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Right Rate

January 4, 2019

The 4 Rs of Manure Management - Right Rate from Central Valley Ag on Vimeo.

The 4Rs of Manure Management:  Right Rate

Last month we introduced the concept of the 4Rs of nutrient management (right source, right rate, right time, & right place) and how they relate to manure. We talked mainly about source and the differences between different manures from different animals and how those differences can be matched up to your fertility needs. This month I want to talk about the right rate of manure to apply.   

As an example, I am looking at some feedlot manure from a field stockpile I sampled about a month ago as we were trying to decide on an appropriate application rate for one of CVA’s growers. There are some key things we need to consider when we make a recommendation for manure application. First, we need to know the nutrient value of the manure we are going to apply. We all know that these values can vary substantially from one area of a pile or pit to another. Because of that variation, it’s a good idea to grab two or three samples and average their values coming from the testing lab vs. relying on only one sample. My samples are averaging 5 pounds of first year available nitrogen with one pound being ammonia and 4 pounds being organic. There are actually 12 lbs of total organic nitrogen, but the laboratory is telling me I should only count on 4 lbs being available that first year. These samples also average 30 lbs of P2O5 (21 lbs assumed to be available the first year), 22 lbs of K2O (20lbs first year available), 6lbs of Sulfur (3 first year aval), and 0.4 lbs of Zinc (0.3 lbs available this year).  

Normally I tell growers they can plan on the total amount of nutrients for nutrients like P & K where you are more focused on long-term soil fertility than the nutrients needed for that crop that year. In the case of nitrogen and sulfur the focus should be more on nutrients available for this crop year. 

Of course, the nutrients available in the manure are only half of the equation when choosing an application rate. The other half is composed of the nutrient needs of the crop you are growing and the needs of the soil you are trying to improve. I am looking at a field with widely varying soil test phosphorus levels, and I want to make sure I am applying a correct rate to maximize yield but minimize the chances of P levels getting extremely high and possibly creating large amounts of nutrient runoff during rain events in future years. I want my soil test levels to average around 25ppm P but what I currently have in this fields are levels ranging from only 4 ppm to over 40 ppm. The field lends itself well to splitting into three fertility zones based on P values. Zone 1 is very low in fertility with an average P value of 5 ppm and K value of 128 ppm. Zone 2 has better fertility with P levels averaging 16ppm and K values averaging 166 ppm. Zone 3 has better P values with P tests averaging 24 ppm, but K levels are still low at 135 ppm.  

On the crop side, we have yield variation, but we will keep it simple and say we are shooting for 240 bu corn and 70 bu soybeans. We want to make a manure application that will cover both crops and allow us to build soil test levels to at least 20 ppm P.  We would also like to see K levels increase to a minimum of 160 ppm in each field. We can assume the 240 bu corn will remove approximately 90 lbs of P2O5 and 65 lbs of K2O.  The soybeans will take off around 55lbs of P2O5 and 100 lbs of K2O. To cover just the crop removal of P will take five tons of this feedlot manure.  Covering the crop removal of the K will take eight tons of manure.  

What we decided to do in this case is do a variable rate application by zone. In zone 3 with good P levels but low K levels we are going to apply a 10-ton rate to raise P values slightly and also increase the K levels in the soil. In zone 2 we are going to apply a 15-ton rate to build the P values a little more. In zone 1 we are going to apply a 25-ton rate to increase these very low P levels in that area. This will also help address the very low levels of K in that area. These rate changes will be built into a prescription map that will change the spreader application rate as the applicator crosses the field to match the fertility needs of the different zones.

After we finish this application we will need to adjust our nitrogen rates in subsequent applications to match the variation in what we applied with our manure. We are surface applying this manure, so we will only take credit for the four pounds of first year available organic nitrogen and not the ammonium that we expect will volatize off. In zone 3 with a 10-ton application we are getting 40 lbs of N, in Zone 2 with 15 tons we are applying 60 lbs of N and in Zone 1 with 25 tons we are applying 100 lbs of N.  We will take credit for those N applications and adjust our follow up applications accordingly.  

Variable rate application is not just for commercial fertilizer. We can build variable rate prescriptions for manure application based on your soil’s fertility needs. These VRT prescriptions allow CVA to apply the right rate in the right place for your needs.   
Tim Mundorf
Nutrient Management Lead
Central Valley Ag

Posted: 1/4/2019 11:39:25 AM by Kelli Reznicek | with 0 comments

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