How does spring weather affect yield response to added nitrogen?

Tuesday, January 6, 2015

The answer depends on the texture of your soil and the amount and distribution of the rainfall. Now a soil scientist has produced a handy index to help you evaluate your soil's nitrogen needs

by KEITH REID

Winter is the time to prepare for spring and part of that is planning a fertilizer program for each field. It would be convenient to know exactly how much fertilizer to buy in advance, but we have learned that, for nitrogen in particular, the type of spring weather influences how much nitrogen is available from the soil. This, in turn, affects how much yield increase will result from added nitrogen and indirectly, the amount of nitrogen needed to optimize that increase.

Dr. Nicolas Tremblay from Agriculture and Agri Food Canada in Saint-Jean-sur-Richelieu, has collaborated with scientists across North America to research the impact of spring weather conditions on corn crops grown in nitrogen-enriched soils. This is what he learned:

– The greatest impact of weather occurred in fine-textured soils, and the amount and distribution of rainfall had more influence on these soils than the accumulation of Crop Heat Units (CHU);  
– In medium-textured soils, variations in the accumulation of CHUs had a greater impact, almost equal to the impact of rainfall.  

What is happening in these two types of soil for these differences to occur? Two factors simultaneously influence the response to nitrogen: one is the growth of the crop – in other words, its capacity to respond to added nitrogen – and the second is the net supply of nitrogen from the soil. Both of these factors are intimately related to the weather and, during the early part of the growing season, the impact weather has on soil conditions.  

Lack of water will reduce both crop growth and nitrogen release from the soil, so it is not surprising that one of the major factors affecting corn's response to added nitrogen is the amount of precipitation. In this case, the old corn trader's adage that "rain makes grain" is true.

Too much rain at one time, however, can be harmful by saturating the soils and leading to nitrogen losses through denitrification. The risk of saturation is much greater in fine-textured soil than in a medium-textured soil because it takes longer for the excess water to drain away.

Dr. Tremblay has accounted for the distribution of rainfall as well as the total quantity of rain with an index of "Adequate and Well-Distributed Rainfall" (AWDR) covering the period from 20 days before side-dress to 30 days after. Rainfall that comes in a few cloudbursts is much less favourable than the same amount of rain that falls in many small showers.

In fine textured soils, this translated into a yield increase of 4.5 times the unfertilized control when high nitrogen rates were applied and the AWDR was high. In contrast, the same treatments in low AWDR sites yielded only about half again as much as the untreated check. It is safe to assume that the low AWDR affected the growth of the crop, either through a deficiency or excess of moisture, as well as the nitrogen supply from the soil.

While too much moisture overwhelmed the fine-textured soils, the picture was different with the medium-textured soils. These soils are much less prone to saturation, since even a heavy rain can drain away relatively quickly. Temperature, however, had a much greater influence on the medium-textured soils. Warmer temperatures not only increased nitrogen mineralization rates, but also the growth of the crop, so therefore, with medium-textured soils, the response to nitrogen was greater under warm conditions.

These findings imply that cutting back on nitrogen rates slightly could be justified under cool spring conditions or low AWDR, since there will be less crop response (yield) to pay for the extra nitrogen. However, this does not mean that large increases in nitrogen rates will result under warm temperatures and high AWDR, since these conditions also favour nitrogen release from the soil.

Taking advantage of this variability in nitrogen response will mean delaying nitrogen application as late as possible. When all of the nitrogen is applied pre-plant, there is not enough time for much nitrogen mineralization to occur or for crop growth, so there is no opportunity to assess the impact of weather on the crop. Delaying the bulk of the nitrogen application until side-dress time will also reduce the risk of nitrogen losses if the weather does turn wet after planting. BF

Keith Reid is Soil Scientist with Agriculture and Agri-Food Canada, Guelph.