Is it thawed or unthawed?
My humble reader, spring is officially here. Temperatures are warming up nicely during the day, but activity in the farm fields remains anxiously quiet with the perpetual cold evenings; and never-ending snowfall. With that being said, the ‘sugarers’ tapping trees have been benefiting as sap bags continue to fill.
Looking past the trees for the fields, soil conditions can seem “all-surface.” We are looking at the outward appearance of the ground without any consideration of what is going on deeper. And this is a fair observation. It isn’t every day that someone has access to mechanisms that allow them to see what’s happening several feet below the soil surface. We react to what we see, and between our gut feeling and access to information, we form our best rationale. A lot of chatter has come across my desk regarding the capacity and condition of farm fields, and whether they are taking in moisture. If we are not seeing pooling water on the fields, can we assume that snowmelt and rain soaked into the ground? It must have gone somewhere, right? The rate or ability for fields to take in winter precipitation isn’t consistent, and is impacted by several intriguing factors that we must further explore.
To compare precipitation amounts, we are going to use the daily data collection of soil moisture and precipitation from the University of Wisconsin Agricultural Research Station in Stratford, and compare it to historical averages of the site. I am also going to look at snowfall, rainfall, soil moisture at the 4 and 8 inch depths, as well as a few select soil temperature data points. In prior articles, we have discussed water in regards to soil properties, but to refresh ourselves for this conversation; visualize scooping up a coffee cup worth of soil. In that cup, typical soil would consist of 25% air, 45% minerals, 5% organic matter, and 25% water.
The month of January saw a total of 3 inches of snowfall and 0.23 inches of rain, with an average of 1 inch of snow covering the ground at any time. Leading into January, temperatures were mild, and soil moisture was high; 48% at 4 inches and 40% at 8 inches. But things took a turn and soil moisture dropped to 9% in the 4 inches and 30% at 8 inches by mid-January. Air temperatures from early to mid January averaged -3 to 6 degrees (Fahrenheit) with hardly any snow to cover the fields. The frost was setting in.
Once February hit, the chill was on, with 20 days averaging 8 degrees, and soil temperatures stuck at 30-31 degrees. There was a jump in snowfall, with 13 inches and a little rain at 0.7 inches. Average snow cover was 5 inches. Despite the higher precipitation, soil moisture remained unphased throughout the month, staying around 7% in the top 4 inches and 25% at 8 inches, despite 5 inches of snow melting off after the 24th thanks to 40 degree weather. But even though 5 inches of snow disappeared, the soil moisture never moved. Indicating that all the potential moisture collected on the fields never penetrated the soil profile.
March began to see an increase in soil moisture, but nothing significant until March 22nd when soil moisture rose to 25% at 4 inches and 36% at 8 inches. The month was marked by a rapid melt off, with 0 inches of snow observed until the time of this article. The rising air temperatures warmed the soil to 32 degrees (at both 4 and 8 inches) and we had rain and major snow events from March 15th, 17th, and 20th, that accumulated 7 in. of snow and 0.5 in. of rain.
While this observation of less moisture in the soil despite an increase of precipitation may seem unusual, it is quite commonly caused by a phenomena called “concrete frost.” This occurs when rain or snow-melt get into the soil profile and take on a hard freeze, filling every gap with ice. This concrete frost takes longer to thaw than regular frost. When early-to-mid-January temperatures dropped, following a wet fall, and there was little to no snow cover to stop the frost from driving deep, it made any precipitation, or even nutrients applied (fertilizer, manure) during that time frame to infiltrate into the soil.
Whether seasonal precipitation is soaking into the ground or the majority of it is running off has major logistical, financial, and environmental impacts on field systems and farming operations. Talking about soil conditions to this level of detail may seem trivial, but these details can have major implications for farms. Soils must be the proper moisture for successful germination and establishment of crops, to be able to support heavy equipment without getting stuck, to maintain fertility in the topsoil, and to retain applied-nutrients and manure in the fields where they are most valuable to the farmer.
THE SOIL
SOUNDOFF
BY
MATT OEHMICHEN AGRONOMIST
