Features Agronomy Fertility and Nutrients
Manage pulse fertility to gain the most from crop rotations

Think of growing pulses in a rotation like someone returning from a two-week holiday in the sun. They come back healthier, more efficient, and maybe with some increased microbial activity. The same goes for pulses. “Pulses are effective crops to use in a rotation to biologically access nutrients from air and soil and render them available, for both the pulse and those crops that follow,” says Dr. Jeff Schoenau, strategic research chair for the Saskatchewan Ministry of Agriculture and a professor at the University of Sasksatchewan. Schoenau presented a summary of rotational benefits at the Alberta Agronomy Update in 2010.

To get the most out of pulse crops and their rotational benefits, Schoenau says a good understanding of where those benefits come from is necessary to help plan a good fertility program for pulses and subsequent crops. He says the goal is to maintain soil fertility as efficiently and effectively as possible, and that is where pulses come in.

Pulse contribution to soil nitrogen
Pulses can fix large amounts of nitrogen from the air. About 70 to 80 percent of N in pulses can be derived from the atmosphere through biological fixation in the nodules on the pulse crop roots. The actual amount depends on the success of nodulation, soil and environmental conditions, the crop, soil available N and other nutrients.

However, when the pulse crop is harvested, much of the plant N is taken off the field in the pulse seed; about 70 percent of the N in the total pulse crop. Schoenau explains that the amount left behind can be equal to what was taken up by the crop from the soil, leaving a neutral N balance.

For example, a 35 bu/ac pea crop can fix 100 lbs of N per acre. At harvest, 70 lbs of the fixed N is removed, leaving only about 30 lbs of fixed N per acre on the field. “The total amount of N in harvested seed is just about equal to the amount of N fixed from the atmosphere. The N in harvested pulse seed is obtained “free” from the air through fixation,” explains Schoenau.

Still, a small N benefit is left for the subsequent crop due to microbial decomposition of surface residues, roots and old nodules. Research by Stu Brandt at Agriculture and Agri-Food Canada showed greater spring available N following field pea or lentil than after wheat. Spring available N on wheat stubble was 34 kg per hectare (30 lbs of N per acre), lentil was 45 kg per hectare (40 lbs per acre) and field pea was 42 kg per hectare (37.5 lbs per acre). Research has reported the N credit from pea stubble to vary from 0.5 to 1 lb of N per acre for every bushel per acre of pea harvested.

Some soil fertilizer recommendations provide a credit for N based on the previous year’s pulse yield, with higher yields providing higher credits. The Plant Root Simulator probe approach provides a direct assessment of available N release rate that includes a mineralization component. Schoenau cautions that less than 10 percent of the aboveground pulse residue is typically made available to the crop in the following year. He says another factor to consider is that nitrogen uptake by crops grown on pulse stubble is typically enhanced by a greater amount than what can be attributed to coming directly from pulse residue. “Possible explanations include better conditions for root growth, stimulated soil biological activity after pulses, and other factors not understood.

The N benefit to a following crop can be measured as the N fertilizer replacement value, explained as how much extra fertilizer N needs to be added to bring yield of a non-pulse crop on a non-pulse stubble to the same yield as the crop grown on pulse stubble. This can be equated to the fertilizer N savings by growing a pulse the year before but depends on the stubbles compared. Wheat on pea stubble may have a 20 lbs of N per acre fertilizer replacement advantage compared to wheat on canola stubble, while wheat on pea stubble may have an advantage of 100 lbs of N per acre compared to wheat on wheat stubble. “Making generalized statements on the N residual benefit of pulse crops is difficult,” says Schoenau.

Non-N benefits also help explain the rotational benefits of pulse crops while taking a holiday from cereal and oilseed crops. These include breaking pest cycles, improving soil tilth and moisture utilization, promotion of beneficial soil biological activity and uptake of other nutrients. 

Wright et al, measured these non-N benefits in a study in 1990 at Melfort, Saskatchewan. He compared barley on barley stubble to barley on field pea, lentil and faba bean. The difference between barley on barley stubble and barley on a pulse crop stubble (with the same fertility program) was as much as five bushels per acre.
Source: Wright, Melfort, Saskatchewan.

Enhanced phosphorus fertility
Schoenau says pulse crops also have been shown to enhance phosphorus (P) fertility and uptake in subsequent crops. Pulse roots can acidify the root zone, which helps solubilize calcium phosphates common in prairie soils, making P more available for plant uptake. “This may explain why pulses are sometimes not highly responsive to P fertilization,” says Schoenau. 

For crops following pulses in rotation, P uptake by the non-legume crop is generally increased following pulse crops versus non-pulse crops.

Research has ruled out increased P availability from pulse residues. In a greenhouse study at the University of Saskatchewan, while pulse residue resulted in less P and N tie-up than cereal residue, the amounts of P released from aboveground residues were calculated to be only 0.9 kg of P per hectare (0.8 lbs of P per acre) for chickpea, 1.2 kg of P (1.1 lbs of P) for pea and 1.0 kg of P per hectare (0.9 lbs of P per acre) for wheat. “There was less than 1.0 kg of P per hectare contribution from aboveground residues. That’s not very large,” explains Schoenau. “However, there is greater P uptake by wheat on pea stubble than wheat on wheat stubble or fallow. Pulse crops don’t appear to increase the concentration of available P in the soil to a great extent, but they can promote greater plant access to soil P and other nutrients.”

Schoenau says the possible reasons include increased colonization of roots by beneficial arbuscular mycorrhizae (AM) fungi, and improved non-pulse root growth and healthier roots. He says this P benefit is not detectable in soil tests and a P credit is not typically given for fertilizer recommendations. 

Optimize fertility for sustainability
While pulse crops provide both fertility and non-fertility rotational benefits, Schoenau recommends managing the fertilizer program throughout the crop rotation for long-term sustainability.  Recommendations include using the correct species and superior strains of N-fixing rhizobium inoculants, and observing safe seedrow P fertilizer rates to ensure healthy stand establishment and nodulation; more than 15 lbs P2O5 per acre in the seedrow for peas has been observed to reduce germination and emergence in some cases.

Schoenau cautions that pulse crop rotational benefits should not be used to mine the soil of nutrients. Just because pulse crops are efficient at P uptake and cereals also have improved P uptake on pulse stubble does not mean P fertilizer rates should be cut back. Rather, P fertilizer should be applied at a rate to meet immediate crop needs and also improve maintenance of soil fertility during the long term.