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.
Frost and freeze damage affect countless fruit and vegetable growers leading to yield losses and occasionally the loss of the entire crop. Frost damage occurs when the temperature briefly dips below freezing (32°F).With a frost, the water within plant tissue may or may not actually freeze, depending on other conditions. A frost becomes a freeze event when ice forms within and between the cell walls of plant tissue. When this occurs, water expands and can burst cell walls. Symptoms of frost damage on vegetables include brown or blackening of plant tissues, dropping of leaves and flowers, translucent limp leaves, and cracking of the fruit. Symptoms are usually vegetable specific and vary depending on the hardiness of the crop and lowest temperature reached. A lot of times frost injury is followed by secondary infection by bacteria or opportunist fungi confusing with plant disease.
Most susceptible to frost and freezing injury: Asparagus, snap beans, Cucumbers, eggplant, lemons, lettuce, limes, okra, peppers, sweet potato
Moderately susceptible to frost and freezing injury: Broccoli, Carrots, Cauliflower, Celery, Grapefruit, Grapes, Oranges, Parsley, Radish, Spinach, Squash
Least susceptible to frost and freezing injury: Brussels sprouts, Cabbage, Dates, Kale, Kohlrabi, Parsnips, Turnips, Beets
More information:
Interested in staying up to date on the latest robotic ag technologies? FIRA USA and a number of other entities are organizing a 3-day forum focused on autonomous farming and agricultural robotics solutions. The event will be held October 22-24 in Woodland/Sacramento, CA. The program includes top-level keynote speakers, breakout sessions, a trade show and field demos. Over 35 robots will be demoed and/or on display including 8 machines designed for weeding vegetable crops. Some of the latest technologies for in-row weeding will be featured including lasers (2 companies) and high precision spot spraying (3 companies). If you are interested in ag tech, FIRA 2024 promises to be a quality event and one well worth attending. For more information, visit https://fira-usa.com/.
Fig. 1. Robotic technologies on display and being demoed in the field at
FIRA USA 2024. The event will be held October 22-24 in Woodland/Sacramento,
CA. (Photo credits: FIRA USA).
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.
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 remain at low levels in all areas, well below average for this time of year.
Beet armyworm: Trap increased areawide; above average compared to previous years.
Cabbage looper: Cabbage looper counts decreased in all areas; below average for this time of season.
Diamondback moth: DBM moth counts decreased in most areas. About average for this time of the year.
Whitefly: Adult movement beginning at low levels, average for early spring.
Thrips: Thrips adult counts reached their peak for the season. Above average compared with previous years.
Aphids: Aphid movement decreased in all areas; below average for late-March.
Leafminers: Adults remain low in most locations, below average for March.
