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
This study was conducted at the Yuma Valley Agricultural Center. The soil was a silty clay loam (7-56-37 sand-silt-clay, pH 7.2, O.M. 0.7%). Spinach ‘Meerkat’ was seeded, then sprinkler-irrigated to germinate seed Jan 13, 2025 on beds with 84 in. between bed centers and containing 30 lines of seed per bed. All irrigation water was supplied by sprinkler irrigation. Treatments were replicated four times in a randomized complete block design. Replicate plots consisted of 15 ft lengths of bed separated by 3 ft lengths of nontreated bed. Treatments were applied with a CO2 backpack sprayer that delivered 50 gal/acre at 40 psi to flat-fan nozzles.
Downy mildew (caused by Peronospora farinosa f. sp. spinaciae)was first observed in plots on Mar 5 and final reading was taken on March 6 and March 7, 2025. Spray date for each treatments are listed in excel file with the results.
Disease severity was recorded by determining the percentage of infected leaves present within three 1-ft2areas within each of the four replicate plots per treatment. The number of spinach leaves in a 1-ft2area of bed was approximately 144. The percentage were then changed to 1-10scale, with 1 being 10% infection and 10 being 100% infection.
The data (found in the accompanying Excel file) illustrate the degree of disease reduction obtained by applications of the various tested fungicides. Products that provided most effective control against the disease include Orondis ultra, Zampro, Stargus, Cevya, Eject .Please see table for other treatments with significant disease suppression/control. No phytotoxicity was observed in any of the treatments in this trial.
Last week, we initiated our first on-farm demonstration of soil steaming of the season with our self-propelled steam applicator. The machine is designed to inject steam into the soil and raise soil temperatures to levels sufficient to kill weed seed and soilborne pathogens (140°F for > 20 minutes). After the soil cools (< ½ day), the crop is planted into the disinfested soil.
In this trial, we are examining the viability of soil steaming for controlling weeds in organic carrot at the field scale level (plot size > ½ ac). The machine performed well in that it was able to reach target soil temperatures at reasonable travel speeds (> 0.4 mph), provide uniform temperature distribution across the bed and form nicely shaped beds suitable for subsequent planting. Stay tuned for reports of weed control efficacy, crop yield and overall profitability as compared to the grower standard.
We are seeking collaborators to conduct similar field-scale trials/demos in Yuma, AZ. The primary objectives are 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 trialed 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. I’d be happy to work with you.
Fig. 1. On-farm demonstration of a self-propelled steam applicator for weed and
disease control
Acknowledgements
This project is sponsored and funded in part by the Arizona Specialty Crop Block Grant Program and the Propane Education and Research Council (PERC). We greatly appreciate their support.
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 all traps over the last month; about average for December.
Beet armyworm: Moth trap counts decreased in all areas in the last 2 weeks but appear to remain active in some areas, and average for this time of the year.
Cabbage looper: Moths increased in the past 2 weeks, and average for this time of the season.
Diamondback moth: Adults increased in several locations last, particularly in the Yuma Valley most traps. Below average for December.
Whitefly: Adult movement remains low in all areas, consistent with previous years
Thrips: Thrips adult movement continues to decline, overall activity below average for December.
Aphids: Winged aphids still actively moving but declined movement in the last 2 weeks. About average for December.
Leafminers: Adult activity down in most locations, below average for this time of season.
