Boron toxicity – a follow up from Strawberry Hydroponics Y6 W16.

The previous post can be found here.

I was waiting for tissue analysis results to come through for reference before posting this. I am also comparing my results to Haifa's recommended tissue levels which can be found here, specifically section 4.2.3.

Before diving in, please also note that my tissue analysis was done on leaves which are ~8 weeks old. Typical guidance on leaf tissue analysis is to sample the most recent mature leaves which are often ~2 weeks old (give or take a few days). As such, there are a lot of values which are just outside of optimal range, but that's not the focal point of this post as we are looking for things that are WAY out to give us deficiency or toxicity issues. As typical guidance suggests, the most recent new mature leaves are the ones to look at for optimal numbers.

On the last image included in this post, you can see my most recent tissue analysis results for the oldest leaves. N is low, but these are old leaves, and as I selected some leaves for analysis which were naturally dying off the plants, N will translocate as the old leaves senescence. Quickly going through the rest of the "good" values, P is a touch high, K is fine, Ca is a touch high (but I've found this amount up in other fruit plant types to be okay), Mg is a touch high, Cu is fine, Fe is fine, Mn is fine, and Zn is a touch high (this because in the younger leaves there is the odd start of chlorosis which I added a little bit of Zn into based on the tissue analysis and return water analysis results from week 6). What is the stand out value is B. Boron is so far past the upper boundary it's causing toxicity issues. What this in turn then does to younger leaves and flowers is shorts P, Ca, and Zn predominately. This also gives the leaves their burned appearance that we've seen over my last few posts (see the links to more reference articles in my previous post, Year 6 Week 16, which is linked at the very top of this post).

As I mentioned earlier, I was using fertilizer that had a good amount of boron in it. Working out the amount of fertilizer I added into the roughly 400L of water the entire system holds between the nutrient reservoir and the coco coir, this would amount to about 0.9 mg/L in water. That then is added to my base water which already has a further 0.6 mg/L in it after reverse osmosis, and you get a value of ~1.5 mg/L (water analysis in Year 6 Week 6 showed this to be around 1.6 mg/L which is close to the above calculated value). In week 6, old leaves had a boron level of around 240 ppm, but it's only been the past week or two that berries have started to take a visual hit, and I suspect once B passed 300 ppm in leaf tissue, that's when the berries started to visually decline.

I have since swapped fertilizers out roughly two weeks ago which should now lower my boron levels to ~0.7 mg/L in water, and as the new leaves continue to age over the next 6 weeks, we should start to see healthier looking leaves again. 0.7 mg/L is still too high as table 5.4 suggests we should be closer to 0.15 mg/L in water. I may keep this slightly higher than 0.15 mg/L in time, but for now the goal is to bring boron down in the starting water (I may start melting a bunch of snow yet). As we have just come through the end of year holiday period, I am waiting on some more water samples from the tweaking I've done to my RO production in the past week, and I am still waiting to hear back from some of my industry contacts regarding boron removal options – all of which will be pending water analysis results from recent tinkering which I should have next week.

Turning attention now to the remainder of the pictures, even with excessive boron, the berries are still doing overall well. Over the last 8 days I harvested a total of 13kg of berries which is a record. My previous harvest maximum over a ~week period was ~11kg in the third bloom cycle (we are currently finishing up the second bloom cycle now) with the same amount of plants. There are still a lot of good quality berries in the most recent harvests, but getting to the fourth picture onwards is where you can see some visual differences on some berries.

Note in the fourth picture, there are three cut berries horizontally in line along the bottom, and three other cut berries vertically. The vertical ones are fine in appearance without any visible boron stress, while the ones horizontally along the bottom do have boron stress. Internally to the berries, there's almost no visual difference. The middle one of the horizontal line is a little darker, but it also was a little more overripe than the other berries in general.

Where you can see the difference is on the outside of the berries (picture #5-8). Picture 7 has a boron excess affected berry at the bottom, and a more regular looking berry at the top for comparison. Physically, the tips of the berries where you can see the most difference becomes softer. Soft doesn't do the description justice either, it's almost like the tips of the berries get a little bit "Play-Doh-y" You can push the tip of the strawberry in a little bit, and the part you push just stays there without further damaging or rebounding back to the original shape.

As mentioned earlier, the second bloom cycle is winding down, and I expect the berry quantity to drop off in the coming week or two. I should have more normal looking berries and hopefully leaves as we get into February, and once I can further reduce boron in my starting water.