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.

Leaf analysis is the preferred method of guiding a fertilizer program for fruit tree crops.  Soil testing is less important, since the tree has the capacity to store nutrients in its various parts – roots, trunk, stems and leaves.  However, soil testing is a component of a plant nutrient management program and has been standard practice for growers to aid in adjusting fertilizer applications.  Soil testing is performed not only to improve plant growth, but also to reduce over-application of fertilizers that may lead to nutrient toxicities, excessive leaching and consequent economic losses.

For maximum accuracy and benefit, soil testing must be conducted using reliable methods on correctly-sampled soils (if the user is not trained in obtaining representative soil samples, test results even from the same soil can vary greatly). Test results must also be properly interpreted for a specific crop.  Interpretative guidelines are readily obtainable for many agronomic and horticultural crops, as well as landscape trees.  Cost for laboratory analysis for pH, NO₃-N, P₂O₅ (Olsen), and extractable K₂O are typically under $20 per analysis, but frequently results take from 1-4 weeks to get back to the grower.

By contrast, many retail garden centers offer commercial test kits, ranging in cost from $10 to $50 for multiple tests, so that the cost per test can be relatively low.  These commercial kits are also advantageous because results can be obtained within one to two days.  Commercial kits typically use a colorimetric method for indicating macronutrient and pH levels.  Soil is measured into a sample container, extractant is added, and after a specified time for the reaction, the user compares the color obtained to a color card corresponding to categorical nutrient and pH levels.

We have always wondered how well these kits performed, so we purchased  five commercially-available test kits and  compared their results to standard laboratory analysis of NO₃-N, P₂O₅ (Olsen), extractable K₂O and pH from the same soil type with three distinct cropping histories (Soils 1, 2, and 3).  The objectives were to identify differences in accuracy, if any, among test kits and to suggest a kit that most closely corresponds to analytical lab results.

Four of the kits, “La Motte Soil Test Kit” (La Motte Co., Chesteron, MD); “Rapitest®” (Luster Leaf Products, Woodstock, IL); “Quick Soiltest” (Hanna,Woonsocket, RI); and “NittyGritty” (La Motte Co. Chesteron, MD) measured nitrate-N, P₂O₅, K₂O and pH.  “Soil Kit” (La Motte Co., Chesteron, MD) measured only nitrate-N, P₂O₅ and K₂O.  The kit results for macronutrients were categorical (high, medium, and low); pH results were numeric, rounding to half pH units for the Rapitest® and one pH unit for the other three kits. The manufacturers’ instructions for each kit were followed for soil testing.

Results show that pH measures from LaMotte Soil Test Kit and Rapitest consistently matched lab results.  Soils 1 and 3 proved to be in the pH 6.5 range, but the pH of Soil 2 was 7.8, technically beyond the capacity of Rapitest (pH 4.5-7.5).  NittyGritty did not match lab results at all.  Quick SoiltTest generally indicated lower pH values than the analytical lab. Results from LaMotte Soil Test Kit, Rapitest, and Quick Soiltest consistently matched the analytical lab results for nitrate-N and P₂O₅, while Soil Kit and NittyGritty did not. Soil Kit and NittyGritty analyzed K₂O content with greater accuracy than for the other nutrients; the commercial tests in total corresponded with the analytical lab 82% of the time for this test.  For Soil 3; all the commercial test results matched the analytical lab results 100%.

Precautionary measures for these commercial kits may increase their accuracy.  For Soil Kit and Nitty Gritty, the extracting powders that came with the kits dissolved poorly; these kits generally yielded inaccurate results, but pulverizing the tablets or powders may increase extraction potential.  Interpretation of color development should be made only within the time specified by the kit instructions because color intensity could vary within minutes.  Also, interpretation can occasionally vary depending on the user.  In this study, the observers independently interpreted the same result for 91% of the tests; this would probably be an acceptable proportion for a home gardener or farmer individually conducting tests, but occasional independent interpretation by another source may change the result.

