Starter Fertilizer with Pulses

F. Walley¹and G. Hnatowich²

¹Saskatchewan Centre for Soil Research, University of Saskatchewan,

Saskatoon, Saskatchewan, S7N 5A,

² Research Development, Saskatchewan Wheat Pool,

201-407 Downey Road, Saskatoon, SK, S7N 4L8

Summary

A three-year field study was initiated in 1996 on soils with relatively low levels of available N and P to investigate the N and P fertility requirements of desi- and kabuli-type chickpea. At the time of preparation of this paper, results from the 1998 field season were unavailable; thus, only results from the 1996 and 1997 field seasons will be discussed. In general, application of starter N resulted in increased vegetative yields at midseason (early pod-fill); however, starter N typically reduced N fixation and did not result in significant seed yield advantages. Application of phosphate (P2O5) did not predictably increase seed yields at all sites; however, it is known that application of P₂O₅ can play an important role in enhancing seed maturity, drought stress and frost tolerance of pulse crops.

Research Question

Pulse crop acreage continues to expand in Saskatchewan. As acreage increases, there is a need to investigate and develop appropriate agronomic practices for pulse crop production. In particular, producers want to answer the questions: "Should I be using starter N?" and "How much phosphate should I be applying and should the phosphate be placed with the seed or away from the seed"? The impact of N fertilizer on pulse crop production is of particular interest because the application of starter N in low N soils is recommended although it is well known that high rates of N can adversely affect inoculant performance (i.e., N fixation). Moreover, many pulses, such as lentil and chickpea, have an indeterminate growth habit and may need to experience an N or moisture stress to induce seed set. Our work has focused on the N and P requirements of chickpea; however, research by others suggests that chickpea responses to N and P are similar to that of other pulses.

Materials and Methods

Experiments were conducted with desi- and kabuli-type chickpea at eight study sites over the past three field seasons on soils low in available N and P. At each location, 12 fertility treatments were used as follows: "starter N" as 46-0-0 at 0, 15, 30 and 45 kg/ha N ha and P applied as 11-54-0 at 0, 20 and 40 kg/ha P₂O₅. The P₂O₅ treatments were applied both in the seed-row and as a side-band application whereas N was sidebanded in all treatments.

Major Findings

Increasing the rate of starter N led to increased vegetation at midseason (early pod-fill stage) (Table 1). In some instances, P₂O₅ application increased midseason vegetative yields, particularly when the phosphate was banded away from the seed; however, the effect of P₂O₅ application was not consistent between sites or chickpea varieties.

Table 1. Vegetative yield of desi- and kabuli-type chickpea determined at midseason (i.e., early pod-fill), in response to starter N and P₂O₅ fertilizer in 1997.

Means within each column followed by the same letter are not significantly different according to the LSD test (P=0.05).

We also were interested in determining the concentration of N and P in the plant tissue at the early pod-fill stage. The concentration of nutrients in the plant tissue at midseason can be an early indication of a fertilizer response. We observed that application of starter N enhanced the concentration of N in the plant tissue (data not shown). Generally, the highest rate of fertilizer N resulted in the highest concentration of N in the plant tissue at early pod-fill.

Although P₂O₅ application influenced the concentration of P in the plant tissue at midseason, the effect of the fertilizer was not consistent between sites or chickpea varieties (data not shown). Our data suggested that the highest concentrations of P in the plant tissue occurred when the P₂O₅ was applied as a sideband treatment, although it is important to note that the effects of fertilizer placement (i.e., seed-placement versus sideband) were not always consistent.

Although starter N application enhanced both midseason vegetative yield and tissue N concentration, final seed yield was unaffected by starter N, irrespective of the rate of N application (Table 2). Moreover, even relatively low levels of fertilizer N significantly reduced N fixation (data not shown). The observation that N fixation was limited by increasing amounts of fertilizer N application is in keeping with the observation that starter N typically did not enhance final seed yields. Thus, there was no net gain in N supply for seed development from the fertilizer N because N derived from N fixation presumably compensated for lower fertilizer N rates.

