Many commonly used rooting media are derived from naturally occurring substances; therefore, they contain some, or many, of the essential plant nutrient elements. Here, samples of rockwool, perlite and coir were heated in aqua regia (a mixture of concentrated hydrochloric and nitric acids). Perlite did not go into solution, but the obtained digests of rockwool and coir were assayed for their elemental contents by ICP spectrometry. The results are given in the Table 1:
Being natural products, these rooting media will have differences in elemental content depending on the composition of the source material. Therefore, there is probably a “batch effect.” For rockwool, for example, the chemical composition of the source mineral, as well as the fluxing agent (limestone), will determine the final product’s elemental content; while for coir, the chemical environment associated with the production and processing of the coconut fiber will determine its element content. How significant the batch effect is needs to be determined by assaying various batches and sources.
Experience would suggest that a portion of these contained elements are available for root absorption. However, what portions of these elements are available for plant utilization is not generally known nor easily determined due to their varying parameters of use. Indeed, how to make that determination is the challenge. There are two extraction methods that can be used for making that determination: water-equilibrium extraction and extraction using a soil reagent.
Here, the Mehlich No. 3 soil extraction method was chosen since it is a volume-based procedure, the volume ratio being 1:10. The obtained extract was assayed for element content by ICP spectrometry. The element extract concentrations are expressed to pounds per acre (lbs/A) so that the Mehlich No. 3 interpretation values can be used. In addition to rockwool, perlite and coir, two other commonly used rooting medium—pinebark and peatmoss—were included. The results are given in Table 2.
These results do not necessarily verify what portion of the element content of the rooting medium is indeed “available” for root absorption, but it does provide as basis for comparison among rooting medium. The assay results would suggest that all but perlite could be considered a “fertile soil.” It might also suggest that one needs to match a fertilizer or nutrient solution formulation with the available elemental contents of the plant rooting media. In addition, these results would suggest that when using these rooting medium, including the micronutrients would not be necessary in the selected fertilizer or nutrient solution formulation.
The next issue when it comes to rooting media is, “What occurs when a nutrient solution is brought into contact with a rooting medium and allowed to come to equilibrium?” To answer this question, a nutrient solution was added in an equal volume to rockwool, perlite and coir. The mixtures were stirred intermittently for 30 minutes, and then the liquid phase removed by filtration and the filtrate assayed for its elemental content by ICP spectrometry.
There are three possible outcomes: there is no change in elemental content from that in the initial nutrient solution, there is an adsorption that results in a decrease in concentration or there is an increase in the elemental content of the recovered nutrient solution as result to a release.
In the Table 3, the elemental content of the nutrient solution in parts per million (ppm) is given in the first column and the elemental content of the recovered nutrient solution after equilibrium with rockwool, perlite and coir, respectively, after filtration follow in the next three columns.
For perlite, the only elemental change was for the element Cu. For rockwool, elemental change occurred for Ca—not surprising since Ca is a major constituent (see Table 1). Iron is also a major constituent, but no change occurred. The most significant changes occurred with coir, with increases in elemental contents for the elements P, K, Mg, Na, Fe and B, and a decrease in Ca.
Some of this might be due to the fact that in the interaction process, the filtrate was colored with colloidal organic material—the elemental results might have been different if it had been removed. These results suggest that rockwool and perlite could be considered inert in their interaction with an applied nutrient solution, while coir is not.
Based on the physiochemical nature of a rooting media, the elements in an applied nutrient solution can potentially interact with the rooting media, either being physically adsorbed or chemically bonded to form complexes, thereby resulting in their accumulation. To determine the degree of elemental accumulation that can occur, samples from a rockwool slab and perlite from BATO buckets were collected for elemental analysis following the hydroponic growing of greenhouse tomatoes using a drip irrigation system for nutrient solution delivery.
Since there occurs an accumulation of applied elements during the growing season, which is observed as an increase in the electrical conductivity (EC) of the residue solution, growers are advised to monitor the EC of the retained solution and to water leach when the retained solution reaches a certain EC level. For both collected samples, the growers were following is routine of periodic water leaching.
The gathered rockwool slab and perlite samples were first water leached, and then extracted using the Mehlich No. 3 soil extraction method, the results are expressed as pounds per acre (lbs/A) so that the assay results can be interpreted using established Mehlich No. 3 interpretation values. The assay results are given in Table 4.
The water-soluble assay results confirm that there is an accumulation of elements that remain in the rooting media in solution, while the Mehlich No. 3 extraction results indicate that there exists another form of these elements, probably as precipitates of calcium sulfate and phosphate that either entrap other elements or form chemical complexes.
The elements in these precipitates are probably available for root absorption since the plant root surfaces are acidic and when in physical contact with a precipitate particle, some degree of dissolution could occur. These results also suggest that there needs to be an evaluation of the nutrient solution formulation so that both water-soluble and precipitate accumulation is minimized in order to avoid essential element insufficiency from occurring in the growing crop.
Elemental content in a rooting media can be a significant factor that affects the nutritional status of a growing crop. There is an interaction that can occur between an applied nutrient solution formulation and the rooting media, suggesting that matching media characteristics with a nutrient solution formulation is important in order to avoid an elemental insufficiency from occurring.
Nutrient element accumulation in a rooting media can be due both to a residue increase, as well as a possible formation of precipitates, combining to significantly affect the nutritional status of the growing crop.
A nutrient solution formulation excessive in its elemental content will result in a significant accumulation of an element, or elements, with the potential to adversely affect the nutritional status of the growing crop. However, when one knows what the elemental rooting media content is, one can match it with an appropriate fertilizer or nutrient solution formulation in order to avoid the potential of a plant nutrient insufficiency.