Spotlight on SWEEP in Citrus

Shulamit Shroder, UCCE climate smart agriculture specialist - Kern County

In 2014, Bruce Kelsey in Kern County received a grant through the California Department of Food and Agriculture's State Water Efficiency and Enhancement Program (SWEEP). He used the funds to set up 8-foot-wide plastic weed mats underneath his mature organic citrus trees. He also decreased his electrical consumption by about 30% and installed soil moisture sensors, a water flow meter, and a pressure-sustaining device.

Benefits

Labor: The installation of the weed mat was a labor-intensive process, but it ended up paying off in the long term. It diminished weed populations so that he no longer has to weed under his citrus trees. Now he only mows with a small mower in the lanes between his trees.

Water usage: His overall water usage decreased by about 10%. The weed mat decreased evaporation and weed pressure while the other devices allowed him to better manage and schedule his irrigation. 

Drawbacks

Pests: Bruce experienced an increase in earwigs in the weed mat orchard. The plastic covering provided the perfect humid environment for the insects.

Organic certification: The weed mats will eventually start to disintegrate, which could contaminate his soil. To maintain his organic certification, he will have to rip them up once they start to break down. Smaller, younger trees do not protect the plastic from the sun, which quickly destroys the plastic. For this reason, he recommended against using weed mat in immature orchards.

Figure 1. Weed mat in place.

How to irrigate is probably the most common question in irrigated agriculture, even with 10,000 years of cultivation knowledge to guide us. The complexities of irrigation and the unique situation for each grower makes this question so difficult. Not enough water, and plants have diminished growth or the propensity for disease and disorder ¹. Too much water leads to root disease and nutrient problems ². So, it can't be too much or too little, but just right. There are times when citrus can handle a little more water stress than other times, which can lead to water savings ³, especially in a drought year or in areas where water costs are crucial. Salinity further compounds the question of irrigation where striking a balance determines the health of your tree. Staying in tune with your orchard and using appropriate methods to measure water need, water use, environmental water demand, and soil water-holding capacity will help inform irrigation management decisions.

There are all kinds of ways of estimating tree water need ⁴ , a valuable piece of information for irrigation decision making. An inexpensive and often overlooked method of estimating tree water requirements is grower observation in the orchard to assess leaf color, leaf size, the look of the leaves, and canopy fullness. Pure observation and knowledge of your trees yields a lot of valuable information regarding irrigation management. Beyond observation, a direct measure of the tree with a porometer, pressure gauge (bomb), sap flow meter, dendrometer or other device gives an absolute or relative number of tree performance. Technological advances, such as telemetry and imaging with drones or satellites, holds promise, but are still being perfected for general irrigation use. In general, technological devices yield informative data, but tend to be expensive, delicate, and require manual monitoring to account for tree-to-tree variation in the orchard.

Soil moisture sensors can be an effective method of evaluating water use by the tree. The most basic way to measure soil moisture is with a human powered shovel or soil tube ⁵. While it requires an operator who knows what they are doing, the technique is easily learned and repeatable. A human and shovel can move around an orchard checking out different suspicious spots that are not easily done with fixed-in-place sensors. Installation of soil moisture sensors systems range in cost and capabilities, yet provide specific data on water use. Integrating certain systems into communication relay systems allow for the monitoring of multiple sites at once. Some sensors can measure soil salinity, as well as soil moisture, to give a sense of whether the water in the soil will be useable by the tree. If soil moisture sensors are used, correct placement of where roots are taking up water is imperative to get an accurate assessment of water uptake. Overall, it is critical to keep the entire orchard in mind and understand that fixed sensors only take a specific location's reading.

Another great technique to inform irrigation scheduling is an estimate of the demand that drives water use. An evapotranspiration estimate either by CIMIS, a private weather station with ET-calculation or atmometer gives not only an amount to apply but also when to apply that amount based on the water holding capacity of the soil and the rooting depth of the crop. Soil moisture holding volume can be complicated, but can be estimated from the NRCS table in the previous paragraph⁵ or from tables in the Web Soil Survey ⁷.  

