Soil Nitrogen
Nitrogen (N) is the essential nutrient that is required in largest amounts by plants.  Following water, N is the most limiting factor in the growth and development of non-leguminous crops. 

In most places around the world, sunlight is the first most limiting factor in terrestrial ecosystems, including crop production systems.  This is followed closely by water as a limiting factor to plant growth and biologically available N (NO3- -N) is commonly the third most limiting factor.

Nitrogen goes through many natural transformations in the soil and cycling of N can take many routes and forms. Thus, the management of N is also one of the most challenging plant nutrients to work with efficiently.

Even though N is often a limiting factor in terrestrial ecosystems and crop production systems, N is ubiquitous in the atmosphere and biosphere.  For example, 78% of the Earth’s atmosphere is made up of N gas or N2, a molecule made of two nitrogen atoms bonded together by a strong, stable, triple bond.  As a result, N gas is biologically inert.

Nitrogen is the mineral element required by plants in the greatest amount and it serves many functions in plant physiology.  Nitrogen is an integral component of amino acids, which are the building blocks for proteins.  

Proteins are present in the plant as enzymes that are responsible for metabolic reactions in the plant.  Because N is so important, plants often respond dramatically to plant-available N, which is nitrate-nitrogen (NO3- -N), (Havlin, et al. 2014; Thompson and Troeh, 2005; Warren, et al., 2017; and Weiland Brady, 2017).

Nitrogen is central to global crop production.  Many parts of the world do not have enough to achieve food and nutrition security, in other cases excess N from fertilizer leaks into the environment with damaging consequences.

Though it makes up a large portion of the air we breathe, most living organisms cannot access N in this form.  Atmospheric N must go through a natural process called “nitrogen fixation” to transform before it can be used for plant nutrition.

In both plants and humans, N is used to make amino acids, which make the proteins that construct cells, including the building blocks for DNA.  It is also essential for plant growth because it is a major component of chlorophyll, the compound by which plants use sunlight energy to produce sugars from water and carbon dioxide(photosynthesis).

The Nitrogen Cycle
The N cycle is the multi-faceted process through which N moves from the atmosphere to earth, through soils and organisms, and is released back into the atmosphere, with conversions in and out of organic and inorganic forms (Figure1).

Figure 1. The nitrogen cycle.

A good point to begin the review of the N cycle is with biological N fixation, the process of converting biologically inert N2 gas into an organic compound or an inorganic form such as NO3- -N.  Nitrogen fixation can take place through basic routes: 1) biological fixation or 2) conversion of N2 gas to NO3--N by lightning.  The basic routes of N fixation are shown in the upper left-hand side of Figure 1.

Biological fixation occurs when naturally occurring N-fixing symbiotic and some non-symbiotic bacteria convert N2 gas from air into forms like ammonium-nitrogen (NH4+ -N) and then into nitrate-nitrogen (NO3- -N).  A very important form of biological N fixation is carried out by symbiotic bacteria that live in the root nodules of legumes converting N2 gas into ammonium (NH4+) and then nitrate (NO3-), which are commonly incorporated very quickly into organic forms.

Plants preferentially absorb nitrate -N (NO3- -N) from the soil through the root hairs and use it in their physiological systems to create the N forms they need (amino acids, proteins, enzymes, complex compounds, etc.).  Some ammonium-N (NH4+-N) can be taken up by some plants.  The preferential form of N for plant uptake and utilization is nitrate-N (NO3--N).

Organic forms of N are not taken up by the plant and incorporated into the plant physiology.

Denitrifying bacteria convert excess nitrate back into inorganic N which can be released back into the atmosphere in gaseous forms (N2O and N2).

Nitrogen fixation can also begin with lightning, the heat from which ruptures the triple bonds of atmospheric nitrogen (N2 gas), freeing its atoms to combine with oxygen and creating nitrous oxide gas (N2O), which dissolves in rain forming nitric acid (HNO3) which then can be absorbed by the soil.

Excess nitrate in the soil can be lost through leaching, the process where nutrients mobile in the soil, including nitrate-N, can pass through the soil profile and into groundwater and potentially polluting streams.

Because N is so important and plant-available forms are often limiting, plants often respond dramatically to available N.  There are no substitutes for sufficient plant-available N and management is a critical part of a crop production system.

References
Havlin, J.L., Beaton, J.D., Tisdale, S.L. and Nelson, W.L. 2014.  Soil Fertility and Fertilizers; An Introduction to Nutrient Management.  6th Edition, Prentice Hall, Upper Saddle River, NJ.

