In a recent edition of this University of Arizona Vegetable Integrated Pest Management (UA VIPM) Newsletter, Vol. 14, No. 18, I provided a brief review of some basics related to soil physical properties and soil health.  Physical properties have strong impacts on some basic functions of healthy soil.  In irrigated agriculture one important physical feature is that of water holding capacity and the plant-available water (PAW) a soil can hold (Baver et al., 1972).
 
Soil serves as the storehouse or reservoir of water available for plant uptake.  As noted in the article regarding soil physical properties, soils vary significantly in terms of texture (relative proportions of sand, silt, and clay) and water holding capacity.  The relationship between total water in a soil and the portion of that water that is “available” to plants is shown in Figure 1 and Table 1.
 
In review of Figure 1, it is important to note several points: 1) The total amount of water that can held by a soil increases with finer textured soils.  2) The PAW held in a soil generally increases with finer textured soils but only to a certain extent with a salient bulge of maximum proportions of PAW in medium textured soils, such as loams and silt loams.  3) Soils with higher clay content, e.g. clay loams and clays, have high total water holding capacity but lesser amounts of PAW (Kirkham, 2023).  
 
Water that is held very tightly by the soil particles is referred to as “hydroscopic” water and the finer soil particles, such as clay, exert the strongest hydroscopic forces, and holding that soil-water unavailable to most plants.
 
The lower boundary of PAW shown in Figure 1 is a general line of demarcation at -1500 kilopascals (kPa) of potential energy and is referred to as the permanent wilting point (PWP).  The PWP was originally considered as a more universal limit among plant species (Briggs and Shantz, 1912).  However, the PWP as defined in Figure 1 is very general since all plant species vary considerably in their capacities to extract water from the soil.  Many native desert plant species for example can extract some hydroscopic and most crop species will suffer water stress long before soil-water content approaches the PWP line shown in Figure 1 (van Lier et al. 2023).  

Figure 1.  Soil-water holding capacity as a function of soil texture.  Source: The
COMET program.  University Corporation of atmospheric research.



Figure 2.  Soil-water relations and energy levels with soil-water content,
kilopascals (kPa) of potential energy.

Figure 2 describes a close-up view of two adjacent soil particles and the pore space between them.  Soil-water is held as a film around the soil particles leaving some air space in the pores after the gravitational water has drained away, leaving the soil at field capacity (FC).  As the soil dries the water film on the soil particles become thinner and held more strongly by the physical forces associated with the soil particles.
 
Following irrigation with the gravitational water drained away (dependent upon good internal soil drainage), which commonly takes about 24 hours, a soil will be at FC.  A crop will extract water most easily at FC and can continue to do so until the soil dries to a point where the crop plant can no longer pull water from the soil against the forces of the soil exerting holding that water.  
 
That critical point below which plants begin to encounter water stress is unique among crop plants but for most leafy green vegetable crops this limit is commonly about the 65-70% PAW level.  Other common desert crops like alfalfa, cotton, wheat, and melons can maintain adequate water levels before stressing and starting to show signs of wilting at approximately 55% PAW.
 
This means the actual range of soil-water that is available to plants is in the range from FC to the specific point of crop stress level, that is unique to each crop species, and the corresponding wilting point (WP).  Thus, PAW = FC – WP.
 
One of our challenges in managing crop production systems is identifying the WP for the crop in question and FC for the soil in that field.  In addition, with irrigation management we strive to maintain crops in a non-stressed condition and manage the fields in that range of PAW between the FC of the soil and the WP for the crop.  
 
This is a good example of a healthy soil that does not limit water availability due to physical or chemical restrictions and benefits the growth of a healthy crop.  
 
The Web Soil Survey in the following link can be used to assess soil textures dominant in a given field.  https://websoilsurvey.nrcs.usda.gov/app/

 

 
Table 1.  Available Water Capacity (AWC), range, and typical levels for various soil textures. Source: Chapter 2 “Soils”, Irrigation Guide, Natural Resources Conservation Service- National Engineering Handbook.
 

 
References 
Baver, L.D., Gardner, W.H., and Gardner, W.R. 1972. Soil Physics. 4th ed. Wiley: New York.
 
Briggs, L.J., and Shantz, H.L. 1912. The wilting coefficient for different plants and its indi-rect determination. USDA Bur Plant Industry Bull No. 230. U.S. Dept. Agr: Washington, DC.
 
Kirkham, M.B. 2023.  Field capacity, wilting point, available water, and the non-limiting water Range. Ch. 8. In: Principles of Soil-Plant-Water Relations. 2023 Elsevier Ltd.
 
“Soils”, Irrigation Guide, Natural Resources Conservation Service- National Engineering Handbook.https://directives.sc.egov.usda.gov/OpenNonWebContent.aspx?content=17837.wba
 
van Lier, Q. J., S.D. Logsdon, E. A. Rodrigues, and P.I. Gubianid,  Plant-Available Water.  Ch. 12.  In: Principles of Soil-Plant-Water Relations. 2023 Elsevier Ltd.

