Zinc (Zn) is an essential micronutrient for plant growth; it plays a critical role in the function of enzymes and in nitrogen metabolism of plants. Although it is essential for their growth, crops take up relatively small amounts of zinc (generally < 0.5 lb/acre/crop). Zinc is a divalent cation (Zn²⁺) and its radius is about the same size as iron and magnesium, which allows it to substitute for these ions in soil minerals such as hornblende and biotite. Most soil Zn occurs in mineral structures, but Zn also occurs as salts of varying solubility (e.g. ZnS, ZnCO₃ and ZnO) and on the exchange sites of clay minerals and organic matter. Historically, Zn deficiency was widespread in California, particularly on weathered soils.  However, due to the widespread application of Zn fertilizers over years, Zn deficiency is now much less common.

The bioavailability of Zn is determined by several factors, the most important of which is pH. The solubility of Zn decreases with increasing pH. For instance, in the range of 5.5 to 7.0 the concentration of Zn in the soil solution may decrease 30 to 45 times for each unit increase in soil pH. Other factors that decrease the availability of Zn in the soil solution include: high clay content, high phosphorus and low soil temperatures. However, the bioavailability of Zn at a given pH may also depend on the quantity of natural chelates from organic matter, and other factors.

Zinc deficiency causes distinct symptoms such as interveinal yellowing, stunting and leaf distortion. On lettuce zinc deficiency appears as severe stunting and yellowing along the edges of the leaves that turn brown and “papery”. Zn sufficiency levels for annual crops range from approximately 15 to 30 ppm in leaf tissue, with values above 200 rarely observed.  Leaf levels of 300 to 400 ppm can be toxic to crop plants.

The most common soil Zn test is DTPA extraction, a non-aggressive extractant that gives a reasonable measure of plant-available Zn. The following chart shows the DTPA soil test levels at which Zn fertilization would be likely to improve crop growth.

In 2013 we conducted a survey of over 50 fields throughout the Salinas Valley. Levels of DTPA extractable soil Zn varied from 1.3 to 4.3 ppm, with an overall average of 2.4 ppm. These data indicate that most soils in the valley have adequate quantities of Zn for optimal crop growth.

In that survey the average levels of Zn in head lettuce, romaine and spinach were 27, 30 and 73 ppm, respectively; levels of Zn in spinach varied greatly and ranged from 41 ppm to 112 ppm. The highest Zn levels were on heavy soils in the Blanco, while the lowest were along the river on the sandy soils. All of these fields appeared healthy and the observed levels of leaf Zn appeared to be adequate, based on published tissue sufficiency levels for these crops.

When choosing a Zn fertilizer the primary considerations are solubility and cost (see the table below). Highly soluble Zn forms are likely to be more plant-available for the first crop after application than less soluble forms. However, over time Zn in soil solution will combine with other minerals to form less soluble compounds, the exact compounds formed depending on soil pH. Less soluble Zn fertilizers can be effective over time, and are generally less expensive per unit of Zn than highly soluble forms.

Source: International Zinc Association, www.zinc.org

Nitrate test strips are an affordable tool for quickly measuring nitrate (NO₃) in soil and water, and can help farmers and crop advisers adjust fertilizer inputs to match the nitrogen (N) needs of various types of crops. There are now a variety of brands of nitrate test strips available, many of which are manufactured for testing the quality of aquarium water, but may also be suitable for soil testing.   All of the brands of test strips are used in a similar fashion: the strip is briefly dipped into an extractant solution (for soil) or in water, and allowed to develop color during a standard interval of time, usually ranging between 30 and 60 seconds.  After color develops on the strip, a color chart, calibrated to either parts per million (ppm) of NO₃ or expressed in ppm equivalents of nitrogen (NO₃-N), is used to determine the NO₃ concentration of the sample. Multiplying Nitrate-N concentration by a factor of 4.43 converts the reading to NO₃ concentration.  Because the strips may continue to develop color with time, it is important to always read the strips at a standard time interval, or the measurements will not be accurate or repeatable.  More detailed information on using the nitrate test strips for monitoring soil nitrate levels was presented in several of our past bulletins, newsletters, and blogs.

Depending on the soil type and crop nutrient requirements, vegetable farmers need test strips that are accurate for soil NO₃-N concentrations ranging between from 5 to 30 ppm, which would roughly correspond to a range of 10 to 60 ppm of NO₃ in the nitrate quick test extract solution.  For strawberry production, and other crops that have a slower N uptake rate than vegetables, growers need test strips that are accurate over a narrower range of soil NO₃ concentrations (5 to 15 ppm NO₃-N in soil).  Past studies have demonstrated that the Merckoquant test strip are accurate for measuring soil NO₃-N in the range of 10 to 40 ppm.   Because more brands of test strips have become commercially available in recent years with varying ranges of sensitivity, and the need to identify test strips that are accurate for measuring low concentrations of soil NO₃-N (0 to 15 ppm), we evaluated the accuracy and ease of using six commercially available brands of test strips over a range of nitrate concentrations found in commercial agricultural fields.

