Nutrient Mobility Concept
In two recent articles published in this newsletter on 27 November 2024, Volume 15, No. 24 and 25, I presented the Bray Nutrient Mobility Concept (Bray, 1954) in relation to mobile and immobile nutrients (Silvertooth,2024a and Silvertooth, 2024b). This article provides a summary of the nutrient mobility concept and the implications of mobile and immobile nutrient behavior in soil-plant systems and plant nutrition management.
In 1954, Dr. Roger H. Bray at the University of Illinois proposed a nutrient mobility concept that has proven to be very important inthe management of nutrients for optimum efficiency (agronomically, economically, and environmentally). Bray essentially simplified all soil nutrient chemistry to the fact that some plant nutrients are mobile in the soil, and some are not (Bray, 1954; Raun, 2017; Warren et al., 2017, Havlin et al. 2014; Troeh and Thompson, 2005).
Mobile Nutrients and the Root System Sorption Zone
Mobile plant nutrients in the soil move with the soil water. Thus, plants can extract mobile nutrients from a large volume of soil beyond the direct root system. Accordingly, plants take up mobile nutrients from a “root system sorption zone” (Figure 1). This gives plants the capacity to utilize most of the mobile nutrients in the root system sorption zone as those nutrients will move to the plant roots with soil water as it is taken up by the plant (Silvertooth, 2024a).
We consider the mobile plant nutrients to be nitrogen (N), sulfur (S), boron (B), and chlorine (Cl). These mobile plant nutrients are taken up by the plant in the following form: nitrate-nitrogen (NO3--N), sulfate-sulfur (SO42- - S), boric acid (H3BO3) and borate ions (BO33- - B), and chlorine is taken up as the chloride ion (Cl-).
Figure 1. The root system sorption zone and an illustration of the large volume of soil
from which plants extract mobile nutrients.
Root system sorption zones of adjacent plants commonly overlap, and plants compete for water and mobile nutrients (Figure 2) in the soil and for light at the canopy surface. This is one of the reasons plant populations are important for optimum yield.
Figure 2. Competition among plants brought about by increasing yield goal.
Immobile Nutrients
Plant nutrients that are immobile in the soil include phosphorus(P), potassium (K), calcium (Ca), magnesium (Mg), iron (Fe), zinc (Zn), manganese (Mn), copper (Cu), and molybdenum (Mo). Immobile nutrients do not move as freely in the soil solution as the mobile nutrients do. These nutrients interact more directly with soil colloids and root surfaces.
Immobile nutrients are absorbed by the plant from the soil and soil solution that is directly next to the root surface. Plant roots must grow through the soil volume to come into direct contact with the immobile nutrients.
Figure 3 describes this soil volume and plant root interface as the root surface sorption zone.
Figure 3. The root surface sorption zone and an illustration of the small volume of
soil from which plants extract immobile nutrients.
Since only a thin layer of soil surrounding and in direct contact with the plant roots are involved in supplying immobile nutrients to the plant, there is little or no competition among plants for immobile nutrients. Competition among plants only occurs at points where roots from adjacent plants come in direct contact with one another (Figure 4).
Figure 4. Limited competition among plants for immobile nutrients.
Plant Nutrient Management Implications
The nutrient mobility concept and these basic illustrations can help us understand the basis for some common observations and resultant crop management practices.
Fertilizers with immobile plant nutrients are more effective when they are incorporated into soil and particularly in soil zones where there is a high probability of plant roots encountering the immobile nutrients.
Banded applications of immobile nutrients are generally more effective than the same rates broadcast and incorporated into the soil. In contrast, mobile nutrients like nitrogen (N) can be broadcast and moved into the root system sorption zone by water.
Soil concentrations of immobile nutrients do not usually change rapidly but they can be slowly mined out of the soil by a series of crops without proper fertilization. On the other hand, continued or over-applications of immobile nutrient fertilizers, such as phosphorus (P), will cause a buildup of that nutrient in the soil since only a small fraction (commonly 15-20% for most crops) of the nutrient or fertilizer comes into direct contact with the plant roots.
Appropriate soil tests that are properly correlated and calibrated with crop-specific response categories are important in evaluating immobile plant nutrient status.
References:
Bray, R.H.1954. A Nutrient Mobility Concept of soil-plant relationships. Soil Sci. 78(1), p. 9-22.
