How much Nitrogen is too much?

This picture doesn’t have anything to do with farming, but it makes me think of spring. It’s a photo of Frank Sinatra catching a baseball outside a film studio. The other neat thing about this is that Sammy Davis Jr. took the picture. When I’m done blogging today I’m going to head outside to play some ball with my boys. It’s 68 degrees here today!

Year 2 of the Blank Slate field will be corn on corn. Friends tell me I shouldn’t do this on ground with low levels of organic matter and fertility. I think it’s the perfect place for it. One of the initial challenges to deal with this spring will be determining what rate of Nitrogen to use and how to apply it.

The first thing we do on our farm when determining how much Nitrogen to apply is look at the Cation Exchange Capacity of our soil, which is commonly abbreviated CEC on soil tests. On a trip to Minot ND and Sidney MT yesterday, I got the chance to look at a number of soil tests from growers in those areas. NONE of them had a CEC test or base saturation. Frankly, the tests were barely worth the price of the paper they were written on. You see, the CEC test tells you the holding capacity of the soil. It’s a measure of 3 things:

1. The type of clay soil you have
2. The amount of clay your soil contains
3. The amount of organic matter your soil has.

By understanding the holding capacity of my soil at the Blank Slate, I can make an informed decision about how much Nitrogen I can responsibly use at one time. Without that test, I’m shooting in the dark. Soil has a negative electrical charge and Nitrogen, when it converts to the nitrate form, also has a negative charge. If you apply more than your soil can hold the result will be Nitrogen loss that could eventually end up in someone’s water. Here’s an example of how important that test is.

My CEC on the hilltops is as low as 12.2. In the valleys it’s as high as 22.3. We use a rule of thumb for Nitrogen application rates. The soil can hold about 10 times as many pounds of Nitrogen in a single application as the CEC.

12.2 CEC X 10 = approximately 122 pounds of Nitrogen my soil can hold on hilltops.
22.3 CEC X 10 = approximately 223 pounds of Nitrogen my soil can hold in valleys.

If you’re new to my blog, the Blank Slate field is a 60 acre field with 70 feet of drop from its highest point to the lowest. With that kind of slope, there has been a great deal of soil erosion over the years. While the field has terraces, they were installed far too late to do much to help the situation. Years of conventional tillage and the management of cash renters on a 12 month contract has been tough on the soil that I just purchased in the fall of 2009. As a result, much of my topsoil is either in the valleys of the field or washed down the hills into the neighbor’s fields.

So what am I doing with Nitrogen rates and application methods this year? I’ll get there in a minute. I have one more problem in addition to my highly variable CEC levels, I also have to deal with last year’s cornstalks. So far we’ve managed the cornstalk residue by using a chopping head on our Case IH combine in the fall and following up by strip-tilling an 8 inch wide path down the middle of the 30 inch rows. Cornstalks have a Carbon to Nitrogen ratio of 60 to 1. Soil bacteria need that ratio to be around 15 to 1 in order to begin breaking down those stalks. As a result, corn on corn and especially 1st year corn on corn needs a little extra Nitrogen in order to get that Carbon to Nitrogen ratio in line. Most guys in our area that are successful with corn on corn add another 50 pounds of Nitrogen per acre to account for the breakdown of cornstalks. That Nitrogen will eventually free back up for you, but not in time for helping this year’s crop.

With my 150 bushel yield from last fall, I will have at least 3 tons of cornstalks per acre to break down. This should allow me to safely place a little bit extra Nitrogen. I’m planning on putting 50 gallons of liquid 28% on this spring in a broadcast application on the soil surface with my pre-emerge herbicide. Each gallon of 28% contains about 3 pounds of Nitrogen.

3 lbs/gallon X 50 gallons = 150 pounds of applied Nitrogen.

In addition, I’m planning a sidedress application later so I can feed my crop as it begins to use up my pre-plant application of Nitrogen. I have another 25 gallons of 28% purchased, so I can add another 75 pounds of Nitrogen for my crop. I was just reading an article about how in the first 50 days, corn uses about 43% of the Nitrogen it needs for the year. If I wait to apply the rest of the Nitrogen rather than putting it all on at one time, my soil will then be able to hold it. Here’s a ridiculously long link if you’d like to read the article (which also talks about P, K and micros) that I mentioned above.

http://us1.campaign-archive1.com/?u=8bb09d67da2fac3b5bc596e17&id=f3209948ca&e=93f2a71e97

Nitrogen is a key nutrient. I’ll discuss the other nutrients I’m using as we get closer to planting. By the way, if the weather stays good we’ll begin planting on our farm in about 4 weeks.

Darren