Reduced Stands in Sweet Corn Can Be Caused by Fertilizer Injury

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

Soil insects, birds, seeding diseases, and poor quality seed can cause reductions in sweet corn stands. Another possible cause is fertilizer injury.

We see fertilizer injury in sweet corn each year. Injury is often more pronounced in certain rows and is most prevalent in areas of fields with light textured soils (sandy loams, loamy sands). Commonly, fertilizer injury will be seen as gaps in rows with no seedling emergence. Where plants have emerged they will be stunted. Fertilizer injured seeds will often be intact and with no outward signs of damage. The most common symptoms of fertilizer injury on seedlings will be browning (death) of seminal root tips, entire shoots or shoot tips (coleoptile/mesocotyl), or a section of the shoot (mesocotyl). Affected plants that emerge will be lighter in color than healthy plants with necrotic edges on the leaves (the leaf margins will be brown and dried).

Fertilizers can injure germinating seedlings by salt effects. Inorganic fertilizers are manufactured in the form of salts that disassociate into ionic forms in water. As salts, fertilizers have high osmotic potential when concentrated in solution. When a high concentration of dissolved salts surrounds a seedling, moisture moves out of the seedling into the dissolved salt solution in the soil through osmosis. Osmosis is the passage of water from a region of high water concentration through a semi-permeable membrane to a region of low water concentration. Plant cell membranes are thin layered semi-permeable membranes and will allow water to pass through. When excess fertilizer is put near the seed, the soil will have a lower concentration of water than the cell (i.e. a very concentrated salt solution in the soil) and the cell will lose water by osmosis. Water crosses the cell membrane in both directions, but more water leaves the cell than enters it. Therefore, cells will shrink and eventually can loose enough water to disrupt normal activities, and, in severe cases, die.

The second way that fertilizers can injury seedlings is by ammonia (NH₃) toxicity. Starter or popup fertilizers that contain urea can release ammonia under certain soil conditions (high pH, dry soils, low organic matter) in the process where urea-N fertilizer is converted in to ammonium (NH₄). Ammonia (NH₃) is a toxic gas (the odor you smell when opening a bottle of household ammonia or that you smell in urine) that can kill plant cells if it is in high concentration. Ammonium (NH₄) is not toxic and is one of the forms that plants take up nitrogen in.

Fertilizers vary in their salt effects with some having greater effects than others. This is given as a salt index. Salt index values for various fertilizer materials is given below:

Table 1. Fertilizer Salt Index of some materials

Starter and popup fertilizers should be made from lower salt index fertilizer forms and avoid urea nitrogen forms to limit potential salt and ammonia injuries.

Fertilizer burn in seedling sweet corn is most common in dry, sandy soils and during periods of drought. In these conditions, salts in the soil solution become more concentrated. Often fertilizer burn arises from problems with application equipment on the planter. The most common problem is getting fertilizer application rates too high. Improper settings, lack of or improper calibration, and poor distribution on liquid systems (some rows receiving more than others) can lead to over application of fertilizer and salt injury. This is extremely critical in popup fertilizer application where fertilizer is being put directly in the seed furrow at low rates. A small increase in rate can lead to fertilizer injury in seedlings. Too high of rates of starter fertilizer, even placed 2 inches to the side and 2 inches deep, can also lead to fertilizer burn. With starter fertilizer, injury can also occur if the fertilizer opener is closer than 2 inches, placing the concentrated fertilizer band too close to the seed.

The Case Against Popup Fertilizer in Sweet Corn
Popup fertilizer is fertilizer at a low rate that is placed in the seed furrow in contrast to starter fertilizer which is commonly placed in a band 2 inches to the side of the seed, two inches in depth. Many studies have been done on popup fertilizer over the years with inconsistent results. Commonly, no yield benefit is found. The risk of salt injury, especially in dry, sandy soils, out weighs any potential benefit from the fertilizer. Starter fertilizers have a much lower risk of injury to seedlings.