Organic and inorganic fertilizers are both valuable tools in crop production systems. For optimal soil health management, it is best to understand how organic and inorganic fertilizers are processed and function in the soil and contribute to soil health.
Plants absorb nutrients in specific inorganic ionic forms such as nitrate and ammonium (NO3−, NH4+); phosphate. Plants primarily absorb phosphorus (P) as inorganic orthophosphate ions, which are either hydrogen phosphate (HPO42−) or dihydrogen phosphate (H2PO4−), depending on the soil pH. Potassium (K) is taken up by plants as the monovalent potassium ion (K+).
The ionic forms of nutrients taken up by plants are identical whether they originate from organic or inorganic industrial fertilizers (Havlin et al., 2014). The source of the nutrient does not determine the form of the nutrient taken up by plants nor does the source of the nutrient determine crop health. The transformations of soil nitrogen (N) into plant-available forms serves as a good example, illustrated in the N cycle, shown in Figure 1.
Figure 1. The nitrogen cycle. Source: Stevenson, 1982.
Crop performance and soil health are influenced by the timing of fertilizer applications and the quantity of nutrients applied and delivered to best match plant needs (Fageria, 2009). The fertilizer source is important in terms of the reactions and time required to convert it into plant-available forms in the soil.
The major differences between organic and inorganic fertilizers are found in the way they impact soil properties. Organic fertilizers provide nutrients in complex carbon-based matrices and contribute organic material to the soil that can contribute to the stable soil organic matter (SOM) as they decompose.
Increased organic material inputs (such as animal manure, crop residues, etc.) can improve soil aggregate stability, which contributes to improved soil structure and the soil-water-holding capacity. Improved soil structure enhances aeration and internal soil-water movement and drainage. These factors also serve to stimulate a diverse soil microbial community (Drinkwater and Snapp, 2007).
Long-term application of organic fertilizers or amendments has been shown to enhance soil carbon stocks, biological activity, and nutrient cycling (Fließbach et al., 2007; Clark et al., 1998). These biological and physical improvements from organic fertilizer applications are usually found in direct proportion to the natural tendencies of the soil-plant system to accumulate and maintain higher SOM levels (Figure 2).
Figure 2. Soil organic matter content across the continental United States.
Inorganic fertilizers have definite advantages that include the ability to supply nutrients in concentrated, soluble forms designed to maximize availability, improve fertilizer efficiency, and support full crop yield potential. Strengths of inorganic fertilizers include precision, predictability, and the ability to match nutrient supply to crop demand (Fixen et al., 2015).
Because inorganic fertilizers do not contain or supply organic material, crop production systems that rely exclusively on inorganic nutrient sources may exhibit gradual declines in soil structure and biological activity. This is directly remedied when organic materials are incorporated into the soil on a regular basis. For example, important sources of organic materials that can be incorporated into soil include crop residues, green manures, or added amendments such as animal manure (Powlson et al., 2011).
Crop rotation systems are also important in managing organic material additions and incorporating them into the soil. For example, including a legume crop (i.e., alfalfa, clover, soybeans, etc.) into a crop rotation system provides large inputs of organic material through above ground crop residues, extensive root systems, and residual N from the natural symbiotic N fixation that legume plants provide in combination with specific Rhizobium bacteria species.
Negative impacts from inorganic fertilizers can include soil acidification (particularly in soils that have neutral or low pH and are poorly buffered), increased salinity, or nutrient losses. Each of these negative features are not directly due to the inorganic fertilizer source but these are the typical results of over-application and poor management.
However, it is also important to note that increased salinity and nutrient immobilization can occur from the application of many organic fertilizers, particularly animal manure. An analysis of any animal manure amendment applied to the soil is important for soil health management.
The evidence from several major long-term field experiments evaluating sources of organic and inorganic nutrient sources supports a balanced interpretation. For example, numerous long-term experiments have been conducted at the Rothamsted Experiment Station in England, which is one of the oldest agricultural research institutions in the world, founded in 1843. These Rothamsted studies have shown that organic amendments can increase soil organic carbon but often provide lower nutrient availability relative to inorganic industrial fertilizers (Johnston et al., 2017).
