An awareness of the nutrients contained in manure is an important part of managing crop fertility. It is one thing to know what nutrients are in the applied manure, but another to know what nutrients will actually be available for the crops to utilize.

Often, a comment is made that a producer applied manure to a corn crop, but then did not seem to get the value from the manure nitrogen (N) that was expected. The reason given is typically cool conditions that slowed mineralization, or wet conditions that increased denitrification loss. It may be time to re-evaluate potential nitrogen volatilization loss, based on some comparisons looking at N losses from different application practices where pH appeared to influence loss.

Manure nitrogen is composed of organic N and ammonium N (NH4+). The ammonium portion is similar to commercial fertilizer in that it is quickly available to a crop, but is also at higher risk for loss due to volatilization. Volatilization occurs when the "free ammonia" (NH3) in manure is lost to the atmosphere. The rate of loss depends on temperature, humidity, soil moisture, wind speed, pH and vegetative cover (example in Figure 1) as well as the rate of application and infiltration capacity in the soil. Losses increase with surface exposure which makes incorporation the best method of reducing ammonia loss and retaining the nitrogen for crop utilization.

Figure 1. Available nitrogen from plots where high pH material was surface applied versus injected is evident in in oat cover crop growth.

The trend for ammonia loss is shown in Figure 2. The sooner the material is incorporated, the lower the ammonia loss. Incorporation equipment varies in its ability to completely cover the manure, and temperatures and moisture levels vary from season-to-season and from year-to-year. This makes it difficult to put an exact number on available nitrogen.

Figure 2. Timing of application and incorporation on ammonia loss

However, the impact of manure pH on losses (and other organic amendments) was accidently discovered from plot work where ammonia loss was being measured in manure applied immediately after forage harvest. Normal pH in manure is in the 6.5 to 7 range. When a digestate material from an anaerobic digester had a pH of 7.8, the ammonia loss measured in the dosimeter tube was immediate and much higher than for a liquid dairy manure with normal pH. The relative difference between a high pH material surface applied and injected is shown in Figure 3. The trend is the same for both high pH and normal pH materials, however the immediate loss is dramatic with the high pH material. Ammonia loss increases as pH increases, however the losses shown in Figure 3 were at pH levels of 7.8 to 8.2

Since pH is not normally tested in manure samples, many producers will not know if the material they are applying has a high pH. Where the manure has a high pH and is applied, but not incorporated for even 1 day, the "expected" nitrogen credit from manure may be much lower than anticipated, and may result in the need for additional commercial nitrogen to maximize economic yield.

Take home message:

  1. If you have manure (especially liquid) and are sending a sample to the lab for analysis, ask to have the pH tested. The cost is about $10 per sample. While you're at it, it is a good practice to have the sample tested for sulphur.
  2. If the pH of the material being applied is high, it is more important to inject or immediately incorporate the material. Waiting 24 hours to incorporate the manure may result in up to 100% loss of the ammonium portion of the manure, which will cost you in yield and/or additional commercial nitrogen if you are applying to corn. Temperature, soil moisture and wind speed will also influence potential loss.
  3. Materials such as some those from anaerobic digesters or products such as Lystegro, have high pH levels as well as high NH4 content. Municipally-sourced digestate and Lystegro are injected by the companies to help ensure maximum nutrient availability for crop production.

Figure 3. The graph shows the rapid loss trend (hours after application) of ammonium-N from materials that were surface applied with a high pH compared to surface applied materials with normal pH.