Field Notes April 1, 2010; DD50, drill seeding, heat units

If you visit the AgCenter’s web site, you can access weather data that offers some explanation for the slow movement of rice this year.  On the left side of the home page is a block with the title Services.  Weather is within that block.  I looked at growing degree days.  A growing degree day (GDD) for rice is calculated by adding the daily high and daily low then dividing by 2 (this provides a type of average temperature for the day) then subtracting 50 from that number.  For example if the high was 71 and the low was 46 there would be 8.5 GDD accumulated that day. [(71 + 46) ∕ 2 = 58.5.  58.5 – 50 = 8.5.]   

I took a look at weather data from March 2009 and 2010.  In 2009 there were 5 days in March where no heat units were accumulated, 9 days when 15 to 19 heat units were accumulated, and 5 days when more than 20 GDD units were accumulated.  In contrast, this year there have been 5 days of no heat units, 1 day between 15 and 19 units, and 0 days with more than 20 heat units.  I repeat, it’s just too cold. 

Below is a photograph I took in one of our verification fields last year.   Note the seed furrow on the right.  It is not closed properly and this was made by one of the best drills on the market.  The problem is in certain soil conditions we are asking too much of the machinery.  When the soil moisture is at a certain level the soil will not crumble so the press wheels cannot close the furrow properly.  

The next photograph on the next page shows what often happens when silt loam soils are prepared in a flooded state.  They crack or “potato chip” as some refer to it.  

It happens because a thin layer of sand forms about an inch from the surface as sand particles settle out before the silt and clay fractions.  The very fine sand will not prevent the movement of water downward due to the pull of gravity, but it can prevent moisture movement UPWARD due to capillarity.  As the upper half inch of so of soil dries by evaporation from the surface the barrier of sand prevents upward movement of water.  Soil below the sand can only dry by gravity while the upper layer dries by both gravity and evaporation.  That causes the upper layer to contract more than the layer below and cracks develop.  If it continues to shrink the edges curl upward causing “potato chipping”. 

The following photograph was sent in by Stuart Gauthier, county agent in Vermilion parish.  It shows a field worked in water and drained.  I’ll try to get a photograph of the sand layer I described above.

Cracking of the soil surface


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