Simply running an irrigation system for a specific amount of time and probing for depth of water penetration and extent of wetted area is the best way to get an estimate of soil moisture holding capacity. This knowledge is needed in order to decide whether the active rooting volume is getting wetted sufficiently or too much is being applied. Emitters are rated by gallons per hour, but that 1 gph, 5 gph, 20 gph emitter output might differ according to water pressure that can vary over an irrigation period. On the flip side, monitoring soil moisture depletion over time can give an approximation of how depletion compares to ET estimates. Soil moisture depletion can be measured by soil moisture sensors or by shovel and feel. This estimate of applied water compared to output and ET only needs to be done once at a given growth stage of the orchard. If the orchards is young, it will need to be done each year as the trees fill out. An estimate of canopy growth can also be used to better approximate young orchard ET.

All of these methods suppose that a grower has the capability to irrigate when, where and for how long they need to. If water delivery is on a fixed schedule and the amount of water can be controlled it is valuable to understand specific water needs. Knowing the rated applied amount of an emitter is important, but that amount shouldn't be assumed, especially considering natural wear and tear, damage from harvest, poor filtration, clogging, or damage by wildlife. Maintenance to insure good distribution uniformity is critical to the operation and the correct application of water to trees and for the maintenance of tree health. Low-pressure systems are wonderful but they should be evaluated on a yearly basis and tuned up in preparation for every irrigation season. Many growing areas have mobile irrigation labs that will evaluate system performance and make recommendations for improvement.

All said, knowing the orchard and evaluating tree health will inform irrigation management decisions. Applying technology where technology is appropriate will help. Using it to help advise irrigation decisions is valuable, but new tools will not always be the answer.

It's important to know what is being applied.

Trust but verify.

Drought Tips & Video: https://www.youtube.com/watch?v=LKSQRuHAnYA ; https://anrcatalog.ucanr.edu/pdf/8549.pdf

Irrigated agriculture must always contend with salts. Five years of drought and its effects can magically disappear, but it will be back again. Low rainfall is the norm for California. We rely on winter rainfall to leach the salts from root zones that have accumulated salts from previous irrigations.  Salinity affects plant growth and understanding what it is and how it is measured and evaluated need to be understood.  Just having wet soil that is full of salts is not going to help a plant, it's going to add stress and eventually physiological and disease problems - https://ucanr.edu/blogs/Topics/index.cfm?start=28&tagname=disease

All waters, even rain water, have some salts dissolved in them, so all waters could be called saline.  The term saline is restricted to waters with concentrations that could cause harm to plants or people.  Seawater is highly saline, many wells are moderately saline.  But unlike humans that excrete salts, plants are often affected by salt levels that have very little health impact on humans. Well waters used for irrigation can often exceed standards for plants that are fit for human consumption.  However, with proper management many waters can be used on plants, depending on the plant species.  Domestic water supplies from cities typically have better quality than some well waters because they are monitored and often blended to meet human consumption.  Most domestic water supplies have low concentrations of salts and are not considered to be saline.  However, using even domestic water in growing subtropicals does not mean that we should not be concerned about salinity.

Before going any further it is worth remembering that salt is not just the sodium chloride that's on the table.  Salts are combinations of electrically charged ions.  These ions separate from one another when a salt dissolves in water.  Water with dissolved sodium chloride and potassium nitrate contains sodium, potassium, chloride and nitrate ions.  The most common ions in natural waters are:

            sodium (Na+)              chloride(Cl-)    sulfate (SO42-)

            calcium (Ca+)             boron (H3BO3)

            magnesium (Mg+)      bicarbonate (HCO3-)

Different waters can have very different proportions of these ions and these proportions can change with time.  Some typical analyses of City of San Buenaventura water can be seen in the following chart (2015 Annual Report of the City of San Buenaventura).

Ionic composition of some wells in Ventura

Total dissolved solids (TDS) and electrical conductivity (EC) are two different ways of measuring the total amount of salts in water.  The old way of taking a specified volume (l for liter) of water and boiling it down to the residue which is weighed (mg for milligram) gives TDS.  The more modern technique is to measure the electrical current a water will carry (umhos/cm or micromhos/cm), which is in proportion to the number of ions in the water.

Natural waters also contain low concentrations of many other elements.  For most, the amounts are too low to be either harmful or beneficial to plants.  The main exception is boron which can be a problem for sensitive plants, such as citrus and avocado and probably for cherimoya as well, when in excess of 1 mg/l.  Many well waters in Santa Barbara and Ventura Counties contain potentially harmful levels of boron for plants.  This is not as common a problem in San Diego County.

In addition to the ions mentioned, there are also those that come from fertilizers and the soil.  The main extra ions are potassium, ammonium, nitrate and phosphate.  The concentrations of these will depend on the type of soil and the amounts and kinds of fertilizers applied, minus the amounts taken out by plants, held by the soil and lost by leaching or erosion.

