Aug 9, 2023Protecting Crops at Stand EstablishmentTo contact John Palumbo go to: jpalumbo@ag.Arizona.edu
Arizona consistently produces the highest yields and quality of crops found anywhere. This is due to the skill and expertise of Arizona farmers and the management of the valuable natural resources that we depend on, which includes the soil, water, and climate we have in this region. Soil serves as the foundation of the agricultural systems that we all depend on. In agriculture, we recognize that healthy soil translates to healthy crops and farms (Brady and Weil, 2008 and Parikh and James, 2012).
There are three primary aspects of soil systems that are important in healthy soil function that include physical, chemical, and biological factors (Figure 1). These three aspects are of soil systems operate in an integrated manner, but we can consider them individually, beginning with the soil physical properties.
Figure 1. Soil health factors and physical properties.
Soil texture determines the coarseness or fineness of a soil (Figure 2). Soil texture is defined by the relative proportions of sand, silt, and clay particles (Figures 3 and 4). The soil particles are defined by diameter size with sand (0.05–2.0 millimeters (mm)), silt (0.002–0.05 mm), and clay (less than 0.002 millimeters or the 2-micron (µ) fraction (10-6 m)).
Soil texture is a product of the geologic process of basic parent materials weathering, both physically and chemically, over long periods of time in each location making soil texture a prominent characteristic of any field or location. Soil texture cannot be changed in a practical manner, but we can utilize appropriate management practices suited for soils of varying textures.
Particles that are larger than 2 mm, such as rock fragments (pebbles, cobbles, stones, and boulders), are not considered in defining the soil textural class because they are relatively inert or non-reactive.
Soil particles provide the basic building blocks of the soil system and structure. The pore spaces between the particles and soil aggregates are extremely important since that is where most physical, chemical, and biological processes take place.
Figure 2. Soil textural triangle.
Figure 3. Relative size of soil particles.
Figure 4. Proportional relationship of soil particles.
Soil particles provide the basic building blocks of the soil physical structure, but the way soil particles are assembled in clumps or aggregates. Aggregation is basically the arrangement of primary soil particles (sand, silt, clay) which are bound by soil organic matter and through other forms of particle interactions and associations. Aggregate stability is a good indicator of soil health.
As a result of the aggregation of soil particles, the sizes of soil pores can vary tremendously. The proportion of large, medium, small, and very small pore spaces in soils govern the important processes of water and air movement. In addition, soil pores provide the spaces where soil organisms and plant roots live, grow, and function.
Clay and finer textured soils (generally, soil textures in the upper ½ of the textural triangle) have smaller soil particles and aggregate structure that contribute to small pore spaces (often less than 0.002 mm or 2 µ). In contrast, sandy soils are dominated by pores that are larger (macropores). As a result, finer textured soils with high silt and clay content will have much higher surface areas within in given soil volume (Figure 5) and are more reactive physically and chemically.
Figure 5. Soil particle size and surface area relationships.
We can see and feel soil aggregates when we pick up a handful of soil and see the clumps or pieces of soil, which are the soil aggregates. Soil aggregates consist of soil particles of varying sizes that are held together by both the attraction of soil particles and the binding capacity of organic matter between soil particles. There are several general types of common soil aggregates (Figure 6).
Figure 6. Examples of soil aggregates.
Aggregates provide a very important function of soil structure. Aggregates also serve to hold and supply organic matter in soil; however, they also have structural functions. Soil aggregate structures provide large and small pores with large soil pores allowing faster water infiltration into the soil. Smaller soil pores provide the capacity of soil to hold more plant-available water. A healthy soil will have a good soil structure, often recognized in the field as good soil “tilth”.
Poor soil structure, due to a breakdown of soil aggregation and the dispersion of individual soil particles can be a problem for water infiltration into a soil. We can often see this in the field with soil crusting surface compaction (Figure 7a and 7b), which can also be symptom of high sodium (Na) concentrations in the soil.
Consequently, soil structure is critical to our efforts to manage soil salinity since we need good water infiltration and percolation to accomplish adequate soil leaching to remove soluble salts from the crop root zone (Figure 8).
Figure 7a. Importance of good soil structure.
Figure 7b. Example of unfavorable soil structure, such as a dispersed or
compacted soil structure.
