Evaluating the Fertilizer Value of Alternative Nutrient Sources

Published: March 28, 2008

High fertilizer prices have many farmers considering alternative sources of nutrients for their crops. The most common alternatives are animal manures and sewage sludge. A wide variety of other agricultural and industrial by-products can find their way onto farmer's fields for use as a nutrient source.

Before we discuss alternative sources of fertilizer, it is helpful to appreciate the qualities of most commercial fertilizers. The fertilizer products you purchase from your local fertilizer dealer typically have high concentration of available plant nutrients, and are highly uniform products. Commercial fertilizers are guaranteed to meet the nutrient content listed on the label and a statewide inspection program monitors compliance of labeled fertilizer products.

Some alternative nutrient sources including some sources of manure have gone through the process of being certified as a labeled fertilizer with a guaranteed analysis. But many farmers are considering the use of products that are not registered as labeled fertilizer products. The following tips should be helpful as you evaluate some alternative nutrient sources.

Is there a nutrient analysis?

For most materials test results that include total nitrogen, ammonium nitrogen, phosphorus and potassium are essential.

Soil scientist John Kovar plants corn seedlings into a silt loam soil amended with turkey litter. Kovar and soil scientist Thomas Sauer will compare the effectiveness of the litter and commercial fertilizer as phosphorus sources for corn and evaluate depletion of available soil phosphorus under intensive cropping. Photo by Peggy Greb. http://www.ars.usda.gov/is/graphics/photos/

How available are the nutrients?

This can be a difficult question to answer. Working with a nutrient management planner can help. Here are a few tips:
Phosphorus and potassium:
With conventional organic by-products, such as animal manure and sewage sludge , the phosphorus and potassium is typically available to plants. In less conventional products , availability can be a real issue. Phosphorus in organic sources can be tied up if aluminum (e.g., alum) or iron has been added. Some poultry operations add alum to their litter to reduce ammonia in their barns.

Some of the phosphorus in ash products also can be tied up in forms unavailable to the plant. If the product is a mineral source of nutrients (e.g., ash) you can have it tested using the standard fertilizer methodology for measuring available phosphorus.

Nitrogen:
Nitrogen value of organic sources of nutrients can be difficult to predict. Most predictable is injected unagitated lagoon effluent which is nearly 100 percent available. Nitrogen availability in most surface-applied manure sources is closer to 50 to 60 percent. Sludges and composted materials typically will have lower nitrogen availability. Be cautious with materials that have recently had high amounts of carbon such as wood chips and sawdust added to them. These products can actually tie up nitrogen in the soil for awhile unless they are composted first.

How concentrated and uniform are the nutrients?

Low nutrient concentration products cost more to transport and it takes longer to apply the needed nutrients to a field. Many sources of organic nutrients can be quite variable unless steps have been taken to mix the materials during handling. Most uniform will be liquid sources of manure such as unagitated lagoon effluent and agitated manure slurries. An exception to this rule can be sludges pumped directly from a lagoon because of the difficulty of thoroughly mixing this material. Composting and/or stacking solid manure can significantly reduce variability in solid manure.

When will they deliver?

Frequently the timing of alternative nutrient sources is driven by when the product is generated at the source, not when you want it applied to a field. Ask when it will be applied and be clear about the field conditions, and when you do not want equipment traveling on your field.

Manure and other by-products can be excellent nutrient sources for your crop. They require more awareness on the part of the buyer to determine their value. Such products can be an excellent value for the knowledgeable buyer.

In Missouri, the fertilizer law states that a product cannot be sold with a fertilizer claim unless it is a labeled product under the Missouri fertilizer law. There are also detailed regulations for land application of sewage sludge. More information on land application manure and of sewage sludge is available through searches of the extension publications Web site (http://extension.missouri.edu/explore).