- Author: Ben Faber
I was recently in an orchard looking at what appeared to be avocado root rot. I was checking on the symptoms and quizzing the grower on the other cultural practices. The grower was prepared with soil, leaf and water reports. I asked how much nitrogen was being applied and it was something in the 50 pounds nitrogen per acre. When I saw the water report it listed nitrate in the water at the level of 84 ppm. Rarely do you see a yield response in plants when soil nitrogen exceeds 90 ppm nitrate. The soil nitrogen would pretty rapidly take on the nitrogen level of the water. So this grower was very close to the level at which no yield improvement would occur, and in fact would be increasing vegetative growth and hence increasing pruning problems. I looked at the leaf nitrogen levels and they were quite high, as well. I suggested that it would be a good idea to cut back significantly the amount of fertilizer applied, if not stopping application all together. There are many areas along the coast where avocados are grown where there are high nitrates in the water. Water can be a significant source of nutrients, as well as toxics, such as boron, chloride, sodium and total salts. Learn to read all your reports – soil/leaf/water – and figure out the puzzle of fertilizing appropriately.
- Author: Ben Faber
Along the coast, it is very common to see windbreaks protecting the citrus and avocado groves. Invariably the first two rows next to the eucalyptus trees are shorter and less thrifty than the citrus further away from the windbreak. This is due to competition primarily for water, but somewhat due to light, as well. Often by putting emitters on the windbreak, the completion stops. Growers will also root prune between the windbreak and the first row of citrus. Those roots inevitably grow back and pruning must be done again. This also occurs in areas where there are oak trees or other natives that are planted in or around the orchard. Growers will frequently plant right up to the canopy or even under the canopy of the native tree(s), with a similar result seen with windbreaks.
It is important to remember the architecture of roots. Not all trees are exactly alike, but a general rule of thumb is that the active roots go out one and half times the height of the tree. So a 40 foot tree will have competitive roots out 60 feet away from the trunk. That’s why it is best to keep a distance away from a competing tree, because avocados and citrus are just not as competitive as an oak or eucalyptus.
In low rainfall years, this competition is even more intense. Significant defoliation of the crop plant can be seen. The grower then thinks that it is some disease and ponders what to spray, when they should actually be spraying more water.
- Author: Neil O'Connell
By the beginning of the irrigation season, the entire root zone is usually wetted by winter rainfall. Under low volume irrigation during the irrigation season only fifty percent or less of the root zone is wetted with each irrigation on most soil types. Soils with slow infiltration do not allow enough water to penetrate into the root zone to meet the plant’s water requirement. During an irrigation the water puddles while the soil beneath remains dry. Less than ten percent of the soil in the root zone may be wetted during an irrigation when water infiltration is a problem. Water storage in such a small volume of soil may amount to only two to three days of evapotranspiration. The tree may be under stress even though the amount of applied water exceeds the amount lost by evapotranspiration (ET). An infiltration problem is often associated with irrigation water low in salt and/or soils with inherently slow infiltration rates. Soil particles contain sites occupied by electrically charged ions such as calcium, sodium, and magnesium. In an optimum situation, a sufficiently high percentage of these sites are occupied by calcium which results in an aggregating or clumping effect among soil particles allowing water to penetrate. When the percentage of sites occupied by calcium is low and sodium predominates there is a repelling or dispersion of particles and water penetration is reduced. With increasing numbers of the exchange sites occupied by sodium ions the soil particles swell and repel each other creating a dispersion or loss of aggregation resulting in single particles. As this happens the porosity (or pore space) is reduced and the ability of water to enter is reduced. On the other hand as the exchange sites become more occupied by calcium the particles move closer together and aggregate or clump resulting in an increase in pore space. Therefore, soils that have a high percentage of the exchange sites occupied by sodium ions are dispersed and deflocculated and resist the entry of water while those with a high percentage of calcium ions are flocculated and favor water infiltration. With the use of low salt water over time, such as snow melt water, calcium may be removed from the soil particles exchange sites and these sites may then become occupied by another ion such as sodium.
