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.
Interested in more information about mechanical weed control in vegetable crops? Check out this seminar given by Sam Hitchcock Tilton, Lakeshore Technical College. Topics covered include weed ecology, systems-based strategies for weed control, new and emerging machines on the market, individual tools and implements and best-use strategies for cultivating equipment. Although the target audience is small-scale vegetable growers, Sam gives many practical tips and insights that are applicable to large-scale vegetable farms. Even if you’re very knowledgeable about mechanical cultivation, this entertaining seminar is a good refresher on the basics of integrated weed management and one I think you’ll find interesting and informative.
Fig. 1. Sam Hitchcock Tilton presenting seminar entitled “Mechanical Weed
Control and Weed Ecology in Vegetable Production”. Click the video above to view a recording of the presentation. (Photo credits: Practical Farmers of Iowa,
Ames, IA).
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.
Resistant varieties are crucial for effective insect pest management, more specifically in organic crop production. When growing crops organically, resistant varieties should be the first line of defense against insect pests. Compared to nonresistant varieties, insect-resistant crop varieties are less vulnerable to insect attacks or yield well despite insect attacks.
Resistance in crops can be expressed through different mechanisms, including antibiosis, antixenosis, and tolerance. For the antibiosis mechanism, the resistant crops act on the life history traits of the pest, such as affecting fecundity, growth, development, and survival. Thus, this reduces the pest population. On the other hand, antixenosis is characterized by the ability of the crop to affect the behavior of the pest by deterring it after initial tasting or probing, which is non-preference. For the tolerance mechanism, however, the crops can sustain a high density of pests and still maintain their yield potential. In most cases, the overall resistance of a variety is a combination of all these three mechanisms.
There is also a type of resistance called phenological resistance, field resistance, or pseudo-resistance, which is when the crop appears to be resistant because it is at a less susceptible stage when a pest attack occurs. This may happen either because of the rate of development of the crop or due to management practices implemented, causing the crop to escape the pest pressure. Thus, the implementation of proper agronomic practices plays a significant role in improving the ability of crops to resist pest attacks.
In certain scenarios, additional insect control measures such as the use of bioinsecticides (in organic production) and natural enemies may be necessary in conjunction with an insect-resistant variety for comprehensive pest control. However, even in cases where additional control measures are required, the cultivation of a resistant variety can significantly delay and reduce the amount of insecticide applications needed. This underscores the economic, ecological, and environmental benefits of using insect-resistant crop varieties.
Previous studies conducted at Yuma Agricultural Center demonstrated that resistant lettuce varieties can provide suitable control of aphids affecting lettuce in the region. Therefore, it is very important to consider cultivating insect-resistant lettuce varieties for suitable management of aphids in organic lettuce production since biological insecticide options are currently limited. My lab is planning to evaluate several lettuce varieties to determine lettuce varieties that carry resistance against aphids in an attempt to generate applicable information on varieties that could fit well within the production practices and environmental conditions of Arizona’s vegetable-growing regions.
