Whether you’re producing woody or herbaceous plants, it would naïve and downright economically foolish to assume that you can judge your fertility at a glance.
Once you add fertilizer to your container substrates, your job of supplying your crops nutrients is not complete. Yet there are plenty of nurseries that do nothing more than add the fertilizer (via incorporation, dibbling, fertigation or topdressing) and wait for their plants to grow.
More often than not, those plants stop growing too early in the production season. The growers are often aware that more growth could have been obtained had they paid more attention to available nutrients.
Many variables affect the rate at which fertilizer -- an imperative addition to soilless substrates -- is depleted. Plant uptake is the most obvious. But the amount of leaching due to rainfall or irrigation, along with air temperatures and the type of fertilizer applied, are also important.
Don’t forget more subtle nutrient depleters such as container blow over, which causes the loss of topdressed granules, and competing weeds.
Get on the ball
A grower shouldn’t wait for visible symptoms of nutrient deficiency before applying fertilizer or changing some part of the production system (perhaps decreasing irrigation volume). With many crops, once growth is halted during the growing season, it’s impossible to jump-start vegetative growth again.
But how is nutritional status best monitored? Traditional soil tests don’t satisfactorily give the answer.
Neither does tissue analysis, which reveals what is or isn’t contained in a plant, It is useful if good baselines have been established for comparison, but those tests are expensive and don’t give immediate results.
In the future, we may have genetically engineered crops that give visual symptoms when particular elements run out. Or we’ll have new technology that provides chlorophyll fluorescence that allows growers to better detect elemental depletion. Until that time, other testing methods are needed -- especially as container production continues to gain popularity over field production.
Pour-Thru and others
Most methods developed are based on substrate-solution extraction methods and determine the electrical conductivity (EC) of the soluble salts in the substrate. Three common techniques are the 1:2 dilution and saturated media extract method, the Virginia Tech extraction method and the Pour-Thru (or leachate collection) method developed in the 1980s by Robert Wright from Virginia Tech and Tom Yeager from the University of Florida.
The first two methods are cumbersome and slow. The latter allows for a more rapid collection of substrate solution that can be immediately tested in the field with an appropriate meter.
Biweekly, or at least monthly, monitoring is desirable during the growing season. Given this, and that growers generally need to test multiple crops in varying substrates with different formulations and/or fertilizer rates, field testing is most convenient.
But growers run into problems with the Pour-Thru method when it comes to large containers. As the size of containers used at nurseries increases, especially with more growers using these big pots in pot-in-pot systems, the Pour-Thru method becomes difficult to administer.
While 15-gallon containers may have been the norm with early pot-in-pot systems, the more typical sizes are now 35s and 45s.
Containers of that size can’t readily be pulled from the ground to collect substrate leachate. This means we need another substrate-solution testing method in lieu of the Pour-Thru method.
New ideas
In 1997, two more Virginia Tech researchers, Ron Walden and Alex Niemiera, reported their research involving the use of suction cup lysimeters -- sealed tubes with ceramic tips that draw substrate when suction is created.
These lysimeters were inserted into the substrates of pot-in-pot containers for solution extraction without lifting the production containers from their in-ground socket containers. When weighed against the Pour-Thru method, the data the lysimeters collected regarding total nitrogen and pH were comparable.
Lysimeter use was further refined at Virginia Tech and researchers now recommend 1.9-inch-diameter lysimeters for large containers. Details are available at www.ext.vt.edu/resources. Search the Agriculture and Natural Resources publications for “Monitoring Nutrients in Large Nursery Containers.”
The use of lysimeters is certainly not restricted to just large pot-in-pot containers. It’s also a convenient way to test nutrient levels in above-ground containers.
The next frontier
The latest development might be some newly developed probes that can be directly inserted into substrates to gather EC levels. They don’t require the solution extraction required by other methods.
These probes work as long as the volumetric water content of the substrate is above 35 percent.
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So now -- regardless of container size, irrigation or fertilization regime -- no one has an excuse for not regularly monitoring nutrient levels. There’s a relatively quick and convenient method available for determining substrate EC and making appropriate adjustments to help provide optimal conditions for plant production.