In evaluating a water for its potential to harm plants, it is necessary to look at total salinity, as well as the specific ions.  Waters with a TDS in excess of 1000 mg/l or an EC greater than 1500 umhos/cm might pose problems for sensitive subtropical plants, and none at all to tolerant plants like figs, apricots or pomegranates.  Waters with an excess of sodium and/or chloride (more than 100 mg/l) can induce symptoms that are similar to high levels of salinity.

In most cases, plants respond by initially having their leaf margins turn yellow and die.  This happens first on older leaves because they have had the longest time to accumulate the ions.  Annual plants are often less affected than perennials, since they do not grow long enough to accumulate sufficient ions to cause damage.

As trees remove water from the soil, the concentration of salts in the remaining soil water increases.  Plants adapt to moderate increases, but if the plant is sensitive (and most subtropicals are), it will slow growth in response.  If the salt increase is small, the growth reduction will be small and acceptable.  But if the level of fertilizer use is high, the water quality poor, or the soil has not been properly leached, the increased soil salinity could reduce growth seriously.

The effects of salinity are usually gradual on plants, unless too much fertilizer has been suddenly applied or strong, dry winds causes rapid drying.  Also, with some domestic water there is variation in concentration and kinds of salts in the water with time.  The 200 mg/l of sodium in water sample 1 on the chart would be a problem if this were what the homeowner continuously received.  However, according to city data, this house does get 94 mg/l at times (not on the chart).  The better quality water serves to flush out the higher concentration salts.  And this is how to practically deal with poorer quality water, occasionally leach the soil with a volume of water in excess of plant need.  When there are no leaching rains, we need to be more aware of the potential for salt accumulation in the soil.  With proper plant selection and water management even extremely saline waters can be used.

Water Terminology

The ions in water are measured as parts per million (ppm) or milligrams per liter (mg/l), terms which are interchangeable.  This is like saying a percent, but instead of the ions' weight per 100 weight of water, it is the ions' weight per million weight of water.  The ion concentration also can appear as milliequivalents per liter (meq/l).  A milliequivalent is the ppm of that ion divided by its atomic weight per charge. 

            Example:  Ca2+ with atomic weight of 40 and a solution concentration of possibly 200 ppm.  Ca2+ has two charges per atom, so it has an atomic weight of 20 per charge.  200 ppm divided by 20 = 10 meq of calcium for a liter of water.

Total Dissolved Solids (TDS): measure of total salts in solution in ppm or mg/L

Electrical Conductivity (EC):  similar to TDS but analyzed differently.

            Units: deciSiemens/meter(dS/m)=millimhos/centimeter (mmhos/cm)=

                                    1000 micromhos/cm (umhos/cm).

            Conversion TDS to EC:  640 ppm=1 dS/m=1000 umhos/cm

Hardness:  measure of calcium and magnesium in water expressed as ppm CaCO

pH:  measure of how acid or base the solution

Alkalinity:  measure of the amount of carbonate and bicarbonate controlling the pH, expressed as ppm CaCO3.

Sodium Adsorption Ratio (SAR):  describes the relative sodium hazard of water

                        SAR= (Na)/((Ca+Mg)/2)1/2, all units in meq/l

            There is also an Adjusted SAR which considers the carbonate and bicarbonate present, but does not do much better in predicting plant response.

General Irrigation Quality Guidelines

(U.C. Leaflet 2995, 1979)

Measurement                          No problem                 Increasing                   Unsuitable

Effect on plant growth

EC (dS/m)                                  3

Na+ (SAR)                                   9

Cl- (ppm)                                 140                               140-350                         >350

H3BO3 (ppm)                               2

Effect on soil permeability

EC (dS/m)                                    >0.5                       

SAR                                               9

1.5 feet of water with EC of 1.6 dS/m adds 10,000 # of salt per acre

WATER NEEDS TO BE APPLIED NOT ONLY FOR THE PLANT NEED,

BUT ALSO TO LEACH THE SALTS

Plants lose water through their leaves and it's called transpiration.  People lose water off their skin and it's called evaporation or sweating.  When a plant stops losing water and when people cant produce enough sweat to cool off, both overheat.  The weather influences that drive this water loss - water that needs to be replaced or the bodies begin to go into heat stress - are the amount of light (day length, cloud cover), relative humidity (it dries faster when air is dry and slower when humid - think desert versus Florida),  and windy (more wind, more drying).  Temperature is important, but not as much as these other drivers. Think freeze-drying  - a very successful process for removing water from food.  Often humans respond more to temperature than these other factors and figure, when it's cool. it's not necessary to water their plants, themselves or their workers.

