Features Agronomy Pulses
Soil moisture, nitrogen fertility after pulses

Field pea is a viable cropping alternative to chemfallow. Photo by Bruce Barker.

On the semiarid Prairies of southern Saskatchewan, where wheat/fallow rotations were common, chemfallow was the first step to improving sustainability. Now, producers are growing pulse crops instead of chemfallowing in a further effort to enhance soil productivity.

“One of the features with pulses is to replace conventional fallow, providing a ‘greening’ opportunity with more positive environmental impacts. However, little is known about how much soil water and nitrogen are left post-harvest in pulse crops in comparison with chemfallow,” says Yantai Gan, research scientist with Agriculture and Agri-Food Canada (AAFC) at Swift Current, Sask. “Also, a quantitative assessment of soil water and nitrogen recharged from post-harvest to the next spring would help land managers to make sound decisions on crop choice and fertilization.”

Gan, along with colleagues Chantal Hamel, John O’Donovan, Con Campbell and Lee Poppy, conducted a three-year crop sequence study to quantify the amounts of soil water and nitrogen (N) available in the preceding pulse crops in comparison with preceding barley and chemfallow, in southwestern Saskatchewan.

The study was conducted from 2007 to 2011 at AAFC’s Semiarid Prairie Agricultural Research Centre (SPARC) in Swift Current. Gan summarized and presented his findings in a poster presentation at the University of Saskatchewan’s Soils and Crops Workshop in 2015.

In the first year, spring wheat was grown under no-till management. At harvest, wheat stubble was cut to 15 centimetres and left standing in the field. In the second year, 10 types of pulse crops were grown between the rows of standing wheat stubble. The pulses, along with barley and the chemfallow check, were arranged in a randomized, complete block with four replicates. In the third year, durum wheat was uniformly grown on the different residue types.

The three-year cropping sequences were repeated for three cycles between 2007 and 2011. In each year, soil samples at five different depths (0-120 cm) were taken post-harvest and again at planting the following spring. Detailed measurements were taken to quantify soil water and available N (NO₃-N and NH₄-N).

Rainfall storage inefficiencies
The precipitation during the growing season totalled 176 millimetres in 2009, 354 mm in 2010, and 287 mm in 2011. On average for all crops, water left in the 0-120 cm soil profile at harvest was 180 mm in 2009, 191 mm in 2010, and 240 mm in 2011.

During that period, chemfallow fields had more stored water at harvest than the cropped land: 52.3 mm more water in the 0-120 cm soil profile in 2009, 52.2 mm in 2010, and 29.7 mm in 2011. However, chemfallow did not efficiently store in-season rainfall.

“About 70 per cent of the total precipitation in 2009 was lost through evaporation from chemfallow fields during the period from crop seeding to harvest,” Gan says. “In 2010 it was about 85 per cent and almost 90 per cent of precipitation was lost in 2011.”

In 2009, the fields with dry pea and lentil had 17.6 mm (9.7 per cent) more water left unused in the 0-120 cm soil profile than the fields with barley crops. In 2010, no differences were found between crop types post-harvest. In 2011, however, dry pea had 42.3 mm (19 per cent) and 30.2 mm (12.9 per cent) more water left unused in the 0-120 cm soil profile than chickpea and lentil, respectively.

Soil water was also recharged from post-harvest to the next spring. In the 2008-2009 fall and winter, the soil was recharged with 25.8 mm of water in the cropped field and 22.3 mm in chemfallow. In the 2009-2010 and the 2010-2011 fall and winters, soil water in the cropped fields was increased by 24.2 mm and 27.2 mm, respectively, whereas soil water content did not change in the chemfallow field. Over-winter recharge helped to narrow the gap in the 2010 and 2011 years, resulting in a deficit of about 28 mm in 2010 and 2.5 mm in 2011 (see Fig 1).  

 Pulse crops improve soil N
Gan says the quantity of soil N (NO₃-N, NH₄-N) in the 0-120 cm depth post-harvest varied between years and among crops. In 2009, post-harvest soil N averaged 51.2 kg N/ha in pulse fields, similar to that in chemfallow fields (46.3 kg N ha^-1); they were significantly greater (by 78 per cent) than that in barley fields. In 2010 and 2011, pulse fields had the same amounts of N as chemfallow; they were 30 per cent and 88 per cent greater compared to barley, in the two years.

“Among the 10 different pulse crops evaluated, yellow pea was outstanding. The total N in the fields with yellow pea was 74.4 kg N/ha, 65.9 kg N/ha and 52.0 kg N/ha at crop harvest in 2009, 2010 and 2011,” Gan says.

These values were 61 per cent, 124 per cent and 33 per cent greater than that in the chemfallow, and 160 per cent, 181 per cent and 194 per cent greater than that in the barley-grown fields, in the three respective years.

On average, pulse fields increased soil N in the 0-60 cm depth by 15.3 kg N/ha, 15.4 kg N/ha and 8.4 kg N/ha from post-harvest to the following spring in 2009, 2010 and 2011, respectively. These values were 39, 67 and 149 per cent greater compared to barley fields; and 57, 11 and 69 per cent greater compared to chemfallow, in the three respective years (See Fig 2).

The results of the trial show that pulse may be a viable alternative to chemfallow. Greening of chemfallow with pulses may provide environmental benefits, but more research is required to quantify these effects, Gan says.

“The use of annual pulses to replace chemfallow can utilize the precipitation effectively and increase total grain production over a number of years,” he notes. “Growing pulses significantly increased soil N through symbiotic N-fixation from the atmosphere, as well as through the decomposition of N-rich root and straw of pulses from post-harvest to the next spring as well as during the next year’s growing season, providing a nutrient-richening effect. By contrast, chemfallowing can also enrich soil N but it does so through the mineralization of soil organic carbon, which is a ‘mining’ soil process, depleting soil carbon over time, and is unsustainable.”

February 4, 2016  By Bruce Barker