La Motte Soil Test Kit results corresponded to those from the analytical lab for pH and all nutrients (86% of the tests matched).  This kit is suitable for growers because it proved to be very accurate even over a range of pH values and is housed in a hard-sided, padded container.  Rapitest yielded accurate results 92% of the time for all nutrients and pH less than 7.5, and was comparatively easy to use and interpret. Quick Soiltest matched the analytical lab results only 64% of the time because pH and K₂O values were inaccurate.  Interpretation of values from this kit may have resulted in application of potassium in excess of the needs of Soils 1 and 2.

An important limitation of all commercial test kits is the approximate or categorical value of nutrient content (i.e., low, medium, high).  Analytical labs must be used when precise values are required.  Nevertheless, commercially-available kits such as Rapitest and La Motte Soil Test Kit have shown to provide accurate, fast, and economical results and can help growers improve nutrient management. 

Biological control of Phytophthora cinnamomi in avocado through the use of mulches was identified by an Australian grower and later described as the "Ashburner Method" by Broadbent and Baker. The technique uses large amounts of organic matter as a mulch along with a source of calcium. Control of avocado root rot in the Ashburner method was attributed to the presence of Pseudomonas bacteria and Actinomycetes. Multiple antagonists are more likely the cause of biological control, since no single organism has been found to be consistently associated with soils suppressive to P. cinnamomi.

The use of organic mulches has multiple effects, such as altered soil nutrient and water status and improved physical structure. Any improvements in plant status resulting from improvements in the growing environment can improve plant health. The effect of organic amendments on soil physical and chemical properties can vary considerably depending on soil texture and the environment. One of the most consistent effects of organic amendments is an increase in biological activity. Increases in organic substrate lead to increased fungal and bacterial populations. In numerous cases, this increase in biomass has been associated with disease suppression. This biological control can be ascribed to several mechanisms: competition, antibiosis, parasitism, predation and induced resistance in the plant.

The microbial biomass is responsible for release of enzyme products and polysaccharides in soils. The microbially-produced enzymes cellulase and glucanase have been demonstrated to have a significant effect on Phytophthora populations. This mechanism of antibiosis is possible because the microbes are releasing these enzymes to solubilize organic matter. Unlike other fungi, Phytophthora have cell walls that are comprised of cellulose and in the process of decomposing organic matter with enzymes, an environment is created that is also hostile to the pathogen.

In order to see if there might be potential differences in organic materials being better at combating avocado root rot, a little field trial was established with 23 different types of materials. The mulch materials were obtained from nearby hedges and chipped or obtained from commercial sources of mulch. Some of these materials would be difficult to get in large amounts, such as manuka (Leptospermum scoparium), but others are commercially available chipped greenwaste. The materials were then spread on the ground to a depth of five inches, in separate plots that were 36 X 36 inch squares. Decomposition was measured over a two year period and then cellulase was measured in the mulch, at the soil / mulch interface and at a two inch depth in the soil at the end of 2 years.

Since cellulase production is part of the decomposition process, the rate of decomposition should be a partial indicator of the amount of cellulase present. After a mulch application there is generally settling due to rainfall-caused compaction, but much of the decline by the second year is due exclusively to decomposition. The more recalcitrant materials, such as bark, wood chips and sawdust have barely lost half their depth after two years, while others such as shredded eucalyptus, manuka, avocado and willow are less than 20% of their initial depth. Much of the shredded/chipped material, such as eucalyptus had a significant fraction of leaves in the mulch. The wool disappeared a little after one year. The greenwaste + chicken manure compost is nearly the same depth as the wood chips, since it is a material that had gone through a decomposition process prior to its application and much of the easily digestible materials had already been decomposed.

The rate of decomposition has some bearing on the rate of cellulase production. Eucalyptus and manuka had the two greatest rates of decomposition and show the highest levels of cellulase production. The cellulase levels were consistent with all the different mulch materials. Using decomposition rate alone is not a complete indicator of cellulase production since, poplar, willow and avocado had high rates of decomposition, but their cellulase rates were half those of manuka and eucalyptus.

It is clear that the cellulase effect is limited to the layer of mulch and not to depth within the soil. There is some effect at the soil surface, but at 5 cm (2 inches) cellulase activity drops to background levels. There is earthworm activity at the test sites and one idea was that earthworm incorporation of organic matter would move the cellulase production into the soil. Maybe with further time this would occur. As it is, when mulches are applied to avocado, the roots tend to proliferate in the mulch, out of the soil where the cellulase activity is the least.

