In Arizona agriculture, we have the benefit of generally working with good soils that exist in alluvial valleys or the terraces immediately adjacent to the alluvial valleys, e.g. the mesa areas. Arizona soils are geologically young and fertile but often have high levels of salinity and often sodicity. When reclaimed and properly managed with adequate leaching, we can reduce the salinity to manageable levels to support crop production systems. In the case of sodic conditions, appropriate amendments are needed then followed by adequate leaching.
In the process of applying an irrigation in the field, it is important to recognize that not all soils are created equal. Soil types vary across the landscape and they also vary by depth for any site or location. This is particularly true for alluvial soils which originate from water deposition over time, such as from the Gila and/or Colorado River systems. The soils of the lower Colorado River valleys are great examples of alluvial soils and the high degree of variability we commonly experience in the field. With some crops, particularly more deeply rooted crops, we can sometimes nearly map the soil types across a field based on crop growth patterns. Accordingly, this type of soil variability creates some challenges for in-field management, including irrigation management.
Many of the rotation crops common to the lower Colorado River Valleys, such as cotton, wheat, and sudan; are excellent examples of crops that can express growth patterns as a function of soil texture, which is clearly demonstrated in response to water stress. The courser textured parts of the field will stress earlier and consistently have reduced plant vigor. Anyone driving a tractor for medium to heavy tillage operations in the field will literally feel soil textural changes and anyone harvesting those fields will see it as well. The GPS field mapping systems can detect and record these areas of soil type differences through yield monitors as well in response to crop growth and vigor.
Soil textures vary in terms of water holding capacities and it is important to understand the dominant soil textures in the field, not only on the surface but also through the depths of the soil profile and the effective rooting depth of the crop, Tables 1 & 2 and Figure 1. To manage a complete field or set of fields, it is necessary to determine a functional “average” of soil texture and water holding capacity.
In the process of irrigation, we are attempting to replenish the soil-water extracted by the crop through evapotranspiration (ETc). In previous articles, the determination or estimation of crop ETc has been discussed.
Therefore, with irrigation management it is important to know the fields we are working with in terms of the dominant soil textures present, the degree of variability that exists, and the general water-holding capacity of the soils. Matching irrigation timing and volumes for each event to replenish the plant-available water for each field is important in our efforts to avoid water stress and achieve and maintain irrigation efficiency agronomically, which is providing the amount of water necessary to replenish the soil-water to field capacity with some degree of additional water needed for the leaching of soluble salts.
With the high degree of variability that is common among soils in the lower Colorado River Valleys, it is both important and challenging to know the soil characteristics common in each field, the water holding capacity of the dominant soils, and the level of soil-water depletion that is being replenished with each irrigation event.
Table 1. Soil texture and water holding capacity.
Table 2. Depths to which the roots of mature crops will deplete the available water supply when grown in a deep permeable, well-drained soil under average conditions. Source: Chapter 11, "Sprinkler Irrigation," Section 15, Natural Resources Conservation Service National Engineering Handbook
Figure 1. Soil volume, soil texture, and water holding capacity relationships.
Engineering Handbook.
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.
The 2nd AgTech Field Demo: Automated Weeding Technologies will be taking place TOMORROW, October 21st at the Yuma Agricultural Center. Eight of the latest weed control technologies ranging from autonomous in-row weeding machines, to “smart” cultivators to smart spot sprayers to band-steam applicators will be demonstrated in the field. Registration begins at 7:00 am and the program starts at 7:30 am. CEU credits – 1.0 CA, 1.5 AZ and 2.5 CCA are available. Looking forward to seeing everyone there!
Click here forAgenda or see below.
Clovers can be very difficult to control weeds here, but it is also a major crop and common ornamental. Clovers can survive under poor growing conditions and are not controlled with glyphosate and seem to get worse every year. There are more than 50 types and 300 species of clover and they can be easily misidentified. They are all in the legume (Fabracea) family and can use a bacterium (rhizobium) in the soil to convert nitrogen in the atmosphere to a form that they and other plants can use for fertilizer. There are only 4 or 5 clover species that are agricultural pests here. The ones we get the most questions on are white and yellow sweet clover. These are in the Melilotus family. White sweet clover (Melilotus albus) is tall for a clover and can get 3 to 5 foot in height. The leaves are thinner than most clovers and this difficult to control weed lives at least 2 years and sometimes longer. Glyphosate and most of the contact herbicides do not control it. The plant growth regulator herbicides work best. Yellow sweet clover (Melilotus officinalis) is less common here. The flowers are yellow, and it is not as tall and vegetative as white sweet clover. Yellow is more common at higher elevations. California burclover (Medicago polymorpha) and Black medic (Medicago lupina) are in the same genus as alfalfa and are more of a problem in landscapes, parks and golf courses than in agricultural fields here. They do not grow upright and spread below the crop or turf. The true clovers are in the Trifolium genus and include white and strawberry clover. These creep along the ground and root at the nodes of the stem. These are more of a urban landscape weed and not considered an agricultural problem. Creeping woodsorrel or Oxyalis looks like a clover but it is not related. It is a turf weed that spreads rapidly along the ground and can live for several years. Preemergent herbicides are effective against all these clovers before they become established. The postemergence herbicides that are most effective in controlling these clovers are the plant growth regulators. Contact herbicides and glyphosate are generally ineffective.
