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Soil Biology 101

Soil biology is the study of life in soil. Yes, soil is alive. Just like you care for and protect the crops you grow to get the most out of it, you must also nurture and protect your soil to get the most from it. But it also should be thought of as a natural resource that needs to be preserved for the future.

There is more life and diversity in the soil than there is above ground in a healthy agricultural field. Like any ecosystem, energy flows from the sun, through plants, and finally to the living organisms in the soil. These soil organisms can be divided into algae, bacteria, fungi, protozoa, nematodes, arthropods, and earthworms. Each has their own role to play in the soil ecosystem.

The reason soil biology is so important is that these soil micro-organisms are responsible for decomposition of organic matter, mineralization of nutrients, storage and release of nutrients, degradation of chemicals, nutrient cycling, and finally disease suppression. The basis of a sustainable soil ecosystem is the maintenance of a healthy soil food-web, which begins with routine application of organic matter. Higher organic matter content supports thriving communities of the decomposers and predators that make up the soil's "digestive system." These tiny microbes are the driving force for all of these important processes of a healthy and productive soil ecosystem.

There is a significant amount of evidence showing that adding organic matter to the soil reduces soil borne pest problems. This is likely due to increased populations of fungal, bacterial, and nematodes that are parasitic or antagonistic towards soil borne pests. Soil biological activity also improves soil structure and fertility. Reduced soil borne pest damage from increased organic matter in soil is likely a combination of these interactions.

Soil biological activity greatly improves the soil’s physical structure including soil pore size, soil aggregate stability, and formation of macropores. The soil microbes also partially determine the soil's ability to recover from soil disturbances, such as compaction, tillage, etc. Fungi and bacteria secrete substances that bind soil particles and stabilize soil aggregates. Tiny soil arthropods feed on organic matter and their fecal pellets become part of these stable soil aggregates. Large borrowing invertebrates such as earthworms, ants, termites, beetles produce larger macropores that allows water flow through the soil. These are all important soil physical properties for a healthy and productive soil.

As these soil microbes consume dead organic matter and each other, including plant parasites, they release nutrients that were once bonded to living organisms. Without these decomposers, we would be surrounded by dead matter and nutrients would be tied up forever. However, these nutrients are re-released and made available for new living things to grow including crops. These microbes are also responsible for the degradation of chemicals and pollutants that may be found in the soil. Thus nutrient cycling is a very important process that relies on a healthy soil ecosystem.

It is important to actively manage your soil biology using minimum-tillage practices, compost, animal manures, green manures, cover crops, and crop rotations. These practices help promote the growth of beneficial organisms while suppressing plant parasites, increase soil physical characteristics, and the cycling of nutrients. This subject is too large to be completely covered here. Please read more about it in various books, web sources, or visit an extension office.

W. Murcott Mandarin May Need More Babying Than Navel Orange

The mandarin variety ‘W. Murcott’ (also known as ‘Afourer’) is relatively new to San Joaquin Valley citriculture. In fact, other than a relatively long history of growing the Satsuma mandarin, most mandarin varieties are relatively new to the San Joaquin Valley. Unlike the expertise in growing navel and Valencia oranges and lemons, most citrus growers are still on a learning curve in producing mandarins commercially. The W. Murcott mandarin is relatively late maturing variety, with good color, good production, good appearance and good taste. If grown sufficiently far from other pollen-producing citrus, it is mostly seedless. So what’s the problem?

Some of the oldest W. Murcott mandarin trees in the San Joaquin Valley are located in a randomized planting of numerous varieties of mandarins established in the mid-1990s at the UC Lindcove Research and Extension Center. In early December 2007, most of the W. Murcott mandarin trees in this trial had an unthrifty appearance with very thin leaf canopies and branch dieback. These trees had either very low or very high fruit yields regardless of whether on C-35, Carrizo or Trifoliate rootstock. The many other mandarin varieties in this trial on the same rootstocks appear to be growing well. Apparently there are older W. Murcott trees than these trees at Lindcove in the San Joaquin Valley that are not exhibiting the problems seen in this trial.

Experts with far more experience in mandarin production than this author have stated that the reason for the poor appearance of the W. Murcott trees at Lindcove is that they have not been managed as intensively as this variety requires. W. Murcott has a strong, alternate bearing fruit-production pattern. As has been previously observed, especially in hot, dry environments such as the Coachella Valley, alternate bearing mandarins can exhaust carbohydrate resources in producing fruit in the high-yield year. High fruit production can seriously weaken the tree.

