The point of this post is not to get into all of the various reasons why water drains quickly from some areas and not so quickly from other areas. That diagnosis is best undertaken by the farmer who has the most intimate knowledge of soil types found on the farm and past history, in terms of crop rotation, tile drainage, traffic patterns etc. If you are still confused, then consultations with drainage contractors and agronomists may shed some additional insight. Many times it is still difficult to pinpoint the exact reason why water accumulates here but not over there. My observation over the years are that farmers tend to be experts about why their neighbours crops suffer from too much rain, but seem to be less insightful when their own field is poor. That is another topic in itself.
To help shed light on what causes corn to turn yellow and more importantly is there anything you can do to immediately fix the problem, it helps to understand how soil biology and crop physiology is affected by excess water.
Healthy productive soil for corn production is composed of 50% solids, 25% air and 25% water. The 50% solids made up of the sand silt and clay particles plus organic matter stay pretty much constant. The focus will be on the 50 % that is split between water and air.
When there is too much air and not enough water, soil biology slows to a crawl and the plant withers due to drought stress. The cure is to add water, by irrigation or rainfall. Soil life will bounce back fast because water will infiltrate dry soil very easily, reinvigorating the bacterial and fungal life that drives the soil processes. The drought relief though may not occur in time to repair the drought damage done to the corn plant. The loss in yield due to drought has more to do with damage to the plant itself and less to do with any sustained damage to soil life.
It gets more complicated when the reverse occurs, too much water and not enough air.
Inter row cultivation will help to break the surface crust which allows some air back into the soil, but cultivation is neither a quick cure nor a practical solution.
Two main reasons. As already mentioned corn roots do not function well in water logged soils. With no root growth, leaf growth slows to a crawl. The second reason is nitrogen becomes unavailable due to denitrification. Denitrification occurs in our soils because soil microbes that need oxygen to breathe will attack the nitrate form of nitrogen. Nitrate nitrogen is a nitrogen molecule attached to three oxygen molecules.
The microbes strip the oxygen molecules away. Without the oxygen molecules the nitrogen turns into a gas which escapes into the atmosphere. The amount of nitrogen loss is dependent on the duration of the water saturation in soils.
If you listen to the debate raging on Twitter, you would think flexible drop tubes and dry fertilizer spreaders were a brand new invention. These ideas have been around for years. Recently there is a renewed drive to apply nitrogen later in the growing season to boost yields adding fuel to the debate. Simple drop vs Y-drop, skip row vs every row, 28% vs urea, agrotain vs no agrotain and on and on. I am here to remind everyone that the efficiency of surface application of nitrogen is lousy and there is the constant risk of crop injury. We have known this for a very long time and no amount of agronomic marketing glitz and glam will change it. If July turns as dry as June has been wet, surface application of nitrogen will remain exactly where it has been put, which is not in the right place I might add. This nitrogen is also subject to volatilization losses as well. Research has proven this. There is no guarantee this rescue mission will be successful. But we have to try,
I keep thinking to myself, but with no proof to base it on, that the greatest return may be in the best parts of the field. Applying a small amount of additional N may pump enough air into the tires to carry the corn further through the grain fill process that is just around the corner. Pioneer keeps telling us about the amount of nitrogen that the plant takes up post tassel. Can this late season application of nitrogen make up for hidden losses and give a return on the dollars spent? We intend to find out. But don't count on us broadcasting the results on Twitter.