately 75–80% of full evapotranspiration (ET) replacement (Su et al., 2022; Klocke et al., 2007; Zhao et al., 2019). However, when irrigation levels are more restricted, such as under reduced well capacities, there can be substantial yield losses and diminished economic returns. The magnitude of yield reduction varies with region, hybrids, and growth stage at which water stress occurred. For example, in the Central High Plains the corn ET demand is highest in Texas Panhandle and decreases as we move north towards Nebraska. Zhao et al. (2019) found that applying 75% ET in the Texas Panhandle produced corn yields equivalent to full irrigation, whereas reducing irrigation to 50% caused significant yield reductions. Similarly, Klocke et al. (2007) reported that limited irrigation at roughly 50% of full ET replacement in Nebraska achieved 80–90% of fully irrigated yields across multiple crop rotations. Therefore, the irrigation strategies which work in one region may not work the same way in other regions with different crop water demand and must be tested for the region-specific climatic conditions.
The current study was conducted in 2025 at the Oklahoma Panhandle Research and Extension Center in Goodwell, OK. Four Pioneer brand corn hybrids including P13777 (113 day maturity), P10625 (110 day maturity), P05810 (105 day maturity), and P14346 (114 days maturity) were planted at 22,000 and 28,000 seeds per acre. The hybrids were irrigated with a center pivot fitted with variable rate irrigation system at 200, 300, 400, and 500 GPM well capacities. The well capacities were simulated by adjusting the frequency of irrigation events.
Results & Discussion
The crop received 12.1 inches of rain from planting until physiological maturity, while total rainfall from April till September was over 15 inches. Manual probing of the field showed near 4 feet soil profile at the time of planting which can hold up to 2 inches of plant available water per foot. The well capacities 200, 300, 400, and 500 GPM treatments received 7.4, 8.9, 10.8, and 12.0 inches of irrigation, respectively. The data showed no significant effect of population on corn yield across hybrids for any well capacity. However, the hybrids showed significant interaction with well capacities, which indicated that hybrid yield response varied at different capacities (Figure 1). In general, the average yield declined from longest maturity to shortest maturity hybrids irrespective of the well capacity, but was only statistically significant at for 200 GPM (Figure1). At this irrigation level, the shortest maturity hybrid P05081 yielded significantly lower yield than longest maturity hybrid P14364, while P13777 and P10625 were not different from either of these two hybrids.
Although there was no statistical difference among the hybrids at 500, 400, and 300 GPM, when compared across well capacities, yield reductions were most pronounced at the 200 and 300 GPM irrigation levels for each individual hybrid, indicating that irrigation capacity was the primary yield limiting factor under restricted water availability (Figure 2). While the exact causes of this abrupt decline are not yet understood, as mentioned in the beginning of this blog, previous literature has suggested that severe yield decline in corn can be expected when irrigation is reduced to 60% ET replacement in the study region. Both 300 and 200 GPM well capacities met 60 and 65% crop ET demand, while 400 and 500 GPM met 71 and75% crop ET demand, respectively. More data will be needed to ascertain these threshold levels of well capacities for corn production in this region.
All the hybrids showed a positive yield response to Irrigation+Rain with different yield gains per inch of water applied (Figure 3). Hybrid P10625 registered highest yield gain of 14.1 bushel per inch of water applied, followed by P13777 (12.0 bu), P05081 (11.9 bu), and P14364 (11.6 bu). The stronger coefficient of regression (>80%) for two short maturity varieties indicated that irrigation was stronger yield limitation factor for these hybrids, in comparison to 114 and 113-day maturity hybrids for irrigation explained on 67 and 69% variability, respectively. This suggests that besides irrigation there might be other factors which could contribute to filling the yield gaps for given irrigation levels in longer maturity hybrids.
Planting population did not significantly affect grain yield across irrigation capacities. When pooled across the hybrids for individual planting populations, 28,000 seeding rates resulted in gain of 0.1, 2.6, 5, and 12 bushels per acre for 200, 300, 400, and 500 GPM, respectively. This indicates that higher planting populations at well capacities of 400 or above should be considered, while reducing population at 300 GPM or lower might be more cost-effective option.
