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.
I hope you are frolicking in the fields of wildflowers picking the prettiest bugs.
I was scheduled to interview for plant pathologist position at Yuma on October 18, 2019. Few weeks before that date, I emailed Dr. Palumbo asking about the agriculture system in Yuma and what will be expected of me. He sent me every information that one can think of, which at the time I thought oh how nice!
When I started the position here and saw how much he does and how much busy he stays, I was eternally grateful of the time he took to provide me all the information, especially to someone he did not know at all.
Fast forward to first month at my job someone told me that the community wants me to be the Palumbo of Plant Pathology and I remember thinking what a big thing to ask..
He was my next-door mentor, and I would stop by with questions all the time especially after passing of my predecessor Dr. Matheron. Dr. Palumbo was always there to answer any question, gave me that little boost I needed, a little courage to write that email I needed to write, a rigid answer to stand my ground if needed. And not to mention the plant diagnosis. When the submitted samples did not look like a pathogen, taking samples to his office where he would look for insects with his little handheld lenses was one of my favorite times.
I also got to work with him in couple of projects, and he would tell me “call me John”. Uhh no, that was never going to happen.. until my last interaction with him, I would fluster when I talked to him, I would get nervous to have one of my idols listening to ME? Most times, I would forget what I was going to ask but at the same time be incredibly flabbergasted by the fact that I get to work next to this legend of a man, and get his opinions about pest management. Though I really did not like giving talks after him, as honestly, I would have nothing to offer after he has talked. Every time he waved at me in a meeting, I would blush and keep smiling for minutes, and I always knew I will forever be a fangirl..
Until we meet again.
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).