Havlin, J.L., Beaton, J.D., Tisdale, S.L. and Nelson, W.L. 2014. Soil Fertility and Fertilizers; An Introduction to Nutrient Management. 6thEdition, Prentice Hall, Upper Saddle River, NJ.
Silvertooth,J.C. 2024a. Soil Health - Bray’sNutrient Mobility Concept and Mobile Plant Nutrients University of ArizonaVegetable IPM Newsletter, Volume 15, No. 24.
Silvertooth, J.C. 2024b. Soil Health - Bray’s Nutrient Mobility Concept and Immobile Plant Nutrients University of Arizona Vegetable IPM Newsletter, Volume 15, No. 25,
Raun, W.R. 2017. In: Warren et al. 2017. Oklahoma Soil Fertility Handbook, Id:E-1039
Troeh, F.R. and Thompson, L.M. (2005) Soils and Soil Fertility. Sixth Edition, Blackwell, Ames, Iowa, 489.
Warren, J., H. Zhang, B. Arnall, J. Bushong, B. Raun, C. Penn, and J. Abit. 2017. Oklahoma Soil Fertility Handbook. Id: E-1039
Weil, R.R. and Brady, N.C. (2017) The Nature and Properties of Soils. 15th Edition, Pearson, New York.
At events and in the halls of the Yuma Agricultural Center, I’ve been hearing murmurings predicting a wet winter this year…
As the Yuma Sun reported last week, “The storms of Monday, Aug. 25 [2025], were the severest conditions of monsoon season so far this year in Yuma County, bringing record-rainfall, widespread power outages and--in the fields--disruptions in planting schedules.”
While the Climate Prediction Center of the National Weather Service maintains its prediction of below average rainfall this fall and winter as a whole, the NWS is saying this week will bring several chances of scattered storms.
These unusually wet conditions at germination can favor seedling disease development. Please be on the lookout for seedling disease in all crops as we begin the fall planting season. Most often the many fungal and oomycete pathogens that cause seedling disease strike before or soon after seedlings emerge, causing what we call damping-off. These common soilborne diseases can quickly kill germinating seeds and young plants and leave stands looking patchy or empty. Early symptoms include poor germination, water-soaked or severely discolored lesions near the soil line, and sudden seedling collapse followed by desiccation.
It is important to note that oomycete and fungal pathogens typically cannot be controlled by the same fungicidal mode of action. That is why an accurate diagnosis is critical before considering treatments with fungicides. If you suspect you have seedling diseases in your field, please submit samples to the Yuma Plant Health Clinic or schedule a field visit with me.
National Weather Service Climate Prediction Center: https://www.cpc.ncep.noaa.gov/
National Weather Service forecast: https://forecast.weather.govEarlier this year, we completed fabrication of a prototype commercial scale steam applicator for injecting steam into the soil prior to planting. The concept behind soil steaming is similar to soil solarization - heat the soil to levels sufficient to kill soilborne pathogens and weed seeds (typically 140 °F for >20 minutes). The self-propelled machine is principally comprised of a 100 BHP steam generator mounted on tracks and a steam applicator sled (Fig. 1). Steam is applied via shank injection as the machine travels through the field. After cooling (< ½ a day), the crop is planted into the disinfested soil.
The device has been demonstrated in several on-farm, field-scale (>1-acre plots) tests in Salinas, CA this summer. Although the trials are still in progress, preliminary results indicate that the machine is performing well and similar to our previous steam applicator prototypes. In those trials, we found that soil steaming provided excellent weed control (>90%), suppressed problematic soilborne diseases (Fusarium wilt of lettuce >50%, lettuce drop >70%), reduced Pythium spp. counts in soil assays (>93%) and increased crop yields (>24%).
For this upcoming season, we are seeking collaborators to conduct similar field-scale on-farm demonstrations in Yuma, AZ. The primary objectives would be to assess the viability of soil steaming at the field-scale level and obtain grower feedback on the device’s commercial potential. The machine can be adjusted to work with most bed configurations including narrow (40”, 42”) and wide (80”, 84”) beds, and is suitable for use in conventional or organic crops (soil steaming is organically compliant). To date, the device has been successfully tested in iceberg lettuce, romaine lettuce, baby leaf spinach and carrot crops.
If you are interested in an on-farm demo of soil steaming, please let me know. We have resources to conduct 3-4 on-farm demos, so space is limited. I’d be happy to work with you.
