Compared to conventional agronomic crops, modern production of ornamental (nursery and greenhouse) crops is undoubtedly a much more intensive, demanding and risky agricultural activity. Ornamental crops were originally grown in extensive field or greenhouse operations relying on mineral soils, which allowed for simpler and more benign cultural management programs and cheaper costs of production per plant or unit area. The shift of production to containerized systems allowed for shorter production cycles that yield a larger number of high-quality (saleable) plants per unit area (high planting density).
Careful use of fertilizers can maximize plant growth and quality.
IMAGES AND CHARTS COURTESY OF RAUL I. CABRERA
Container plant production, however, relies on intensive and heavy, and at times excessive, use of water, fertilizer and pesticide inputs to produce quality crops. The use of rather limited volumes of soilless substrates in containers intrinsically implies very limited water and nutrient reserve capacities, compared to field soils, and these need to be continuously supplied and monitored, striving to minimize stresses and maximize plant growth. Technically, it’s more like forcing or pushing for maximum growth.
To get an idea of the differences in fertilization inputs between field and container production methods, consider that field nursery stock usually is typically fertilized with nitrogen (N) at 100 to 250 pounds per acre per year, whereas containerized nursery crops can receive 1,000 to 2,000 pounds per acre, or more! A concomitant result of high fertilizer application rates in container-grown crops are the relatively low fertilizer use efficiencies, which usually do not surpass 50 percent. This means that at least one-half, or more, of the applied fertilizer is not used by the crop but leaves the container in the drainage water (≥ 1,000 pounds per year in the case of N), which, if not recycled, becomes an undesirable economic loss and an environmental threat to below- and aboveground water bodies.
Growers might argue, and rightfully so, that cutting down on their current fertilizer applications will negatively impact their plant growth and quality. This is true if they were to continue to use the same fertilizer and water applications and management practices. Adjustments to these practices, based on some basic tenets of plant biology and nutrition could, however, significantly enhance fertilizer use efficiency, while sustaining maximum growth and quality. Let’s explore some of these basic concepts and their practical applications to ornamental plants, in particular those being grown in intensive soilless production systems, such as substrates and hydroponics.
Figure 1. Nitrogen uptake rates in ‘Royalty’ rose plants as affected by NO3-N concentration in the nutrient solution. Plants were growing in a continuously recirculating hydroponic system. Unpublished data from R. I. Cabrera.
As a starting point, be reminded that plants require 14 essential mineral elements to fulfill all of their biological functions of growth, development and reproduction (flowering and fruiting). Essentiality means each element is required for the plant to complete its life cycle, and each one has specific biological functions or roles and cannot be substituted by another. These include the macronutrients nitrogen, phosphorus, potassium, calcium, magnesium and sulfur (N, P, K, Ca, Mg and S) and micronutrients chloride, iron, manganese, boron, zinc, copper, molybdenum and nickel (Cl, Fe, Mn, B, Zn, Cu, Mo and Ni), defined as such by their relative content in plant tissues. This means that the average concentration of N in plant tissues (30,000 mg/kg) does not make it more essential or important than molybdenum (Mo,
Figure 2. Schematic movement of ions (mineral nutrients) to root surfaces.
A major tenet of plant nutrition is that the uptake of mineral nutrients (ions) from the soil solution by the roots is regulated by the plant, and it is an active physiological process for most elements, meaning the plant has to spend energy (in the form of carbohydrates) to first absorb and then metabolize them – or incorporate them into compounds, or use them for biological processes. To illustrate this point, observe the N uptake behavior shown by the roots of rose plants (Rosa × ‘Royalty’) growing in a continuously recirculating hydroponic system, inside of a growth chamber at optimum environmental conditions (see Figure 1), and with the plants at the stage preceding the harvest of their flowers. Nitrogen uptake rates increased asymptotically (in curvilinear fashion) as the concentration of N (provided in the NO3– form) increased in the solution that continuously bathed the root surfaces. Uptake rates reached a plateau region where they literally do not change any more with increases in the concentration of N in solution, in this case at about 300 μM, which translates to ~4 ppm N. A theoretical implication of this observation is that the maximum N uptake capacity of a rose plant (or crop) could be successfully satisfied with a minimum NO3-N concentration of 4 ppm if, and only if, their roots were to be bathed continuously with such solution. Please bear in mind that these uptake rates represent the amount of N absorbed per hour in a fixed volume of roots (1 cm3), so that the actual total amount (like grams, ounces, pounds) of N absorbed over a certain period of time (a day, a week, a season) will depend on the actual uptake rates multiplied by the time interval and by the total volume of roots found in that interval. (Figure 1)
How is it, then, that a commercial rose crop (for cut flower production), growing in containers with peat or coir-based substrates, is typically fertigated with solutions containing 50 to 75 times this concentration of 4 ppm (200 to 300 ppm)? The answer lies primarily in the presence of a solid substrate matrix and the logistical limitations of the irrigation/fertilization infrastructure, both of which make it practically impossible to deliver a continuous (uninterrupted) supply of a fixed nutrient concentration to the root surfaces over time and space.
