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. 

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.

High levels of salts can accumulate in the soil with each irrigation.

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?

University of California (UC) researchers and private industry consultants have invested much effort in correlating optimal citrus tree growth, fruit quality and yield to concentrations of necessary plant nutrients in citrus (especially orange) leaf tissue. The grower can remove much of the guesswork of fertilization by adhering to UC recommendations of critical levels of nutrients in the tissues of appropriately sampled leaves.  Optimal values for elements important in plant nutrition are presented on a dry-weight basis in Table 1.  Adding them in appropriate rates by broadcasting to the soil, fertigating through the irrigation system or spraying them foliarly may correct concentrations of nutrients in the deficient or low range.  Compared to the cost of fertilizers, and the loss of fruit yield and quality that can occur as a result of nutrient deficiencies or excesses, leaf tissue analysis is a bargain.   At a minimum, the grower should monitor the nitrogen status of the grove through tissue sampling on an annual basis.

Leaves of the spring flush are sampled during the time period from about August 15 through October 15.  Pick healthy, undamaged leaves that are 4-6 months old on non-fruiting branches.  Select leaves that reflect the average size leaf for the spring flush and do not pick the terminal leaf of a branch.  Typically 75 to 100 leaves from a uniform 20- acre block of citrus are sufficient for testing. Generally, the sampler will walk diagonally across the area to be sampled, and randomly pick leaves, one per tree.  Leaves should be taken so that the final sample includes roughly the same number of leaves from each of the four quadrants of the tree canopy.  Values in Table 1 will not reflect the nutritional status of the orchard if these sampling guidelines are not followed. Typically, citrus is able to store considerable quantities of nutrients in the tree.  Sampling leaves from trees more frequently than once a year in the fall is usually unnecessary.  A single annual sample in the fall provides ample time for detecting and correcting developing deficiencies.

Table 1.  Mineral nutrition standards for leaves from mature orange trees based on dry-weight concentration of elements in 4 to 7 month old spring flush leaves from non-fruiting branch terminals.

*California and Florida recommendations for K are sufficiently different that they are presented separately.  The California standards are based on production of table navels and Valencias, and those for Florida were developed primarily for juice oranges like Valencia.

The sampled leaves should be placed in a paper bag, and protected from excessive heat (like in a hot trunk or cab) during the day.  If possible, find a laboratory that will wash the leaves as part of their procedure instead of requiring the sampler to do this.  Leaf samples can be held in the refrigerator (not the freezer) overnight.  Leaves should be taken to the lab for washing and analysis as quickly as is feasible.

Often separate samples are taken  within a block if areas exist that appear to have special nutrient problems.  The temptation encountered in sampling areas with weak trees is to take the worst looking, most severely chlorotic or necrotic leaves on the tree.  Selecting this type of leaf may be counter-productive in that the tree may have already reabsorbed most of the nutrients from these leaves before they were sampled. A leaf-tissue analysis based on leaves like this often results in a report of general starvation, and the true cause of the tree decline if the result of a single nutritional deficiency may not be obvious.  Often in weak areas, it is beneficial to sample normal appearing or slightly affected leaves.  If the problem is a deficiency, the nutrient will, generally, be deficient in the healthy-looking tissue as well.

Groves of early navels that are not normally treated with copper and lime as a fungicide should include an analysis for copper. Copper deficiency is a real possibility on trees growing in sandy, organic, or calcareous soils.  For later harvested varieties, leaves should be sampled before fall fungicidal or nutritional sprays are applied because nutrients adhering to the exterior of leaves will give an inaccurate picture of the actual nutritional status of the tree.

Usually leaf samples taken from trees deficient in nitrogen will overestimate the true quantity of nitrogen storage in the trees. Trees deficient in nitrogen typically rob nitrogen from older leaves to use in the production of new leaves.  Frequently, by the time fall leaf samples are collected in nitrogen deficient groves, these spent spring flush leaves have already fallen.  Nitrogen deficient trees typically have thin-looking canopies as a result of this physiological response.  Since the spring flush leaves are no longer present on the tree in the fall when leaves are sampled, younger leaves are often taken by mistake for analysis. These leaves are higher in nitrogen than the now missing spring flush leaves would have been and provide an inaccurately higher nitrogen status in the grove than actually exists.

Critical levels for leaf-nitrogen for some varieties of citrus, like the grapefruits, pummelos, pummelo x grapefruit hybrids and the mandarins, have not been investigated as well as those for oranges.  However, the mineral nutrient requirements of most citrus varieties are probably similar to those for sweet oranges presented in Table 1, except for lemons, where the recommended nitrogen dry-weight percentage is in the range of 2.2- 2.4%.

A complete soil sample in conjunction with the leaf sample can provide valuable information on the native fertility of the soil with respect to some mineral nutrients and information on how best to amend the soil if necessary to improve uptake of fertilizers and improve water infiltration.

Iron deficiency of citrus