All fertilizers are not created equal
Price per unit of plant nutrients, availability, ease of storage and application, potential for nutrient losses and site specific factors are most important considerations when deciding on a fertilizer type.
Fertilizers come in different forms, grades and formulations. Physically, they can be solids, liquids or gaseous. Some fertilizers can be applied in the fall, some in the spring and others during the growing season. The majority of fertilizers are synthetic but there are organic sources such as livestock manures. Most synthetic fertilizers sold today release their nutrients to the soil rapidly, but some fertilizers are designed to release their nutrients slowly. All fertilizers irrespective of their origin or form are meant to serve one basic purpose – to supplement the essential nutrients in the soil. Some fertilizer nutrients, particularly nitrogen (N) and phosphorus (P), if improperly used could move away from the application site and degrade water quality.
For handling purposes, there are two major classes of fertilizer: solid and liquid. Anhydrous ammonia is the most common gaseous form but it is stored as a liquid under pressure. Blended dry fertilizers are generally much less expensive in bulk and easier to store compared to liquid. Liquid fertilizer use in Michigan has steadily increased in the past 50 years and today liquids account for about 50 percent of the market. Liquid fertilizers are popular because of the ease of handling and application. With liquids, a custom blend and greater uniformity in the application rate can be achieved. In dry blended fertilizers, particles such as micronutrients can segregate during mixing and handling, resulting in uneven application.
Just like their solid counterparts, liquid fertilizers contain one or more available plant nutrients. They can be true solutions where all the N-P-K nutrients are dissolved in water or held in suspension. Suspension agents such as colloidal clay material are typically used to keep the fertilizer particles from settling out. Suspension fertilizers should not be stored for long periods. True solution fertilizers can be stored as long as the temperature stays above freezing but salting out in cold weather may be an issue.
In terms of crop response, there is no agronomic difference in the efficiency of liquid and dry fertilizers when the same rate and placement are used under satisfactory growing conditions. Fertilizer recommendations are the same regardless of the fertilizer source. When placed in moist soil, dry fertilizers absorb water and undergo chemical reactions similar to liquid fertilizers. Both solid and liquid fertilizers, when properly diluted with water, can be applied directly on foliage or with irrigation water. Both forms can be used as starter or pop-up applications. Farmers generally take advantage by using both solid and liquid fertilizer forms; solid fertilizers for heavy pre-plant applications and liquids preferred for starters and in-season applications. Certain pesticides are compatible for mixing with liquid fertilizers. Most liquid fertilizers weigh between 10 and 11 pounds per gallon. This density is used to determine how much liquid fertilizer to apply per acre. About 9 to 10 gallons of the liquid form are needed to provide the same quantities of nutrients as 100 pounds of dry fertilizer of the same grade.
A drawback of the liquid fertilizer system is that it requires special storage and application tanks and pumps, which greatly increase costs. Most farms use special poly tanks to store liquid fertilizer. Michigan regulations require secondary containment facilities to prevent leaks.
Organic fertilizers, such as chicken litter, biosolids and compost, in addition to providing plant nutrients, also contribute organic matter to the soil. Because of their low nutrient content, large quantities are needed. Their nutrient content also tends to be variable. Just like the synthetic counterparts, the N and P of organic fertilizers are susceptible to environmental losses if improperly managed.
Introducing polymer-coated urea fertilizers helped to reduce N leaching and runoff losses because of their controlled release action. Their use is particularly beneficial on fields close to surface water or on fields that are coarse-textured where N leaching is predictable. They provide flexibility in application timing and higher N use efficiencies that may offset their higher cost.
A plethora of new fertilizer-related compounds have emerged recently. These products are advertised as soil amendments, nutrient enhancers or growth stimulants. When trying new products, Michigan State University Extension advises farmers to use them on small acreage first and have untreated check strips side by side to compare yields and economic returns. A pre-plant soil test will serve to establish baseline nutrient levels in the treated plots. Adding new fertilizer products will further increase the cost of production. In the meantime, the search for the perfect fertilizer, one that is freely available and easy to use with high plant recovery and low nutrient losses to the environment, will continue well into the future.