CHAPTER III
T.L.C. GROVE CARE AND MAINTENANCE
As we have seen in the previous chapter, grove planning and development for even a small ownership requires much skill, thought, and attention to detail. Grove care is no different, and must be applied with Tender Loving Care (T.L.C.).
The preferred approach is to have the grove developer complete his work and the first irrigation of the planted trees by the first of May. While completions later in the year are successful, the young trees will stand a better chance of surviving the following winter if they are set out early in the spring. If they are in place with a long growing period, they are more apt to “harden off” before the first likely chance of cold damage, which is approximately December 5 in northern San Diego County.
After planting, the young orchard needs regular attention. The first order of business is irrigation. Unlike many other plants, even older avocado trees will not seek moisture. It is up to the grower to supply water when and where the tree wants it.
If your orchard is in a windy area, extra water is required, both in amount and frequency of application. This is especially true with orchards on hilltops and hillsides, where soil may be shallow and coarse with a low capacity to hold water. In San Diego County, trees exposed to strong, desiccating winds suffer damage from water stress. This is not uncommon during the fall and winter Santa Ana winds and with the strong westerly winds that occur regularly during spring and summer, and has a drastic effect on water use of plants. The effect is more than most growers realize. Tests with grass plants growing in areas where winds were dry (10-30% relative humidity) show water use was three times the normal use on clear, calm days.
Soil acts as a reservoir for the moisture necessary to satisfy the plant’s needs. The influence of gravity on soil moisture causes it to drain downward. Soil texture and structure determines the rate at which water moves to lower levels. The attentive grower or grove manager will take pains to acquaint himself with the behavior of irrigation water on and in the soils of the growing areas of the ranch.
Over-irrigation is a frequent problem in avocado orchards. Excess moisture causes a lack of aeration, resulting in root rot, or accentuating the spread of root rot in infected soils. Good irrigation water management will prevent either insufficient or excessive soil moisture. In years of heavy rainfall, buried drain lines may solve the problem of excessive soil moisture and prevent tree loss.
Careful attention to irrigation is necessary to produce good results with avocado trees. During the summer and fall, when temperatures are high with a strong, dry wind blowing, it will be necessary to irrigate continuously with a drip system to supply enough water to replace the water lost through leaf transpiration. Under Santa Ana conditions, the tree roots may not be able to absorb the water fast enough to prevent wilting of the leaves and stems of new, young growth, even when the water supply is adequate. This wilting is not so evident on older growth that has hardened off, but trees will still be under stress. Hass trees are the most sensitive to these conditions, and wilting may be evident by the early afternoon. By the next morning, however, the moisture has been restored and the new growth appears normal.
During the winter rainy season, drip irrigation may be reduced considerably when rainfall is sufficient to provide for the tree’s needs. After a 3” rain it may not be necessary to irrigate for two weeks, providing temperatures remain cool with no strong, drying winds. Read your tensiometer or use your soil tube to determine the moisture level of the soil.
Tip burn on leaves appears as a dying back on the leaf tips and margins, and is the result of excess chloride in the irrigation water. This accumulation of salts in the root zone is transmitted to the leaves, particularly in the fall of the year before beneficial rains take place. You can reduce the problem by applying extra irrigation water at least three times during the irrigation season to leach salts below the root zone. Usually an extra one-half of an irrigation treatment will suffice.
Determining Soil Moisture
There are several ways to determine loss of soil moisture.
A. Tensiometers are favored by many growers and some university researchers. These instruments measure the soil moisture condition rather than the quantity of water in the soil. The instrument consists of a ceramic cup, a body or tube, a vacuum gauge, and a reservoir. Colored water is placed inside the tube through the opening at the top of the reservoir to ease monitoring of water level in the tube.
Movement of moisture in and out of the ceramic cup causes tension on the column of water in the tube. This is shown on the dial of the vacuum gauge. Most gauges are calibrated in centibars from 0-100, the lower readings indicating wet conditions. Readings above 80 are not reliable.
For avocados, a tensiometer station should have a 12” and a 24” instrument placed within 6” to 1’ of each other inside the wetted area. Stations should be set up in both the driest (usually the highest) and the wettest areas to balance the irrigation. When properly placed, these instruments will measure the amount of energy that a plant exerts to extract moisture from the soil. They should be placed in an area where the tree roots are active.
