Justus von Liebig was a 19th century German scientist (1803-1873) who is regarded as one of founders of organic chemistry. He is also commonly considered as the “father of agricultural chemistry and the fertilizer industry.”
Liebig developed several important analytical methods used in a broad range of applications as well as many other important contributions and developments. Liebig was one of the great chemistry teachers of the 19th century that served as a foundation for the robust German chemical industry of the 20thand 21st centuries.
In 1840 Justus von Liebig presented a concept that is known as the “Law of the Minimum”, which was built upon a theory that had first been developed by Carl Sprengel, a German botanist (1787-1859). The Law of the Minimum basically states that a plant’s rate and extent of growth and overall health is dependent on the amount of the scarcest of the essential nutrients that are available to the plant (Figure 1; Liebig, 1840 and van der Ploeg et al., 1999).
The Law of the Minimum has been further applied into a general model of all organisms and biological functions, including the limiting effects of other environmental factors i.e., sunlight and water in terrestrial ecosystems, as well as excesses of nutrients and other environmental factors (Bruuselma and Nigon, 2023; Davidson, 2016; and Mosaic, 2023).
An important way to consider the Law of the Minimum is that the growth of plants, or crops, is not dependent on the total amount of nutrients available, but rather by the scarcest nutrient or resource (i.e., water). This is particularly important in relation to nutrients such as nitrogen, which is the nutrient required in largest amounts by plants, and it is the most common limiting plant nutrient. In desert agriculture, water is commonly the first most common limiting factor in plant growth and development, closely followed by bio-available nitrogen.
The Law of the Minimum is important to understand in managing fertilizer and other agronomic inputs to a crop. This has been demonstrated when fertilizer prices are high, particularly for nitrogen and phosphate fertilizers, and growers may be inclined to reduce or eliminate fertilizer applications.
Liebig’s Law of the Minimum is most applicable for nutrients and plant growth factors that are mobile in the soil. This is particularly relevant for nitrogen, which is available to plants in the nitrate form (NO3--N), which is mobile in the soil. Thus, (NO3--N) moves with soil-water.
Accordingly, the Liebig Law of the Minimum also pertains to water in a soil-plant system. Other growth factors or nutrients will not compensate for a deficiency in a given nutrient or plant growth factor. Plant-available nitrate-nitrogen (NO3--N) or water are good examples, there are no substitutes.
The Liebig Law of the Minimum is an important concept in soil fertility and plant nutrition and overall agronomic crop management in an irrigated production system.
Figure 1. Graphic illustration of the Law of the Minimum with shortest stave in
the barrel representing the most limiting nutrient in the soil-plant system.
References:
Bruulsema, T, and Nigon, L.L. Crops & Soils Magazine, November–December 2023 American Society of Agronomy. pp. 54-59.
Davidson, D. 2016. Nutrient Management Magazine.https://www.no-tillfarmer.com/articles/5648-no-till-notes-how-to-plan-your-summer-fertility-program?v=preview
Liebig, J. 1840. Die organische Chemie in ihrer Anwendung auf Agri- Sprengel, C. 1831. Chemie fu¨ r Landwirthe, Forstma¨nner und Cameralisten (Chemistry for agronomists, foresters, and agricultural econo-cultur und Physiologie (Organic chemistry in its applications to agri-culture and physiology). Friedrich Vieweg und Sohn Publ. Co., mists). Volume 1. Vandenhoeck und Ruprecht Publ. Co., Go¨ ttingen, Germany. Braunschweig, Germany.
Mosaic. 2023. How Law of the Minimum Impacts Crops' Nutrient Use. In: No-Till Farmer. https://www.notillfarmer.com/articles/12637-how-law-of-the-minimum-impacts-crops-nutrient-use
Vander Ploeg, A.R; Böhm, W.; and M. B. Kirkham. 1999. On the Origin of the Theory of Mineral Nutrition of Plants and the Law of the Minimum. Soil Sci. Soc. Am. J. 63:1055–1062.
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.
Earlier this year, we completed fabrication of a prototype commercial scale steam applicator for injecting steam into the soil prior to planting. The concept behind soil steaming is similar to soil solarization - heat the soil to levels sufficient to kill soilborne pathogens and weed seeds (typically 140 °F for >20 minutes). The self-propelled machine is principally comprised of a 100 BHP steam generator mounted on tracks and a steam applicator sled (Fig. 1). Steam is applied via shank injection as the machine travels through the field. After cooling (< ½ a day), the crop is planted into the disinfested soil.
The device has been demonstrated in several on-farm, field-scale (>1-acre plots) tests in Salinas, CA this summer. Although the trials are still in progress, preliminary results indicate that the machine is performing well and similar to our previous steam applicator prototypes. In those trials, we found that soil steaming provided excellent weed control (>90%), suppressed problematic soilborne diseases (Fusarium wilt of lettuce >50%, lettuce drop >70%), reduced Pythium spp. counts in soil assays (>93%) and increased crop yields (>24%).
For this upcoming season, we are seeking collaborators to conduct similar field-scale on-farm demonstrations in Yuma, AZ. The primary objectives would be 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 tested 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. We have resources to conduct 3-4 on-farm demos, so space is limited. I’d be happy to work with you.
