Look at our tomatoes!

As you’ve probably gathered, I’m not one to miss an opportunity for some agricultural science, whether it’s at work, social occaisons or at home. Recently an opportunity presented itself so I thought I’d share.

During the summer school holidays, Boy persuaded me to get some of those ‘kitchen garden’ kits from a popular retailer. They come in a plastic cup with two pellets of compressed coconut fibre and a packet of seeds. We bought tomatoes (a dwarf variety), chillies, coriander and chives. Being a good little scientist, I encouraged Boy to follow the instructions to the letter, and we planted all the seeds in the cups. I didn’t count how many there were, but there were way more than we needed and the instuctions said that you could thin them out afterwards (and you never know what the germination percentage is going to be like with these things).

Within a couple of weeks we had cups full of seedlings, so it was back to the shop to get some potting mix. I filled a few pots with the cheap potting mix I bought but still didn’t have enough, so I mixed the soil from some old pots I wasn’t using with composty soil from the bottom of the compost bin.  I had mixed success with some of the other plants, but one of the pots of tomato seedlings just went off!

The tomatoes in the two pots were transplanted at the same time. The pot on the left is the cheap potting mix and the pot on the right is the compost mixture. Yes, there are actually some seedings in the pot on the left!

Admittedly the cheap potting mix had the consistency of shredded bark, but I didn’t expect it to be that bad. I also didn’t expect my compost to be that good, although I’ve been using the same bin for years, adding unused fruit/veg (naughty!), scraps & peels and the dirty bedding from our  two guinea pigs.

I asked Boy what he thought was happening.

He suggested that there could be more nutrients that the tomatoes needed in the compost mix and also thought that there could be bigger air spaces in the commercial potting mix which let the water through too quickly. He also thought that the tomato roots might find it easier to get through the compost mix.

While all of these are valid hypothesis, I didn’t really have the equipment to test them at home. What I did have was a pH testing kit. Doesn’t everyone? These kits aren’t expensive and are available at most places where you buy garden stuff. Although Boy didn’t mention pH as a possibility, I thought it was worth a try. I had done some pH stuff with Boy before (the old red-cabbage indicator trick – I still have the photos somewhere) so when I mentioned that I thought we should see how acid or alkaline the soils were, he knew basically what I was talking about.

So we put a small sample of the soils on a white tile, added the indicator and then the white powder to make the colour show up. There was quickly an obvious difference according to the colour chart, but we needed to wait a couple of minutes to read the result, so we decided to look up what soil pH tomatoes liked.

According to a site we found, tomatoes do best at the slightly acidic pH of 5.8-6.8.

To our surprise, the potting mix (the lower sample in the picture) has a pH closer to 8 or even 9. No wonder our tomatoes weren’t happy!

We’ve now transplanted the seedlings from the potting mix into another mixture of old soil and compost to see if they’ll recover. And I’ll definitely be mixing the new potting mix with compost and checking the pH before I use it again.


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