Fig. 1. Self-propelled steam applicator principally comprising a a) 100 BHP steam
generator mounted on tracks and a b) steam applicator sled that applies steam via
c) shank injection as beds are formed.
As we continue to be impacted by the drought in Arizona with a reduction in the Colorado River water allocation, we need to reconsider every option for water conservation in our agricultural operations.
We know that weeds compete with our crops for water, nutrients, and space causing yield reductions. However, how much water are we loosing due to high weed infestations?
Some researchers have concluded that weeds use more water than various crops and consider them “water wasters”. Therefore, good weed control can contribute to raise available water for our crops. Transpiration of some of the most common annual weeds is approximately four times higher than crop plants. It has also been reported that weeds use up to three times the amount of water to produce a pound of dry matter.A study showed “common lambsquarters (Chenopodium album) requires 658 pounds of water to produce one pound of dry matter, common sunflower (Elianthus annus) requires 623 pounds, and common ragweed 912 pounds, compared with 349 pounds for corn and 557 pounds for wheat1.” It has been reported that increase from 0 - 8 plants / row meter of Palmer amaranth (Amaranthus palmeri) densities in corn decreased soil water available and the water use efficiency (WUE) of corn.
Uncontrolled weed growth can add direct irrigation costs of more than $50/ha while even weed densities below economic thresholds can add ~$20/ ha in production costs depending upon the cropping system and water cost (Norris,1996).
Under stress condition such as we experience yields can be reduced more 50% just by moisture competition. Other factors that influence water loss are weed densities, transpiration rate, other weed characteristics like root system and depth. For example, perennial weeds with a well-established root system are more drought resistant because they can explore better the soil profile.
Some report that weeds can potentially cause 34 percent of crop loss worldwide. We have seen how weeds cut the water flow in irrigation ditches and cause more evaporative loss. We believe weed control is essential for water conservation purposes and further research is needed in this matter.
A species' persistence requires the ability to adapt to the biotic and/or abiotic conditions present within its environment. This adaptation may occur through natural selection, which is the process by which organisms that are better adapted to their environment tend to survive and have better fitness. Natural selection is believed to be the motor of evolution. Evolution has occurred within all groups of organisms, including plant and animal populations.
Plants have evolved resistance in many circumstances, including resistance to pests and pathogens attacks. In response to insect herbivory attacks, plants have been using chemical defenses to resist. Consequently, to maximize their fitness, plant-feeding insects have co-evolved with plants to overcome plant defenses utilizing an array of strategies. Insects have evolved the ability to detoxify plant chemicals used for defenses and use the compounds as cues that favor the detection of the plant host. Insects have also evolved an adjusted sensory system allowing host cues detection and a nervous system that is able to integrate inputs from sensory neurons. The enhancement in the sensory and nervous systems allows the detection and avoidance of toxic plants as well as the excretion, sequestration, and degradation of plant toxins. Additionally, herbivory insects utilize target-site mutation, cuticular, humoral, and cellular defenses against plant chemical defenses. Moreover, insects have evolved to resist predation, parasitism, and pathogen attacks by means of a series of mechanisms, including cuticular adjustment, adaptive behavior, and chemical defenses.
With the intensive use of pesticides to manage agricultural pests, insect pests have evolved resistance to an array of insecticides using a variety of mechanisms. This type of evolution has been described as field-evolved resistance, which is a “genetically based decrease in susceptibility of a population to a toxin caused by exposure to the toxin in the field”. This is due to strong selection pressure that favors rapid evolution of resistance. For example, the widespread adoption of Bt crops in the U.S. has led to field-evolved resistance of corn earworm, also known as cotton bollworm, against Bt toxins. In some regions, Texas, for example, cotton bollworms being exposed to the Bt toxins in both corn and cotton throughout the year have been subjected to a high selection pressure, causing the pest to become quickly resistant to Bt toxins.
The use of beneficial arthropods to manage insects can favor a decrease in insecticide use and consequently reduce the selection pressure caused by pesticides. Although the evolution of resistance to predators and parasitoids tends to be prohibited by some factors (special and temporal refuges from enemies’ attacks, reciprocal evolution by control agents, and contrasting selection pressure from enemy species), the evolution of resistance to biological control agents has been reported for several insect pests including Argentine stem weevil, greater wax moth, and fruit fly. This is likely due to reduced plant and natural enemy diversity caused by intensive large-scale agriculture.