An important caveat to the Rothamsted experiments is to note that it has a temperate maritime climate (also known as oceanic) characterized by mild winters, cool summers, moderate year-round rainfall, and a lack of extreme temperature. Average annual rainfall is approximately 30 inches with a mean annual temperature of ~ 50° F.
The Pacific Northwest of the United States is a similar climate to Rothamsted, England. That is a climatic environment that is naturally conducive to higher SOM accumulation (Figure 2). It is not appropriate to make direct extrapolations from experiments on soil health conducted in climate and soil conditions such as in Rothamsted, England, to the desert Southwest. However, general patterns of response can be deduced from field experiments conducted in one region and applied in a very different region of application.
Several experiments from the Kellogg Biological Station in Hickory Corners, Michigan and the Rodale Institute in Kutztown, Pennsylvania (also two very dissimilar regions to the desert Southwest) report enhanced soil biological functioning in organic systems, but consistently lower yields compared with conventional systems unless organic nutrient sources are applied at very high rates (Robertson et al., 2014; Seufert et al., 2012), which becomes impractical in most cases.
Summary
Collectively, a review of these studies indicates that integrated nutrient management, which combines organic inputs for soil health with inorganic fertilizers for precise and efficient nutrient supply, provides the most robust and sustainable outcomes in crop production systems. This can be accomplished in conventional production systems with good crop rotations and the inclusion of organic amendments, and very importantly the appropriate management of the timing and application rates of inorganic fertilizers.
It is not accurate to assert that organic fertilizers inherently produce healthier crops or soils. It is correct that organic fertilizers can be important tools in improving soil health by increasing organic material inputs, which can contribute to maintaining or improving SOM content and supporting soil biological processes. It is also correct to state that inorganic fertilizers most efficiently deliver plant-available nutrients to the soil.
An integrated management system that properly utilizes both organic and inorganic fertilizers provides the best results for crop and soil health and agroecosystem sustainability. The most resilient and productive crop production systems strategically use both types within a comprehensive soil fertility program. Proper management is the key factor in soil health development.
References:
Clark, M.S., W.R. Horwath, C. Shennan, and K.M. Scow. 1998. Changes in soil chemical properties resulting from organic and low-input farming practices. Agron. J. 90:662–671.
Drinkwater, L.E., and S.S. Snapp. 2007. Nutrients in agroecosystems: Rethinking the management paradigm. Adv. Agron. 92:163–186.
Fageria, N.K. 2009. The Use of Nutrients in Crop Plants. CRC Press, Boca Raton, FL.
Fixen, P., F. Brentrup, T. Bruulsema, F. Garcia, R. Norton, and S. Zingore. 2015. Nutrient/fertilizer use efficiency: Measurement, current situation and trends. In: P. Drechsel et al., editors, Managing Water and Fertilizer for Sustainable Agricultural Intensification. IFA, IWMI, IPNI, and IPI. p. 8–38.
Fließbach, A., H.-R. Oberholzer, L. Gunst, and P. Mäder. 2007. Soil organic matter and biological soil quality indicators after 21 years of organic and conventional farming. Agric. Ecosyst. Environ. 118:273–284.
Havlin, J.L., S.L. Tisdale, W.L. Nelson, and J.D. Beaton. 2014. Soil Fertility and Fertilizers. 8th ed. Pearson, Upper Saddle River, NJ.
Johnston, A.E., P.R. Poulton, and K.S. Coleman. 2017. Soil organic matter: Its importance in sustainable agriculture and carbon dioxide fluxes. Adv. Agron. 142:1–63.
Powlson, D.S., C.M. Stirling, M.L. Jat, B.G. Gerard, C.A. Palm, P. Sanchez, and K.G. Cassman. 2011. Soil organic matter, food security, and climate change: A review. Agron. J. 103:351–363.
Robertson, G.P., S.K. Hamilton, D.A. Philpott, A.M. Schmidt, and S.K. Applegate. 2014. Long-term ecological research in agricultural landscapes at the Kellogg Biological Station LTER site. Bull. Ecol. Soc. Am. 95:292–312.
Seufert, V., N. Ramankutty, and J.A. Foley. 2012. Comparing the yields of organic and conventional agriculture. Nature 485:229–232.