Research addressing this problem of low infiltration was conducted in citrus under low volume irrigation by University of California researchers Peacock, Pehrson and Wildman. The soils type, at the experimental site of mature navel oranges, was a San Joaquin sandy loam characterized by a low infiltration rate. Canal water with a low salt content was used for irrigation. The trees were irrigated with a drip system every week day. Treatments began in June when soils typically begin to exhibit a reduced infiltration rate and were continued until mid-August but measurements continued until September. Simple devices for measuring the infiltration rate, called infiltrometers, were made from 12 inch PVC pipe and installed in the orchard. Chemical treatments and water were applied and rates of water infiltration were measured within these infiltrometers. Gypsum was applied weekly to the soil surface to maintain a slight excess continually on the soil surface and watered in resulting in gypsum application with each irrigation. Calcium nitrate and CAN-17 were each injected into the irrigation water. Calcium nitrate was introduced into the irrigation water at the rate of ten pounds per acre per irrigation. Calcium nitrate was applied daily, biweekly and in a single application. CAN -17 was applied daily, biweekly and in a single application. With these injections into the irrigation water, calcium was being introduced into the water at the rate of 3 milliequivalents per liter. Adding calcium continuously to irrigation water doubled infiltration rates over that of untreated low-salt water. It took 2-3 weeks before a treatment difference could be measured. However, the occasional additions of calcium nitrate or CAN-17 were not effective in maintaining infiltration rates. There were concerns that nitrogen application from these treatments could result in the nitrogen level in the tree being in excess of the tree’s nutritional requirements. Following this research equipment was made available on a commercial basis for regulated injection of materials into low volume irrigation systems.
- Author: Gary Bender
- Author: David Shaw
OK! Let's Strategize. There are four steps for everybody to consider, it doesn't matter if you have a backyard lawn and landscape or if you have 700 acres of avocados.
1. Maintenance: Irrigation System and Cultural Practices
2. Improve Irrigation Scheduling
3. Deficit Irrigation
4. Reduce Irrigated Area
1. Maintenace
a. Irrigation System.
- Fix leaks. Unfortunately, there are almost always leaks for all kinds of reasons. Pickers step on sprinklers, squirrels eat through polytube, branches drop on valves, coyote puppies like to chew….the system should be checked during every irrigation
- Drain the lines. At the beginning of each year every lateral line should be opened in order to drain the fine silt that builds up.
- Maintain or increase the uniformity of irrigation so that each tree or each area gets about the same amount of water. Common problems include different sized sprinklers on the same line or pressure differences in the lines. Where there are elevation changes, every line should have a pressure regulator, they come pre-set to 30 psi. Having all of your lines set up with pressure regulators is the only way you can get an even distribution of water to all of the trees, and it solves the problem of too much pressure at the bottom of the grove and not enough at the top.
- Clean the filters often. You don't have a filter because you think that the district water has already been filtered? Hah! What happens if there is a break in the line in the street and the line fills with dirt during the repairs? All of your sprinklers will soon be filled with dirt.
- Is water flow being reduced at the end of the lateral line? It could be because scaffold roots are growing old enough to pinch off the buried line. The only cure is to replace the line.
-
b. Cultural Management.
- Control the weeds because weeds can use a lot of water.
- Mulch? Mulching is good for increasing biological activity in the soil and reducing stress on the trees, but the mulch will not save a lot of water if you are irrigating often….the large evaporative surface in mulches causes a lot of water to evaporate if the mulch surface is kept wet through frequent irrigation. Mulches are more helpful in reducing water use if the trees are young and a lot of soil is exposed to direct sunlight.
2. Improve the Irrigation Scheduling.
- CIMIS will calculate the amount of water to apply in your grove based on last week’s water evapotranspiration (ET). You can get to CIMIS by using several methods; for avocado growers the best method is to use the irrigation calculator on the www.avocado.org website. If you need further instruction on this, you can call our office and ask for the Avocado Irrigation Calculator Step by Step paper. You need to know the application rater of your mini-sprinklers and the distribution uniformity of your grove’s irrigation system.