Procedures:

A stock solution of a known NO₃ concentration was prepared by dissolving a measured weight of sodium nitrate (NaNO₃) into 1 liter of distilled water.  This stock solution was further dilluted with distilled water to standard nitrate concentrations that matched the values of the color chips of the various test strips evaluated in this study.    The NO₃ concentration of each standard solution was confirmed by spectrophotometric analysis.

Each brand of strip was evaluated at NO₃ concentrations corresponding to the color chips provided by the manufacturer.  The Hach Aquacheck and Lamotte Instatest NO₃/NO₂ strips differed from the other brands because the color chips were calibrated in  equivalents of  NO₃-N rather than NO₃.  For convenience of displaying and comparing the data, results for these two brands were converted to NO₃ (by multiplying the NO₃-N values by 4.43).  The Merckoquant NO₃/NO₂ test strip was the brand originally tested by UC Cooperative Extension for use with the soil nitrate quick test, and was considered the standard in this evaluation.  This strip measures to a maximum of 500 ppm NO₃, but was only evaluated up to 250 ppm NO₃ (56 ppm NO₃-N) for this test. 

Each brand of test strip was evaluated 4 times for each standard NO₃ solution corresponding to the manufacturer's chip color chart.  The procedure that we followed to determine NO₃ concentration was to dip the strip briefly in solution, and hold it horizontally after removing it, allowing color to develop for the interval specified by the manufacturer.  Most strip manufacturers recommended a 1-minute time interval between wetting and reading the strip color.  The manufacturer for API 5-in-1 and LaMotte Instatest 5-Way recommended reading test strips after 30 seconds, but results appeared to be more accurate after a 60 second interval, therefore all results reported for these strips are from readings taken 60 seconds after placing the strip in the test solution.   After waiting the specified interval, the color of the test strip was compared to the color chips provided by the manufacturer.  If the test strip color matched one of the chips, then the value of the chip was recorded. In many cases, the color of the test strip was between 2 of the standard chips, and in these cases an estimate was made based on comparing the intensity of the color development with the 2 closest matching chips.  Because this method relies on visual observations, all tests were made in a room with ample lighting and by one observer. 

Results:

The mean NO₃ values measured using different brands of test strips were compared to the standard solution values in Table 1.  Some brands of test strips appeared to be accurate at specific ranges of NO₃ concentration.  The Merckoquant NO₃/NO₂ brand was the most accurate for the full range of NO₃ concentrations (Table 1).  The next most accurate brand over the entire range of NO₃ concentrations evaluated was the LaMotte Instatest NO₃/NO₂.  The Hach Aquacheck was accurate for the range of 10 to 90 ppm NO₃ but measured NO₃ lower than the standard solutions at concentrations above 100 ppm NO₃.  The remaining brands of test strips, LaMotte  Instatest 5-way, API 5 in 1, Tetra 6 in 1 Easystrips, all measured less NO₃ than the standard solutions over the range of 20 to 200 ppm NO₃.  These strip brands should probably not be used for the soil nitrate quick test and for assessing nitrate concentration in irrigation water. 

Although the LaMotte Instatest NO₃/NO₂ also had good accuracy across the range of 20 ppm to 220 ppm NO₃, it did not have a standard color chip for evaluating NO₃ at low concentrations, and therefore may not be suitable for strawberries and other crops where soil nitrate is typically in the 5 to 15 ppm NO₃-N  range.  Both the Merckoquant and Hach brands were accurate for measuring NO₃ at low concentrations (10 to 40 ppm).  Although the Hach Aquacheck strip had a color standard of 5 ppm NO₃, the strip was not able to measure NO₃ at a concentration below 10 ppm (Table 1).

With the exception of the Merckoquant NO₃/NO₂, all test strips were purchased online through Amazon.com.  The price reported for the strips in Table 1 was the purchase price advertised at the time our study was conducted (January 2014).  Some strips were available in larger quantities or from other vendors, for different prices. The Merckoquant NO₃/NO₂ can be purchased from Cole-Parmer (http://www.coleparmer.com) or at EMD Millipore (http://www.emdmillipore.com).

Summary

We identified 3 brands of test strips that accurately measured NO₃ and can be used to quickly assess the concentration of NO₃ in soil or water.   Both the Merckoquant NO₃/NO₂and the Hach Aquacheck strips were accurate for measuring concentrations of NO₃ as low as 10 ppm, which would roughly correspond to 5 ppm NO₃-N in soil.  No brand of test strip measured NO₃ accurately below 10 ppm.  Several brands of strips that measure NO₃ in addition to other constituents in water were found to under estimate NO₃ concentration, especially at high values.   While laboratory analysis of NO₃ is generally more accurate than using colorimetric test strips, the strips tested in this study appear to be sufficiently accurate to estimate the level of residual mineral N in soil samples and for determining the NO₃ contribution from irrigation water, and should be useful for quickly assessing soil N status before making a fertilizer decision. 

Table 1.  Comparison of nitrate-test strip and standard solution values.