Although the reasons for the decline of these trees is not known for sure, W. Murcott growers in the San Joaquin Valley (and there are now thousands of acres of these trees) need to be aware that controlling alternate bearing appears to be advisable. Most growers, currently, manage alternate bearing by limiting the fruit-bearing area of the tree through annual pruning. Less fruit in the high-bearing year translates into more fruit in the off-year. As well, it has been suggested that fruit harvests in heavy bearing years should proceed in a timely fashion, so that fruit does not remain on the tree for excessive time periods. Irrigation and fertilization practices may also need to be more tightly controlled than in other citrus varieties so as not to encourage alternate bearing, yet ensure that water and nutrition is adequate to minimize stress when fruit load is higher. Knowledge of what to do and when to do it to W. Murcott mandarins is lacking. Not in doubt is the observation that California W. Murcott mandarin growers will likely be learning as they grow.

Cypress Tree Problems

Several cypress tree species are frequent in Kern County landscapes and windbreaks. In recent years significant numbers of cypress appear to be declining, which is likely due to the inherent adaptability of these trees to the local climate. Certain opportunistic pest problems may accelerate their demise.

Arizona cypress, Cupressus arizonica, and Leyland cypress, Cupressocyparis leylandii, have been extensively planted throughout Kern County. These species are often found in windbreak plantings around houses or corrals. Arizona cypress has been widely established in Mojave desert landscapes in the Ridgecrest, Rosamond, and Mojave areas, while Leyland cypress has been planted in Tehachapi and on the San Joaquin Valley floor. Both species possess cold tolerance sufficient for almost any Kern County location. Arizona cypress is considered to be drought tolerant based on its performance in desert areas. When vigorous, these cypresses have few pest problems.

In recent years a number of these trees appear to be declining or have died. In almost all situations plants have apparently been adequately irrigated. For Arizona cypress, tree ages at decline are about 20-30 years. These trees appear to be nearing the end of their natural life span in the desert, but their decline may be accelerated by bark beetle attack. Although trunk sprays of insecticides may forestall beetle attacks, insecticides are a distant third line of defense – after keeping plants vigorous (especially by supplying sufficient irrigation) and preventing injury (such as from sunburn) which are far more effective strategies.

For Leyland cypress, the fundamental problem appears to be an inherent lack of adaptation to the warm, dry climate of Kern County. This weakness renders the species susceptible to attack by a canker disease, resulting in a life expectancy of 12-15 years. It has not been uncommon to see entire rows of Leyland cypress turn brown en masse, almost as though a clock had struck their death knell, and this despite sufficient irrigation and appropriate maintenance. Fungal pathogens such as cypress canker, Seridium or Coryneum cardinale, causes lesions to form on small branches, resulting in death of branch tips, often followed by colonization of larger-diameter wood. Fungicides cannot be expected to provide any control of this disease, and are not recommended. Although Leyland cypress trees grow rapidly and provide an excellent screen, their short life expectancy must be considered if they are selected for planting.

2008 San Joaquin Valley Grape Symposium

The San Joaquin Valley Grape Symposium will be held at the C.P.D.E.S. Hall, 172 W. Jefferson Avenue, Easton, California on Wednesday, January 9, 2008. This year’s meeting, presented by the University of California Cooperative Extension and sponsored by the California Raisin Marketing Board, will begin with a continental breakfast at 7:00 am followed by a technical program from 7:45 am to 2:00 pm. Topics will cover new rootstocks for raisin production in the SJV, irrigation management for DOV raisin vineyards, vine mealybug management, nutritional characteristics of new raisin cultivars and the potential use of abscission agents for raisin production. Lunch and proceedings are $10 per person if registration is completed by December 31, 2007. Register online at http://ucanr.org/sjvgrapesymposium.

Growing Pistachio Trees

No scientific trials have been conducted to give the pistachio growers guidance on the best way to prune pistachio trees. So what is a pistachio grower to do?

Observation made in some older orchards on the west side of the San Joaquin Valley suggests that this lack of information is translating into a reluctance of some growers to control tree height for fear of losing yield. Standard industry practice for pruning branches in the upper canopy of pistachio trees is to make the pruning cut at a point on the branch located two or three vegetative buds above the last flower bud. These vegetative buds will provide locations for new growth the following year.

Of course, continuing this practice indefinitely will result in ever larger pistachio trees. In some orchards, excessive tree height is resulting in excessive shading of the lower tree canopy resulting in loss of fruit wood and leaf area. Shading results in increased humidity in the tree canopy and increased problems with fungal pathogens, like Alternaria species. Tall canopies make harvest less efficient. Long limbs dampen the energy that the harvest shaker is able to transfer to nut clusters, and results in nuts being slung outside the catching screen.