Frost and freeze damage affect countless fruit and vegetable growers leading to yield losses and occasionally the loss of the entire crop. Frost damage occurs when the temperature briefly dips below freezing (32°F).With a frost, the water within plant tissue may or may not actually freeze, depending on other conditions. A frost becomes a freeze event when ice forms within and between the cell walls of plant tissue. When this occurs, water expands and can burst cell walls. Symptoms of frost damage on vegetables include brown or blackening of plant tissues, dropping of leaves and flowers, translucent limp leaves, and cracking of the fruit. Symptoms are usually vegetable specific and vary depending on the hardiness of the crop and lowest temperature reached. A lot of times frost injury is followed by secondary infection by bacteria or opportunist fungi confusing with plant disease.

Most susceptible to frost and freezing injury: Asparagus, snap beans, Cucumbers, eggplant, lemons, lettuce, limes, okra, peppers, sweet potato

Moderately susceptible to frost and freezing injury: Broccoli, Carrots, Cauliflower, Celery, Grapefruit, Grapes, Oranges, Parsley, Radish, Spinach, Squash

Least susceptible to frost and freezing injury: Brussels sprouts, Cabbage, Dates, Kale, Kohlrabi, Parsnips, Turnips, Beets

More information:

https://www.extension.iastate.edu

https://www.farmprogress.com

Over the last couple of years, we developed a prototype 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 > 20 minutes). The device is principally comprised of a 63 BHP steam generator mounted on an elongated bed shaper (Fig. 1). The apparatus applies steam via shank injection and from rectangular ports on top of the bed shaper. After cooling (< ½ a day), the crop is planted into the disinfested soil.

Trial results have been very promising and reported in previous UA Veg IPM articles. In brief, the multi-year studies have shown that soil steaming provides excellent weed control (>90%), suppresses problematic soilborne diseases (Fusarium wilt of lettuce> 50%, lettuce drop > 70%) and increases crop yields (>24%).

This season, we would like to demonstrate the technique to interested growers. In addition to obtaining grower feedback on the viability of soil steaming, a second objective would be to validate our small plot research results at the field scale level. The machine can be adjusted to work with most bed configurations including 40”, 42”, 80” and 84” beds, and work with any crop, including organic crops (soil steaming is organically compliant). So far, the device has been successfully tested in iceberg lettuce, romaine, baby leaf spinach and carrot crops.
If you are interested in an on-farm demo of soil steaming, please let me know. I’d be happy to work with you.

Fig. 1. a) Band-steam applicator principally comprising a 63 BHP steam generator
mounted on a bed-shaper applicator sled. Steam applicator sled b) top view and
c) bottom view. Click here or on the image above to see the device in action.

With every irrigation we continue having new generations of weeds germinating. Herbicides with a good soil residual are very useful for extending good control for a longer period. The ideal scenario would be that the herbicide is present only when we need it and dissipate when we rotate to a sensitive crop. This can become a challenge when we have products that have a long soil residual. Sometimes herbicides have a low use rates and long residuals. As an example, we have some of the sulfonylureas and imidazolinones that are highly effective but have to be carefully used for some scenarios. Some of the factors affecting the persistence of herbicides are the soil characteristics, herbicidal characteristics, and environmental conditions¹.

The University of Nevada, Reno explained that problems can occur when soil where sterilant products has been applied is moved or blows from the application site to other part of a field². Interestingly recently the IPM Team received a report of a field of radishes with injury in a pattern that was consistent with the passes of the disk. It appears that accidentally a high rate of DCPA and metholachlor was applied previously. Even after land prep and irrigations the crop presented phytotoxicity symptoms. Also, a celery field located South of a treated, dusty non-agricultural area presented injury consistent with the wind blow. The damage was more severe closer to the treated area (North) and gradually disappeared as walking South into the celery field. Similarly, it has been reported that compost can be contaminated with herbicides such as clopyralid when treated plants are added to the compost mix causing symptoms to lettuce.
 
 

Reference:
1. Retrieved from: https://ag.arizona.edu/crops/vegetables/advisories/more/weed31.html
2. University of Nevada Reno:   https://extension.unr.edu/publication.aspx?PubID=3322

Results of pheromone and sticky trap catches can be viewed here.

Corn earworm: CEW moth counts remain at low levels in all areas, well below average for this time of year.

Beet armyworm: Trap increased areawide; above average compared to previous years.

Cabbage looper:  Cabbage looper counts decreased in all areas; below average for this time of season.

Diamondback moth: DBM moth counts decreased in most areas. About average for this time of the year.

Whitefly: Adult movement beginning at low levels, average for early spring.

Thrips: Thrips adult counts reached their peak for the season. Above average compared with previous years.

Aphids:  Aphid movement decreased in all areas; below average for late-March.

Leafminers: Adults remain low in most locations, below average for March.