Soil, temperature and moisture

I started collecting soil temperatures in a randomly selected part of the back yard in August 2012 in an attempt to understand why beetroot was reluctant to germinate in April and May, but was quite reliable during July and August.  For no better reason than it was a convenient time to sit in the garden, the temperatures were measured around sunset on a Sunday afternoon.  Whilst this was convenient and consistent with regard to forming a time series, it was poor experimental design, by sunset the soil was usually warm, so on an April afternoon it was probably 10 deg. C, warm enough for some native english species to germinate.

One day in April, I had occasion to make frequent visits to the garden from early morning to late evening to make use of the time I decided to measure the soil temperatures over the course of the day.  The graph from this day's activity was quite instructive:

This shows that the variations in soil temperature during the day are much greater than that of the air above it. A crude extrapolation suggests that the temperature of the topsoil (0.1m) might have been from 2 - 16 deg. C, and for the two centimetres where seeds are sown could have been much greater.  During the day, the soil is warmed by the sun, during a clear night it experiences radiative cooling.

I had an attempt at taking soil temp measurements during the day on 25 July when the weather had been warm and dry for several days.  This exercise came to an abrupt end around 14:00 when a lone shower of heavy rain arrived.  The temperatures that were collected at a depth of 0.1 are shown in the graph below:

The maximum air temperature was around 25 deg. C when the soil was noticeably warmer than the air above it, it looks unlikely that the soil temperature fell to less than 10 degrees overnight.  When the surface temperature is high, it is clear that the top .01m of soil starts to loose moisture rapidly.

Do the low night time temperatures during April halt germination?

I'm new to gardening and a little lazy, so my method of sowing is to rake the soil and remove the building rubbish that abounds in the garden and then plant the seeds in a two centimetre groove formed with my index finger.  Over a few weeks it became clear that there were significant variations in moisture content.  The human body is a good measuring device for all sorts of things, e.g. solar irradiance, wind speed, humidity, air quality and soil moisture content to name but a few, the downside is that it does not produce graphs.

Over a few days I collected three soil samples from a bit of the garden that escapes watering, each sample was about 200gm (a half dog food tin), these were left to dry in the sun.  The moisture content being estimated from weight of soil at the time it was collected and its weight after drying, the resulting numbers were then turned into a graph:

The sample collected after a hot day may have been inadvertently watered, so it could be an over estimate.  So as for temperature, there are considerable fluctuations in soil water content over a short time interval.  And the lesson from this, make sure newly sown seeds are frequently watered until the plants are robust enough to withstand the turbulent environment that is the topsoil.  I think I was taught that at school.