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Climate Extension

Soil Temperature Shifts

March 15, 2011

UNL Study Finds Soil Temperature Thresholds Shifting Earlier

My last CropWatch article on soil temperature addressed the importance of soil temperature as a guide to planting agronomic crops in Nebraska. This article addresses the question of whether there has been a shift in when soil temperatures reach optimum levels for planting spring and summer crops.

 Nebraska soil temperature maps

Figure 1. Deviations in planting dates observed in this decade (2000-2009) compared to the previous decade (1991-1999). Red indicates how much earlier the area reached the specified soil temperature in 2000-2009 compared to 1991-1999. Shifts of one to seven days were included under the one week earlier/later label.


Comparing Two Decades of Spring Soil Temperatures Trends

To identify potential soil temperature trends, the average dates when soil temperature reached values between 40⁰F and 60⁰F were compared for the last two decades, 2000-2009 and 1991-1999 (Figure 1). Red indicates areas where the average time when soil temperatures reached the cited values was earlier in 2000-2009 than in 1991-1999. In other words, red indicates earlier shifts in soil temperature and a potential for earlier planting dates, and blue indicates the reverse.

The maps for 50⁰F, 55⁰F, and 60⁰F show large areas (red) that reached this soil temperature planting threshold earlier than in the previous decade. There is a prominent spatial trend of areas reaching the 50⁰F and 60⁰F thresholds one to three weeks earlier than in the previous decade. The mean values (Table 1, below) from all 29 Automated Weather Data Network stations showed more than a week earlier advancements for 55⁰F and 60⁰F soil temperatures. Optimum soil temperatures for germination of corn, soybean, and sorghum are in the 55⁰F to 60⁰F range, indicating great potential for shifting planting earlier in the spring.

This study also shows that the shift isn’t consistent for all temperature thresholds. The date when average soil temperature reached the 40⁰F planting threshold used for barley was slightly later in 2000-2009 than in 1991-1999, but only by one day in most areas (Table 1).

Can This Shift be Attributed to Climate Change?

The earlier planting dates in the most recent decade were due to an increase in soil temperatures. This short-term trend is consistent with climate change projections and indicators, but does not distinguish itself from natural climate variability. In order to conclude that increased soil temperature is an impact of climate change, we would need to evaluate data over a longer time, such as 100 years. However, identification of this soil temperature trend can be used to alert decision makers of a possible evolving trend.

Effects of a Spring Soil Temperature Shift

This data indicates that spring planting season could occur earlier in the year for warmer season crops like corn, soybean, and sorghum than it did a couple decades ago. The spring soil temperature threshold for planting spring wheat, barley, and oats is coming slightly later than it did a couple decades ago. If climatic conditions are closely monitored and farmers follow the temperature shift and plant earlier, benefits would include being able to plant a longer season hybrid with a correspondingly higher yield potential, and minimizing the risk of freeze prior to harvest.



Table 1. Day on which soil temperature averages reached select values from 40-70⁰F across 29Automated Weather Data Network (AWDN) soil temperature stations in Nebraska. (Negative signs indicates earlier shifts. Positive sign indicates later shifts.)
    Date when average soil temperature reaches 
the temperatures shown for the years indicated.
  Year 40F 45F 50F 55F 60F 65F 70F

Average (day of year)  1991-1999  3/9 3/22 4/6  4/24  5/7  5/18  6/1
   2000-2009  3/10  3/23  4/3 4/15  4/28  5/24  5/26
 Difference    1  1  -3  -9  -9  -4  -6



Tapan Pathak, Extension Educator in Climate Variability
Kenneth Hubbard, Climatologist and Professor, School of Natural Resources
Martha Shulski, Director, High Plains Regional Climate Center