Stevenson, F.J. 1982, Origin and distribution of nitrogen in soils. In: F.J. Stevenson, Ed., Nitrogen in Agricultural Soils, American Society of Agronomy, Madison, WI, pp. 1-42.
First, I want to thank everyone who participated in last week's Vegetable Pest Losses Survey.
This year's survey included the return of the lettuce disease losses section. While several diseases were present and managed last season, downy mildew accounted for the majority of disease management costs by a wide margin. This really underscores the
impact that last spring's unusually rainy weather had on disease development across the Yuma lettuce production region.
No one can predict exactly what this upcoming spring will bring, but there has been discussion about the possibility of a strong El Niño leading to an extended monsoon season. If that proves true, the conditions would once again support spring
downy mildew development. The pathogen only needs about 3 to 4 hours of continuous leaf wetness to infect lettuce, so periods of overnight moisture, prolonged morning dew, or frequent rainfall when inoculum (spores) are present increase disease risk.
With that in mind, this seems like a good opportunity to review what is known about downy mildew and discuss strategies for its management.
Resistance in lettuce to Bremia lactucae, the causal oomycete pathogen behind downy mildew, is inherited in a gene-for-gene fashion, meaning one major gene product in the plant host interacts with one major gene product in the pathogen. When
resistance is present, this leads to an incompatible interaction between plant and pathogen and results in complete immunity to infection. Resistance genes in these types of interactions most often encode a protein molecule that acts like a burglar
alarm. These molecular sensors in the host bind to proteins secreted specifically by the pathogen, and when that happens a storm of defense responses is activated in the plant that excludes further infection. This is not the only mode of genetic resistance
found in plants, but it is often the most drastic and effective against obligate parasites like downy mildew.
But this simple gene-for-gene interaction often puts incredible selection pressure on the pathogen populations to change such that they can get around the resistance. By losing the molecule that the plant detects in order to initiate a defense response,
the pathogen becomes unrecognizable to the resistance genes a plant variety may have. Just like spraying the same mode of action over and over again leads to a pest population developing resistance to a pesticide, the same selection applies to genetic
resistance. The longer a resistance gene is deployed in a region, the more likely the pathogen population is to change in response until that resistance gene is no longer effective at managing the disease.
One of the biggest challenges with lettuce downy mildew is that B. lactucae is constantly changing over time. It exists as many different races, where each race has a different reaction to the resistance genes bred into lettuce varieties. You
can think of these races as different versions of the same pathogen. A lettuce variety that resists one race may still be susceptible to another.
These races are identified by testing them against a panel of lettuce varieties with known resistance genes. In the western United States, races are named by the International Bremia Evaluation Board-U.S. (IBEB-US) and are given names with a number followed by the country’s abbreviation, such as 8US, 9US, or 10US. The populations found in the western U.S. are different from those found in Europe, so each region uses its own independent naming system.
The downy mildew population has changed considerably over time. Earlier races (1US through 4US) are now rarely found in commercial lettuce production. During the 2000s and 2010s, races 5US through 8US became the most common. Race 9US was recognized after being detected repeatedly between 2015 and 2017, and the newest officially recognized race, 10US, was designated in 2025. Below is a pie chart showing the relative frequency of the races identified from 114 Yuma County downy mildew samples between 2023-2024:
Figure 1: Pathotyping results of 114 lettuce samples from Yuma County collected between 2023 and 2024. Data source: https://bremia.ucdavis.edu/bremia_database_main.php
The results show that much of the downy mildew population found in Yuma County is made up of novel strains of Bremia lactucae that have not yet been officially classified as a race. An official race is only recognized after it has been shown to be stable and widespread over multiple locations and growing seasons. These newer strains may disappear over time, or they may eventually become established and earn an official race designation. In the meantime, this means growers and lettuce breeders in Yuma County are often dealing with strains that can dodge the resistance in some lettuce varieties before those strains are common enough to be officially recognized. It also highlights why relying on resistance alone is not enough to manage the disease.
Table 1: Pathotyping and fungicide sensitivity results of samples from Yuma County collected in 2025.
This trend appears to be continuing. All of the downy mildew samples sent for race testing last season were identified as novel strains rather than known, officially designated races.