- CIMIS tells you how much water to apply, but you need tensiometers, soil probes or shovels to tell you when to water.
- “Smart Controllers” have been used successfully in landscape and we have used one very successfully in an avocado irrigation trial The one we used allowed us to enter the crop coefficient for avocado into the device, and daily ET information would come in via a cell phone connection. When the required ET (multiplied automatically by the crop coefficient) reached the critical level, the irrigation system would come on, and then shut down when the required amount had been applied. Increased precision can be obtained by fine tuning these devices with the irrigation system precipitation (application) rate.
-
3. Deficit Irrigation.
- Deficit irrigation is the practice of applying less water than the ET of the crop or plant materials. Deficit irrigation is useful for conserving water in woody landscape ornamentals and drought tolerant plants where crop yield is not an issue. Water conserved in these areas may be re-allocated to other areas on the farm or landscape.
- There hasn’t been enough research on deficit irrigation of avocado for us to comment. We suspect, however, that deficit irrigation will simply lead to dropped fruit and reduced yield.
- Stumping the avocado tree could be considered a form of deficit irrigation. In this case, the tree should be stumped in the spring, painted with white water-based paint to reflect heat, and the sprinkler can be capped for at least 2 months. As the tree starts to re-grow, some water should be added back, probably about 10-20% of the normal water use of a mature tree.
- Regulated Deficit Irrigation for Citrus is an important method for saving water, and in some cases will reduce puff and crease of the peel. In one orange trial done by Dr. David Goldhammer in the San Joaquin Valley, an application of 25% of ETc from mid-May to Mid July saved about 25% of applied water for the year and reduced crease by 67%, without appreciably reducing yield.
- 4.
- 3. Reduce Irrigated Area.
- Taking trees out of production. Trees that are chronically diseased and do not produce fruit (or the fruit is poor quality) should be taken out of production during this period. Also consider: trees in frosty areas, trees in wind-blown areas, trees near eucalyptus and other large trees that steal the water from the fruit trees.
- Changing crops. You may want to take out those Valencias during this period and replant to something that brings in more money, like seedless, easy-peeling mandarins. The young trees will be using a lot less water.
- Fallow Opportunities. You may decide to do some soil preparation, tillage or cultivation, or even soil solarization of non-irrigated areas.
We have found that this four step process is a logical way to achieve water cutbacks with least impact. It is possible to achieve a ten percent reduction in water by only improving irrigation system uniformity and scheduling procedures. Often, these two measures also result in better crop performance and reduced runoff. Reducing irrigated area or taking areas out of production should be a last resort and a well thought out decision. Plan for the future, hopefully water will be more available in future years.
- Author: Ben Faber
Saline Waters - A Growing Problem
Ben Faber
Irrigated agriculture must always contend with salts, but two years without rain and a dry winter forecast, salt is an even more important issue. We rely on winter rainfall to leach the salts from root zones that have accumulated from previous irrigations. Salinity affects plant growth and understanding what it is and how it is measured and evaluated need to be understood.
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 ofSan Buenaventurawater can be seen in the following chart (2005 Annual Report of the City ofSan Buenaventura).
Ionic composition of some wells in Ventura
Sample |
Na+ |
Ca+ |
Mg+ |
Cl- |
SO42- |
TDS |
EC |
|
|
|
(mg/l) |
|
|
|
(umhos/cm) |
1 |
200 |
259 |
70 |
92 |
839 |
1668 |
1990 |
2 |
45 |
92 |
191 |
44 |
210 |
645 |
874 |
3 |
28 |
59 |
21 |
20 |
140 |
316 |
580 |
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 inSanta BarbaraandVenturaCountiescontain potentially harmful levels of boron for plants. This is not as common a problem inSan DiegoCounty.
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 pomegrantes. 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 TDSEC: 640 ppm=1 dS/m=1000 umhos/cm
Hardness: measure of calcium and magnesium in water expressed as ppm CaCO3
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