The weed science program at UC Davis has a long and storied history which set the program on its course to develop practical weed management options for growers. Much of the progress has been built on use of herbicides to control weeds in the wide diversity of California crops. Undoubtedly California growers have much better weed management options today than they did in 1940 or 1950 thanks to translocated herbicides like glyphosate which enable us to suppress the most difficult perennial weeds like field bindweed. Much of this progress in weed management is owed to the agrochemical industry which through private investment in weed management research, developed some very effective products. However, conditions are not static, and the status of private investment in weed research today has diminished much from what it was as recently as the 1970's or 1980's. This essay will explore some of the implications of the relationship between the agrochemical industry and California vegetable crops. Suggestions for new weed control tools will be explored.

Herbicides are different than fungicides or insecticides in the eyes of industry. The agricultural chemical industry since the 1990s has placed much more emphasis on developing new fungicides and insecticides and less on herbicides. There are many reasons for this: 1. Higher potential for injury to high value crops like vegetables with herbicides than for fungicides and insecticides, i.e., the liability exposure for registrants is higher than the value of potential sales; 2. Glyphosate tolerant corn, soybean and cotton has reduced incentives for development of herbicides in major crops, and fewer herbicides are being introduced than say 30 years ago; 3. Herbicides are much more likely to persist in soil and injure rotational crops than fungicides or insecticides, hence there are few potential herbicides available for vegetables. Proof that industry has spent much more time on fungicides and insecticides is easy to find. According to the California Dept. of Pesticide Regulation data base, and using head lettuce as an example, there are 6 major fungicides, 8 major insecticides and 1 major herbicide, with “major” defined as use on more than 40,000 acres of lettuce. The average age of the fungicides is 17.2 years, insecticides 18.9 years and herbicide 27 years. With constant development of disease and insect pest resistance there is need for new products. In contrast, there has been little development of herbicide resistant weeds in California vegetables due to the integrated weed management system used in those crops.

Better options for weed management in California vegetable crops. For years California vegetable crops have been hand weeded to supplement the partial weed control provided by herbicides like DCPA, and pronamide. However, with a growing agricultural and industrial economy in Mexico, California is competing with Mexican farmers and factory owners for labor. The result is less labor available for California farms. However there is considerable interest in Europe for development of intra-row cultivators, as high labor costs and interest in physical weed control tools is much further along there than here. I am aware of three commercial intra-row cultivator models available: Ferrari from Italy, Stekettee IC-cultivator from the Netherlands, and Visionweeder from Denmark. These cultivators use machine vision to weed in the plant line by crop recognition based on pattern analysis, i.e., it sees the plant line. The cultivators remove weeds by pushing cultivator knives into the seedline to uproot weeds and withdrawing the knives to protect the saved crop plant. There also has been quite a bit of work in the area of robotic lettuce thinning, and since 2013 some commercial adoption has occurred in Arizona and California lettuce production districts. These devices spray an herbicide or fertilizer solution to reduce direct seeded lettuce stand from say 3 inches to 9 to 10 inch spacing. Standard practice for decades has been to use laborers with hoes to thin lettuce. Machine lettuce thinners have the potential to replace the hand thinning operation. Lettuce thinners also have the potential to remove intra-row weeds, but more research must be done in that area. Most of these robotic cultivators and lettuce thinners have been developed in the past several years, which is in stark contrast to the “old” herbicide situation. There are many new robotic options for weed management in vegetable crops, there are few or no new herbicides.

 A future direction for UC Davis weed science. Given the complexities of the California pesticide regulatory environment, a high urban population with strong anti-pesticide sentiments, the diverse set of crops in which to manage weeds, a shrinking pesticide industry, and a strong technology sector in Silicon Valley, it is very easy to make the argument that the future for robotic and physical weed control is bright in California and the future for herbicide development likely to remain stagnant. I would suggest that a weed science research focused on engineering of weed removal devices would be very productive here and would draw students from around the world. There is considerable need for better and faster crop and weed recognition systems, new devices for killing weed and weed seed banks with heat, sprays, steam or electrical impulses. In the past weed management has used a passive model delegating the creation of new weed removal tools, i.e., herbicides, to the agrochemical industry. When the agrochemical industry was active in the 1970's new weed control tools were created. When the flow of new herbicides diminished, little new development has occurred in the area of vegetable weed management. Meanwhile some useful products like diethatyl have been removed from the market, and for a time cycloate and DCPA were temporarily unavailable as there was no commercial supplier, and pronamide is now only registered on head lettuce not leaf lettuce. I argue for an active model in which robotic devices are created in collaboration with engineering partners, and made available commercially. The advantage to this approach is to develop weed control “devices” like intelligent cultivators, and avoid the regulatory quagmire in which pesticides operate.

In closing, the development of glyphosate resistant crops has led to the widespread violation of the principles of IPM – with disastrous results. We must forge a different path that maintains the excellent integrated weed management systems used for decades in California vegetable crops. However, the high labor inputs for hand weeding and other operations are unsustainable in the long run. We must find more sustainable weed control tools for California growers and accept the fact that there is little the agrochemical industry can do to save us. We can and must create our own weed management solutions. We in California have the chance to take a leadership role for the US in the development of robotic weed removal systems – let's grab this opportunity with both hands and not let go.