It's impossible to predict exactly how these new strains will respond to the resistance genes found in today's commercial lettuce varieties. However, because they have not been previously characterized, they are more likely to overcome existing genetic
resistance than the races we already know about.
New strains develop naturally over time. They can arise when different strains exchange genetics (i.e. intermate) or through random mutations. When growers plant varieties with similar resistance packages over large areas, the pathogen population
is placed under strong selection pressure. Any strain that happens to acquire the ability to infect those resistant varieties gains a major advantage and gets to reproduce without competition where other strains cannot. Over just a few disease
cycles, those successful strains can become much more common in the population until they are the predominant strain overall.
An important point to remember is that the resistance bred into commercial lettuce varieties is not wearing out or becoming weaker over time. The genetics in the lettuce remain just as effective as when the variety was released. What changes is the
pathogen. As the downy mildew population evolves new strains emerge that can bypass resistance genes that previously worked very well.
That means that varieties carrying resistance to races 5US through 10US are still doing exactly what they were designed to do. They continue to suppress those known races and help prevent them from becoming widespread in commercial fields. So, if
you experience significant downy mildew in a field planted with a variety that has a strong resistance package, the culprit is most likely one of these newer, uncharacterized strains rather than a failure of the variety itself.
Unfortunately, Bremia lactucae can evolve much faster than scientists can identify new races and breeders can develop and release resistant varieties. That's why no resistance package should be viewed as a stand-alone solution.
This is also why extension, researchers, and the seed and crop protection industries place so much emphasis on the integrated pest management (IPM) concept. Genetic resistance is an essential tool, but it works best and remains the most sustainable when combined with other management practices. For novel strains that can slip past host resistance, timely fungicide applications and other disease management strategies become especially important for maintaining control.
Figure 2: Mean disease severity by treatment. Disease severity was determined by rating 10 plants within each of the five replicate plots per treatment using the following rating system: 0 = no downy mildew present; 1 = downy mildew present on bottom leaves of plant; 2 = downy mildew present on bottom leaves and lower wrapper leaves; 3 = downy mildew present on bottom leaves and all wrapper leaves; 4 = downy mildew present on bottom leaves, wrapper leaves, and cap leaf; 5 = downy mildew present on entire plant. Disease severity is displayed as the mean of five replicates across all three lettuce varieties and bars show a 95% confidence interval around the mean calculated from individual treatment data. Compact letter display (CLD) above boxes show significantly different treatments (Kruskal-Wallis ANOVA and Dunn’s test). Boxes sharing the same letter(s) are not significantly different from one another. Fb = “followed by” in the rotation. Not all products are registered yet for use in lettuce. The inclusion of specific fungicide products or formulations in these trials does not constitute an endorsement or recommendation over other labeled products.
The most effective way to manage lettuce downy mildew is to use an integrated approach. Plant varieties with a strong resistance package against races 5US through 10US, and pair that resistance with timely, full-label-rate fungicide applications when environmental conditions favor disease. This combination provides the broadest and most reliable protection against both known races and the novel strains that continue to emerge in Yuma County.
If you have any concerns regarding the health of your plants/crops please consider submitting samples to the Yuma Plant Health Clinic for diagnostic service or booking a field visit with me:
Christopher Detranaltes, Ph.D.
Cooperative Extension – Yuma County
Email: cdetranaltes@arizona.edu
Cell: 602-689-7328
6425 W 8th St Yuma, Arizona 85364 – Room 109Last week, we initiated an on-farm demonstration of soil steaming of the season with our self-propelled steam applicator. The machine is designed to inject steam into the soil and raise soil temperatures to levels sufficient to kill weed seed and soil borne pathogens (roughly 150°F for > 20 minutes). After the soil cools (< ½day), the crop is planted into the disinfested soil.
In this trial, we are examining the viability of soil steaming for controlling weeds in spinach at the field scale level (plot size > 1/3rd ac). The machine performed well in that it was able to reach target soil temperatures at reasonable travel speeds (> 0.25 mph) in cold soil (roughly 50°F), provide uniform temperature distribution across the bed and form nicely shaped beds suitable for subsequent planting. Stay tuned for reports of weed control efficacy, crop yield and overall profitability as compared to the grower standard.
If you are interested in seeing the machine operate at the field scale level on a commercial farm, we are conducting additional trials this week! Please contact me for further information.