Take Home
- Irrigation capacity remains the primary determinant of yield potential under limited well capacities in the Central High Plains.
- Pre-irrigation and recharging the soil profiles will be critical to support crop water demand for limited well capacities.
- Short maturity hybrids appeared to have consistently lower average yield and more vulnerable for yield losses at limited irrigation. However, one must consider that the growing conditions were more conducive for corn production in 2025 which generally favor long maturity hybrids. Therefore, long-term data will be required to assess the performance of short maturity hybrids during inclement growing seasons.
- Even though population didn’t significantly influence the grain yield. The 28,000 seeding rates overall had higher average yield at 400 and 500 GPM. Therefore, producers should consider the higher population at these well capacities or more.
- Overall, irrigation is the most important factor for yields, but there is a need for long-term agronomic data on hybrid maturity and population along with economic analysis to ascertain these findings.
Using soil moisture trend values from moisture sensors for irrigation decisions
Sumit Sharma, Extension Specialist for High Plains Irrigation and Water Management
Kevin Wagner, Director, Oklahoma Water Resources Center
Sumon Datta, Irrigation Engineer, BAE.
Sensor based and data driven irrigation scheduling has gained interest in irrigated agriculture around the world, especially in semi-arid areas because of the easy availability of commercial irrigation scheduler technology such as soil moisture sensors and crop models. Moisture sensing has particularly gained interest among the agriculture community due to ease of availability of the sensors to the producers, affordable costs, and easy to use graphical user interface. Economic potential of sensors in saving irrigation costs, data interpretation training through extension education programs, and policy initiatives have also helped with adoption of the sensors, especially in the United States. However, sensor adoption and efficient use can still be challenging due to poor data interpretation, steep learning curves, overly high expectations and subscription costs. This blog briefly discusses scenarios where sensors can be helpful in irrigated agriculture. For moisture sensor types, functioning and installation, readers are referred to BAE-1543 OSU extension factsheet.
Irrigation Scheduling
Irrigation scheduling with soil moisture sensors follows traditional principles of field capacity (FC), plant available water, maximum allowable depletion (MAD), and permanent wilting point (PWP). Figure 1 shows the transition of soil moisture level from field capacity to MAD, and to permanent wilting point in a typical soil. The maximum amount of water that a soil can hold after draining the excess moisture is called field capacity. At this point, all the water in soil is available to the plants. As the moisture content in the soil declines, it becomes more difficult for the plants to extract moisture from the soil. The soil moisture level below which the available moisture in soil cannot meet the plant’s water requirement is called the MAD. The water stress that occurs once moisture level goes below this moisture level can cause yield reductions in crops. Therefore, irrigation should be triggered as soon as the soil moisture level approaches this point (MAD) to avoid any yield losses (for detailed information on MAD, its value for different soils and crops, and irrigation scheduling, readers are referred to BAE-1537). Modern soil moisture sensors can come self-calibrated and are equipped with water stress threshold levels for different crops to avoid water stress or overwatering (Figure 2). These decisions are useful in furrow and drip irrigation systems where irrigation triggers can be synchronized with MAD values.
Figure 2: Screenshots of graphic user interface of three sensors a) GroGuru b) Sentek c) Aquaspy (Top to bottom) with threshold levels for soil moisture conditions. Aquspy and Sentek credits: Sumit Sharma. GroGuru image credits: groguru.com
Soil Moisture Trends and Irrigation Depths
Soil moisture sensors can help make data-informed decisions about scheduling irrigation. Previous studies have shown that the moisture values may vary from one sensor to the other and may not represent the exact moisture levels in soil. However, all soil moisture sensors exhibit trends in recharge and decline in soil moisture conditions. These real time soil moisture trends can be used to make informed decisions to adjust irrigation and improve water use efficiency. In high ET demand environments of Oklahoma, pivots are usually not turned off during the peak growing season, yet sensors can help in making decisions for early as well as late growing periods.
