AgriGold

The Nitrogen Trainwreck of 2010

by Jack Hardwick, CCA, March 4, 2010

Most growers are going to try to forget the harvest of 2009, but in reality it is going to keep showing up for several years to follow.  The growing season of 2009 was one of the coolest, wettest on record, and that trend carried on through harvest.  The corn crop was planted almost a month behind normal and most locations were around 200-300 growing degree units (GDUs) behind normal.  To make matters worse, Illinois growers experienced rainfall totals from August 1st to December 1st that were 150% to 250% higher than normal precipitation.  All of these factors equated to wet grain moisture, wet field conditions, late harvest, and limited nitrogen and tillage applications.  With winter hopefully drawing to a close, farmers must now make a plan of attack on how they are going to deal with some of the harvest issues that continue to loom into spring. 

One of the many issues that is going to influence corn on corn in 2010 is how large amounts of residue are going to affect nitrogen availability.  Soil microbes are going to have to decompose tremendous amounts of residue and they are going to require some help to get this accomplished. The two main sources of fuel behind the microbial engine are heat and nitrogen.  Microbes start breaking down residue when soil temperatures near 50°F and up, with activity increasing as soil temperatures rise.  When temperatures drop below 50°F very little microbial decomposition occurs, if any.  Microbes also require a nitrogen source to complete this process, survive and multiply.  They will either use nitrogen from soil organic matter or, if available, applied forms of N such as most commercial fertilizers including urea, anhydrous ammonia, and others.  The main goal of these microbes is to break down residue and as a result of this, they manage the soil's carbon to nitrogen ratio.  The carbon to nitrogen ratio (C/N) is a relationship between how much carbon and nitrogen a given piece of material contains.  Listed in this table are several common forms of residue and their corresponding C/N ratios.   As far as crop production is concerned, the magic number in dealing with C/N ratios is 20:1.  When C/N ratios fall below 20:1 there is a release of plant available nitrogen to the soil.  This process is called mineralization.  When C/N ratios rise above 20:1 there is a tie-up of plant available nitrogen due to microbes "robbing"  the nitrogen supply to break the residue down.  This process is called immobilization.  The higher the C/N ratio, the longer amount of time net immobilization occurs.

Understanding C/N ratios helps predict nitrogen problems that can occur.  There is a huge cause for concern when looking at nitrogen for the 2010 corn crop, especially corn on corn due to three main factors.  The first strike is a lack of fall heat.  The cool growing conditions and abundance of field moisture resulted in a later than normal harvest with only 19% of the Illinois corn crop taken off before November 1st.  According to the Illinois Climate Network Data, Northern and Central Illinois locations recorded only 4-5 days of soil temperatures above 50°F (average at a 4 inch depth) with the highest only around 52-54°F for the entire month of November.  Southern Illinois locations recorded 10-20 days of 50°F (average at a 4 inch depth) with the highest around 54-58°F for November.  There were not any days in December where temperatures reached these levels at any locations.  The 2004-2008, 4 year corn harvest average for Illinois is 86% by November 1st.  A more normal harvest would have permitted fields to be harvested and tilled with warmer soil temperatures allowing for more decomposition. 

Strike number two is a lack of fall nitrogen (N).  With a late, wet harvest many growers did not get a chance to apply fall fertilizer.  In previous years large numbers of farmers applied dry fertilizer to corn stalks directly before tillage.  A portion of that fall fertilizer more than likely included a phosphate compound.  The most common form used is diammonium phosphate (DAP).  The key part of DAP, for this discussion, is that it contains 18% N per 100 pounds of product.  In a 200 pound DAP application there are 36 pounds of actual N that are included.  This nitrogen serves a great fuel source for decomposition.

The third strike is a lack of tillage.  Tillage serves two separate purposes in decomposition.  First it sizes the residue into smaller pieces which allow the microbes to work faster and easier.  It also gets the residue mixed into the soil which exposes it to the full microbial population.  It is safe to say that there was very little decomposition that occurred in the fall of 2009.

Field conditions will someday become favorable for tillage applications, and farmers doing seedbed preparation will incorporate vast amounts of residue into the top 5-6 inches of the soil profile.  Soil temperatures will soon rise much above   50°F and the soil microbes will have a spring feast like they haven't seen in years.  Knowing that most all decompostion will occur this spring, growers should expect a longer, more aggressive period of nitrogen tie-up (immobilization) by the microbes.  They will rob the upper soil profile of any plant available N and a young corn plant will suffer a nitrogen deficiency unless a countermeasure is applied.

The most efficient countermeasure is surface applied nitrogen.  Placing a N source in the upper 4 inches of the soil profile plays two different roles.  It supplies the microbes with an easily accessible N source so they can effectively decompose residue, and it also feeds a young corn plant that does not have enough root mass early in its life cycle to reach deep placement N, primarily anhydrous ammonia.  When a corn plant first emerges, it is relying heavily upon the seed for energy to grow.  When it reaches the V4 stage (4 visible leaf collars) it transitions from relying on the seed to depending upon it's nodal root system for water and nutrients.  A V4 corn plant with a small root system is limited on what it can access through the soil.  To make matters worse, if there are compaction issues from wet field applications, the root system is smaller yet.  Getting early nitrogen to this plant is crucial for it's early vigor and survival.