Fig. 1. On-farm demonstration of a self-propelled steam applicator for weed and
diseasecontrol video. We are conducting additional trials this week. Contact me if
you are interested in seeing it.
Acknowledgements
This project is sponsored and funded in part by the Arizona Specialty Crop Block Grant Program and the Propane Education and Research Council (PERC). We greatly appreciate their support.
Why This Matters to Us
If you've been in agricultural research or extension work for any length of time, you know the drill. There's always another trial to set up, data to analyze, field work waiting, workshops to organize, and emails piling up. We push through because the work matters, and it absolutely does. But somewhere in that push, we often forget something equally important.
Lynn Sosnoskie, an Assistant Professor at Cornell University working in weed science, posted something on LinkedIn that really stayed with me: "My friend, your science is important. So is your health. Take care of yourself so you can take care of others."
It's straightforward advice, but it's worth sitting with for a moment. Because she's right
The Real Talk on Health
When it comes to wellness, there's something important to acknowledge upfront: not everyone can take care of their health in the same way. Some people don't have the same resources, financial or otherwise, to exercise like others do. Some people face real barriers to time, accessibility, and capability that make wellness look different from person to person. And that's okay.
The point isn't to shame anyone into exercise routines or perfect eating habits. The point is to recognize that when we invest in our own well-being, however that looks for us personally, we're building resilience. We're improving our mental and physical capacity to handle the demands we face. We're investing in our ability to do the research better, mentor our colleagues better, and serve our communities better.
When we do find ways to take care of ourselves, moving regularly, eating real food instead of grabbing whatever's convenient, our thinking is clearer. Our problem-solving improves. We're genuinely better at what we do. But that wellness journey is personal, and it looks different for everyone.
As we head into Thanksgiving week, that feels particularly relevant. Taking care of yourself during the holidays—and being thoughtful about it—isn't about restriction or missing out. It's about showing up for yourself in ways that let you enjoy the season without derailing the health habits you've built.
Simple Moves Make a Real Difference
Remember: wellness doesn't have to be complicated. Simple physical activity can go a long way. Something as basic as tapping your body—literally tapping different areas of your body—can increase your energy and reduce the stress that builds up from sitting at a desk for hours. The point is this: wellness doesn't have to be complicated. It doesn't have to cost money. It doesn't have to take up your whole day. Sometimes it's just about recognizing what your body needs in the moment and giving it that, whether that's movement, a break from the screen, or a few minutes to breathe.
An Unexpected Connection: Weeds and Wellness
Here's something interesting that bridges weed science with personal wellness, some of the weeds we deal with in the field actually have real nutritional value.
Take Purslane, for instance. Most of us have seen it popping up where it shouldn't be, especially with the recent rain events. But in fact, this common weed is packed with antioxidants, vitamins A and C, and actually contains plant-based omega-3 fatty acids. Research suggests it may help with blood pressure, diabetes management, and even bone health. It's not a miracle plant, but it's legitimate food.
In many cultures around the world, purslane, or Barbeen (bar-BEEN)/Barbeer (bar-BEER) as it's called in Arabic, is very common. Used for its tangy and slightly tart flavor, often used in Middle Eastern cuisine to add zing to rice dishes. It's not exotic or special; it's just food that's nourishing and readily available. What's fascinating is that traditional food knowledge aligns with what modern nutrition science validates: the omega-3 fatty acids, the heart-healthy properties, the minerals, it all checks out.
Back when Marco Peña was in this position, he created a video called "If you can't beat em, eat em!" that explores this exact idea. The title's catchy, but the concept is real understanding that the plants we work with can have dimensions beyond our immediate management concerns. Also, check the article from the UC Master Gardener Program of Alameda County. Sometimes it's educational, sometimes it's practical, and sometimes it just reminds us that nature's more complex than we give it credit for.
Resources We Can Count On
We're fortunate to have our Associate in Extension, Maria Doten. She works as a Family, Consumer, and Health Science (FCHS), basically helping people navigate the real-world stuff that matters, nutrition, family health, financial wellness, that kind of thing. She's not just spouting theory; she's focused on what actually improves people's quality of life in our communities. Check out her posts on the Yuma County Cooperative Extension Facebook webpage.
