Nutrient Uptake Rate

Orchid roots absorb nutrient solutions fairly quickly, but it happens in two stages with very different speeds:

First is absorption by the velamen, that silvery-looking, spongy, dead outer layer on orchid roots. Its job is to wick and store water and dissolved nutrients. It begins absorbing those solutions almost immediately, and is fully saturated within about a minute of exposure. That happens through capillary action, so is simply a physical process, like wetting a sponge.

The second stage of the process is the transfer of those solutions into the living tissues of the roots. Under the velamen are the exodermis and cortex, that actually absorb nutrients via osmosis, diffusion, and active transport – cellular “pumps” that move fluids through membranes, if you will – moving them into the vascular tissues (xylem) for transport throughoout the plant. The initial extraction from the velamen can start within minutes of watering, and a significant volume is transferred over a 15-60 minute period.

Environmental factors can affect those rates. Faster uptake is favored by warmer temperatures, active root tips (primarily because the nearby velamen is fresher) and with moderate (~75-150 ppm N) fertilizer solutions. Slower uptake occurs at cold temperatures, with old velamen – espcially if there is a lot of mineral buildup – and in dehydrated plants (one reason it’s best to never let your plants totally dry out between waterings).