The shallow instrument tells when to irrigate. The deep instrument tells how long to irrigate. The length of time to irrigate is determined by the number of hours required to cause the deeper instrument to respond to the water being applied. When the deep instrument’s reading drops to 10, there is sufficient moisture 24” down. Using drip irrigation, tensiometer readings of O to 15 indicate adequate moisture is available for good tree growth. When the shallow instrument shows a reading of 15, it is time to irrigate, because the drip system does not provide as large a reservoir of moisture for the tree as a sprinkler with its larger coverage area. Using a sprinkler system, a reading of 0 to 30 indicates adequate moisture, but when the tensiometer reads 30, it is time to irrigate.
Poor quality
Figure 19. Instruments used to measure moisture contents in soil. Shown are 12- and 24-inch tensiometers and a soil tube for taking core samples.
These instruments are easily damaged because they are rather delicate. Covering them with an old bucket or a picking box may protect them from grove operations and also from any possible cold damage that might occur during an unusual freeze. As the trees grow and emitters are added or sprinklers are changed to increase the wetted area, tensiometer stations will need to be relocated appropriately.
If you plan to invest in and use tensiometers to measure soil moisture, it is probably prudent to ask the tensiometer company field man to participate in planning the location and installation of the instruments at the selected stations, even though you know that they will have to be relocated several times until the trees mature. You will also want to acquire a service kit to enable you to check the vacuum and to renew the colored water in the instrument. By regularly taking readings, charting the data, and servicing the instruments, a grower can come to a better understanding of his or her land.
B. A soil tube is a T-shaped instrument made of quality steel and available in any good grower supply house. On its tip end it has an extra hard cutting edge to remove a core sample from the soil into the lower segment of the tool (the tube) so that it can be examined. A word of caution concerning this fine tool: growers often damage their soil tubes by forcing them into dry soil. There must be some moisture in the soil to permit the use of this instrument.
To determine the soil moisture remaining before irrigation, use a shovel, auger, or other method of determining soil moisture, and sample the soil at six- to twelve-inch intervals in the root zone. Table 8 may be used to estimate the percent of water depleted, based on the soil texture. The amount depleted is the net amount that should be replaced by irrigation. The gross amount of water to be applied is determined by dividing the net amount by the irrigation application efficiency for the field based on the irrigation system and management.
Twenty-four to 48 hours after irrigating, the depth of water penetration can be checked at various points throughout the field by probe or auger to determine the uniformity of the application and the adequacy of irrigation.
The most common way to estimate available soil moisture is by the “feel” method.
Because the core sample will reveal moisture conditions as well as clay pockets in the soil, the soil tube can be used to identify problem areas for special water management.
In avocado growing areas, the high ground is likely to be thin and porous, while on the lower levels and in the swales the soil is apt to be tighter and less permeable. In any event, with avocados, we are primarily concerned with the available moisture in the first two feet of soil. Remove the soil from the tube and squeeze it in the palm of your hand. If one can make a satisfactory “ball” of the sample, in much the same way that children’s modeling clay can be balled, it indicates that there is sufficient moisture in the soil for good tree growth. If the soil crumbles when compressed, the tree has insufficient moisture available and should be irrigated. Samples should be taken at both the one- and two-foot depths, using the same procedure for each sample. With a little practice and careful observation, the grower can soon map his grove for the different types of soil that are found on his land.
This writer favors the use of a soil tube for measuring soil moisture, especially coming into or leaving the rainy season, when there is sufficient moisture to avoid damaging the tool. During the wet season, the soil tube should be used on the sunny side of the tree because this soil dries out faster.
C. The soil augur is also a T-shaped instrument, and is similar in appearance to the soil tube described previously. It is seldom found in stores but is a more versatile tool inasmuch as it can be used in dry soils without injury to the tip. It is easy to make, or have made, is lightweight, and can be conveniently carried and stored. It is used in the same way as the soil tube, described previously.
To make an augur, take a 4’ length of ˝” pipe or reinforcing steel and another 18” length of the same material. Weld the short length in the center to one end of the long piece. Buy a 3/4” steel bit (for boring holes in wood) and grind off the 3/8” tip used to draw the bit into the wood. Weld the shaft to the other end of the long piece. To use it, simply twist the bit into the ground to the required depth, remove the soil from the bit, and test as indicated for the soil tube.
Note that the ˝” pipe is lighter to carry than the solid steel shaft but is not as strong and more easily bent.