Fig. 1. Self-propelled steam applicator principally comprising a a) 100 BHP steam
generator mounted on tracks and a b) steam applicator sled that applies steam via
c) shank injection as beds are formed.
One commonly used product has been Roundup, and the label recommendations are the following: “This product provides weed control when applied prior to harvest of feed barley and wheat. For feed barley, apply after the hard-dough stage and when the grain contains 20 percent moisture or less. For wheat, apply after the hard-dough stage of grain at 30 percent or less grain moisture” 3.
There are concerns of the presence of glyphosate in shipments from US and Canada which they attribute to “the practice of pre-harvest spraying”. Therefore, some growers are looking for alternatives to this herbicide2.
Sharpen (saflufenacil) is a quick acting burndown product for broadleaf weed control in field crops and it has been labeled and approved in the US for this purpose.
Consulting with a BASF representative we learned that Sharpen is labeled in Arizona (not CA) for this use. The recommended rate is 1-2 fl oz., adding Ammonium Sulfate adjuvant and Methylated Seed Oil. Also, apply at the hard dough stage with no more than 30% moisture, with 10 GPA by ground or 5 GPA by air.
Knezevic (2009) stated “MSO was the adjuvant that provided the greatest enhancement of saflufenacil across all species tested. COC was the second-best adjuvant and provided control similar to MSO on many weed species. NIS provided the least enhancement of saflufenacil4.
An Agronomy Update from Kansas State University recommended to “Consult grain buyer to see if they will accept Sharpen treated wheat because of export restrictions”5.
References:
1. http://canola.okstate.edu/cropproduction/herbicides/labels/Roundup%20PowerMax%20Label.pdf
2. https://sustainablepulse.com/2019/08/29/unique-hair-testing-project-reveals-high-levels-of-glyphosate-and-ampa-in-members-of-the-japanese-parliament/
3. https://www.farmprogress.com/weeds/controlling-broadleaf-weeds-in-winter-wheat-is-essential
4. Knezevic, S. Z., Datta, A., Scott, J., & Charvat, L. D. (2009). Adjuvants influenced saflufenacil efficacy on fall-emerging weeds. Weed Technology, 23(3), 340-345.
5. https://cropwatch.unl.edu/2019/ksu-pre-harvest-weed-control-wheat-sharpen-update
Integrated pest management (IPM) involves the utilization of a combination of several tactics for the effective management of pests. This concept was developed by entomologists and is currently adopted by pest managers to target many kinds of pests, including insects, weeds, and pathogens. Most IMP tactics fit well in both conventional and organic crop production. It is not uncommon that most pest management techniques that are approved for organic crop production are not very effective as a stand-alone tactic. Therefore, it is essential to use a combination of pest management techniques that will complement each other to control the pests adequately. This is like a many little hammers approach, where each of the management tactics is a little hammer hammering on the pests. When planning your IPM programs targeting pests in organic crop production, it is important to consider planting resistant/tolerant varieties, scouting regularly, implementing economic thresholds (when possible), practicing cultural control, physical/mechanical control, and biological control, and applying biopesticides when necessary.
• Resistant varieties: Resistant varieties are crucial for effective IPM in organic crop production. When available, the use of resistant varieties should be the first line of defense against pests. Resistant varieties help save on production costs and mitigate environmental impacts associated with insecticides and their applications.
• Cultural control: Cultural control includes cropping systems (trap cropping and push-pull), planting date (early planting or late planting), crop rotation, and growing of crop varieties with early maturity traits. Implementing trap cropping and/or push-pull systems can cause the diversion of insect pests. Early or late planting, crop rotation, and variety with early maturity traits can favor the avoidance of damaging insect pest pressures.
• Scouting: Proper and timely scouting helps determine the levels of pest infestations, allowing the pest manager to trigger control action in a timely fashion. Scouting also helps to prevent unnecessary insecticide applications.
• Economic thresholds: The economic threshold determines the pest or injury level at which control action should be taken. The economic threshold works side by side with scouting. This helps to determine when a management action should be triggered, allowing a reduction of unnecessary insecticide utilization, which will consequently help in delaying or mitigating resistance.
• Physical/mechanical control: This method includes establishing physical barriers, plowing, and sanitation (elimination of volunteer crops and other potential hosts). Plowing can help to bury soil insect pests deep into the ground, directly kill them, and expose soil insects to adverse weather conditions, birds, and other predators, which will adversely impact these pest populations.
• Biological control: This method involves using insect pests’ natural enemies, including predators such as spiders, lady beetles, syrphid fly larvae, big-eyed bugs, pirate bugs, lacewing larvae, and parasitoids such as parasitic wasps and flies.
• Biopesticides: Biopesticides are based on botanical extracts, entomopathogenic fungi, entomopathogenic bacteria, or entomopathogenic viruses that have adverse effects on insect pests. Entrust, Bt, Pyganic, AZA-Neem, M-Pede, Celite, and Venerate are commonly used insecticides for insect pest control in organic crops grown in Arizona.
The implementation of IPM permits to manage pests economically while preserving the environment and reducing negative impacts on human health. In other words, IPM aims at managing pests in an economically viable, socially acceptable, and environmentally safe manner. It is important to note that all the IPM tactics are not always viable in all situations (IPM is not a one-size-fits-all). Therefore, the management techniques choice for an IPM program should be done on a case-by-case basis.
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.
