Should I have used more N this year?

Monday, November 1, 2010

Many fields that showed N deficiency suffered little, if any, yield loss. Those that did – and had yellow, stunted leaves – were few in number

by KEITH REID

A significant number of corn fields this year showed typical nitrogen deficiency symptoms on the lower leaves. In some cases, the entire plant was pale instead of the dark green colour we associate with healthy corn plants, but more often it was simply the yellowing starting at the tip of the leaf and extending down along the mid-rib, followed by browning of the affected tissue. (See photograph, above)

This leads to three questions. Why did it happen? How much did it hurt my yield? And should I change my fertilizer program for 2011?

The corn plant can run short of N either because the demand from the plants increased, or the supply from the soil was inadequate. Both factors came into play this year. Good moisture, lots of heat and sunshine resulted in vigorous corn growth and high yield potential in many fields. This meant that the plants were pulling more N out of the soil than normal to support their growth, so fields where there was lots of N for normal growth began to run short.

The same weather that created conditions for excellent growth in some cases also created conditions for N loss. April was warmer than normal, so planting and fertilizer application were up to three weeks ahead of normal. The soil conditions also favoured the conversion of organic and ammonium N to nitrate.

When wet weather moved in during May, this nitrate was lost, either by denitrification on the fine textured soils or though leaching on the sandy soils. Losses of N were greater than normal because less of the N remained in the ammonium form, which would not have been subject to loss.

Many fields that showed N deficiency suffered little, if any, yield loss. Nitrogen is mobile in the plant, so it is normal for the plant to pull N out of the lower leaves and move it to the upper leaves and the cob during maturity. A plant that doesn't show any N-deficient leaves by maturity suggests that the fertilizer program was much too generous.

Deficiency symptoms that get up to the level of the ear leaf, however, indicate that the supply to the cob ran short and so the plant did not yield the maximum it could have, even though the overall yields for the field could have been much above average.

Plants that were yellow and stunted, or where the firing on the leaves extended up above the ear leaf, have probably suffered significant yield losses. The few fields I have seen with these severe symptoms also had very poor cob development and low yield potential.

So should you make changes for next year? Fertilizing to make sure you never run short of nutrients is a guarantee that you will spend far more on fertilizer most years than you need to, as well as increasing the risk of losing unused fertilizer to the environment. Most of the fields that showed N deficiency because of the high demand for N are close to their long-term optimum N rates, so the only justification for changing fertilizer rates would be for bragging rights at the coffee shop.

On the fields that lost N, changing the timing of N application from pre-plant to side-dress (particularly in years when planting is early) can help to ensure that more of the fertilizer you apply actually gets to the crop. BF

Keith Reid is soil fertility specialist with the Ontario Ministry of Agriculture Food and Rural Affairs, based in Stratford. Email: keith.reid@ontario.ca