Nutrient management following the 2009 Corn Crop

A large portion of the 2009 Manitoba corn crop was not harvested for a number of reasons linked to the cool growing season.  Such factors causing growers to till down corn included immaturity, excessive kernel moisture and excessive mould growth on the cob.  Questions now arise whether this mass of material should cause changes in fertility management for 2010 crops – particularly nitrogen (N) needs. The following article relies upon a number of information sources to offer suggestions to growers and agronomists.

First of all – the yield potential of the 2009 Manitoba corn crop was high.  Demonstration plots I maintained at the UM farm in Carman peaked at 200 bu/ac and growers were routinely harvesting 120+ bu/ac crops, albeit at high moisture.  High yields of kernel corn mean a corresponding high yield of stalk, leaves and cobs.  The breakdown of these crop components depends upon their volume and “decomposition rate” which is dictated both by physical structure and chemical make up. In 2003 the MB corn growers sponsored a study where I measured nutrient uptake by corn throughout the season.  The grain yield was high and individual corn components were weighed and analysed chemically (Table 1).

Table 1.  Crop biomass and composition at harvest (2003)

(Heard and Hay, 2006)

Typically if the C:N ratio exceeds 20-25:1, then nitrogen is consumed by soil microbes to decompose the residue.  This process is called immobilization.  This nitrogen may come from the soil or from applied fertilizer if it is not managed optimally.  One might then suspect from the above numbers that the stalk and cob will therefore cause the most immobilization – due to their high C:N ratio and high mass volumes.  But immobilization is not simultaneous, but sequential in the crop.  Actually the initial immobilization will be from the kernels that have been returned, since they are in a physical form and sugar content that most vulnerable to microbial breakdown.   Because the C:N ratio is rather low for the kernels, this decomposition will be rapid, and that N liberated back to the soil for use by the crop or further immobilization to breakdown remaining plant components.

This sequential breakdown of corn components was wonderfully described by Quebec researchers (Burgess et al. 2002. SSAJ 66:1350-1358).  Previous researchers often ground plant material to dust and then showed large and almost immediate immobilization of high levels of nitrogen.  But these researchers left plant parts largely intact (4-6” long pieces), placed them on the surface, at 2 inches or 6 inch depths and measured decomposition for 2 years.  Indeed cobs, husks and stalks all immobilized N at some point.  However, N immobilization was counterbalanced by simultaneous N release from other residues.  No net immobilization was observed for the residues overall.  In fact the cobs immobilized little N at all due to their very slow decomposition.  Likewise Manitoba growers often observe old cobs for 2-3 years following a corn harvest.  So would expect our 2009 corn crop to decompose in the order of kernels followed by leaves and lastly by stalk and cobs.

So although our corn crop does have potential to immobilize N, but this is spread over a rather large timeframe and growers may manage to minimize any impact.

Straw and fertilizer management influences the amount of immobilization.  In our 2-day MB Soil Fertility Workshops we always refer to the classic Manitoba crop residue and N placement study conducted in1981 (Table 2).

Table 2.  The fate of applied nitrogen based on straw and fertilizer placement.

* Incorp = incorporated into the soil.

(Tomar and Soper, 1981)

From this table it is clear that the greatest immobilization is when both residue and fertilizer is incorporated together (3 times as much N ended up in the residue than in the crop).  In such a situation one may certainly need to increase N rates since the crop will be standing in line behind the microbes at the dinner (N) table.  However – if the grower were to separate the straw from the nitrogen, either by leaving the crop residue on the surface or sub-surface banding the N (or both), the nitrogen efficiency is increased dramatically.  Banding N efficiently separates the nitrogen from the crop residue.

As shown in Table 2, one might also consider the removal of the residue however this also has fertility implications.  Stover from a 100 bu/ac corn crop contains about 50 lb N, 20 lb P₂O₅, and 100 lb K₂O/acre (from Manitoba’s Soil Fertility Guide).  When stubble is left in the field after maturity a large portion of this K may leach out into the soil and be left for following crops.  However removal of the reminder will remove much of the nitrogen, phosphorus and other nutrients.  Now some growers may feel they need to burn the residue in order to create a suitable seedbed for 2010 crops.  Our burning studies of cereal straw show that all the N and ¼ -1/3 of the P and K is lost through burning (Heard et al, 2001).  So considerable amounts of N and other nutrients can be permanently lost, however, burning is seldom complete in cornfields.

In some traditional corn growing areas, nitrogen was routinely spread to corn in the fall to promote microbial breakdown of corn stalks.  I remember an elderly Ontario  fertilizer dealer fondly reminiscing about these good-old days, which came to an abrupt end in Ontario with the release of research findings that if corn was adequately fertilized to optimize yield potential, then sufficient N remains in the plant for orderly decomposition.  So the message is that one should not short the nitrogen to the growing crop.  That nitrogen applied to the growing crop is far more profitable to the grower than making the application to its straw.

If growers are unable to manage straw and N optimally, they may experience temporary immobilization this spring and a shortage to their crop.  New NDSU guidelines for fertilizing spring wheat consider supplementing more N to the crop if the previous straw return has been excessive.  For each 1 ton/ac straw returned above normal amounts, 30 lb N/ac is suggested.  Again, this N is not applied to the previous straw to promote breakdown but as a supplement for the current crop.

In summary, growers will want to manage their “crops after 2009 corn”.  Growers should consider:

• Planting N fixing soybeans on those fields where immobilization may be expected.
• Banding nitrogen fertilizer away from crop residue to minimize immobilization
• Use the new MB Nitrogen Rate Calculator.  This bases N rates according to crop and fertilizer prices and with current prices these application rates are generally high for wheat, barley and canola. 
• Monitor crop for any N shortages and be prepared for a supplemental application.  A “planned overlap” or an N-Rich Strip will show if N is limiting growth.
• Fully fertilize your 2010 corn crop, knowing this ultimately aids in stover decomposition
• Plan to be pleasantly surprised in the future.  Reluctantly returning all this N in an unharvested corn crop usually pays off as soil N release in years when yield potential is high.  An example is the yields of cereals and canola in the years since working down much of the 2004 crop.