Nov 18, 2011
Do you need tissue and petiole sap testing?

Getting high yields, excellent quality and long shelf life out of tomatoes and peppers requires intimate knowledge of soil nutrient levels, plant tissue nutrient concentrations, irrigation water characteristics and specific varietal requirements. The only way to remain competitive and thus sustainable is by applying precise amounts of nutrients when plants need them. This precision directly increases nutrient efficiency through waste reduction and helps to prevent the movement of nutrients offsite and into the water table. A combination of pre-plant soil testing, timely plant tissue analysis and sap testing for N and K levels gives growers the tools they need to extract the greatest yield of packable fruit.

Pre-plant soil analysis: Knowledge of the extractable quantities of P, K, Ca, Mg, plus soil pH, is the beginning of producing a great crop. Usable soil analysis begins with a well-collected sample. For produce, your sample should be collected from at least 20 points per acre at a depth of 6 to 8 inches. Use a collection pattern that is representative of the entire field, unless soil types vary, then break the sample into sections. Since each crop’s demand for nutrients varies, be sure to break the field into crops – or the recommendations that come with most soil tests will be too vague to be of any use. “Knott’s Handbook for Vegetable Growers, 5th edition,” is an excellent reference for individual crop nutrient requirements. Newer, higher yield-potential varieties require greater nutrient levels than older varieties, so be sure to factor in varietal characteristics when planning your fertility program.

Depending on your fertigation timing and frequency, you will want to apply 30 percent to 50 percent of your nutrient needs at bed or soil preparation. The remainder should be applied based on crop requirements, as indicated by plant tissue and petiole sap analysis.

Plant tissue analysis

Timely plant tissue analysis is a powerful tool. It is the single best method to stay on top of crop nutrient requirements and can be an excellent tool in diagnosing plant problems. There is no tool that provides a grower with more information than a well-collected tissue sample. Most laboratories will measure N, P, K, Ca, Mg, S, Fe, Mn, B and Cu levels, as well as indicate where your crop falls based on accepted levels for the current growth stage.

Here again, the information you get back is no better than the sample collected. For example, a good tissue sample for tomatoes requires 12 to 15 whole, most-recently mature leaves. What many believe to be tomato leaves are actually leaflets; the leaf consists of many leaflets plus the petiole. For getting the most recently matured leaves, start at the growing point and count down four to five petioles, and snap off the petiole at the main stem. Always use paper bags for tissue sampling in order to avoid sending in moldy samples. Use a laboratory that can turn a sample around in two to three days. If the information is much older, it is old news as you’ve probably applied nutrients by the time the results get back to you. If you are applying foliar nutrients, collect your sample before you apply. Collect your samples from average plants. Do not collect samples from plants that are larger, smaller or discolored compared to your average plants.

Note on plant parts for testing: The reason that you use the most recently matured leaves is that they give the best picture of plant nutrient levels. Younger leaves are hungry and will indicate deficiencies, while older leaves “bank” nutrients and are likely to show plenty of everything. Also, the comparison levels that come back from your laboratory are based on most recently mature leaf samples.

Once you get into the growing season, be sure to compare your most current results with prior returns to see where the crop nutrient levels are trending. Often with tomatoes and peppers, potassium levels will begin dropping shortly after the first fruit set. By identifying the trends, you can make effective adjustments to your nutrient program. Start your tissue analysis program no later than at the first blossoms. Two-week intervals between testing have proven to be sufficient for fertigation planning.

Petiole sap analysis: While few growers use this method of nutrient management to date, I’ve been using the Horiba NO3 and K+ meters for more than 10 years, with excellent results. If you need immediate information on N and K levels, there is no easier method. The downside is the approximately $700 investment in the two handheld meters, plus calibration solutions. The upside is having information when you need it most: before you fertigate.

To analyze sap, you collect the same samples as in tissue analysis, but remove the leaflets, chop up the petioles, mix them thoroughly, then squeeze out several drops of sap using a very heavy duty garlic press. This juice is applied directly to a sensor on the meters. Your readout is in ppm. For K, you can use the digital numbers directly. For the N meters, you’ve got a quick multiplier to use to get plant N in a usable form.

Several years ago, we compared the results from tissue testing through a lab to the sap meters, and found that the results were very close.

Note on plant tissue and sap analysis: Time of day, cloudiness and temperature all factor into how plants take up nutrients. For the best results, take your samples under conditions that are as similar as possible. Always take your samples at the same time of day. Plants are solar pumps: The longer the sun shines, the more nutrients they’ve pumped.

Water analysis: Knowledge of your water’s pH and alkalinity is required for maximum efficiency in nutrient application. In the clay loam soils that dominate the region I consult in, tomatoes extract nutrients most efficiently at a soil pH of 6.2 to 6.5. As the pH rises, potassium becomes increasingly difficult for the plants to extract. Alkalinity is best pictured as resistance to pH change. At low alkalinities, it takes very little acid to move pH levels. As alkalinity rises, the amount of acid necessary to move the same pH downward increases. There is a great online tool for calculating the amount of acid(s) required to reach your target pH using your water test results.

By Steve Bogash, Penn State University

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