One of the easiest adjustments that could be made using soil moisture sensor data is the adjustment of irrigation depth. In an ideal situation, every irrigation event should recharge the soil profile to field capacity; however, this is often limited by the crops’ water demand and the well/irrigation capacity to replenish soil moisture levels. Each peak in soil moisture detected by sensors shows irrigation or rain, which ideally should be bringing moisture to same level after irrigation. However, reduction in moisture peaks in the soil moisture profile with every irrigation often indicates greater crop water demand than what is replenished with irrigation. In such scenarios, as allowed by capacity and infiltration rates, the irrigation depth can be increased. These trend values are particularly useful for center pivot irrigation systems, where triggering irrigation based on MAD might lag due to time and space bound rotations of the pivots in Oklahoma weather conditions.
Figure 3: A screenshot from Aquaspy agspy moisture sensor showing moisture at 8” (blue) and 28” (red) with each irrigation event. Data and image credit: Sumit Sharma
Last irrigation can be a tricky decision to end the cropping season. For summer crops, this is the time when crop ET demand is declining due to decline in green biomass and cooler weather patterns. Similar moisture trends can be used to make decisions for the last irrigation events, which can be skipped or reduced if the profile moisture is good, or can be provided if profile moisture is low. This is important because in an ideal situation, one would want to end the season with a relatively drier profile to capture and store off-season rains. Additionally, saving water on last irrigation can save operational cost and potentially cover the cost of moisture sensor subscriptions.
These decisions can be illustrated with Figure 3, which shows the trends of declining and recharging in a soil profile under corn at 8- and 28-inch depth. This field was irrigated with a center pivot irrigation system which was putting 1-1.25 inches of water with each irrigation event; however, the peak water recharge rate at both depths was declining with each irrigation. This coincided with peak growth period indicating rising ET demand of the crop than what was replenished by the irrigation. Later, two rain events, in addition to irrigation, replenished soil moisture in both layers. As the pivot was already running at a slow speed, slowing it further was not an option without triggering runoff for this soil type and this well capacity. Further in the season, when the crop started to senesce and ET demand declined, each irrigation event added to the moisture level of the soil. This allowed the producer to shut down the pivot between 70% starch line and physiological maturity for the crop to sustain at a relatively wet soil profile and leave the soil in relatively drier profile for the off-season.
In high ET demanding conditions of Western Oklahoma, crops often rely on moisture stored in deep soil profiles during the peak ET period when well capacities can’t keep up with crop water demand. In the high ET demanding environments of Oklahoma, irrigated agriculture depends heavily on profile moisture storage. Declining soil profile moisture is common during peak ET periods in high water demanding crops such as corn. These observations are useful if one starts the season with considerable moisture in the soil profile, however such trends may be absent if the season is started with a dry soil profile. Dry soil profiles can be recharged early in the season with pre-irrigation or deeper early irrigations (if allowed by the infiltration rate of the soil), when crop ET demand is low, to build the soil moisture profile. As such, sensors can be used in reducing the irrigation depth or skipping irrigation in early cropping systems if one starts with a full profile. This usually allows root growth through the profile to chase the moisture in deeper layers. It should be noted that the roots will grow and chase moisture only if there is a wet profile, and not through a dry soil profile.
Sensor installation and calibration are important for efficient use of these devices in irrigation decision making. Poor installation can often lead to poor data and wrong decision making. Although modern sensors are self/factory calibrated, some do provide the option to adjust threshold levels manually based on field observations. Early installation of sensors can be useful in making informed decisions as soon as the season starts. For a more detailed analysis of proper sensor installation, refer to BAE-1543. Producers are encouraged to integrate other means of irrigation planning with soil moisture sensing, such as a push rob to probe the soil profile or OSU Mesonet’s irrigation planner to further validate the sensor data. Further, the cliente should consider their irrigation capacities before investing in soil moisture sensors, as sensors may always show a deficit in low well capacities which cannot meet crop’s water demand.
