Fertilizing gardens in the fall, winter, and spring can be a bad idea!

Organic matter and synthetic nitrogen fertilizers are often applied to gardens, trees, shrubs, pastures, and field crops in the fall, winter, and early spring months. Losses of up to 70% of the nitrogen as ammonia (NH₃) can occur before next season’s growing season if not done properly.   In addition to the loss of a valuable plant nutrient a reduction in air quality can result. Ammonia creates smog by combining with nitrous oxides (NOx). The result is a hazy view and deposits in your lungs and the lungs of other animals.

Microbial activity of Bacillus, Clostridium, Proteus, Pseudomonas, and Streptomyces bacteria change organic nitrogen into ammonia as soil and air temperatures increase in the spring. To prevent nitrogen loss from organic matter applied to a garden it should be worked into the soil and not left on the soil surface. When applied as top dressing to a lawn, organic matter should be raked into aeration holes to move as much of the product into the soil as possible.

The synthetic fertilizer urea (46% Nitrogen) can start to breakdown as soon as it is applied to the soil. If the soil is totally dry, no reaction happens. If there is a small amount of soil moisture present this fertilizer can hydrolyze and convert to ammonium and carbon dioxide within two to four days. This is more of a problem in high pH soil areas.

CO(NH₂)₂ + H₂O + urease = 2NH₃ + CO₂
(urea)

It was commonly thought volatilization of urea was more problematic when air temperatures were 50 ^oF. or above. More recent research has shown volatilization of urea even occurs when temperatures are below 41 ^oF.¹ The same high loses can occur when urea fertilizer is applied to frozen soil. When at least one-half inch of irrigation water (or rain) occurs after application of urea, losses can be significantly reduced. Working urea into the soil also reduces nitrogen losses.

When Ammonium Sulfate (21% N) is applied to soil, very little or no conversion to ammonia (NH₃) occurs making this a very good nitrogen fertilizer.

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1. Engle, R., and Jones, C. Choosing your Nitrogen Fertilizers based on Ammonia Volatilization. Nutrient Digest. Fall 2011; vol3 (1).