Heat stress is more complicated than this, of course, but below are some helpful guidelines to follow to avoid heat stress:

https://www.dir.ca.gov/dosh/heatillnessinfo.html

Cal/OSHA HEAT ADVISORY

When employees work in hot conditions, employers must take special precautions in order to prevent heat illness. Heat illness can progress to heat stroke and be fatal, especially when emergency treatment is delayed. An effective approach to heat illness is vital to protecting the lives of California workers.

California law requires employers to identify and evaluate workplace hazards and take the steps necessary to address them. The risk of heat illness can be significantly reduced by consistently following just a few simple steps. Employers of outdoor workers at temporary work locations must be particularly alert and also plan for providing first aid and emergency medical services should they become necessary. All workers should be accounted for during and at the end of the work shift. Heat illness results from a combination of factors including environmental temperature and humidity, direct radiant heat from the sun or other sources, air speed, and workload. Personal factors, such as age, weight, level of fitness, medical condition, use of medications and alcohol, and acclimatization effect how well the body deals with excess heat. 

Heat Illness Risk Reduction

1. Recognize the Hazard. There is no absolute cut-off below which work in heat is not a risk. With heavy work at high relative humidity or if workers are wearing protective clothing, even work at 70oF can present a risk. In the relative humidity levels often found in hot areas of California (20 to 40 percent) employers need to take some actions to effectively reduce heat illness risk when temperatures approach 80 F. At temperatures above 90 F, especially with heavy work, heat risk reduction needs to be a major concern.

    2. Water. There must be an adequate supply of clean, cool, potable water. Employees who are working in the heat need to drink 3-4 glasses of water per hour, including at the start of the shift, in order to replace the water lost to sweat. For an eight-hour day this means employers must provide two or more gallons per person.  Thirst is an unreliable indicator of dehydration. Employees often need ongoing encouragement to consume adequate fluids, especially when the workload or process does not encourage breaks.

3. Shade. The direct heat of the sun can add as much as 15 degrees to the heat index. If possible, work should be performed in the shade. If not, employers where possible, should provide a shaded area for breaks and when employees need relief from the sun. Wide brimmed hats can also decrease the impact of direct heat.

1.  Acclimatization. People need time for their bodies to adjust to working in heat. This “acclimatization” is particularly important for employees returning to work after (1) a prolonged absence, (2) recent illness, or (3) recently moving from a cool to a hot climate. For heavy work under very hot conditions, a period of 4 to 10 days of progressively increasing work time starting with about 2 hours work per day under the working conditions is recommended. For less severe conditions at least the first 2 or 3 days of work in the heat should be limited to 2 to 4 hours. Monitor employees closely for signs and symptoms of heat illness, particularly when they have not been working in heat for the last few days, and when a heat wave occurs. 
2.  Rest Breaks. Rest breaks are important to reduce internal heat load and provide time for cooling. Heat illness occurs due to a combination of environmental and internal heat that cannot be adequately dissipated. Breaks should be taken in cooler, shaded areas. Rest breaks also provide an opportunity to drink water.
3.  Prompt Medical Attention. Recognizing the symptoms of heat illness and providing an effective response requires promptly acting on early warning signs. Common early symptoms and signs of heat illness include headache, muscle cramps, and unusual fatigue. However, progression to more serious illness can be rapid and can include unusual behavior, nausea/vomiting, weakness, rapid pulse excessive sweating or hot dry skin, seizures, and fainting or loss of consciousness. Any of these symptoms require immediate attention.

Even the initial symptoms may indicate serious heat exposure. If medical personnel are not immediately available on-site, and you suspect severe heat illness, you must call 911.

Regardless of the worker's protests, no employee with any of the symptoms of possible serious heat illness noted above should be sent home or left unattended without medical assessment and authorization.

7. Training. Supervisors and employees must be trained in the risks of heat illness, and the measures to protect themselves and their co-workers. Training should include:

• Why it is important to prevent heat illness
• Procedures for acclimatization
• The need to drink approximately one quart per hour of water to replace fluids.
• The need to take breaks out of the heat
• How to recognize the symptoms of heat illness
• How to contact emergency services, and how to effectively report the work location to 911.

Photo: Heat Stress to avocado leaves.