If you're looking for solid, research-based guidance on nutrition, stress management, or just practical strategies for balancing work with taking care of yourself, that's who you reach out to. She gets that extension work is about serving real people with real challenges.
The Bottom Line
The bottom line is this: you can't pour from an empty cup. We all know that phrase by now, probably because it's true. When we invest in our own health and wellbeing, in whatever way works for us, we're not being selfish. We're building the foundation to do everything else better.
Your science matters. Your health matters too. And so does self-worth. Taking care of yourself isn't about perfection or doing it exactly like someone else. It's about recognizing that you deserve that investment and doing what you can with what you have.
As we head into the holidays, give yourself permission to enjoy the season. Eat the food that brings you joy. Spend time with family. And then, when the holidays are over, get back to the practices that help you feel your best—moving your body, eating real food, getting rest, and being kind to yourself in the process.
Taking care of yourself isn't about perfection. It's about recognizing that you deserve that investment, and doing what you can with what you have.
Take the time to move your body in ways that feel good. Eat actual food when you can. Talk to someone if you're struggling. Get outside. Rest when you need it. And remember, sometimes the simplest things, a few taps on your shoulders, a walk around the building, a moment to breathe, are exactly what you need in that moment.
These aren't luxuries or distractions from the work. They're part of doing the work well and being the kind of person and scientist you want to be. Happy Thanksgiving!Bagrada bug (Bagrada hilaris) is an invasive stink bug recently established in the desert Southwest of the U.S. It has become a major economic pest of many cruciferous vegetable crops cultivated in fall and winter in the agricultural valleys of Arizona and southern California. Crop injury caused by Bagrada bug feeding has resulted in major economic losses. Depending on the crop species, Bagrada bugs may cause yield losses of 15-35%. Effective options for controlling Bagrada bug infestations in organic crops are limited, creating a significant challenge for organic growers. The objective of this study was to evaluate selective organic-approved insecticides to identify new tools that organic vegetable producers in the desert can use to manage Bagrada bugs effectively.
This fall growing season, we conducted a field trial to evaluate several organic-approved insecticides against the Bagrada bug in broccoli. The results of our trial show that M-Pede and a tank mix of M-Pede and Entrust can provide approximately a 60% reduction in Bagrada bugs relative to the untreated check. Captiva Prime and Neemix also resulted in a nearly 50% reduction in Bagrada bug numbers. Aza-Direct, Entrust alone, and Botanigard each resulted in about a 30% reduction in Bagrada bugs relative to the untreated check (Fig. 1). Our observations indicate that the insecticides evaluated are unlikely to provide quick knockdown. Tank mix of M-Pede and Entrust, Neemix, and Captiva Prime resulted in 39, 35, and 34% reduction in damage caused by Bagrada bug feeding. Entrust alone and M-Pede resulted in Bagrada bug feeding damage reductions approaching 25% (Fig. 2). Previous research trials conducted in Yuma also show that mixture of Entrust and M-Pede, Entrust alone, Aza-Direct, and Pyganic can fairly suppress the pest (Palumbo 2022).
Figure 1. Number of Bagrada bugs per 10 broccoli plants as affected by biological
insecticide applications, Fall 2025. All insecticide treatments included Nu-Film
P at 1 pt./ac.
Figure 2. Percentage of broccoli plants with damage caused by Bagrada bug
feeding, Fall 2025. All insecticide treatments included Nu-Film P at 1 pt./ac.
Additional Reading Material:
Palumbo, J.C. 2024. Bagrada Bug Management Tips - 2024. Veg IPM Newsletter, Vol. 15, No. 18 University of Arizona, Department of Entomology. http://hdl.handle.net/10150/676902
Keith, M., & Calvin, W. 2025. The Evolution of Bagrada Bug Management in Desert Cole Crops: The Legacy of John C. Palumbo (2010–2025). Veg IPM Newsletter, Vol. 16, No. 21. University of Arizona, Department of Entomology. http://hdl.handle.net/10150/678668
Between November 15 and November 25, 2025, the Yuma Valley station recorded 1.81 inches of rainfall. In many places, that amount would read like a simple win. In the Yuma winter lettuce season, it can land very differently, because timing is everything and water is usually something you control.