The movement of ions (mineral nutrients) in solution to the root cells is a passive process driven by diffusion (down a concentration gradient, from high to low), mass flow (moving along with the flow of water) or by a direct physical contact (interception) with the root surface (see Figure 2). The substrate particles represent both physical and chemical obstacles that slow down the movement of water and dissolved ions, shifting the balance toward the diffusion pathway, whereas the more expeditious (or faster) mass flow movement takes precedence only as the irrigation/fertigation application intervals are shortened (or made more frequent). In simpler and broader practical terms, in fine-textured substrates and with relatively long fertigation/irrigation intervals, higher nutrient concentrations (in liquid feeding, LF) or amounts (with controlled-release fertilizers, CRF) will be needed to ensure their adequate movement and supply, by diffusion, to the uptake sites on the root surfaces. On the other hand, the ability to deliver fairly frequent irrigation/fertigation applications will enhance nutrient movement by mass flow, thus allowing for reductions in their concentrations (LF) or applications (CRF) without significantly affecting their net uptake rates. This situation is more likely to apply to growing operations that have the infrastructure that permits pulsed fertigation/irrigation; that is, the ability to do multiple water or solution applications per day.
Figures 3a and b. Water and nitrogen uptake rates in rose plants over the course of a day (A, cv. ‘Erin’) and one fl owering cycle (B, cv. ‘Royalty’). Plants were growing in recirculating hydroponic systems. From Cabrera and Sol_s-P_rez (2009) and Cabrera et al. (1995).
The regulation of nutrient uptake not only is observed at the root surface level, but also over time frames going from hours, days, weeks and seasons, modulated by the overall plant demand, which in turn is dictated by its growth (size) and its developmental (vegetative, reproductive, dormant, and so on) stages. This concept is illustrated by the N uptake rates observed in greenhouse roses over a day and a flush of growth and flowering (43 days). A cyclical uptake behavior of both water and N (also observed for P, K, Ca and Mg, not shown here) is observed over the course of a day, with maximum water and N uptake rates typically observed at midday (see Figure 3A). Interestingly, however, a closer evaluation of all the areas under the uptake rate curves for N and other ions indicated that a larger fraction of the total daily ion uptake (mg/plant) tends to occur in the afternoon hours (from 2 p.m. to 8 p.m.) compared to the morning hours (from 8 a.m . to 2 p.m.).
A cyclical – rhythmical if you will – behavior of water and N uptake was also observed over the course of every growth/flowering flush in roses, which in a typical U.S. greenhouse takes six to eight weeks. Transpiration (see Figure 3B) pretty much followed the crop’s leaf area development, with a rapid drop observed right after pinching (harvesting the flowers) from the previous flush (day 0 in the graph). Bud break occurred at day 6 and the new leaf area generated by the developing shoots produced a mirroring pattern of water uptake. The uptake of N dropped right after the harvest of the flowers from the previous flush (day 0), and reached its lowest point at the time when the shoots were elongating at their fastest rate (days 16 to 18). Once the shoots slowed down their elongation rate, the N uptake rates increased significantly and reached their maximum around harvest time (days 35 to 40).
This cyclical pattern of water and N uptake, also observed for the other macronutrients (not shown), was repeated over the course of the year, albeit the overall uptake rates were lower in the winter months compared to the summer. These uptake patterns were found across all the evaluated rose cultivars and rootstocks, and were also observed when imposing mild salinity stress conditions. Similar cyclical behavior of water and ion uptake has been reported for other woody ornamentals exhibiting strong episodic growth (multiple flushes per year), like euonymus (Euonymus japonica Thunb.), holly (Ilex crenata Thunb. ‘Helleri’) and Japanese privet (Ligustrum japonicum Thunb).
Figures 4a and 4b. Cumulative dry weight and fl ower yields, and leaf nitrogen status in ‘Royalty’ rose plants fertigated for one year with increasing nitrogen concentrations. From Cabrera (2000).
