We have been growing a lemon tree from seed for a few years now and it’s getting pretty big but we nearly lost it, all because of some run-of-the mill, every day fertiliser. I hope our story will help you avoid the pitfalls of the fertiliser market.
As you can see by the yellow leaves in this photo, our lemon started suffering from one or more nutrient deficiencies (most likely nitrogen, iron and molybdenum). It started as just a general yellowing which we assumed was a results of reduced nitrogen uptake due to the cold weather (the pH of the potting mix was neutral and it really is too cold for citrus here but we’re giving it a go in our greenhouse because we really want fresh lemons) so I thought I’d follow the conventional wisdom and give it some nitrogen (just a little because it was winter) as well as the nutrients citrus most commonly lack (iron, magnesium and zinc) because if it was struggling to uptake nitrogen, chances are other deficiencies would show up soon and just a little bit of extra nutrient can help a plant along until the weather warms up.
We had some Epsom salts (for magnesium) but didn’t have an iron or zinc supplement so I purchased some iron chelate and a trace element mix (some people recommend putting a galvanised nail into the trunk of citrus trees to provide zinc but I don’t like the idea of damaging the plant that way and ours was way too little anyway so a trace element mix would provide zinc and all other nutrients the lemon would need.)
The instructions on the packet suggested adding an awful lot of both the iron chelate and the trace element mix so I started off with a half strength dose of those but the correct dose of Epsom salts. After a week or so of particularly cold weather, the plant started looking worse rather than better so I gave it another half strength dose of the iron and trace elements. The plant just kept getting worse though and developed blotchy chlorosis on some of the leaves and interveinal chlorosis on others.
Then I checked the pH of the potting mix. It was 5.5 which is too acidic for citrus. So I added some lime and monitored the situation. As the pH rose the plant started to look better so I kept fertilising the plant according to the instructions. Every time the pH dropped I added some more lime but the pH kept dropping and I couldn’t figure out why. In the end we decided to replace some of the potting mix in case that was the culprit.
We bought some potting mix and checked it had a neutral pH then mixed in some soil from our chicken coop (for nitrogen) as well as a general purpose, organic fertiliser and some of the Epsom salts, iron chelate and trace elements as we figured this would give the lemon everything it needed. We then checked the pH again and it was down around 5 again. Clearly something we had added had changed the pH.
So we went through each additive to figure out what was acidifying the potting mix. The chicken soil and the fertiliser were pretty much neutral and the Epsom salts were only slightly acidic. The iron chelate dissolved in water had a pH of 5.5 however and the trace elements in water had a pH of less than 3! (that’s the lowest pH our probe will measure). So then we knew what was causing the pH to drop! I triple checked the packaging and there wasn’t any information on either pack to suggest that either the iron chelate or the trace elements mix would acidify the soil. I was very disappointed.
So we mixed up a new batch of potting mix with soil from the chicken coop and the organic fertiliser then replaced what we could of the potting mix in the lemon’s pot. I also checked the pH of our seaweed concentrate (it was thankfully neutral) and then watered the lemon in with a dilute solution to help settle the plant roots and boost the ‘immunity’ of the plant. We’ll be monitoring the lemon carefully over the coming months but hopefully that will solve the problem and it will green up soon.
The moral of the story is that you should always check the pH of the soil/potting mix around your plants regularly and especially when they’re looking unhealthy or when you’re applying any amendments. It’s also a good idea to check the pH of the amendments themselves just in case. It’s also worth noting that this sort of problem is much less likely to occur in an organic fertiliser because common organic amendments are pretty close to neutral the majority of the time.