Several factors may play a role in the development of resistance. Large-scale homogenous agricultural systems do not allow enough refuges to sustain the susceptible strains, which would then mate with the resistant strains to dilute the resistance genes and maintain the susceptibility of the pest populations. Additionally, low biodiversity within the natural enemy population may favor the selection pressure. Coevolutionary arms races may play a significant role in that this may favor one participant in mutation and recombination rates.
Mechanisms of resistance:
Physiological resistance: Insects use physiological processes to become resistant to enemies and insecticides. In a pesticide use context, physiological resistance is defined as the capacity of an insect population to survive after being exposed to a concentration of insecticide that is known to be able to kill the totality of the population completely. However, the physiological process can also favor resistance against non-pesticide control methods. For instance, the fruit fly (Drosophila melanogaster) uses encapsulation to protect itself from koinobiont endoparasitoids. The encapsulation is a cellular immune response that follows three major stages, including the recognition of the parasitoid eggs as foreign, increasing the amount of circulating hemocytes that are produced by the lymph glands, and the lysis of the crystal cells allowing the release of prophenoloxidase which results in the melanization of the capsule surface.
Another way insects become resistant is through mutation in the target site of the toxicant. This physiological process can lead to resistance in insects against both plant defenses (toxic compounds released by the plant to protect itself from herbivory) and insecticides. This mutation can lead to target site insensitivity, meaning that even though the insect is being exposed to the toxic molecule, there will be no or reduced binding of the molecule to the target site, making the molecule ineffective. This mechanism of resistance is very common in many insecticide-resistant insect pests. Insects can also become resistant to toxic compounds from plants and to insecticides by evolving the ability to undergo detoxification of certain toxicants after exposure. This ability is also conferred by a series of mutations allowing the resistant insect to increase their enzyme production, which consequently increases their enzymatic activity and causes a rapid degradation of the toxicant into a nontoxic compound. This mechanism is also known as metabolic resistance.
Behavioral resistance: Many insect species have become resistant to certain host plants that use defense compounds to prevent herbivory through their plant selection and feeding behavior. For this behavior to occur, they must evolve the ability to detect toxic plants, which can be determined genetically or through a series of learning processes. Some other insects evolved the ability to deactivate or suppress the toxin produced by the plant hosts. For instance, the cotton bollworm uses its saliva, which is a gluco-oxidase, to cause a reduction in the level of nicotine produced in tobacco leaves. Other insects, when they feed on toxic plants, can excrete a significantly large amount of the accumulated toxic compound. Some insects even sequester the toxic compounds and use them for their own defense against predators and pathogens. Some insects that are hosts for parasitoids use a very effective behavioral resistant strategy by avoiding parasite contact or detection by choosing to niche away from the parasitoids or by choosing to locate themselves near a deterrent. Using this behavior, these insects are not directly resistant to the attackers but use what is present in their environment as tools to resist parasitism. Some other insects use alternative strategies, such as cryptic coloration or masquerade, to prevent their detection by predators and/or parasitoids. In this situation, they disguise themselves as something dangerous or unwanted to avoid being prayed on or parasitized.
Cuticular resistance: Insects depend heavily upon cuticular defenses to resist pathogens, parasitism, predation, and insecticides. For instance, to resist insecticide penetration, they develop a barrier in the outer layer of the cuticle either by changing the composition of the cuticle or by thickening it. This causes the toxicant to be penetrated slowly, consequently slowing the absorption of the contaminants to the insect bodies, where actions will take place.
Although the development of resistance is mostly beneficial for insects, there are some fitness costs associated with that. Physiological resistance, behavioral resistance, and cuticular resistance require the use of a large amount of energy; some energy that would have been allocated for growth, development, and reproduction is likely to be reduced, which would consequently reduce the fitness of the insect. Thus, fitness cost may cause an evolutionary constraint, which may reduce the rate or even prevent the evolution of resistance from occurring. Given that, an increase in resource availability is likely to favor the rate at which evolution occurs within a population.
In conclusion, resistance in insects can occur in a diversity of forms, and several factors may cause resistance to occur within insect populations. Additionally, while insect populations are more likely to be resistant to insecticide in large-scale agricultural systems, they can also become resistant to biological control agents, which underscores the importance of integrated pest management programs. The rate at which resistance occurs in a population closely depends on the intensity of selection pressure to which the insect populations are exposed. Thus, the more intense the selection pressure the quicker the populations will evolve resistant.
