By Dr. Chad Penn
USDA-ARS, National Soil Erosion Research Laboratory
Studying the details of crop uptake of phosphorus (P) and other nutrients that have a strong affinity for minerals is very difficult in soils. Since plants take up nutrients from the solution phase and not directly from the soil, the use of soils in nutrient uptake experiments can be confounding.
Phosphorus must first be relinquished from the soil “warehouse” into solution before it can be taken up by a root. Therefore, nutrients in solution are potentially 100% bioavailable. To control solution concentrations is to control the root environment, and thus bioavailability. However, it is impossible to control solution P concentrations in the presence of soil because nutrients are dynamically sorbing from (i.e. adsorption, precipitation, and immobilization) and desorbing/dissolving/mineralizing into solution. The resolution to this experimental problem has been to utilize soil-less hydroponics, where the nutrients are 100% bioavailable as applied in the growing solution. While useful, it created another problem: hydroponic-grown plants are not similar to field-grown, especially with regard to roots. In addition, it is impossible to grow a realistic corn (maize) plant to maturity in such systems
Figure 1. Illustration of the dynamic between soil and solution-phase phosphorus (P): P must first be released by the soil into solution before it can be taken up by a plant.
Similarly, environmental conditions such as water availability, light, and temperature have a strong impact on nutrient uptake and vary dramatically in the field, making nutrient uptake experiments difficult to control and interpret. For this reason, many nutrient uptake experiments are conducted in greenhouses and growth chambers for environmental control. Still, both have some important disadvantages compared to field experiments; as a result, they do not produce plants that are similar to field-grown plants, making it difficult to practically apply the results of a nutrient study.
At the USDA-ARS National Soil Erosion Research Laboratory, we developed an indoor growth room, capable of growing 96 corn plants to maturity (R6) under fully artificial conditions with semi-automation. This system produces realistic maize plants with corresponding grain yields, thereby achieving all the benefits without the disadvantages of field, greenhouse, growth chamber, and traditional hydroponics studies, with regard to nutrient research.
After testing several non-soil media, we found that silica-sand was the best for two reasons: inertness and physical similarity to soil. With silica-sand, P (and all other nutrients) were neither sorbed or desorbed, allowing total control of the root environment and nutrient bioavailability. Nutrients were simply added with irrigation water. Figure 2 shows the major components of the growth-room.
Figure 2. Several components of the grow room. (a) Oscillating fan; (b) irrigation pipe containing drip emitters; (c) six rows of sloped trays for holding pots and draining water; (d) astronomical timer; (e) four rows of LED lights and power sources mounted on struts; (f) air duct for incoming air plus temperature, humidity, and CO2 sensors hanging from ceiling; (g) pulleys for raising/lowering LED lights; (h) controls for activating air handler and humidifiers; (i) air handler with filter located in “lung room” connected to air duct; (j) eight small nutrient injectors for applying treatments; (k) containers for treatment concentrate solution; (l) timer/controller for irrigation; (m) large nutrient injector for all non-treatment nutrients; (n) container for non-treatment nutrient concentrate; (o) drip emitter, one per pot; (p) humidifiers; (q) pots containing silica sand and drip rings; (r) mature and dry maize ready for harvest. Not shown: “Watch Dog” daily logger station for radiation, relative humidity and temperature, air conditioners, and louvers for exit air.
Nutrients were automatically dosed into irrigation water using nutrient injectors coupled with irrigation timers and solenoids. One single nutrient injector was used to inject all plant nutrients with the exception of the experimental nutrient, which in this case was P. After receiving non-P nutrients, the irrigation water was split into eight different nutrient injectors: one for each concentration of P to be tested. In addition to maintaining realistic diurnal variations in temperature and humidity, a changing photo-period was simulated with an astronomical timer which controlled LED lights. These lights produced a spectrum and intensity similar to the sun (unlike greenhouse lights) when maintained at a 16-inch distance from the plant. To achieve that, lights were attached to pulleys that allowed them to be raised upward as the plants grew. Sensors were used to monitor the atmosphere and trigger a whole-room ventilation system; all air entering the room was filtered to prevent any pests or disease. Oscillating fans were used to simulate the wind.
Compared to field-grown maize, maturity occurred earlier, likely due to more ideal growth conditions than what plants typically see in the field, including constant N addition and better weather. Plant parts and nutrient concentrations were nearly identical to field-grown corn. Plant growth was responsive to changes in P fertigation concentrations, illustrating the highly inert behavior of the silica sand growth media and the efficiency and bioavailability of the nutrient delivery system; that is what makes this system so valuable for conducting nutrient uptake and plant physiology studies. Based on a statistical analysis of the P study, maximum grain yield was achieved in this system with a P concentration of 7.8 mg/L, but total biomass continued to significantly increase with increasing P concentration until 14.4 mg/L, suggesting that genetic potential limited grain yield (Figure 3).
Figure 3. Grain (15.5% moisture content) and total biomass (dry basis) plant yield from harvested maize plants grown synthetically in the indoor grow room during a fertility trial. Values represent averages among three replications and three different hybrids grown using six different concentrations of P in fertigation (4, 8, 12, 15, 20, and 22 mg P L−1). Error bars indicate standard deviation.
Using this solution-culture methodology, plants can be grown with precise control of nutrient bioavailability due to the use of inert silica sand coupled with nutrient application in solution form, as demonstrated by its ability to not sorb any added P, whereas traditional hydroponics media were fairly sorptive. In practice, this means that the solution environment of plants could be altered within a matter of minutes in order to study the effects on plant physiological function and processes. This feature alone gives this solution-culture method immense potential for plant nutrient timing studies. Growing plants in this type of artificial environment has immediate applications for nutrient and physiological studies. The system described also allows for precise manipulation of light, specifically light intensity, quality, and photoperiod duration. Additionally, one of the most important features of this growth environment is that regardless of study type, a relatively large number of plants (96 in this instance) can be grown and observed at one time. In theory, this number could be scaled up where there is adequate space. For more details, please see Weithorn et al. (2021).
References:
Abendroth, L.J.; Elmore, R.W.; Boyer, M.J.; Marlay, S.K. Corn Growth and Development; Iowa State University Extension Publication PMR1009: Ames, IA, USA, 2011.
Wiethorn, M., Penn, C., and Camberato, J. (2021). A Research Method for Semi-Automated Large-Scale Cultivation of Maize to Full Maturity in an Artificial Environment. Agronomy, 11(10), 1898. Online at: https://doi.org/10.3390/agronomy11101898
Opinions expressed here do not necessarily reflect those of the Sustainable Phosphorus Alliance.