Plant nutrient levels change throughout the course of the growing season, with some nutrients decreasing in concentration, and others accumulating. At different points in the year, nutrient concentrations may be changing rapidly or be relatively stable. Stability in nutrient levels is often the basis for recommended sampling times.
We had monitored nutrient status on six grower farms back in the spring of 2003 for ‘Chandler’ strawberries in plasticulture production. Strawberry samples were taken about every other week from the time the leaves were large enough to sample through harvest. At each sampling, the most recently fully expanded leaves were selected. Results for the three farms with the highest yields are presented here and show what we could expect nitrogen levels to look like in a well-managed planting. All three farms were fertigating with recommended rates, but nitrogen concentrations still decreased rapidly throughout the course of the spring. Nitrogen concentrations for the other three farms with lower yields varied, but they generally ran 0.3 to 0.8 percentage points lower depending on the time of sampling.
From this graph, it’s easy to see that it would be difficult to interpret results reliably without knowing the date or crop growth stage. When samples are submitted to Penn State’s Agricultural Analytical Lab for plasticulture strawberries in the spring, we recommend that samples are taken around the time of first bloom or a few days earlier to allow time for the analyses to be completed and to correct problems.
With blueberries, PA recommendations have been to sample leaves during the first week of harvest or immediately after harvest. Now late varieties exist that give new meaning to the term “late”, so harvest covers a larger part of the year. One question that arose was “Does sampling time need to be adjusted because of the expanded harvest season that now exists with late varieties?”
To answer this question, a study was done at Oregon State University under organic and conventional production using six early to late season blueberry cultivars. The results are presented in the graphs below. In both types of production, nitrogen levels changed throughout the year, dropping rapidly at the beginning of the growing season for all cultivars except ‘Legacy’. Sampling leaves at the beginning of the growing season usually resulted in high nitrogen concentrations and more variability among cultivars, so adjustments would need to be made to levels used for interpretations. Nutrient levels were more stable later in the year. What would make sampling time more straight-forward would be to specify a mid-to-late summer window for all cultivars. We likely will update the sampling instructions in the near future to indicate this more clearly.
NWREC: North Willamette Research and Extension Center, Oregon State University, under certified organic production. Grower: 6 sites under conventional production. Adapted from Strik and Vance, 2015. Courtesy: B. Strik. All of the above explains the “why” behind our recommendations for tissue sampling, and some of the work that goes into establishing standard levels for interpretations. Here are some answers to the common questions regarding the “Hows”.
How should samples be taken?
These instructions pertain to samples sent to Penn State’s Agricultural Analytical Laboratory. Because some nutrients are mobile in the plant (they can move to another plant part if needed), and others aren’t (where they get used first is where they stay), a leaf-age is needed that isn’t very old or very young. For all berry crops (and most other crops) this is usually the most recently fully matured leaf. Start with the newest leaves and work your way down until you encounter a leaf that looks mature. Select that leaf unless your selection has some obvious defect (bug holes, spider mites galore, or disease symptoms), in which case you’ll need to find another. Take off the petioles, as they have different nutrient levels than the leaves, and if ground up with your sample, will affect results, especially for strawberries where the petiole can make up a large part of the sample. For example, nitrogen will appear to be lower than it really is, and potassium will appear to be higher.
Plasticulture strawberries: Take samples of the most recently fully expanded leaves early in the harvest year, preferably right around first bloom.
Matted-row strawberries: Samples should be taken after renovation once enough leaves have grown back to allow sampling of fully matured leaves. This is a relatively stable time of year for nutrients with strawberries, similar to that for blueberries. In a new planting, samples could be taken at about this same time of the year, but this shouldn’t be necessary if the field was amended according to soil test results before planting.
Floricane-fruiting raspberries and blackberries: Take samples of the most recently-fully matured leaves from primocanes (this year’s canes that are vegetative) between Aug. 1 and Aug. 20th.
Primocane-fruiting raspberries and blackberries: Take samples of the most recently fully matured leaves from primocanes that have reached their full height (flower clusters are separated or just beginning to bloom).
Blueberries: Take samples of the most recently-fully matured leaves at the time of year that would normally be the end of harvest for mid-season varieties like ‘Bluecrop’.
If the plants aren’t growing well and you want to figure out what is going on, take samples at another time of the year. Send in two samples—one from plants that are growing poorly, and one from plants that growing well—but check first for other problems such as root rots (nothing is going to look normal when the plant has few roots) or incorrect soil pH.
How often should I sample plants?
For younger plantings of perennial crops that are getting established, every year would be prudent, but once established, every 2 or even 3 years is fine. Routinely testing over a number of years on your farm gives you insights into your conditions, and adjustments you might need to routinely make for your farm. It’s also very helpful if tissue analysis is conducted along with soil analyses (and sometimes an irrigation water analysis also). All of this info together helps to get plantings on the right track and provides a lot of information that helps with understanding conditions on your farm.
– Kathy Demchak is a senior extension associate for Penn State University.
Acknowledgments: Thanks to Dr. Bernadine Strik of Oregon State University for sending information related to blueberry nutrients. Adapted from: Strik, B.C. and A.J. Vance. 2015. Seasonal variation in leaf nutrient concentration of northern highbush blueberry cultivars grown in conventional and organic production systems. HortSci. 50(10):1453-1466.