D. An evaporation pan is a plastic tray, such as the one used for dishwashing, commonly available in supermarkets, drug stores, and general merchandise stores. It need not have any specific dimensions but should be a minimum of 4” deep to allow for a volume of water large enough that the water doesn’t evaporate completely between readings. The pan should be placed in the grove so that the prevailing winds pass over it. It should be as nearly level as possible and located so that it will be in direct sunlight part of the day and shaded the other part. A piece of 1” mesh hardware cloth should be placed over the pan to prevent animals and birds from depleting the water supply, which would give false readings. By occasionally adding a small quantity of common household bleach, the scum that collects on the pan and the water can be controlled.
The amount of moisture lost in the evaporation pan is a good indicator of the moisture removed from the soil by foliar evaporation. The moisture loss in the pan should be checked at regular intervals, preferably once a week on the same day at the same time. The water loss from the pan can be determined in two ways:
1. By making marks ˝” apart on the inside edge of the pan. For example, if 1” of water has evaporated, the soil might require a two-hour irrigation to replenish the soil moisture. The type of irrigation system used and the soil type will make considerable differences in the length of irrigation time.
2. Using a standard measure, determine how many cups of water are needed to refill the pan to the required level. If, say, 8 cups are needed, it might indicate a 4-hour irrigation; if 4 cups are needed, a 2-hour irrigation.
Though not quite as accurate as the tensiometer, the evaporation pan is easy to use, and even the most casual observer can soon become familiar with its meaning.
Water Conservation Practices
Our avocado and citrus growers have been practicing water conservation for a long time because of the high cost of water. For many growers, it will be difficult to become more efficient than they are already. However, there is always something we can do to “tighten up” and affect some savings. The following practices can lead to water savings if you do not already have them in place.
1. Control weeds in the area of the trees’ root system.
2. Use mulches (straw, manure, wood chips, sawdust, compost, etc.) to prevent water loss from the soil surface. Mulches also help prevent the runoff of irrigation water and serious soil erosion.
3. Learn the daily water requirement of the plant. Apply only the required amount, either by daily application with drip irrigation, or weekly application with sprinklers.
4. Irrigate in the early morning, late afternoon, or at night to obtain the best watering pattern and uniformity of water distribution. Do not irrigate with spitters or sprinklers during windy periods, as this increases the evaporative losses.
5. Repair leaks in irrigation lines and valves. Replace worn orifice tips of sprinklers.
6. Control gophers. Destroy gopher and mole runs that take water away from the root zones of trees.
7. Leaching for salinity control should be done carefully. Salts in soils and irrigation water decrease the ability of plants to take up water. As salts become concentrated in the soil, the result is poor uptake of water by plants and a toxic effect on plants. Frequent irrigation with periodic leaching is a must for avocados.
8. Maintain good fertilizer practices, as a healthy tree is a more efficient user of moisture.
9. Install tensiometers as a guide to proper and efficient irrigation.
10. Learn how to measure the water applied.
Salt Accumulation and Leaching
The irrigation water used in our avocado groves contains dissolved salts, especially chlorides, which accumulate in the soil and produce the common malady on tree leaves during the late summer and fall known as “tip burn.” These salts impair the normal functions of the plant, but if the salts are in a sufficiently diluted state they are much less apt to be picked up by the plant. Salt accumulation may be reduced by several deep, leaching irrigations that will push the salts below the first two to three feet of the soil where 90% of the roots are located. A rule of thumb for leaching might be a 25% to 50% increase in irrigation time three times during the irrigation season. Winter rains are usually sufficient to leach the salts below the root zone, but anything less than a 2” rain may only move them into the lower portion of the root zone. Unless there is sufficient rain to leach the salts below the root zone, it is best to continue the normal irrigation schedule.
A method sometimes called “water bumping” may be used to more efficiently leach the salts below the root zone. This process is accomplished by giving the trees a normal irrigation, and two days later a second normal irrigation. The first irrigation wets the soil to the usual depth but the second “bumps” the water, with its accumulated salts, down below the root zone. This method has been found to greatly improve water penetration, particularly in the heavier soils.
Measuring Irrigation Water
Efficient water management is very important. To become a better water manager, you must be familiar with water measurements. Learning water measurement terms and how much water plants require will help you: 1) conserve water, 2) produce maximum yields from water used, 3) prevent poor growth because of insufficient water application, and 4) reduce drainage problems caused by too much water.
Irrigation water is measured in volume and rate of flow units. The commonly used volume units include cubic feet, gallons, acre-inches, and acre-feet. Rate of flow is the combination of volume units with a convenient unit of time. Those commonly used are: cubic feet per second, gallons per minute, acre-inches per hour, acre-feet per day (24 hours), and miner’s inches. Because miner’s inches vary in different localities, they may not always be a satisfactory reference. Data given in miner’s inches can be converted to cubic feet per second or gallons per minute.