References:
1- Ali, J. G, and A. A. Agrawal. 2012. Specialist versus generalist insect herbivores and plant defense. Trends in Plant Science. 17: 293-302.
2- Balabanidou, V., L. Grigoraki, and J. Vontas. 2018. Insect cuticle: a critical determinant of insecticide resistance. Current Opinion in Insect Science 2018, 27:68–74.
3- Berenbaum, M.R. 1986. Target site insensitivity in insect-plant interactions. In: Brattsten, L.B., and S. Ahmad. (eds) Molecular aspects of insect-plant associations. Springer, Boston, MA. https://doi.org/10.1007/978-1-4613-1865-1_7
4- Boots, M. 2010. The Evolution of Resistance to a Parasite Is Determined by Resources. The American Naturalist. 178: 214-220.
5- Castagnola, A., and J. L. Jurat-Fuentes. 2016. Intestinal regeneration as an insect resistance mechanism to entomopathogenic bacteria. Current Opinion in Insect Science. 15:104–110.
6- Chareonviriyaphap, T., M. J. Bangs, W. Suwonkerd, M. Kongmee, V. Corbel, and R. Ngoen-Klan. 2013. Review of insecticide resistance and behavioral avoidance of vectors of human diseases in Thailand. Parasites & Vectors. 6: 280.
7- Dang, K., S. L. Doggett, G. V. Singham, and C-Y. Lee. 2017. Insecticide resistance and resistance mechanisms in bed bugs, Cimex spp. (Hemiptera: Cimicidae). Parasites & Vectors. 10:318, DOI 10.1186/s13071-017-2232-3
8- Després, L., D. Jean-Philippe, and C. Gallet. 2007. The evolutionary ecology of insect resistance to plant chemicals. TRENDS in Ecology and Evolution. 22: 298-307.
9- Dubovskiy, I. M., M. M. A. Whitten, O. N. Yaroslavtseva, C. Greig, V.Y. Kryukov, E. V. Grizanova, K. Mukherjee, A. Vilcinskas, V. V. Glupov, and T. M. Butt. 2013. Can insects develop resistance to insect pathogenic fungi? PLoS ONE. 8: e60248. doi:10.1371/journal.pone.0060248
10- Fellowes, M. D. E., and H. C. J. Godfray. 1999. The evolutionary ecology of resistance to parasitoids by Drosophila. Heredity. 84:1-8.
11- Ferré, J., and J. V. Rie. 2002. Biochemistry and genetics of insect resistance to Bacillus thuringiensis. Annual Review of Entomology. 47:501–33.
12- Heidel-Fischer, H., and H. Vogel. 2015. Molecular mechanisms of insect adaptation to plant secondary compounds. Current Opinion in Insect Science. 8: 8–14.
13- Mills, N. J. 2017. Rapid evolution of resistance to parasitism in biological control. PNAS. 114: 3792-3794.
14- Ryan, M. F., and O. Byrne. 1988. Plant-insect coevolution and inhibition of Acetylcholinesterase. Journal of Chemical Ecology. 14: 1965-1975.
15- Tabashnik, B. E. and Y. Carrière. 2010. Field-evolved resistance to Bt cotton: Helicoverpa zea in the US and pink bollworm in India. Southwest. Entomol. 35: 417–424.
16- Tomassetto, F., J. M. Tylianakisb, M. Realed, S. Wrattene, and S. L. Goldsonet. 2017. Intensified agriculture favors evolved resistance to biological control. PNAS.114: 3885–3890.Results of pheromone and sticky trap catches can be viewed here.
Corn earworm: CEW moth counts down in most over the last month, but increased activity in Wellton and Tacna in the past week; above average for this time of season.
Beet armyworm: Moth trap counts increased in most areas, above average for this time of the year.
Cabbage looper: Moths remain in all traps in the past 2 weeks, and average for this time of the season.
Diamondback moth: Adults decreased to all locations but still remain active in Wellton and the N. Yuma Valley. Overall, below average for January.
Whitefly: Adult movement remains low in all areas, consistent with previous years.
Thrips: Thrips adults movement decreased in past 2 weeks, overall activity below average for January.
Aphids: Winged aphids are still actively moving, but lower in most areas. About average for January.
Leafminers: Adult activity down in most locations, below average for this time of season.
