|Urea as a Nitrogen Source For Forage Crops|
By Don Ball
Nitrogen is the fertilizer nutrient needed by plants in the greatest amounts and it is the one that results in the most dramatic increases in forage growth. Anyone who purchases commercial nitrogen fertilizer on a regular basis and who has purchased (or priced) this product recently is painfully aware fertilizer prices have risen sharply in recent months. In view of the fact prices had already risen a great deal in the past few years, the expense of purchasing nitrogen fertilizer can now be described as downright alarming.
Many cattlemen, producers of other livestock and crop farmers are looking for ways they might lower their fertilizer expenses, especially their nitrogen (N) fertilizer bill. Strategies being considered include using legumes to a greater extent to get the benefit of biological N fixation, using animal manures or other organic waste materials as a source of plant nutrients, or using less expensive commercial fertilizer products which they either have not used before or have used very little. Although urea fertilizer is nothing new, there are probably at present more questions being asked about it than have ever been asked before.
As a dry fertilizer material, urea has an analysis of 46-0-0. In other words, one hundred pounds of dry urea contains 46 pounds of actual N and no phosphorus or potassium. By comparison, 100 pounds of granular ammonium nitrate contains 33.5 pounds of N, and 100 pounds of ammonium sulfate contains 21 pounds of N (plus 24 pounds of sulfur). Fluid or liquid urea has an analysis of 28-0-0 or 32-0-0. Traditionally, urea-N is about 2/3 the cost of ammonium nitrate-N.
The cost of N from various fertilizer products varies over time, so it makes sense to calculate the cost per pound of N various sources will provide. In the case of ammonium sulfate, the sulfur this product contains has value because sulfur is also essential for plant growth. However, sulfur has the "downside" of lowering soil pH more quickly than urea or ammonium nitrate, thus resulting in the need to make a lime application sooner than would otherwise be the case.
There also is something else to consider with urea…under certain conditions there can be significantly more volatilization loss (gaseous loss of N into the atmosphere) with this product, regardless of the urea formulation being used. The amount of volatilization loss can vary from none to over 50 percent. With the potential loss being this high, it makes sense for producers to understand what situations lend themselves to significant levels of volatilization.
Several factors affect the likelihood of substantial volatilization loss. If the soil pH is alkaline (above 7.0) the risk is greater than if the soil is acidic (pH less than 7.0). Fortunately, except for certain Black Belt soils, this is unlikely to be a problem in Alabama. If the soil is moist or there is heavy dew, the volatilization risk is increased; on the other hand, if there is a rain within 24 hours after application, there will likely be little volatilization loss. Soils with a low cation exchange capacity are more vulnerable to volatilization loss than soils with a higher cation exchange capacity. Also, the more vegetation present on the soil surface, the higher the potential volatilization loss.
The last and perhaps the most widely applicable factor to consider is temperature. When soil temperature is less than 50 degrees Fahrenheit, there is low likelihood of volatilization. Therefore, urea can normally be applied between around
early October and early May with little concern about volatilization loss. Even in summer when temperatures are high, there is often no more than 15 to 20 percent loss. Thus, if the cost of urea is significantly less than other sources of N, it may be economically justifiable to use this product even in summer.
A common question is whether a urease inhibitor should be added to urea to reduce potential volatilization. These products do work. They may not eliminate N volatilization, but they can certainly reduce the loss when urea must be applied under less-than-favorable conditions. A producer should consider the additional cost of adding a urease inhibitor to a urea or urea/ammonium nitrate solution versus simply applying a little extra N to compensate for anticipated N loss.
Don Ball is an Extension Forage Crop Agronomist with Auburn University.