Although no comprehensive pruning studies have been conducted, related scientific information does exist. Research indicates that pistachio can compensate, to a degree, for bud removal in one part of the tree by producing more nuts or heavier nuts in the remaining clusters in other parts of the tree. In addition, since pistachio is alternate bearing, yield loss as a result of pruning that occurs in one year, has been compensated for by increased yield the following year. Huge trees are not required for huge yields. Relatively small 10th leaf trees in the southern San Joaquin Valley are capable of producing 5000 pounds of dry yield in a single season.

Dr. Louise Ferguson, University of California Extension Specialist, in cooperation with others, has demonstrated that two successive years of mechanical topping mature Kerman trees, on P. atlantica rootstock, mitigated alternate bearing without affecting cumulative yields for the following 5 years. She also noted that cutting into 3 and 4-year old wood during the mechanical hedging of both sides of a tree decreased cumulative yield over the following 6-year period.

University of California Farm Advisor Bob Beede has experimentally pruned an orchard that was initially 16 years old and composed of large, crowded trees on P. integerrima rootstock (planted 17 ft between trees and 17 feet between rows). Farm Advisor Beede found that while hedging every-other row in alternate years did not significantly decrease cumulative yields, moderate mechanical topping (removal of approximately 50% of the fruit buds at the top of tree) prior to the on-year in successive cycles, did. He also reported that biennial hedging both sides of the tree was detrimental to cumulative yield.

Clearly, attempting to control tree size, once trees are mature, may result in significant yield loss. With the above described studies in mind, factors are examined that can influence the rate at which trees grow and influence how trees grow. Later, possible ways to mitigate these factors are discussed in relation to management of tree size, structure and yield.

FACTORS CONTRIBUTING TO OVERSIZED TREES

Factor One: Rapid Tree Growth

Oversize pistachio trees are generally a developing problem on the west side of the southern San Joaquin Valley. Conditions here appear to be perfect for pistachio growth. The soils are deep, boric, and calcareous and heat units accumulate quickly during the summer. As a result of problems with Verticillium wilt fungus, the trees are planted on rootstocks with P. integerrima heritage. Pistachio trees on P. integerrima rootstock or its hybrids are more vigorous than those on P. atlantica rootstock. The result is a bigger tree that produces good yields of nuts more quickly.

Factor Two: Close Tree Spacings

Pistachio trees, historically, have been planted fairly close together in California. There are good reasons for this. Close plantings (usually less than 18 feet between trees and 20 feet between rows) will give higher yields per acre when young, because there are more trees per acre. Also, since trees will eventually be harvested with mechanical shakers and nut catching frames, tree size needs to be managed so nuts do not fall outside of the frame. Since the standard frame is something of the order of 20 feet in length, problems arise in catching the nuts if trees are more than 20 feet apart within the row. However, if trees are planted too close together, mutual shading and crowding occurs more rapidly. Once trees begin to shade each other, the only direction left to grow is up. Shaded branches tend to grow thinner and extend more rapidly toward sunlight in response to shading than unshaded branches.

Factor three: tree training

Branches attempting to grow within trees that are already densely packed with branches will compete for light and productivity. The dense canopies of older trees are often the result of the way young trees were trained initially. Often trees of older orchards are characterized by long, thin interior scaffolds arising low in the tree and that don’t branch. This lack of branching on a scaffold is the result of too many scaffold branches being left in the tree during training within the first few years of planting. Crowded scaffolds have no room to branch as they grow due to competition with neighboring scaffolds. The eventual result is a large tree composed of narrow, weak scaffolds. These scaffolds will tend to sag into the middles once they bear leaves and nuts.

Factor four: high cost of hand pruning

Sending pruning crews through the orchard with pole pruners and other hand tools can be an expensive operation. Skimping on hand pruning can be a temptation, especially in an off-year when income is reduced. Difficulty in finding trained labor for this task is also increasing. The further bad news is that the cost of hand pruning can only go up.

DEVELOPING AN ORCHARD WITH MANAGEABLE TREE SIZE

As discussed previously, no scientifically designed experiments have been conducted comparing possible strategies for maximizing yield while maintaining manageable tree size from planting to advanced maturity. However, based on the identification of factors that have promoted oversize trees above, perhaps, strategies are suggested for limiting tree size of trees in future plantings.