Most years, the region’s advantage is not just that it is dry. It is that it is predictable. When the skies stay clear, field crews can move in a steady sequence, and managers can keep crop development on pace. Irrigation is not only a way to keep plants alive. It is part of how growth is regulated. You add water, you accelerate; you hold back, you slow down. The schedule works because the system is designed for control.
Then the storm arrives, and control shifts.
The part of lettuce production most people never see.
From the road, a lettuce field looks uniform early on, but the planting strategy behind it is not as simple as putting down one seed for one head. In practice, stands are often established with more seed than the final harvest population. It is a practical hedge against uneven emergence. Wind, small differences in seed placement, and moisture variability can all create skips. If a field were planted exactly to the final stand, those skips would translate directly into lost yield.
So the field starts a little crowded on purpose. Later, that crowding is corrected by thinning, removing extra plants so the remaining lettuce can grow into consistent, marketable heads.
The important point is that thinning is not a task you can do whenever you get around to it. It lives inside a narrow window. Plants need to be big enough to identify and strong enough to tolerate disturbance, but not so far along that removal becomes damaging or ineffective. The window is short, and in a normal season, it is manageable because the field pace is steady.
In Yuma Valley, soil moisture is more than a field condition. It is the clock that sets the tempo of operations. With irrigation, that clock is adjustable. With rainfall, it is not.
During the November 15 to 25 period, the 1.81 inches of precipitation did two things at once. It provided a flush of water that pushed plants forward quickly, and it narrowed the timing for field access. When the crop advanced faster than expected, the thinning window tightened. When the ground conditions shifted, the ability to run mechanized operations on schedule tightened too.
That combination is what makes rain at the wrong time so disruptive. It is not only that the crop grows faster. It is that the step needed to manage that growth becomes harder to execute.
In a smooth season, mechanized thinning is one of the pieces that keeps the whole system efficient. When that timing is missed, the work does not go away. It simply changes form.
Manual thinning with crews becomes the fallback. It is slower, more variable, and harder to schedule at scale. The cost rises immediately, but the hidden cost is time. The longer the thinning takes, the more the crop continues to move forward, and the more likely variability shows up later as uneven head size.
As soils dry, the surface can set into a firm crust. Under those conditions, any corrective work that disturbs the surface can become more aggressive than intended. Instead of a clean adjustment, thinning can carry a higher risk of nicking or bruising plants that are supposed to remain. At that point, the field is not only managing spacing. It is managing injury risk, variability risk, and timing risk at the same time.
Rain in the mountains and river basins supports water supply stability. That is water that can be stored and delivered when needed. Rain on the active lettuce ground is different. It arrives without a schedule, and it arrives during operations that depend on narrow windows. It can be beneficial in the long run and disruptive in the moment, all in the same event.
What does this event suggest for next season
The lesson from this November rain is not that storms are bad. The lesson is that systems built on precision need contingency plans that activate early.
One practical approach is to treat the days leading into thinning as a high-sensitivity period. When the forecast shows meaningful rainfall in the next few days and fields are close to the thinning stage, the decision point should come sooner rather than later: thin early if it is safe, or prepare the alternate plan immediately so the first workable day after the storm is productive.
The second takeaway is that after rain, monitoring needs to tighten. Not because growers do not already scout, but because crop speed changes quickly when moisture status changes. A few days of accelerated growth can close an operational window faster than expected.
Takeaway
The November 15 to 25, 2025 rainfall in Yuma Valley, 1.81 inches, was not just a weather note. It was a reminder that desert lettuce is as much about execution as agronomy. When water arrives outside the plan, it can accelerate the crop and compress the operations that keep it uniform. In Yuma Valley, sunshine keeps the system predictable. Rain can still be welcome, but when it arrives at the wrong time, it rewrites the week.
VegIPM Update Vol. 17, Num. 15
July 22, 2026
Results of trap catches below!!
Whitefly: Adult activity remains steady across locations; above average for this time of the year, especially high numbers seen in North Gila Valley. Historically, whitefly numbers peak in July.
Thrips: Adult thrips activity remained low over the last two weeks. About average for this time of the year. Historically, thrips numbers remain low until Sept-Oct.