References:
Taghvaeian, S., D. Porter, J. Aguilar. 20221. Soil moisture-sensing systems for improving irrigation scheduling. BAE-1543. Oklahoma State Cooperative Extension. Available at: https://extension.okstate.edu/fact-sheets/soil-moisture-sensing-systems-for-improving-irrigation-scheduling.html
Datta, S., S. Taghvaeian, J. Stivers. Understanding soil water content and thresholds for irrigation management. BAE-1537. Oklahoma State Cooperative Extension. Available at: https://extension.okstate.edu/fact-sheets/understanding-soil-water-content-and-thresholds-for-irrigation-management.html
For more information please contact Sumit Sharma sumit.sharma@okstate.edu
Pre-plant Irrigation
Sumit Sharma, Irrigation Management Extension Specialist.
Jason Warren, Soil and Water Conservation Extension Specialist.
Pre plant-irrigation is a common practice in Western Oklahoma to recharge soil profile before growing season starts. Pre-plant irrigation is useful when the irrigation capacity is not enough to meet peak ET demand. It can also be important to germinate and provide for optimum emergence of the crop. As such, pre-plant irrigation is not useful when the soil profile is already wet, or soil profile is not deep enough to store moisture, or if planting dates are flexible and can wait until rains can recharge soil profile. Pre-plant irrigation becomes an important consideration if the previous crop had extensive rooting systems, which depleted moisture from deep in the profile. The crops in western Oklahoma especially in the Oklahoma Panhandle depend on stored water in the profile to meet ET demand during peak growth period, especially when well capacities are limited. Deep profiles and excellent water holding capacities of soil found in the region make the storage of a considerable amount of moisture possible. While pre-plant irrigation to recharge the whole profile (which can be 6 feet deep) may not be possible or advised, producers can still use certain tools to assess the stored water in the profile and make decisions on pre-plant irrigation.
A soil push probe (Figure 1) can provide a crude estimate of the moisture in a soil profile. For example, if an average person can push the probe to 2 feet, this means that the first 2 feet of the profile has moisture stored in it. The profile beyond 2 feet is considered too dry to push the probe through. This method does not provide the amount of water stored in the profile. For accurate measurements of soil moisture, soil samples could be collected, weighed, dried and weighed again to determine the water content in the soil. An alternative is to install moisture sensors, however this is usually not practical due to potential damage during planting, although some probes that can be permanently buried are becoming available. On average a clay loam soil in western Oklahoma can hold up to 2 inches of plant available water per foot. The approximate water holding capacity of your soil can be found on the websoilsurvey. Your county extension or NRCS personnel should be able to help you navigate this website if necessary. When the water holding capacity of your soil is known, the use of a push probe can provide a preliminary estimate of soil water content. Probing should be done at multiple locations in the field on both bare and covered (with crop residue) spots. The presence of crop residue reduces evaporation and increases infiltration so the first thing you will notice is that it is generally easier to push the probe into the surface where the ground is covered by residue. If the soil water content is near full the probe will be easy to push into the soil and it may even have mud on its tip when you pull it out. In this case you can estimate that the water content to the depth of penetration is near field capacity and that the current water content is equal to the water holding capacity. For example, if you can push the probe 2 ft into a soil with a water holding capacity of 2 inches/ft then we expect to have 4 inches of plant available water. In contrast if it takes some effort to push the rod 2 ft the estimated water content may be reduced.
