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Choosing your field for peas

Field pea growers usually follow a crop rotation of field pea on a cereal stubble. The theory, aside from breaking disease rotations, is that by planting on stubble that is low in residual nitrogen (N), the greatest economic benefit will be derived from the pea crop’s ability to fix nitrogen. But what happens if the field has high levels of residual nitrogen? Should a grower worry about reduced nitrogen fixation and a negative impact on yield?

“Most growers follow a logical sequence to avoid planting peas on soils with high residual nitrogen, but occasionally, you hit a dry year where the field has higher levels of nitrogen and you don’t want to disrupt your rotations,” explains Guy Lafond, a researcher with Agriculture and Agri-Food Canada at Indian Head, Saskatchewan. “The results of our study found that high residual nitrate nitrogen levels of up to 60kg/ha didn’t affect yield,” (Note: 60kg/ha x 0.89 = 53.5lb/ac).

The research was conducted as part of the Precision Farming Project sponsored by the Indian Head Agricultural Research Foundation (IHARF), which investigated how precision farming could improve crop production and economic returns, based on varying fertilizer rates according to pre-determined management zones in a field. As an offshoot of that work, Lafond analyzed data from soil tests taken on a grid pattern and measured the effects on the subsequent crops using grain yields collected from a New Holland combine equipped with a yield monitor and GPS antenna sponsored by New Holland North America and Markusson New Holland in Regina.

A spring wheat-canola-spring wheat-field peas rotation was replicated twice, with each field or plot approximately 40 acres in size. Three management zones were established from estimates of crop biomass using remote sensing and a specialized classification software, called Fuzzy-k-means. Intensive soil sampling at four samples per acre was conducted each year to establish fertility levels for the variable rate nitrogen management component of the project. In those soil tests lies a wealth of information that was not only useful for the precision farming component, but to investigate other crop production problems as well. The project provided a good opportunity to determine what the potential effects of residual nitrate-N might be on field pea production.

In order to determine the effects of residual nitrate-N on field pea production, Lafond analyzed the soil test and yield data for each year. He observed a positive correlation in 2001 and a negative correlation in 2000 and 2004. In 2000, he observed a small linear decrease in grain yield with increasing residual nitrate-N levels in the range of 10kg/ha to 50kg/ha
nitrate-N. In 2001, Lafond observed an increase in pea yield for nitrate-N levels in a range of 15kg/ha to 35kg/ha. In 2004, a decrease in field pea grain yield with increases in residual N levels was also observed, but most of the effects were due to some values of residual-N levels that were very high. But when the high residual N values greater than 70kg/ha NO3-N were removed from the data set, there was no longer a significant relationship.

Lafond concluded that providing residual nitrate-N levels are less than 60kg/ha, growers should not expect a drop in field pea grain yield.

Field selection helps maximize benefits
Mark Olson, provincial pulse industry development specialist with Alberta Agriculture, Food and Rural Development (AAFRD) at Edmonton, says that as a rule of thumb, growers should try to put peas on a field where the previous crop has drawn the nitrogen down as far as possible. Normally, an average to high yielding cereal crop would leave less than 40kg/ha of nitrate-N. Above that, he says growers might want to consider a different crop that does not fix nitrogen. While the yield might not be affected, as shown by Lafond’s work, that nitrogen might be better used by another crop.

“You can go around and around on the magic level of nitrogen for field selection, but you’ll get the greatest benefit from putting peas on a low residual nitrogen field,” says Olson. “Guy’s research findings support observations in the drought years of 2003 and 2004 in many parts of Alberta, as well as what is seen in the Black soil zone in normal years. The Black soils tend to have higher organic matter levels and under good environmental conditions will breakdown this organic matter, resulting in higher levels of background nitrogen.”

Information from the Saskatchewan Pulse Growers’ Pulse Production Manual also cautions against planting peas on inordinately high nitrate-N soils. Seeding on summerfallow can result in excessive vine growth and lodging, due to excessive nitrogen. Heavily manured fields are also fields to avoid.

“I know a lot of hog growers who want to grow field peas as a high protein feed, but if it is going on manured land close to the hog barn, they’re going to see problems, such as increased plant material or volume, lodging and later maturity if the nitrogen levels are high,” says Olson.

On fields with higher levels of nitrate-N in the soil, nodulation and nitrogen fixation may be reduced. However, as Lafond has demonstrated, at soil residual nitrate-N levels below 60kg/ha, yield should not be affected. That is not to say that the pea seed should not be inoculated, though. A 42bu/ac pea crop uses approximately 184kg/ha of nitrate-N, so pea inoculation is a necessity to maximize yield and returns.