What the data tell us
The overall data from all these studies support the biological concept that source-sink relations in the plants control the internal distribution and competition for carbohydrates and nutrients, in particular nitrogen. During periods of rapid growth, shoots and leaves become major sinks for carbohydrates, diminishing their supply to the roots, thereby reducing root growth and nutrient uptake activities. The resulting low uptake of nutrients from the soil solution, and remobilization of nutrient reserves from stems and roots, eventually cannot meet the demands of the new shoots, resulting in the reduction or cessation of shoot growth. When shoot growth slows down or ceases, then carbohydrates become available again for translocation to the roots, increasing ion uptake and root growth activities, and the cycle repeats itself. This phenomenon is present basically in all perennial plants, regardless of whether they have a single or multiple flushes of growth per year, and thus implies that there is a natural cyclical uptake and demand of nutrients (and water also) over a season, which superimposes over the equally rhythmic uptake that occurs within a day.
Another important concept of plant nutrition to keep in mind is that basically all the fertilizer sources we use in agriculture are based on salts containing the elements of interest (essential nutrients), and thus, even when used judiciously, they are contributing to the overall salinity of the substrates and soil solutions in contact with the roots and could actually reduce plant growth and quality. Consider, for instance, the cumulative dry biomass and flower yield responses of rose plants fertigated with increasing concentrations of N (see Figure 4).
Figure 5. Dry weight yields of American holly and crape myrtle ‘Tonto’ fertigated for nine months with increasing nitrogen concentrations. From Cabrera (2003).
Our specific study
The plants were irrigated for a year with a complete nutrient solution containing all nutrients, and N provided in concentrations increasing from 30 to 220 ppm. The classical asymptotic behavior of crop-yield response to fertilizer applications was clearly observed in roses (see Figure 4a), with both dry weight and cut flower yields increasing rapidly with N concentrations up to 90 ppm, stabilizing in the 90 to 150 ppm concentration range and actually decreasing at the highest applied concentration of 220 ppm (which was in the ballpark of what growers actually used). Leaf tissue N concentration followed a similar behavior, except it remained stable, at 3.3 percent with N applications of 90 to 220ppm (see Figure 4b).
All together, these observations confirmed first that rose plants do, indeed, have a control over the maximum amounts of N absorbed regardless of what is offered to them; second, that the N that was not absorbed remained in the substrate and contributed to the overall salinity of the soil solution, and thus reducing plant growth at the highest applied concentration of 220 ppm.
It should be noted that salinity, measured in practice in electrical conductivity (EC) units, reduces plant growth by three mechanisms: 1) by making it more difficult for the plants to absorb water (known as an osmotic effect); 2) by causing toxicities with specific ions (like sodium, chlorine and boron; and 3) by causing nutrient imbalances in the soil solution that impair uptake of certain elements, causing their eventual deficiency inside the plant. We have observed that excessive N applications (in the nitrate form) in roses, typically in the range of 200 to 300 ppm, can be contributing as much as 50 to 75 percent of the total salinity in soil solution, and the resulting EC values are responsible for the yield reductions observed at those N application rates. We have observed similar results in other woody ornamentals like American holly (Ilex opaca) and crape myrtle (Lagerstroemia indica × fauriei ‘Tonto’), whose response was more detrimentally affected by liquid feed N applications over 60 ppm (see Figure 5). The growth reductions observed at higher applied N concentrations were indeed attributed to increased substrate EC values, as well as a rather significant leaf tissue N to sulfur imbalance, which was definitively provoked by the known antagonistic effect of high N concentrations on the uptake of sulfur from the soil solution.
After all these concepts and information, the likely question that might be asked by a grower could be: What sort of nutrient concentrations (for LF) or fertilizer applications (CRF) are a good starting point, more attuned to the characteristics and/or limitations of current irrigation/fertigation practices in ornamental crops? In the broadest sense, and supported both on research results and anecdotal references from commercial growers, and specifically focusing on nitrogen, typical N concentrations used for fertigation of greenhouse crops (cut flowers and flowering potted plants) oscillate between 100 and 300 ppm, whereas they range between 50 and 150 ppm for containerized nursery crops, with the lower concentrations for slow-growing species and the higher concentrations for fast growing ones. Regarding CRF, and referencing an eight- to nine-month release formulation containing 20 to 25 percent N, typical application rates range from 6 to 12 pounds of fertilizer per cubic yard of substrate, with the actual rates also attuned to the growth rate of each species. Consult with your local extension agency and/or your fertilizer sales representative for the CRF formulations more appropriate to your geographic/climatic region, as the release rates are dependent both on the fertilizer coatings and the average temperatures over the seasons. Once you set on a good starting point for your fertilization program, at least starting with N, then you can try applying some of the principles outlined here to fine tune it, striving to achieve maximum plant growth and quality, and significantly enhance fertilizer use efficiency and its economic and environmental benefits.
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