Features Agronomy Fertility and Nutrients
Soil test versus tissue test

Ever since Justus von Liebig popularized the Law of the Minimum in the mid-1800s, agronomists have been using all the tools at their disposal to develop balanced fertility programs. The Law of the Minimum states that growth is not controlled by the total resources available to the plant, but by the scarcest resource: the limiting factor.  

Soil tests are valuable for developing fertilizer plans. Tissue tests are for evaluating those plans. (Photo by Bruce Barker)

Soil testing and plant tissue testing are two of those tools for helping to understand fertility programs. Yet to use them properly, the prerequisite is an understanding of their value, capabilities and limitations. “Soil testing and plant analysis are complementary: however, they are extremely different,” says Viterra agronomy manager Rigas Karamanos at Calgary, Alberta. “With soil testing, we are trying to assess and predict nutrient availability in the soil, whereas with plant analysis, we are trying to assess nutrient uptake by the plant.”

Soil test labs calibrate their chemical analysis with crop response in the field. The calibration must be done in the region where the soil testing occurs. For example, for nitrogen response in wheat on the Prairies, the soil test calibration should also be done on the Prairies for the recommendations to be reliable. 

Soil tests measure the amount of nutrients available for crop uptake at a given point in time or determine a nutrient supply rate, as is the case with root simulator technology. The labs take the information from the soil analysis and make a nutrient recommendation based on the crop, yield target, estimated plant-available water, geographic location and cropping history.

Understanding the different analysis techniques and recommendation philosophies between soil test labs is also important when choosing a lab.

Karamanos explains that for soil tests to be relevant, accurate recommendations require accurate soil samples. Getting a truly representative sample for each field should be the main focus in soil sampling. Commonly, samples are taken in random locations across the field, or from a smaller area of the field that is considered typical of the entire field (benchmark sampling). Geo-referencing the sampling locations using GPS is another tool that is currently used in soil testing today. All methods are effective if soil samples are taken from areas that best represent the field. “Sometimes soil tests are not appropriate. Nitrate and sulphate tests in areas with high rainfall don’t tell you anything because the nutrients can leach out of the rooting zone. They don’t even talk about soil test for N in Ontario because it is entirely ambiguous,” says Karamanos. “Make sure your soil tests are calibrated for your area.”

Western Canadian soil tests are also only calibrated for preseed, spring or fall, sampling periods. Sampling outside the recommended time will produce unreliable results. “Soil tests are done to predict nutrient availability so that a balanced fertility program can be developed,” he says. Soil tests can also be used for diagnosing pH and salt problems.

Tissue testing for evaluation
On the other hand, plant tissue tests are used for several reasons. A tissue test can be used for monitoring of high-value crop nutrition, such as the petiole analysis in potato. It can also be used for crop logging, which allows determination of critical levels, or for diagnosing crop fertility problems. “The best reason plant analysis is done is to make sure that the fertilizer that was applied at the beginning of the season is adequate to meet crop requirements,” says Karamanos. “It is a good evaluation tool.”

Using tissue analysis to identify an imbalance or deficiency provides an opportunity to improve the fertility plan for the following year. Using plant tissue analysis for this purpose works best when specific plant parts are sampled at appropriate growth stages. Most laboratories can analyze for various combinations of many plant macro- and micronutrients. Growers and agronomists must work with the laboratory to help interpret the tissue test results.

Karamanos says that caution must be exercised if a tissue test is used as a means to help correct nutrient deficiencies in-season. He says there are several factors involved when using a tissue test for diagnostic purpose with a goal to attempt to correct a nutrient deficiency, and that correlating a tissue test result with a recommended foliar application is difficult.

Theoretically, a tissue test is used to establish the concentration of a nutrient in the plant tissue. That level is compared to a critical level required for plant growth. In practice, Karamanos says correlating critical levels to foliar fertilizer recommendations is much more difficult.

One of the confounding factors is the effect of cultivar on fertilizer response. For example, a few years ago, Viterra funded a project at Agriculture and Agri-Food Canada at Morden, Manitoba, that looked at the response of 12 different dry bean cultivars to zinc fertility. Every cultivar behaved differently. “How are you going to determine a critical level with these differences? You have to look at each cultivar. I don’t believe any lab has criteria for every single cultivar in every single crop,” says Karamanos, who set up the plant tissue test database at the Saskatchewan Soil Testing Lab before that. “We take it for granted that if we send in a tissue sample to a lab the calibration is correct. For many labs the tissue criteria are probably irrelevant because the cultivars have changed.”

Karamanos also cautions that in-season foliar application of nutrients to correct nutrient deficiencies is difficult to achieve. Generally, macronutrients are not good candidates for foliar fertilizer application because they require large amounts of the nutrients, and only a small amount can be applied to the leaf without burning it. Foliar application of micronutrients is generally an efficient method of application because of the small amount of nutrient that needs to be applied. However, growers and agronomists should ensure that the foliar fertilizer recommendation is based on a calibrated tissue test. “The bottom line is that there are benefits to foliar fertilization, but you have to consider both the cost and effectiveness of the application,” says Karamanos.

Where that leaves growers is with the understanding that both soil tests and tissue tests have their place in the fertility toolbox. Soil tests are best for preseed predictions of the nutrient-supplying power of the soil to help formulate fertilizer plans. Tissue tests are best for checking to see if those fertilizer plans were correct, which will help correct fertility plans the following year.