Something to keep in mind is that we do not know what levels of cellulase are necessary to control the root rot fungus. It may be that levels seen with pine bark are more than adequate. Also we have measured cellulase production at only one time in a two-year period and it is quite likely that this is not the best snapshot of what is happening before and after. A further reminder is that cellulase is only one of the many byproducts associated with decomposition and many of the antagonistic properties that are associated with the microbial biomass are not being measured in this trial. Having developed this screening procedure what needs to be done next is to take high, medium and low cellulase producing mulches and challenge the fungus to verify that this is a good way to evaluate mulches.

Improving root health with mulches.

We’d like to challenge you to take the following quiz.  Take a minute to place a check mark next to all the practices you regularly employ in your operation.  Go ahead – we won’t be collecting them!

Part 1

Yes/ No I know what the nitrogen requirements (lbs actual N/acre/year or /tree/year) are for my crops

Yes/ No I know what the nitrogen levels are in soil amendments I use in my operation (compost, manure, crop residues, etc.)

Yes/ No I have lab analysis of my well/irrigation water.

Yes/ No I monitor tissue levels of nitrogen in my crops to help with fertilizer decisions.

Yes/ No I have put together a nutrient budget that considers all sources of nitrogen for the crops I produce.

Part 2

Yes/ No When I do apply nitrogen, applications are timed according to crop requirements.

Yes/ No I use fertigation to apply nitrogen.

Yes/ No Applications of nitrogen are split into smaller doses to improve efficiency of uptake.

Yes/ No I use cover crops that help manage nitrogen availability.

Yes/ No I manage irrigations to avoid nutrient loss below the rootzone of the crop.

If you marked yes to these as regular activities, you’ve just taken steps in showing how your production decisions can protect water quality.  The combined activities noted in Part 1 constitute a Management Practice that protects water quality by developing a nutrient budget to help apply only the appropriate amounts of fertilizer.  Activities in Part 2 may alone or in combination constitute Management Practices that help ensure fertilizers are applied efficiently.

Every grower uses ‘management practices’, many of which are meant to generate the best possible product for market.  Depending on who you’re talking with, the term ‘management practice’ can be something your Farm Advisor recommends (i.e., pruning to control tree height), your produce buyer suggests (protect avocados in bins from sun scald), or the term can have regulatory connotations.

You’ve all probably heard the term Best Management Practices.  Best Management Practice (BMP) is defined in the Federal Clean Water Act of 1987, as “a practice or combination of practices that is determined by a state to be the most effective means of preventing or reducing the amount of pollution generated by nonpoint sources to a level compatible with water quality goals.” The term “best” is subject to interpretation and point of view. In recognition of this, the Coastal Zone Reauthorization Amendment (2000) substituted the terms Management Measures and Management Practices.

How can you tell if any individual activity constitutes a Management Practice that meets the needs of a regulatory program to protect water quality?  Ask yourself this question:  Can the activity stand alone and result in water quality benefits?  Just knowing the nitrogen requirements of your crop doesn’t result in any water quality benefits – developing and using a nitrogen budget for your crop can.  A nitrogen budget that takes into account the nutrients applied in amendments, irrigation water, and fertilizers in meeting the requirements of your crop does have the potential to protect water quality from nitrogen pollution from your operation.

Some Management Practices can have water quality benefits as a stand alone activity.  Cover crops are recognized as a Management Practice that can help manage both sediment and nutrients to reduce the potential of pollution when used appropriately.

Water quality protection is being asked of all industries in California.  You have the opportunity to take credit for all of the activities you already do, like the ones listed above, that protect your local water bodies and/or groundwater from nonpoint source pollution from your operation.  Look for additional articles in the coming issues to help you in this effort.

For additional background information on water quality legislation, and nonpoint source pollution from agriculture you can download the following free publications from the University of California’s Farm Water Quality Program:

Water Pollution Control Legislation

Nonpoint Sources of Pollution from Irrigated Agriculture

Can you answer these questions?