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.  Oklahoma Soil Fertility Handbook. Published Apr. 2017; 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/

Given the positive feedback from last week’s article, I thought I’d share with you another video that showcases the cutting-edge advancements in AI technologies. This time, the topic is Digital AI Twins. Reid Hoffman, a renowned expert in AI technologies, has created a digital twin of himself named “Reid AI” using a custom Generative Pretrained Transformer (GPT). Reid AI was trained using content from over two decades worth of Hoffman’s public speeches, podcasts and published books. The result is a digital entity that mirrors Hoffman’s knowledge, insights, and even his conversational style. In the segment, Hoffman interviews his AI counterpart. The conversation is not only entertaining but also very realistic, blurring the lines between human and machine. I was pretty impressed and think you will be too. Given the rapid advancements in these technologies, one can’t help but wonder what’s next in the evolution of AI and how this technology will change the world.

Check it out here or by clicking image below.

Fig. 1. Reid Hoffman meets his AI twin. (Credit: Reid Hoffman).

We did some trials at the University of Arizona Yuma Agricultural Center in broccoli to evaluate and compare Napropamide (Devrinol) liquid formulation 2XT versus the Dry formulation DF-XT. 
This product inhibits the production of fatty acids in plants, which is crucial for plant development. It affects primarily the meristematic cells which are in growing points of the stems and roots. We saw activity especially on seedling development that we show in some pictures at the end of the article. 
Some growers expressed their concern on the safety of different levels of incorporation with sprinkler irrigation. Therefore, we established a test in which we applied the product as a broadcast application after planting. Then we used different levels of incorporation in some sections using 12, 24 and 36 hours of sprinkler irrigation. No difference was observed with the incorporation level in our trial. We observed temporary phytotoxicity from 4-10%. The data was obtained from visual evaluations. Also, a 0.5 to 1” height reduction was exhibited when compared to untreated plots. 
It is common that growers and PCAs make management decisions on herbicide applications in different crops knowing that some injury is expected. Such is the case for alfalfa, wheat, spinach, lettuce and in this case broccoli. Frequently slight stunting it is not noticed because commercial fields don’t have untreated areas for comparison.
We talked to PCA’s and growers at the SW Ag Summit and asked for their experience with napropamide this past season. Some noticed the broccoli exhibited similar levels of phyto, which they considered economically tolerable. 
We noticed that good soil prep was important for before the application of the product and avoiding direct contact of the seed with the product for best results.
On weed control we noticed good activity on nettleleaf goosefoot and lambsquarter. 
At the 45day we collected data counting SMALL and LARGE weeds and at the 60 day evaluation we noticed most of the SMALL goosefoot and lambsquarter in the high rate of napropamide plots stayed small (pic. below).

The push-pull strategy, a stimulo-deterrent diversionary strategy, combines behavior-modifying stimuli that manipulate the distribution and abundance of insect pests and/or natural enemies. When your main crop is intercropped with plant species that can mask the host (main crop) appearance or emit undesirable volatiles (smells) that divert the pests away from the main crop (push), on the other hand, other plants in your intercropping system can be extremely attractive using stimuli that are highly apparent and attractive to the pest, hence trapping the pest (pull) (Fig. 1). Insects use visual, chemical, or tactile cues. Thus, by intercropping the main crop with plants that emit more attractive smells, are more visually appealing, or release undesirable smells, one can cause the pest to be trapped and repelled from the main crops, resulting in effective control of the pests.

Figure 1. Pictorial representation of push-pull strategy. 

In Brazil, the push-pull strategy has been found effective in managing major kale pests. They found that using mustard as a preferred host pulled the pests away from the kale crops, while marigold plants increased the beneficial arthropod population which provided additional control of the pests (da Silva et al. 2022; https://www.sciencedirect.com/science/article/pii/S1049964421003029). My 

lab plans to evaluate the efficacy of similar systems for insect pest management in organic vegetable crops in Arizona. 

In Salinas, California, intercropping lettuce with sweet alysum has favored some measurable aphid control. Sweet alyssum attracts and feeds hoverflies, which then lay eggs in lettuce, producing hoverfly larvae that consume aphids. In this video, Dr. Brennan describes in detail how this system works. This research was conducted about a decade ago, but I believe this could be an important tactic to consider for aphid control in lettuce. We also plan to evaluate this system for aphid management in lettuce in Arizona lettuce growing regions. 

Figure 2. Graphical representation of Lettuce-Alyssum intercropping system for aphids control. (Image source: Brannan 2013). 

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.