Although not consistent at all sites, a statistically significant seed yield response to P₂O₅ was detected at one site in 1997 (Kenaston) (Table 2). Highest yields were achieved when P₂O₅ was sidebanded at a rate of 40 kg/ha, although this treatment was not significantly different than either the 20 or 40 kg/ha P₂O₅ rate applied in the seedrow. A similar response was not observed for kabuli-type chickpea.

Table 2. The impact of starter N and P₂O₅ fertilizer on seed yield of desi- and kabuli-type chickpea in 1997.

Means within each column followed by the same letter are not significantly different according to the LSD test (P=0.05).

Applied Questions

1. How do the chickpea results compare with the results from studies with other pulse crops?

Our results are very similar to the results from a number of different trials conducted on the prairies with other pulse crops. For example, Clayton and his colleagues (1997) conducted a study to examine the effectiveness of inoculant formulation and added starter N on nodulation, dry matter production and seed yield of field pea. Their results revealed that starter N enhanced dry matter production of pea at the flatpod stage. However, the increase in dry matter production did not translate directly into equal seed yields. In fact, the seed yield of pea receiving no fertilizer N but inoculated with a granular peat formulation (i.e., soil implant), typically was greater than the seed yield of pea inoculated with peat powder or liquid, at starter N rates ranging from 0, 20, 40 and 80 kg/ha N. They concluded that an effective inoculant could provide all the N that a pea plant requires, particularly when soil N test levels are low.

Clayton and his colleges (1998) more recently suggested that choosing fields with limited soil N levels will enhance field pea yield stability because the N fixation process will be more effective. They reported that in most of their studies, adding starter N did not increase the seed yield of peas and in studies where yield was increased, the potential yield was limited.

Lafond and Johnston (1998) have been investigating the impact of inoculation formulation and N fertilizer on the yield of field peas at Melfort and Indian Head, SK. According to their findings, adding N as 46-0-0 at 6, 20 40 and 80 kg/ha N reduced plant populations and increased grain yield for some inoculant treatments. However, the N rate response was only observed in the uninoculated check and the treatments receiving liquid inoculants. The N rate response was not observed where a peat powder inoculant or a granular peat inoculant had been used.

2. It takes a few weeks before the root nodules on pulse crops are actively fixing N. Won't the crop be N deficient if I don't use some starter N?

It is possible that a pulse crop that has not received an application of starter N may show early signs of N deficiency. However, a successfully inoculated crop should begin actively fixing N within 3-4 weeks of emergence, alleviating the early N deficiency symptoms. More importantly, the research indicates that although starter N can effectively alleviate early N deficiency symptoms, this does not necessarily translate into improved seed yields.

3. How does fertilizer N or high residual soil N affect N fixation?

Plants will preferentially use fertilizer and soil N before N fixation will occur in the root nodules. Many studies have been conducted to determine exactly how soil and fertilizer N inhibit N fixation. Interestingly, the entire process of N fixation can be disrupted at many different points by an external supply of N.

The process of N fixation gets its start when a legume inoculant is applied. The inoculant contains Rhizobium bacteria, and these bacteria cause N fixing nodules to form on the roots of appropriate pulse crops. Whether the Rhizobium bacteria are applied in a liquid or peat-based powder inoculant form, or in a granular inoculant form, the first step in the infection process is the rapid multiplication of the Rhizobium in the seedbed. Unfortunately, however, it has been observed that high levels of fertilizer and soil N can limit the initial multiplication of the N-fixing Rhizobium bacteria in the soil, thereby limiting the number of Rhizobium available for nodule initiation (Wahab et al., 1996).

The Rhizobium enters the plant roots through tiny root hairs causing a characteristic deformation, or curling, of the root hairs. High levels of N can interfere with the process of root hair curling, thereby limiting the successful entry of the Rhizobium into the root and ultimately limiting the number of nodules that successfully form (Streeter, 1988). If the Rhizobium bacteria are successful in entering a root hair and initiating the formation of a nodule, soil and fertilizer N can inhibit further nodule development. In each of these instances, the end result is a reduction in the number of nodules that are formed - but the bad news doesn't end there. Even if the Rhizobium bacteria struggle on and finally are able to cause the formation of a nodule, external sources of N can interfere with the function of a critical enzyme, called nitrogenase, within the nodule (Wahab et al., 1996). The nitrogenase enzyme is produced by the Rhizobium in the root nodules and it is one of the key components of the N fixing system. If the nitrogenase enzyme is compromised, the entire N-fixing system shuts down.