Photosynthesis
Though not a fertilizer, the plant’s ability to utilize these elements is directly connected with the manufacture of carbohydrates by the chlorophyll containing tissues (chloroplasts) when exposed to the sun’s light and heat rays. In years of exceptionally cool winters and wet springs, we often note a yellowing of avocado leaves, a condition sometimes called winter chlorosis, which is due to the absence of photosynthetic activity because of a lack of sunshine. This condition will remedy itself when the rains diminish, the weather warms with an increase in the solar insolation. This winter chlorosis is pronounced in trees three to seven years of age but is not generally found in older trees.
Avocados enjoy a soil pH in the neutral range of 6.5 to 7.0, which is true for most agricultural crops. Growers have recognized that using only one fertilizing material continuously tends to make the soil unhappy in the pH department. Therefore, the use of only one source of nitrogen should be avoided. For example, growers who use inorganic fertilizers (nitrogen manufactured synthetically by man) can balance their applications by using calcium nitrate in the spring when the soil, water, and temperatures are cool.(Calcium nitrate performs well under cool conditions.) In the summer, growers can switch to urea because the warmth tends to speed up its biological conversion to the nitrate form. The cost of rotating fertilizers is about the same as using ammonium nitrate all through the growing season. If the soil in a grove has become overly acidic by the continued use of urea fertilizer, it may be brought back to neutral by the use of calcium nitrate, which tends to increase the alkalinity of the soil. Conversely, if the soil is too alkaline, ammonium sulfate will acidify the soil. Ammonium nitrate and urea will also produce an acidic condition to a lesser degree, which can bring the soil back to a neutral condition.
This chart shows how soil PH affects the availability of plant nutrients and the relative availability of plant nutrient elements at various pH levels. Growers will note that a pH between 6.5 and 7.5 fosters the maximum availability of the primary nutrients -- nitrogen, phosphorus, and potassium (N, P, and K) -- and a relatively high degree of availability of other nutrient elements.
Essential Plant Elements
Sixteen chemical elements have been proven to be absolutely essential for plant growth and reproduction. The major elements (or macronutrients) are needed in relatively large quantities by plants; the minor elements (or micronutrients) are no less essential but are needed in much smaller quantities. Carbon, hydrogen, and oxygen are the building blocks of carbohydrates and fats. These elements, plus nitrogen and sulfur, are found in proteins. Phosphorus is essential in the energy transfers of cells and also occurs in the cell genetic material. Calcium is a constituent of pectic materials, which help hold cells together. Magnesium is the central atom of the chlorophyll molecule in the cells of green plants. The role of potassium is not yet understood, although it has been clearly demonstrated that plants will not grow without it. (MLA: Is the previous statement about potassium still accurate? Thanks, sm) Iron is required for the formation of chlorophyll and in the oxidation reactions of the cytochrome chain. Several of the other minor elements function in enzymatic reactions necessary for plant growth. The most important of the macronutrients are nitrogen, phosphorus, and potassium, with nitrogen as the most important element in soils. Although some plants fix nitrogen directly from the air, it is not known to be fixed by the avocado. These elements are usually supplied by growers to provide the nutrients for good crop production, either in the form of “complete” chemical fertilizers or from an organic source.
Of the three macronutrients, the principal demand in avocados is for nitrogen. Nitrogen fertilizers come to us in a number of forms and percentages but primarily in the dry form. “Actual” nitrogen refers to the real quantity of nitrogen present after adjustments for form and percentage. For the grower who wants to determine the actual percentages and the cost of the nitrogen, Table 11 gives several sources of nitrogen together with the formula to compute the percentages and cost.
Of special interest to growers are the bacteria found in the soil which oxidize ammonia (NH4) first to the nitrite form (NO2) and then to the nitrate form (NO3) for use by the plants. Nitrate is the form of nitrogen that plants use for growth and development. Nitrates are highly soluble and subject to leaching through the soil profile and therefore need to be replenished regularly through fertilization.
This element promotes root growth, fruit development, and maturity, and seems to contribute to the plant’s general hardiness. Most of the sandy loam and clay loam soils seem to have sufficient phosphorus for the avocado; however, in the inland areas among the coarser granite soils, lack of phosphorus can be a limiting factor for good growth, and fertilizers containing phosphorus should be applied. This element tends to remain immobile in the soil until used by the plant.
The role of this element in avocado nutrition is not clearly understood. It is believed to make the plant more resistant to diseases, cold, and adverse conditions in general. It is relatively immobile in the soil.