A reflection on maximising N provision from Pulse Crops

RB209 recommends reducing N fertilizer of crops following pulses by 30 kg N/ha, similar to crops following OSR, potatoes or fallow. AHDB Report 490 attempted to measure the soil nitrogen supply (SNS) of fields following pulse crops by measuring the “Harvested SNS” of following crops (harvested SNS = the final crop N uptake of the unfertilized crop). Figure 1 shows the RB209 predicted SNS vs harvested SNS for crops following beans. The steep best fit line indicates that RB209 is underestimating the amount of N available for crops following beans. RB209 estimates for peas were more reflective of the final harvested SNS.

Figure 1. Harvested SNS (the final crop N uptake of the unfertilized crop) of crops following bean crops vs the predicted SNS calculated from RB209. From AHDB Report 490

There is likely to be variation of the final amount of N provided to following crops based on genetic, environmental or other factors. Pulse crops may take up some N directly from the soil. The amount of additional N provided by pulses to the following crop is determined by the amount of N fixed by the pulse crop and what remains in the system after; N offtake from the seed at harvest, any removal of N containing residues such as straw offtake, losses to the system during crop growth (e.g turn over of roots and nodules), losses resulting from the turnover of crop residues after harvest e.g from breakdown of residues and turn over through the nitrogen cycle. The remainder of the article explores some of these points.

N fixation by pulses and factors that affect it

The amount of nitrogen that a pulse crop fixes is typically around 150 to 240 kg N/ha, and could be >300 kg N/ha. Factors that affect N fixation are:

• Soil mineral N
  • Typically, 50-60% of the total N uptake for a pulse crop is from biologically fixed N (70-80% in forage legumes)
  • This can be reduced to 35-50% total N uptake from fixed N when SMN is high (200-250 kg N/ha)
• pH
  • N fixation is sensitive to low pH (<5.5). pH 6.5 is optimal
• Temperature
  • N fixation is most effective when soil temps are between 7°C and 20°C
  • N fixing nodules develop 3-5 weeks after plant emergence
• Phosphorus and Molybdenum
  • Ensure adequate supplies
• Sunlight
  • N fixation requires lots of energy and uses about 30% of energy captured by the plant
• Other
  • Dry soil, compaction and salinity can reduce N fixation

Nodulation can be monitored around flowering in pulse crops and nodules with pink interiors can be indicative of healthy nodules that are fixing N. Take care to carefully dig up roots and soak in water to gently remove soil so that nodules don’t break off if assessing.

Amount of N in pulse crop residues

Using standard values for yield, seed protein percentage and N harvest index and assuming straw is incorporated, it is estimated that pea and bean crops could return around 45 kg/ha N to the soil from above ground crop residues after harvest. These standard values can be compared to values measured from the Pulse YENs which provide a similar estimation (Table 1).

Table 1. Estimations of N returned to the soil from above ground pulse crop residues for combining pea and field bean crops. Estimations are based on UK averages and standard values, or values derived from the Pulse YEN database.

aAssumes a N harvest index of 0.83 in beans (White et al 2022) and 0.77 in peas (Weightman et al 2005)

The amount of N provided from below ground residues is harder to measure and less well understood, but will also contribute to the N provided by a pulse crop.

Maximising the return of N to the soil from pulse crop residues

Data analysis of Pulse YEN crops from 2018-2022 highlights a strong association of yield and above ground biomass for both combining peas and field beans (Figure 2; Table 2).

Figure 2. Above ground biomass (t/ha) vs yield (t/ha) of pulse crops analysed via the Pea YEN and Bean YEN from 2018-2022.

Table 2. Yield and total above ground biomass of different pulse crop types entered into the Pea YEN and Bean YEN from 2018-2022. Yield and biomass parameters are partitioned into the values averaged for the bottom and top 25 % of yields within the respective networks.

Assuming that there is no dilution effect of plant residue N concentration with increased yield and biomass in pulse crops, one strategy to increase the amount of N returned to the soil in residues is simply to grow crops with a large biomass. This may also have the added benefit of being associated with high yields. Recent crop reports and stormy weather has highlighted the importance of lodging resistance of pulse crops which should not be overlooked. More information on parameters associated with high yielding pulse crops in the YEN data set can be found on the YEN website  and further information on good practice for growing pulse crops can be found on the PGRO website.

Minimising N losses

Nitrogen from pulse crop residues can be at risk of losses over the winter and this is likely to be greater in high rainfall areas or/and on lighter land. An Anglian Water funded project investigated the impact of catch crops (destroyed 1st October) and companion crops after a vining pea crop on a sandy loam soil site. It was found that a companion crop of buckwheat, linseed, phacelia, sunflower, crimson clover and red clover reduced winter nitrate leaching between October and January, suggesting that companion crop helped minimize N losses due to nitrate leaching.