4. How much N is too much N?

Research results are variable, partly because the initiation of nodules depends on many factors and the level of available N is just one of the factors that can influence the success of inoculation. In general, however, if your soil test indicates levels of soil nitrate-N greater than 30 lbs/acre, additional N is likely to cause some inhibition of the N fixing process. Nodulation is likely to occur, but the number of nodules may be reduced or the initiation of nodules may be set back. If the combined levels of soil and fertilizer N are greater than 50 lbs/acre, the N fixing process, including the initiation and functioning of nodules, likely will be strictly limited.

5. I use 12-51-0 for my phosphate source. Should I be concerned about the N that is going down with my phosphate?

No. As long as your soil test results indicate relatively low levels of available soil nitrate-N (i.e., less than 20 lbs/acre in the top 24 inches), it is unlikely that the small amount of N that is being delivered with the phosphate (less than 5 lbs if phosphate is being applied at 20 lbs/acre) will have any significant negative impact on the N fixing process.

6. I've read that most pulse crops obtain approximately 70 to 90 percent of their N from N fixation. Won't I need to add a little N to make up for the rest of the N that is required?

Although a well inoculated and nodulated pulse crop should derive most of its N from N fixation, there are a number of "hidden" sources of N in most production systems. First of all, the seed itself is one of Mother Nature's N reserve sources. For example, peas seeded at 2.5 bu/acre contribute a little less than 5 lbs/acre of N. In addition, there is the 5 or more lbs/acre of N that will be supplied if a phosphate fertilizer, such as 12-51-0, is used. Add to this the N that's in the soil - for argument's sake, let's say another 15 lbs/acre in the top 2 feet - and we've already accounted for approximately 25 lbs of "hidden" starter N! Finally, it is important to remember that the soil continues to supply N through the process of N mineralization throughout the growing season. The amount supplied will vary from soil to soil, but over the growing season, mineralization can be a significant N source. Thus, even if a pulse crop only fixes 70 percent of the total amount of N that is required, there are plenty of "hidden" sources to supply the rest, without having to apply additional N fertilizer.

7. The use of starter N has been recommended in the past. What's changed?

It's hard to say with certainty - so much has changed in terms of pulse crop agronomy over the past several years. Different varieties, changing tillage practices resulting in better soil moisture management, more diverse rotations, and the list goes on. Perhaps of greatest importance, however, has been the development and introduction of some very effective Rhizobium inoculants.

Clayton and his co-workers (1997) reported that in trials with field pea, the seed yield of pea receiving no fertilizer N but inoculated with a granular peat formulation (i.e., soil implant) was greater than the seed yield of pea inoculated with peat powder or liquid, at starter N rates ranging from 0, 20,40 and 80 kg/ha N. They concluded that an effective inoculant could provide all the N that a pea plant requires, particularly when soil N test levels are low.

8. Does the fact that starter N promotes more vegetative growth early in the growing season mean that the crop will be more competitive against weeds?

This is an interesting question. Certainly the chickpea research and studies with field pea (Clayton et al., 1997) indicate that increasing rates of starter N fertilizer result in increased dry matter production at the early pod-fill and flatpod stage, respectively. However, in both these studies, the increase in dry matter production did not translate to increased seed yields. Recent research by Harker and his colleagues (1998) has indicated that early weed removal helps pea reach their full yield potential. These researchers reported that when weeds were removed later than 2 weeks after crop emergence in weedy plots, seed yields were reduced by as much as 50% (Harker et al., 1998; Clayton et al., 1998). Generally speaking, pulse crops are not very competitive with weeds, and these recent studies indicate that early weed removal, long before the crop canopy is fully developed, is perhaps the best weed management strategy for improving yield stability.

