Fertilizer: Important First Aid for Eroded Soils

Prepared by: John Heard, MB Ag Soil Fertility Specialist

One of our summer students gashed their leg during field operations last spring.  First was a trip to emergency to close the gash with stitches, then some medicine and salve, followed by time for healing.  Many Manitoba fields have suffered similar “gashes” this past spring and summer with overland flow of water.  Farmers must first close the “gash” through backfilling and tillage and then face some rehabilitation.  But don’t forget the nutrients.

Loss of productivity on eroded soils is due to a combination of:

• Loss of soil nutrients
• Loss of soil organic matter
• A decrease in the moisture holding capacity of the soil
• Changes in the soil structure and texture
• Root zone depletion
• Exposure of subsoil

In the 1980s there was considerable research done in Manitoba lead by Dr. Carl Shaykewich to document the loss in crop productivity when top soil was lost (see attached references).  The research was done across some 7 soils types in Manitoba where 2, 4 and 8 inches of topsoil was scraped off.   In an attempt to remediate detrimental effects, fertilizer was applied at three rates: none, the recommended soil test-based fertilizer rate and 2 times this rate.  A summary of the results by soil texture is shown for canola (Table 1) and wheat (Table 2).

Table 1.  Relative Westar canola yields on soils with topsoil removed and supplemental fertilizer applied (from Kenyon and Shaykewich, 1987).

*Actual Westar canola yields were 36, 50 and 34 bu/ac for the sandy loam, loam and clay loam soils, respectively.

Table 2.  Relative Columbus and Benito hard red spring wheat yields on soils with topsoil removed and supplemental fertilizer applied (from Ives and Shaykewich, 1987 and Kapoor and Shaykewich, 1990)

*Actual wheat yields were 31 and 36 bu/ac for the sandy loam and clay loam soils, respectively.

Their conclusions were:

• Yield loss was severe with eroded soils, primarily due to nutrient loss.  Yield depression ranged from 6-66% when soils were fertilized.
• Soils with heavier texture still retained a level of water holding capacity, and so yields were maintained when high rates of fertilizer were applied.  Yields were 86-114% of uneroded but fertilized soil.
• Extra fertilizer applications did not maintain yields on lighter (sandier) textured soils, largely because of the reduced organic matter level and associated water holding capacity.  Yields were 55-86% of uneroded but fertilized soil.

A lesson on this erosion impact was demonstrated at the annual soil and manure management field day held in Portage last August.  Those soil factors exhibiting the greatest change are shown in Table 3.

Table 3.  Impact of topsoil removal on soil characteristics at the Portage CMCDC research station.

Analysis provided by AgVise Labs.

But extra nitrogen, phosphorus, potassium and sulphur cannot mask all the problems associated with topsoil losses.  Exposed subsoil with the associated low organic matter and high pH reduces micronutrient availability. Researchers also observed that even when grain yields were similar, the amount of straw produced was less, meaning organic matter levels are unlikely to be restored.

So growers looking to rehabilitate eroded soils should consider:

• Additional fertility inputs to these sites - this may be in the form of fertilizer or manure.
• Landscape Restoration - this method involves collecting top soil from deposition areas of the landscape and transporting back to the eroded hilltops it originated from.
• Production of high yielding crops which return more straw back to the soil.
• Using a minimum tillage system to reduce further erosion and encourage organic matter buildup.

Fertilizer additions are not the sole cure for these areas with topsoil losses, but are part of the treatment in bringing them back into production. Restoring soil fertility levels are expensive in the short term (up to $100 extra/acre if applying a 2x rate), but can be influenced quicker than the long-term rebuilding of organic matter and soil structure.

References

Ives, A. and C. Shaykewich.  1987.  Effect of simulated soil erosion on wheat yields on the humid Canadian prairie.  J. Soil and Water Cons. 42(3):205-208.

Kapoor, A. and C. Shaykewich. 1990. Simulated soil erosion and crop productivity.  In Proc. 33th Annual Manitoba Society of Soil Sci. Proceedings. P. 125-130.

Kenyon, B. and C. Shaykewich.  1987.  Simulated soil erosion effects on canola yields.  In Proc. 30th Annual Manitoba Society of Soil Sci. Proceedings. P. 79-86.