We grow several different crops. Lettuces, basils, kohlrabi, cilantro, sage, and more. Most everything grows swimmingly. Well, technically, floatingly. Except sometimes, we get some tip burn in particular varieties of lettuce. Mostly, the romaine varieties, but also occasionally on the butter head stuff, too. If we’re eating it ourselves, nobody at the ranch is that picky to care about some brown edges. However, selling these lettuces with brown edges is a different story.
So what’s causing the tip burn? Our greenhouse doesn’t get too hot or too cold being on the coast of Central California. Humidity can fluctuate, but I think the real culprit in this case is too much sodium. Our sodium levels are up to somewhere around a whopping 180ppm and you typically want 40-80ppm. I seriously thought nothing was going to grow in our system when we tested the well water, but it does pretty well. Except for the annoying tip burn which is caused by not having enough calcium at the growing tip of the plant.
The calcium level is about 110ppm. Sodium can displace calcium at high concentrations if I remember from school correctly. Calcium is transported through transpiration and as the water transpires, or “sweats”, out of the plant, it draws more water and along with it, calcium. This is why you see Horizontal Airflow Fans (HAF) in greenhouses raising lettuce. The fans help increase the evapotranspirational flow by blowing air on the leaves.
However, sometimes the outer leaves get a little too much air and transpire faster than the inner leaves. The outer leaves draw more calcium and the inner leaves struggle leading to tip burn as not enough of it can reach the inner growing leaves and the cells begin to fail and brown.
Another rather obvious factor that influences transpiration is light. If there is not enough light, the plant can not draw the calcium up and the tips burn. If there is too much light, sometimes the outer leaves collecting all the light transpire more than the inner shaded leaves who are now not getting enough calcium as mentioned above.
With all the sodium stress, maybe a little too much light is also contributing to tip burn. We installed some 30% aluminet shade cloth and maybe the slightly reduced light will help regulate calcium uptake with our stressful sodium water.
Bottom line is, we need to reduce the sodium in the water. Since we needed to get some shade cloth anyway, I figure what the heck, let’s monitor and see what happens. Until then we’ll being doing a rain dance.