The Mobility Concept
Plant nutrient management is strongly dependent on nutrient mobility in the soil. Nutrient mobility in the soil is different among the essential plant nutrients and nutrient management in the field needs to take this into account.
In 1954, Dr. Roger H. Bray at the University of Illinois proposed a nutrient mobility concept that has proven to be very important in the 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.
A great deal of research has been conducted in the past 70years supporting Bray’s nutrient mobility concept and it is now considered a fundamental feature of soil fertility and soil health management (Raun, 2017; Warren et al., 2017, Havlin et al. 2014; Troeh and Thompson, 2005).
Mobile Nutrients
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 the soil water as it is taken up by the plant.
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 forms: 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.
In a crop field where many plants are growing together, there are root system sorption zones commonly overlap. Therefore, the root system sorption zones for adjoining plants are competing for water and mobile nutrients, Figure 2. This is one of the main reasons that appropriate plant populations are important for optimum yield.
Due to this overlapping and potential competition among plants in the root system sorption zone, we need to fertilize and manage nutrients in direct proportion to the number plants or the potential yield of the crop. We can make an estimate of the mobile nutrient requirements by first calculating the amount of mobile nutrient that will be taken up by the crop.
Knowing the average concentration of the nutrient in the crop and the yield goal for the crop can provide a good estimate of total mobile nutrient demand by the crop. This is important for estimating the crop requirements for nutrients like nitrogen(N) for a crop.
Taking a soil test for plant available N, which is nitrate-nitrogen (NO3--N), will only provide a snapshot of the plant-available N over the season due to myriad N transformations that are constantly taking place in the soil. Thus, it is better to work with a yield goal approach for mobile nutrients such as N.
Figure 2. Competition among plants brought about by increasing yield goal.
Yield goal example: Consider a lettuce yield goal of 30 Tons (fresh weight)/acre. Nitrogen uptake studies on lettuce have shown that for iceberg lettuce a general average of 2.6 lbs. N/Ton is taken up by the crop and 3.62 lbs. N/T for romaine. Thus, using 3.0 lbs. N/Ton of fresh lettuce with a projected yield of 30Tons/acre indicates a total N demand by the crop of 90 lbs. N/acre (Bottomset al., 2012; USDA-ERS, 2013; Doerge et al., 1991).
We can subtract residual nitrate-nitrogen (NO3--N) found in a pre-season soil test from the total N crop demand estimate and identify a target N fertilization rate. For example, 10 parts per million (ppm) residual nitrate-nitrogen (NO3--N) as a preseason level is very common in agricultural soils.
Using an estimate of 2 million lbs. of soil per acre-furrow slice (6-inch-deep layer of soil in an acre area), 10 ppm X 2 = 20 lbs. nitrate-nitrogen (NO3--N) residual in the soil.
90 lbs. N projected requirements – 20 lbs. residual N = 70lbs. N/acre fertilizer requirement.
This would be fine if everything in the field was 100%efficient. Due to inefficiencies that are inherent in a field production system, higher rates of fertilizer N can be required in some cases.
Our management goal is to achieve the highest levels of efficiency (agronomically, economically, and environmentally) in the field as possible (Bottoms et al., 2012 and Doerge et al. 1991). Using the nutrient mobility concept is good to incorporate into our crop management strategy.
References:
Bottoms, T.G., Smith, R.F., Cahn, M.D., Hartz, T.K. 2012.Nitrogen requirements and N status determination of lettuce. Hort Science 47, 1768-1774.
Doerge, T. A., R. L. Roth, and B. R. Gardner. 1991. Nitrogen Fertilizer Management in Arizona. College of Agriculture Doc. 19102. University of Arizona.
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.
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.
USDA-ERS. 2013. U.S. lettuce statistics 2011. http://usda.mannlib.cornell.edu/MannUsda/viewDocumentInfo.do?documentID= 1576 (accessed 21 Feb. 2013).
