How to use tools Best techniques to get the job right with Tim Rumball How to use a soil-testing kit
Testing your soil for acidity and nutrients will encourage healthier growth, says Tim
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F you want plants to grow you have to give them the right food, and the right conditions for them to make use of it. The acidity or alkalinity (pH) of garden soil affects the way food can be taken out of the soil by plants, but the essential nutrients – nitrogen, phosphorous and potassium (NPK) – must be there in the first place. So it’s important to know not just the pH, but also what’s missing so you can add more, and what’s already there so you don’t provide too much! Simple testing kits that just determine soil pH are widely available. You can also buy more elaborate kits from garden centres and online to determine NPK, but for both types the starting point is a good soil sample.
How to take a sample Take soil samples in dry weather when it hasn’t been raining for at least a couple of days. Don’t take samples if you have added lime or humus-rich material to the soil within three months. From a garden flowerbed take three separate samples from different points across the bed. Cleanliness is essential. Wash, rinse, then dry your garden trowel and use it to scrape away the top 2in (5cm) of soil at each sampling point. Take a spit of soil, about one-third of the trowel blade, from the base of the hole and put it in a clean container. Take your other two samples in the same way and mix all three together, removing stones and larger debris. Spread the soil out in a tray under cover and leave it to dry naturally for 24 hours. This soil sample can be used for all four tests.
Add garden lime to make soil more alkaline. It is frequently used on vegetable beds and it is considered organic
It is worth taking the time to understand your soil – it is the key to better results
How to process the sample There are many soil-acidity test kits available. They all work on the same principle but they do vary, so follow the instructions provided. For the one I’m using, soil from the dry sample is put in the test tube up to the 1ml mark, a measure of barium sulphate and then water (use distilled, available from chemists) is added and shaken vigorously. It’s left to settle, then the top liquid is siphoned off and put in a separate tube with a couple of drops of pH test solution. Compare the colour of it against a chart. Dark green shows neutral soil, pale green to blue is alkaline, and varying shades of yellow to red indicate an acid or very acid soil. To test for NPK you’ll find a kit similar to mine at crocus.co.uk. For nitrogen my test kit uses a filter to extract nutrients, but not all do. Again, follow the
instructions in your kit. I put 1ml of dry soil in the filter with 2½ml of N1 test solution and press the solution through the filter.
How to read the sample The resulting liquid is put in a clean test tube with a measure of N2 testing powder, covered, shaken and left to stand for five minutes. The colour of the liquid in the tube is then compared to the nitrate reading chart. A rich-purple colour means the soil has lots of nitrogen, while pale-lavender means it’s low in nitrogen. The same procedure is followed when testing for potassium and phosphorus, but using the relevant testing solutions provided in the kit. How to redress the deficiencies Comparing the colour of a tested soil sample with the appropriate colour chart is not always clear. A bit of interpretation
Testing for nitrogen, phosphorus and potassium
N test: Nitrates leech out of my sandy soil quickly, so it’s no surprise that this test shows low levels. I’ll add well-rotted manure in spring. 46 AMATEUR GARDENING 1 FEBRUARY 2020
P test: The colour chart indicates that the phosphorus level in my soil is at medium, so there is no need to add any more.
K test: Potassium is almost non-existent in this sample, so I will dress the soil with sulphate of potash in spring, or rock potash in winter.
