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).

Increase the effectiveness of glyphosate herbicide by adding nitrogen

Shawn weighs out ammonium sulfate in the Mesa County CSU Lab.

Glyphosate, the herbicide ingredient most commonly known as Monsanto’s Roundup went off patent several years ago. As a result many companies now produce and sell this herbicide sometimes at a lower cost than previously.

While this herbicide is much more effective on grassy weeds than broadleaf weeds (dandelions, purslane, bindweed, etc.) you can increase the overall effectiveness of this chemical by adding nitrogen to the spray. A spray mix containing 0.5 to 1% nitrogen is recommended.

So how do you figure out the amount of a nitrogen fertilizer to add to the glyphosate/water spray?

Let’s say you are mixing up five gallons of spray for a backpack sprayer. One gallon of mixed glyphosate and water weighs about 8.35 pounds, thus 5 gallons of this spray will weigh 41.75 pounds. The steps are as follows:

1. We first need to convert 0.5% to a decimal by moving the decimal point over two places to the left. This gives us 0.005.

2. We next multiply 41.75, the weight of the five gallons of spray by 0.005. (0.005 X 41.75 pounds = 0.209) That is the weight in pounds of nitrogen we need to add to the five gallons of spray.

3. We then can convert 0.209 to ounces. There are 16 ounces in a pound and 0.209 X 16 ounces = 3.34 ounces. Now we know the amount (3.34 ounces) of Nitrogen in ounces we need to add to the each five gallon tank of spray, but not the amount of fertilizer.

4. Fertilizers contain varying amounts of Nitrogen depending on their formulation. Ammonium sulfate contains 21% N; Ammonium nitrate 33%; and Urea 48%. For this exercise I’m going to use Ammonium sulfate.

5. We next have to convert 21% (the percentage of nitrogen in ammonium sulfate) to a decimal. WE do that by moving the decimal point two places to the left. (21% = .21 – there is a decimal between the 21 and the %, you just can’t see it.) We then divide 3.34 ounces (the amount of nitrogen we need) by .21. This gives us the amount of ammonium sulfate needs to be added to every 5 gallons of glyphosate spray. (3.34 divided by 0.21 = 15.9 ounces or one pound).

If you have an eight ounce cup handy, you would need to add two of these, full to the top, to the 5 gallons of spray. This will give you the 0.5% nitrogen mix you need.

To figure out the amount of ammonium nitrate or urea to use, divide the amount of nitrogen needed by the percentage (changed to a decimal) of nitrogen in the fertilizer.