How much surface applied N and type of product used play an important role as well.  There are basically two forms of all nitrogen fertilizers: Nitrate (NO₃^-) and Ammonium (NH₄⁺).  The attached table shows several common nitrogen fertilizers, their formula, and the initial form of nitrogen they react to.  When fertilizers are applied they have an initial reaction to the environment and will break apart into their specific components.  In the case of anhydrous ammonia, it reacts with water to form NH₄⁺.  NH₄⁺ is a more stable form of nitrogen because of it's charge.  It is a positively charged particle which is important because soil has an overall negative charge, and positive particles can attach to them the same way pieces of metal attach to magnets.  This negativity is expressed as Cation Exchange Capacity (CEC).  In general, the higher the CEC, the more negative sites to attach positive particles to.  Over time, depending on environmental conditions, NH₄⁺ is converted to NO₃^- through a process called nitrification. This process is carried out by soil microbes and is very heat dependent.  The warmer the soil temperature, the quicker the conversion.  NO₃^- is a more unstable form because it has an overall negative charge, which means it is repelled by the soil.  Soils and  NO₃^- are both negative, forcing them to push off of each other the same way trying to put two magnets together does. 

Soil water plays a very important role in dealing with the Nitrate (NO₃^- ) portion of nitrogen.  Since NO₃^- cannot be attached to soil particles it moves freely within the water contained in the soil profile.  Heavy amounts of rain can "flush" the NO₃^- down through the soil where it is too deep for the plant to access.  This is called leaching.  Leaching is especially important when dealing with a coarse soil type such as sand where water will infiltrate deeper and faster.  Another process  by which nitrogen is lost is through denitrification.  Denitrification occurs when all of the soil pores are filled with water and the ground is completely saturated.  This leaves no room for oxygen in the soil pores.  Microbes need oxygen to survive, and when they can't find any in the soil pores, they look for other sources.  NO₃^- just happens to be one of those other sources.  They attack the oxygen (O) portion of the NO₃^- molecule and take it so they can breathe.  This causes the NO₃^- to go through a conversion process,  eventually resulting in N₂ gas which is volatized into the air and is lost.

Getting back to applying a countermeasure, the preferred choice of nitrogen for most microbes to decompose residue is the ammonium form (NH₄⁺) and a young corn plant prefers the nitrate form (NO₃^- ).  When thinking about fall residue decomposition, it only makes since to apply an NH₄⁺ form of nitrogen since there is no living crop to feed.  The microbes will use this form to break down residue, heat permitting.  Thirty to fifty units of N is a good range to use for fall residue management using products like DAP and AMS.  Spring residue decomposition is a little different.  There will soon going to be a crop to feed along with the microbes so a product that has a combination of NH₄⁺ and NO₃^- is recommended to feed both parties.  Good products to use for this application are UAN and Ammonia Nitrate.  AgriGold has been recommending 10 gallons of UAN as a carrier for preseason chemical applications in corn on corn, or corn on wheat rotations.  This nets around 28-30 units of N.  With all of the points already discussed about decomposition problems, the 2010 corn on corn crop is going to need more help than 28-30 units of N.  With all of the points already discussed about decomposition problems, the 2010 corn on corn crop is going to need more help than 28-30 units of N.  This is the year where double the "normal" surface applied N recommendation should be used.  Preseason nitrogen needs to net around 60 to 90 units of N to account for the decomposition drawdown of N that is going to happen in corn on corn rotations.  Corn on soybean rotations are more forgiving because the C/N ratio is quite lower than corn on corn.  AgriGold has been recommending 5 gallons of UAN or a total of 14-15 units of N for these situations.  Again, with the troubles expected in decomposition, this recommendation should be doubled as well for 2010.

Supply of nitrogen in 2010 might be an issue to contend with as well.  There was very little fall anhydrous applied in northern areas that typical put on the majority of their N this way.  Weather patterns continue to be wet, and if all areas apply N at once, the pipeline may not be big enough to supply everyone.  Using surface applied N has a big advantage here.  For example if a grower is in a 200 N unit program, has 36 units of N from DAP on, and applies 70 units of N with UAN, he only has to make up 100 units with preplant or sidedress applications.  This greatly reduces the spring workload, leaving more time for tillage and planting.  Hopefully there are no issues with supply and weather, but having several efficient plans of attack will help manage problems if they occur. 

Pay close attention when driving by corn on corn fields early this spring.  If no surface N is applied, there will be some awfully yellow, ugly, uneven corn fields out there suffering N deficits even if there are 200 units 8 inches down in the soil.  The plant just can't reach it yet.

Categories: Nitrogen Application

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