9. I'm considering growing dry beans. Should I be using starter N?

Dry beans have a reputation for being poor N fixers and until we can do something to improve their N fixing status, application of fertilizer N may be advisable. Recently, McAndrew (1998) reported that in twelve out of fifteen site years of trials conducted in Manitoba, there was a positive response to N fertilization. He concluded that N fertilization is a more reliable method of improving seed yield of dry bean than relying strictly on Rhizobium inoculation. If you are considering growing beans, a soil test is well advised.

10. According to the research, application of phosphate fertilizer does not necessarily improve seed yields, even on P deficient soils. Do I really need to be applying it?

The effects of phosphate fertilizer on pulse crop production and seed yields are not always as obvious or as consistent as the effects of other nutrients. One frustrating fact remains - application of phosphate fertilizer does not always result in seed yield increases, even on soils relatively low in available P. To add to the frustration, when a yield response does occur, the increase can be very dramatic and economically rewarding but we can't always predict when or where a P fertilizer response will occur!

We do know that phosphorus is an important nutrient in pulse crop production. It is known to enhance root development, which can result in improved drought tolerance. It also can improve the ability of a crop to tolerate stresses, including early frost damage. Finally, if not supplied in sufficient quantities, a P deficiency can have a negative impact on the N fixing process. In the final analysis, using a phosphate fertilizer is an important component of pulse crop production and should not be ignored.

11. Should phosphate fertilizers be seed-placed or separated from the seed?

Pulse crops generally are sensitive to fertilizer placement. Peas, in particular, are sensitive to seed-placed phosphate. Recently Johnston and his colleagues (1998) presented findings from their research (AFIF Project - SCP #14) regarding the response of field pea to fertilizer placement. Their study, conducted at Indian Head, Melfort, Scott and Outlook, involved the application of a fertilizer blend (13N-20P-10K-10S) at 0, 50, 100 and 150 kg/ha either in the seed row or sidebanded at seeding. Results from this study indicate that field pea is very sensitive to fertilizer applied in the seed row, reducing both seedling establishment and grain yield relative to side band applications.

References Cited

Abdel Wahab, A.M., Zahran, H.H., and Abd-Alla, M.H. 1996. Root-hair infection and nodulation of four grain legumes as affected by the form and application time of N fertilizer. Folia Microbiol 41: 303-308.

Clayton, G., Harker, N., Johnston, A., Rice, W., and Lupwayi, N. 1998. Integrated agronomy research - Does it work for field pea? pp. 43-44, In Proceedings of the Pulse Crops Research Workshop, Progress Reports on Pulse Crops Research in Western Canada, Volume 3 (1998), Saskatoon, SK.

Clayton, G.W., Rice, W.A., Johnston, A.M., Lafond, G.P., Blade, S., Grant, C.A., Harker, N., and Blackshaw, B. 1997. How do I minimize risk and increase yield stability in field pea production? pp. 17-20, In Proceedings of the Western Canada Agronomy Workshop, July 9-11, Saskatoon, SK.

Harker, K.N., Clayton, G.W., and Blackshaw, B. 1998. Early and late removal of weeds in peas. pp. 17-18, In Proceedings of the Pulse Crops Research Workshop, Progress Reports on Pulse Crops Research in Western Canada, Volume 3 (1998), Saskatoon, SK.

Lafond, G., and Johnston, A. 1998. Effect of granular inoculant rate and placement on the yield of field pea. pp. 53-55, In Proceedings of the Pulse Crops Research Workshop, Progress Reports on Pulse Crops Research in Western Canada, Volume 3 (1998), Saskatoon, SK.

McAndrew, D.W. 1998. Nutrient management for dry bean production in Southern Manitoba. pp. 13-14, In Proceedings of the Pulse Crops Research Workshop, Progress Reports on Pulse Crops Research in Western Canada, Volume 3 (1998), Saskatoon, SK.

Streeter, J. 1988. Inhibition of legume nodules formation and N2 fixation by nitrate. CRC Critical Reviews in Plant Sciences 7: 1-23.

Acknowledgements

The financial assistance of the Saskatchewan Department of Agriculture and Food (ADF) is gratefully acknowledged.