- Author: Ben Faber
I just raked up all the leaves under the avocado and it looks so nice. PUT THEM RIGHT BACK. The avocado is shallow-rooted and really depends on the natural leaf mulch to protect its roots. In fact the roots will actually colonize the rotted leaves as if it were soil. This mulch is also a first line of defense against root rot. The decomposing leaves create a hostile environment to the microorganism that causes the disease. The mulch also helps to reduce evaporative loss of water and therefore reduces water needs. Avocado growers will actually spread mulch in cases where trees are too young to produce adequate leaf drop for mulch or in windy areas where mulch has blown away. The key to remember is that the mulch should be kept at least six inches away from the trunk to avoid collar rot which can be causes by keeping a moist mulch against the trunk.
- Author: Ben Faber
It’s that time of year when citrus and avocado growers need to collect leaf samples for nutrient analysis to guide fertilizer applications. Leaves are collected between August 15 and October 15 and sent to the lab for analysis. For perennial crops, leaf analysis is the most important guideline for managing tree nutrient applications. Many growers think that soil analysis is as important as leaf analysis, and is for annual crops, but is much less valuable for tree crops. Because a tree stores nutrients in its various parts, such as roots, trunk, branches, stems and leaves, it does not have to get all of its immediate nutrients from the soil the way a lettuce plant does. Trees also have a root association with beneficial fungi called mycorrhizae (fungus/roots) which aid in the uptake of nutrients such as phosphorus and zinc, and this ability is not reflected in a soil analysis. A leaf analysis integrates everything the tree is "seeing" – weather, soil, in-tree storage, water, crop load, disease – which is then reflected in the leaf analysis.
Leaf analysis is done at this period, because the leaf nutrients are somewhat stabilized. Young leaves are high in such nutrients as nitrogen and potassium, but low in zinc and iron. As the leaf matures it loses nitrogen and potassium, but gains in iron and zinc. A fully expanded four-month old leaf from the spring flush taken at this time of year has been found to best reflect the tree’s nutrient status. For a discussion on leaf sampling, see our fall 2003 edition of Topics in Subtropics - http://ceventura.ucdavis.edu/newsletterfiles/Topics_in_Subtropics3707.pdf.
If leaf nutrients are low or high, it can indicate not only what nutrient is the problem, but also what sort of corrective actions should be evaluated. It may not be the lack of something like iron, but waterlogging from too long or frequent irrigations. Waterlogged soils reduce iron uptake, and this deficiency might be better addressed by correcting the irrigation practice than spending money on iron applications. Zinc deficiency might be a result of root rot killing root hairs that take up zinc and addressing the disease issue is going to have a longer term improvement on tree nutrient status than simply applying zinc fertilizer. And then of course, if leaves are showing toxicities to sodium or chloride, correcting irrigation leaching and infiltration issues is the way to solve this nutrient problem, since this the easiest way to solve the problem.
This does not mean soil and water analyses are not important, on the contrary. A pre-plant analysis for water and soil can tell you before hand what you might be dealing with and allow you to correct the problem before planting. A high pH is best corrected before trees are in the ground. Trying to correct a zinc, iron, manganese, or copper deficiency with the trees in the ground is expensive and can take years to correct. It is easier to apply sulfur or sulfuric acid to the ground before planting and can be done relatively quickly without harm to the trees. The micronutrient availability is controlled by pH and once soil pH is in the 6-7 range, it is less likely for these deficiencies to occur. Trying to lower pH when the trees show iron deficiency, must be done slowly, since adding too much acidifying material at one time can kill the tree and during the process of acidification, some sort of stop gap measure, such as foliar feeding or fertigation must be employed until the soil pH has slowly been corrected. A water analysis too can forewarn you if you will be having problems with such things as high salinity, chloride, sodium, magnesium, boron or pH, and allow you to select appropriate rootstocks tolerant of the problem or again address it with soil amendments pre-plant.
A soil analysis in conjunction with water analysis can also be used for an ongoing determination of how well irrigation is being managed. Soil from trees doing poorly can be analyzed to see if adequate leaching is being accomplished with the frequency and amounts being applied. Generally, though, a soil analysis is a poor indicator of guiding a tree nutrition program and as an ongoing practice should be used for identifying the toxicity problems of salinity, boron, sodium, chloride and pH.
- Author: Ben Faber
- Author: Jim Downer
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.
- Author: Ben Faber
This little mnemonic, or memory aid, in the title is helpful in remembering the critical levels of toxic constituents in irrigation water. The “one” stands for 1 part per million (ppm) of boron (B), the “one hundred” flags 100 ppm of sodium (Na) and chloride (Cl) and the “one thousand” represents the level of total soluble solids (TDS or salts) in water. Levels exceeding the critical values for any of these constituents can present problems for tree growers. The problems typically show themselves as tip-burn and defoliation. The B, Na and Cl are toxic elements at relatively low concentrations, but symptoms appear similar to the damage caused by high salinity.
Water that exceeds the critical levels mentioned in the mnemonic has a greater tendency to cause damage if sufficient leaching is not applied. It doesn’t mean the water is impossible to use, only that greater attention needs to be made to ensure that these salts are adequately leached. High levels of these salts accumulate in the soil with each irrigation. These salts are absorbed by the tree and end up in the leaves where they do damage.
Irrigation is a necessary evil. Every time we apply irrigation water we apply salts, and unless some technique is used to minimize salt accumulation, damage will result. This damage can be more than just leaf drop, but also the stress that induces conditions for root rot. In most years we rely on winter rainfall to correct the salt imbalance resulting from irrigation water.
This year has been a winter largely without rain. Irrigation water was applied throughout the winter, spring, summer and fall and many trees look stressed this spring. Even well irrigated orchards in the spring of 2012 have leaf burn due to the gradual accumulation of salts from irrigation. Avocados, which are generally more sensitive to salts than citrus, drop their salt-burned leaves this spring when flowering begins.
We usually think that it is not necessary to irrigate in the winter, but this winter should change that opinion. To add to the lack of rain problem, it may be necessary to irrigate even if there is rain in the future. The wetted pattern that is created by a drip or microsprinkler emitter also creates a ring of salt in the outer band of the wetted patter. If there is less than an inch of rainfall to push this salt down, this salt tends to diffuse towards the tree where it can accumulate back in the root system. Orchards with even good water quality would find it advisable to run the irrigation system with the first rains. Those with poor water quality definitely should run the microsprinkler system with an equivalent of one-half inch-applied water (13,500 gallons per acre) during or soon after the first events of less than one-half inch rainfall. Growers with water quality exceeding one, hundred, or thousand should be especially alert to the need to manage water in low rainfall winters.
- Posted By: Chris M. Webb
- Written by: Mary Bianchi
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!
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.
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: