SEEDS AND SOIL TEMPERATURES
There’s an old saying that the time to plant seeds is when you can comfortably sit on the ground with a bare bottom. I prefer to use a soil thermometer instead of shocking my neighbours! I have a nifty, dualpurpose one that shows the soil and air temperatures at the same time. In general, the soil in wooden raised beds is warmer than garden beds, and metal and terracotta pots are warmer still. For example, one afternoon, when the air temperature was 15 degrees Celsius; the soil in a wooden raised bed was 12C; the soil in an old copper in full sun was 13C, but heavy, waterlogged clay garden soil only reached 10C.
Moisture and soil consistency affect the temperature too. Light, aerated potting mix that’s well drained warms up much faster than dense, waterlogged clay. Interestingly, the soil temperature is much more stable than the air temperature. Early the following morning, the air temperature was 11C, the raised bed and the copper were 12C and the clay soil was still at 10C. So why does it matter? Seeds need specific temperatures for germination.
You’ll have much better results if you wait until the temperature is in the right range before sowing spring and summer crops. Beans, for example, need soil temperatures of 18-25C for reliable germination. There are some soil warming techniques to speed things up. Try covering a garden bed with a cloche or a sheet of plastic to warm up the soil for a couple of weeks before planting. I’m using the polycarbonate sheets to warm up my soggiest garden bed. where Victorian walled gardens are meticulous recreated, they use marked measuring tapes to line up the vegetables in regimental ranks.i don’t bother to use a ruler as I went ‘‘digital’’ long ago. Nothing electronic, just my fingers, thumbs and whole gumbooted foot. My handspan is 18cm so I allow a tad over a handspan between seedlings that need to be 20cm apart. The width of my hand including the thumb is 10cm and that of my index finger is 1cm. From the tip of my index finger to the second joint is 5cm and my gumboot is 27cm long. Learn the measurements of your own hands and feet as a guide when sowing seeds or thinning out seedlings. When diluting fertiliser, it’s handy to know that a standard plastic bucket is 10 litres. For me, one handful is 75g, so I can work out how many handfuls of fertiliser or gypsum to spread.