Figure 8. Good soil structure and the relationship to leaching
capacity in a soil.
Soil health is affected by several factors including the physical, chemical, and biological properties of the soil system (Figure 1). Of the physical soil properties, aggregation and structure are two of the most important factors affecting soil health. Soil texture, aggregation, and structure are all inter-related and provide a strong influence on the environment that plants deal with in the field. These soil characteristics impact root development as well as water and air movement in soil. Managing soils to enhance these physical properties of aggregation and structure can be a real benefit for soil health and crop production systems.
Brady, N.C. and R.R. Weil. 2008. The Nature and Properties of Soils, 14th ed. Prentice Hall: Upper Saddle River, NJ. Parikh, S. J. and B.R. James. 2012. Soil: The Foundation of Agriculture. Nature Education Knowledge 3(10):2
As we hear towards harvesting season for melons, we might be seeing some bacterial fruit blotch inf the fruits. Though I hope we do not see them, I wanted you to be prepared for your own diagnosis in case you do. Bacterial fruit blotch (BFB) of melon is caused by the bacterium Acidovorax avenae subsp. Citrulli. The bacteria produces large olive green to brown water-soaked lesions on fruit, making them unmarketable.
Symptoms of BFB on seedlings begin with water-soaked areas on the lower surface of the cotyledons and inconspicuous lesions on leaves. BFB lesions will become necrotic often with yellow halos. Lesions are frequently delimited by veins. Infected seedlings collapse and die.
Greenhouse conditions are usually favorable for dispersal and establishment of pathogen. Thus, good greenhouse practices and sanitation is extremely important. Clean transplant trays must be used (disinfect trays if they will be reused) and new soil. Destroy any volunteer seedlings and keep the area in and around the greenhouse weed free. Avoid overhead watering if at all possible, or water in the middle of the day so that the plants dry thoroughly before evening. The bacterium can spread on mist and aerosols. Relative humidity should be kept low through proper watering and good air circulation in the greenhouse. Separate different seedlots, to reduce lot-to-lot spread. Monitor these isolated seedlings daily and destroy trays where symptoms develop. The remaining trays should be sprayed with a labeled bactericide and the applications continued until the plants are transplanted to the field.
The pathogen can be seedborne, so growers should only use seed that has been tested for the presence of the pathogen by a reputable testing facility. Management of BFB includes a combination of preventing the introduction of the pathogen, sanitation to eliminate any inoculum present, and the use of bactericides if the disease appears. There are no commercially available watermelon cultivars that are resistant to bacterial fruit blotch, but there is some variation in susceptibility among cultivars.
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 September 18-21 in Salinas, 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 12 machines designed for weeding vegetable crops. Some of the latest technologies for in-row weeding will be featured including lasers (3 companies), high voltage electricity and high precision spot spraying (3 companies). If you are interested in ag tech, FIRA 2023 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 2023. Event will be held September 19-21st in
Salinas, CA. (Photo credits: FIRA USA).
Postemergence herbicides used for grass control in vegetables include Poast (sethoxydim), Select (clethodim) and Fusilade (fluazifop) and the generics of these active ingredients. When some grasses have escape or have survived the application of Pefar, Kerb and Balan these selective grass herbicides can come to the rescue and have shown to be a great tool for weed management.
The above-mentioned herbicides are all Acetyl CoA carboxylase or ACCase inhibitors. These products work slowly and even slower at lower temperatures and shorter days. Treated grasses should stop growing immediately and will slowly turn yellow, red and gradually die as the crop grows. The process takes about 15-21days. Select (clethodim) works the faster than Poast (sethoxydim), and it isrecommended to add a crop oil concentrate for better activity with the exception of Select Max which can be used either with crop oil concentrate or non-ionic surfactant.
Results of pheromone and sticky trap catches can be viewed here.
CEW moth light in the past 2 weeks, below average for the beginning of the produce season.
Trap counts increasing in all locations, about average for early September.
Cabbage looper numbers remained low consistent with early September.
DBM moths not active in any trap since June; expected this time of the year.
Adult movement increased in the past week, above average for early September.
Thrips adults light in most locations, and trending above average to previous years.
Aphid movement is absent and consistent with what we typically see during the summer and September.
Adult activity tending downward in most locations, below average for early September.