Warren, J., H. Zhang, B. Arnall, J. Bushong, B. Raun, C. Penn, and J. Abit. 2017. Oklahoma Soil Fertility Handbook. Id: E-1039
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.govWe are in the process of organizing The Desert Difference: A Showcase of AgTech Opportunities for Growing in the Desert. The two-day event will be held November 13-14th in Yuma, AZ. The first day will be a Field Day, the second will be a standard conference with keynote speakers and breakout sessions. Details of the conference will be coming soon. The focus of this article is the Field Day. As with our previous AgTech Field Days, the workshop will feature the latest ag technologies being demonstrated in the field. I’ve been reaching out to multiple companies but am sure I’m not aware of all the cutting-edge technologies out there. We would like to showcase as many innovative ag technologies as possible, so please contact me if you are interested in demoing your equipment or know someone that is. It’s an open invitation - private companies, and university and government researchers are all welcome!
Fig. 1. Previous University of Arizona AgTech Field Days.
We are currently doing a trial for Hairy Feeabane control (Conyza bonariensis) with several combinations and wanted to share some preliminary results. Application was done last March 22 when the weeds were approximately 0.5-3” diameter and burn down activity was evaluated 5 days after treated (5DAT). We are sharing the first evaluation with you since the weed is abundant at this time in the Yuma Mesa. This is the initial evaluation, so mortality and final control will be rated later.
In recent conversations with PCAs we talked about Rely (glufosinate) activity and they have seen good performance of the product especially when weeds are small and according to some researchers this product’s works better with high relative humidity1 (Tickes 2010).
This preliminary data shows that Rely and Sharpen both wit AMS (Ammonium Sulfate) and MSO (Methilated Seed Oil) appeared effective at the 5DAT evaluation. There are other PPO herbicides that we are testing like UA850 that looks promising in some combinations such as the combination with Roundup+AMS+MSO. More details will be shared at a later date when additional data is collected.
References:
Aphids are sap-sucking insects that depend on the nutritional content of the sap ingested from the plant hosts for proper growth and development. Nitrogen availability is one of the most important factors in the development of herbivore populations. Excessive application of nitrogen fertilizer to crops is likely to increase insect pests feeding preference and consumption resulting in the survival, growth, and reproduction of the pests. This particularly affects aphids where excessive nitrogen application to host crops such as lettuce, wheat, sorghum, etc. may boost their populations by enhancing their growth and development, thus reducing their generation time, resulting in an increase in the number of generations and density during the cropping season.
Report from a study conducted on Arugula shown that excessive supply of nitrogen increased green peach aphid density. In some situations, high nitrogen levels in plant tissue can decrease resistance and increase susceptibility to aphids’ attacks. Given that, adequate management of fertilizer like nitrogen can tremendously help to manage aphids which are difficult to control pests specifically in organic lettuce production. In addition to pest management, effective fertilizer usage can also result in economic and environmental benefits.
Like fertilizer management, water management is also very important for effective pest control. Water availability around plant roots affects the rate at which nutrients are
absorbed by the plants. Thus, an increase in water availability will increase nitrogen uptake which can affect the population dynamic of aphids. Additionally, with high water availability there is an increase in phloem pressure making food more accessible to sap-sucking insect pests. Supplying the required amount of water using appropriate irrigation methods and irrigation scheduling can be beneficial for pest management. Although this practice is not likely to completely prevent infestation of aphids, it can surely play a role in reducing the density of aphid populations on crops.
Figure 1. Aphid selection of host plants: (a) The migrating aphid’s choice of landing on a particular plant depends on receiving the plant-reflected wavelengths (between about 500 nm and 600 nm); upon landing, antennal receptors detect the plant surface volatiles for initial assessment. (b) After making contact with the plant surface, the aphid briefly and tentatively pierces the epidermis using its stylet (<1 min) and ingests a small quantity of plant sap for further evaluation by a gustatory organ in the epipharyngeal area. (c) If the initial assessment is favorable, the aphid penetrates the epidermis to pierce the mesophyll and parenchyma tissues and briefly ingests more sap from vacuoles for additional evaluation and to determine the appropriateness of further ingestion (<1 min). (d) Upon identifying the host plant, the
aphid pierces the epidermis of the leaf and passes through the intercellular air spaces of the mesophyll cells using its stylet to reach the sieve tube element in plant phloem, releasing salivary enzymes to protect the mouthparts and prevent plant tissue repair, enabling continuous sap consumption. If ingestion in the sieve tube exceeds 10 min, the host plant is deemed suitable (Xia et al. 2023).
Selected References:
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.