Factor one: rapid tree growth

Rapid tree growth, at least early in the life of the tree, is probably worth promoting. Pistachio trees appear to have to reach a certain size before producing enough nuts to be harvested economically. On the west side of the southern San Joaquin Valley, some pistachio blocks reach full maturity by the 9th leaf. Once the orchard reaches full production, it may be advisable to reduce fertilizer and irrigation inputs. Excess fertilizer and irrigation may stimulate unnecessary and unproductive vegetative growth that will cause competition for light among branches within a tree canopy and between canopies of neighboring trees. The objective of this strategy is to find levels of irrigation and fertilization that will support high yield and reduce pruning requirements. Regulated deficit irrigation in conjunction with leaf nutrient analysis may be useful tools to apply experimentally toward meeting this objective.

Factor two: tree spacing

Because trees grow very rapidly on the west side of the southern San Joaquin Valley, growers have less time to enjoy the early yield benefits of closely spaced trees and more time to pay for the pruning required to keep older trees separated for efficient nut harvesting. This observation suggests that pistachio trees could be planted further apart in this area of the pistachio production area.

Although tree spacing is confined by the size of the harvesting equipment, spacing trees 20 feet between rows and 20 feet between trees within the row appear to be economical. Wider in-row spacing could have several advantages. Trees can be hedged not only down the rows, but the wider spacing would provide space for hedging across the rows (called cross hedging). Hedging the four sides of tree could be accomplished on some alternating multi-year schedule. The wider tree spacing, in combination with intervals of light hedging in both directions, would reduce shading between neighboring trees. Less shading would further decrease inter-tree competition for light and the associated production of branches stretching toward light. Moderate mechanical topping, perhaps once every three or four years, would limit tree height, and fruit wood would be encouraged around the entire circumference to the canopy and not just on top, as currently occurs in some crowded orchards. Perhaps a 12 or 13 foot tree height would be the objective instead of 16 feet or higher now seen in some orchards.

Mechanical hedging and topping should probably begin relatively early in the life cycle of the tree, focusing on pruning relatively less plant material more often. The trees would end up being shorter and wider than the tall closely planted trees currently seen in many orchards. Shorter trees would result in similar nut production to taller trees since the productive area of the tree remains the same. With shorter trees the productive area is closer to the ground and a greater surface of the canopy is exposed to high light levels. Shorter trees would be more efficient to shake for harvest and to spray for pest control.

A more frequent mechanical pruning schedule should moderate alternate bearing as well. Trying to play catch-up topping on older trees will only result in the huge yield loss associated with trying to remove 3 or 4 feet of large scaffold branches from the top of the tree in one swoop or 2 feet in each of 2 swoops. Pruning operations of this kind will destroy tree structure, leaving very little intermediate size wood for rapid replacement of lost fruit wood. Growers with excessively large trees do not have attractive options.

Factor three: tree training

Mechanical pruning can reduce the amount of hand-pruning required, but some interior pruning would still be required. Relying more heavily on mechanical pruning has the attendant concern of producing a tree that is nothing more than a cellulose box. The amount of interior pruning required could be reduced by stringent tree training in the first few years after planting. Tree training generally begins during the first dormant season after planting. Training usually involves a combination of thinning cuts (i.e. branch removal) and heading cuts (i.e. branch shortening). Wherever a heading cut is made, two or three vegetative buds below the cut will push, producing two or three new branches. If heading cuts are favored over thinning cuts, a geometric increase in new branches occurs low in the tree during the first few training years. In contrast, by emphasizing thinning cuts during the first few years, and heading cuts later, more of the tree arises higher in the tree. By reducing the number of branches lower in the tree, long, leggy branches that have no space to subdivide are minimized as is competition for light among branches. A more open tree will be produced that is more amenable to mechanical pruning in that fruit wood will be produced at greater depth in the tree canopy, and mechanical pruning operations will not be as destructive to overall yield.

Factor four: high cost of hand pruning

Dependence on outside labor, and at what rate its cost will increase, is a major grower concern and one that is often outside of the control of the grower. Watching trees get taller and taller, and orchards shadier and shadier, is another concern, that appears to have no easy solution once it exists. Greater reliance on mechanical pruning should reduce overall pruning costs compared to the heavy reliance on hand-pruning that currently exists.

IN CONCLUSION

Based on current observations of some orchards on the west side of the San Joaquin Valley, increasing tree height is a problem that facing some growers. Eventually the grower will have to risk losing some yield to maintain trees at a manageable height. The question posed is whether it is better to spend some money and risk some yield loss now or postpone concerns to a much later date. This article suggests that it may be of economic benefit to the grower to begin to think about managing tree size even before trees are planted and to address tree size annually there after. The pistachio tree has demonstrated an excellent ability to compensate for relatively modest annual pruning activities. Waiting until closely-spaced trees are twenty years old before a problem with excessive tree height is first addressed will certainly limit a grower’s options and eventually result in a less productive and more expensive orchard to operate.