Today is soil testing time. A strange thing to be doing on the last day of my apprenticeship, with the soil such an integral part of our growing days. But this is an activity that only needs to be done once every twelve months or so, and Ru leaves it to a time of year when there’s not so much to do and the weather is fairly good. The clear skies mean that we can set up a table outside to make an outdoor lab. First though, we have to collect the soil.
We take samples from two different places on the site. First you check the structure of the soil. You can use a spade to do this, digging one length down into the soil and cutting it out. We have the advantage of a damaged raised bed which needs fixing. We pull away the slab of concrete and look at the how much the soil has been compacted, what life there is down beneath the surface. Our soil looks pretty good. It has a good amount of activity going on – earthworms doing their groovy thing and a millipede rushes through. And being a raised bed which doesn’t get trodden on, it is fairly airy. The addition of lots of yummy compost has helped with that.
The next thing you test for is the texture of the soil. You do this by taking a handful and kneading it into a ribbon or sausage shape. You’re checking to see how well it holds together. Ours has some malleability but is sandy as well. Ru says this means it is a sandy clay loam. ‘Sandy’ and ‘clay’ are pretty self-explanatory, I think. ‘Loam’ means it has a high content of organic matter. There are lots of different types of soil ranging from sandy, loam to clay, with lots of variations in between including loamy sand, silty loam, clay loam etc etc. What this helps to determine is how well your soil retains water, sand will be poor, while clay keeps water well, too well in some cases. This can help you decide what plants to grow but also what you should add to balance it.
Once these two physical tests are done, it is time for the science. Here we have a special soil testing kit to check out how the soil is doing as far as its nutrient content is concerned. We also test the pH or acidity levels of the soil. On the nutrient front we are testing for nitrogen (N), phosphorous (P) and potassium (K). NPK are what are known as the macronutrients. The nitrogen helps with the leaves, phosphorus for the fruit and flowers and potassium for the roots.
All laid out and ready for testing!
For each of the tests, you have to make sure your soil sample is dry. Then you get rid of any sticks and stones and crumble it up a little. Into your test tube you put about 1ml of soil. You add different solutions to test for each nutrient, wait various times (all instructions will come with your testing kit – don’t want to bore you with too much detail) …and then you check colouring for the N and P and the cloudiness of the K solution. Felt just like being back in science class.
And our results – from Bed 1 we got a pH of 6.8, which means it is slightly acid, and from Bed 2 a pH of 7 – bang on neutral. Bed 1 was low on both nitrogen and phosphorus and high to medium on potassium. Bed 2 had low to medium N, a medium/high P and high K. Ru thinks this is pretty good and reckons that this is typical of a site that has been reclaimed for growing food in London. It can only get better with all the compost we are adding!
Grower: 1 | Apprentices: 2 | Volunteers: 4 | Support workers: 1 | Dog: 1
Harvested from the site…
Salad greens & edible flowers: 10kg | Basil: 90g | Tomatoes: 3.5kg | Cape Gooseberries: 300g