When pre-irrigation is applied it can be useful to assess the increase in the depth to which the probe can be pushed into the soil after the irrigation event. For example, if 1 inch of irrigation is applied to the soil in the example above, we may expect that after this irrigation event we can push the rode 2.5 ft. However, in some case we may be able to push the rod 3 ft. The reason being that although we could not push the rod beyond 2 ft before the irrigation event, the soil below this depth was not completely dry. Therefore, the 1 inch of water was able to move to a depth of 3 ft. This is useful information, telling us that the soil below the depth we can push the rod contains some water and that each inch we apply may drain a foot into the profile. Generally, we expect the rooting depth of most crops to be able to extract water from at least 4 ft. Although it is certainly possible to extract water from below this depth, we generally don’t want to pre water our soils to full beyond 4 ft. When we fill the profile with pre water, we are increasing success of the following crop by providing the stored moisture that can offset deficits that may occur in the growing season. However, we are reducing our opportunity to capture and utilize spring rainfall. We must consider this when applying pre-irrigation, because if it is followed by rainfall in excess of ET our irrigation efficiency is greatly reduced by the drainage or runoff that can occur.
Saline and Sodic Seep Renovation A potential Positive Impact of Drought
This article is written by Dr. Jason Warren, OSU Soil and Water Conservation State Extension Specialist.
The drought has caused numerous negative impacts on Agriculture in Oklahoma. However its impact on our ability to renovate some types of Saline and/or Sodic soils has been a positive. Saline and sodic seeps are referred to by many names, including: salt spots, alkaline spots or slick spots. They are all similar in that they contain excessive amounts of salt or sodium that prevent plant growth. However there are various differences that influence how we renovate these sites.
These areas are classified by the amount and type of salt present. Saline soils are those that contain an EC greater than 4000 μmhos/cm and less than 15% Exchangeable sodium. A Saline/sodic soil contains an EC greater than 4000 μmhos/cm and greater than 15% exchangeable sodium. Lastly, the Sodic soils contain less than 4000 μmhos/cm and greater than 15% exchangeable sodium. Given these differences it is important to have soils from these barren areas tested before a renovation plan is developed. The soil tests will provide recommendations for renovation and more detail on these strategies can be found in factsheet PSS-2226.
Beyond the classifications briefly mentioned above there are different ways in which these saline and sodic soils form. Some of these soils are formed from parent material that contained excessive salt or sodium. Others are formed when ground water moves to the surface through evaporation and deposits salt as the water is lost to the atmosphere. The drought conditions we are current experiencing can impact our ability to renovate the latter.
Figure 1: The upper picture was taken in Feb. 2011 and the bottom picture was taken in April 2013.
Hydraulic seeps, those formed from the movement of groundwater to the surface, are often found in low lying areas of the landscape where the groundwater is close enough to the soil surface that water can be conducted through capillary force to the soil surface. These forces are similar to those that allow use to suck water up through a straw but in the case of a saline seep evaporation from the soil surface provides the hydraulic gradient that pulls water from the water table. The drought has caused the water table in many areas to subside and become too deep for these force to pull water to the surface and deposit salts.
Figure 1 shows a saline/sodic soil in 2011 and again in 2013. This site had been treated with Gypsum as described factsheet PSS -2226 in 2007. However, because of a shallow water table that persisted until the onset of drought in 2011 the renovation effort was not successful because there was insufficient movement of water through the profile to leach the salts down out of the soil surface. These soils are in proximity to Stillwater Creek and Lake Carl Blackwell. The water table has declined which allows limited rainfall experienced at this site in 2012-13 to move the salts down out of the soil surface. This in turn has allowed crop establishment further improves water infiltration by protecting the soil from crusting.
The drop in most water tables across Oklahoma, particularly western Oklahoma where these salt spots are most common, provides for a unique opportunity to renovate hydraulic salt spots. Again the first course of action is to collect a soil sample to determine what types of salts are present. You can also make an effort to determine how the salt spot was formed. This information can be found on the soil survey at http://websoilsurvey.sc.egov.usda.gov/App/HomePage.htm. Your county extension educator or local NRCS can help to interpret this information.
We have observed that our success in renovating a hydraulic seep near Stillwater was greatly improved during this period of drought. However, given the fact that our sub soils are generally dry throughout Oklahoma, which improves our ability to leach salts, the drought should improve our ability to renovate those formed from parent material as well.
Down and Dirty with NPK 