Tile Drainage Reduces Salinity – Slowly!

Salinity (salts) has been an increasing problem in many areas since the early 1990’s.  Excessive rainfall and poorly drained soils are a bad combination.  These two factors are a big part of the reason salinity is increasing in our region.  As the water table gets closer to the surface in wet years, water is wicked to the surface.  Once the water evaporates, the dissolved salts are left behind on the soil surface and reduce plant growth and yield.  If this situation occurs for many years, the soluble salts can accumulate on the soil surface to a high level and reduce crop yields.

Surface drainage along with tile drainage and continuous cropping are the only way to reduce the salt level in soils over time.  Keeping plants growing as many days of the year as possible will help keep the water table lower and reduce salinity issues over time.  Including crops like corn in the rotation that use water longer into the season help keep the water table down as well.  There are no magical soil amendments that will reduce the salt level in soil (wish there were!).  With good surface drainage, tile drainage and good crop growth, the water table is lowered.  Once the water table is lowered, this stops the capillary action which brings water to the soil surface and reduces salinity over time.

AGVISE has been monitoring the soil salinity (salts) level on ten sites in a tile drained field near our laboratory in Northwood ND since 2002.   The soil type in this field is sandy loam to loam. Through the wet years of the 1990’s, the salinity level in this field had increased to levels where crop yields were greatly reduced.  In 2002, the farmer/cooperator Grady Thorsgard decided to tile this field.  We established 10 GPS points to monitor the change in salinity over time.  Each fall after harvest we collect 0-6” and 6-24” soil samples and test them for salinity and all other nutrients.

In the pictures and figures you can see that over the past 14 years, the topsoil salinity has decreased significantly.  In the second figure you can see the salt level at site 2 and site 5 have decreased greatly over time.  These two sites had the highest initial salinity of the ten sites we are monitoring.  You can see that the salt level did not decrease every year. In the dry years, the salinity stayed the same or increased a little, but over many years, the salinity at these two site has decreased greatly (see figure on monthly rainfall).  The overall salt level in this field has been reduced, because the tile drainage has lowered the water table and in years with excessive rainfall, some salts have been leached out of the topsoil.  Improving crop growth in all years and including corn in the rotation has also helped remove more water from the soil profile each year.  This field now produces high yields of many different crops without losing yield to salinity. We expect the salt levels in this field to continue to decrease slowly for many years.  As you can see, tile drainage does help decrease the salt level over time, but it is a slow process.  If you install tile in a fine textured soil, the changes will be even slower.  Reducing salinity is only one advantage of tile drainage.  Other advantages include higher yields because of earlier seeding dates, better root development, less soil compaction, less loss to drowned out and less N fertilizer losses due to excessive water in the soil profile.