The effect of phosphorus fertilisers on rhizobial populations and pea yields – Extension of project FV 428



Phosphorus is important for vining pea root growth as well as an essential nutrient for the formation of nodules and nitrogen fixation. The more rhizobia there are in the soil, the greater the potential for root nodulation and the greater the amount of fixed nitrogen available to the pea plant and subsequent crops. This project investigates the impact of different P indices in soils and of starter fertilisers containing phosphorus on root nodulation, rhizobial populations and yield. Results from the 2014 and 2015 seasons showed higher yields in plots that received starter fertilisers. This is true for both starter fertilisers independent whether they contain nitrogen or not. Nitrogen can impact negatively on nodule formation whereas the presence of phosphorus is proposed to impact positively on rhizobia populations. Assessment of nodulation in the field in 2015 did not show any fertiliser effects. Furthermore, only very limited numbers of plants were investigated and assessing nodulation under field conditions is difficult. It is therefore essential to test the effect of starter fertilisers on rhizobia populations in soils using a method that can reliably determine population sizes. A pot test to quantify rhizobia numbers in soils has successfully been developed at PGRO. Three soil samples were tested (one untreated, two fertilised, one with and one without N) and, for this small sub-set, fertiliser application did not influence rhizobial populations. It is crucial to test the full set of soil samples available to this project including a third field season in 2016 to be able to give advice to growers on management strategies beneficial for rhizobial growth and survival.

Benefits to Industry

Vining peas rely on the symbiotic relationship with rhizobia to provide nitrogen to the plant which is vital for plant growth and yield. A result of this relationship is a crop without the need for nitrogen applications and an increase in soil nitrogen for the subsequent crop. Phosphorus promotes root growth and is essential for root nodulation to occur. Applying starter fertiliser containing phosphorus to peas holds the potential to increase yield and help to maintain healthy populations of rhizobia in soils. However, some starter fertilisers contain nitrogen which can inhibit nodulation thereby reducing the benefit of a pea crop to following crops. Results obtained during the last two growing seasons have shown that yield can be increased by the use of starter fertilisers. However, these effects were season dependent and also varied with drilling time. One shortcoming in both 2014 and 2015 was that yields were measured from the whole trial site instead of replicated plots within the treated field strip. This will be rectified in the 2016 field trials thereby not only reducing seasonal influences by getting results from a third pea crop but also allowing statistical analysis on yield data for the first time. These results will help to decide whether phosphorus fertiliser without nitrogen differs in effect on yield from phosphorus fertiliser with nitrogen. Furthermore, the vitally important data on the effect of these fertilisers on rhizobial populations are still missing. The newly developed pot tests will resolve this issue and statistically sound data from three field seasons will be analysed. By being granted a project extension, PGRO will be able to give comprehensive advice to growers on the benefits of using starter fertilisers in pea crops whilst also assuring that rhizobial populations and thereby nitrogen availability to the subsequent crop are not negatively affected.

Project code:
FV 428A
01 March 2016 - 28 February 2017
AHDB Horticulture
AHDB sector cost:
£19, 496
Project leader:


About this project


  • To identify difference in the relative numbers of rhizobia in vining pea crops grown on soils with varying phosphate levels - This aim relates to soil samples taken during project FV 380.
  • To assess the impact of granular fertiliser application at drilling on rhizobial numbers and nodule counts and the effect on quality and yield of the vining pea - The aim of the extension is to carry out a third field season in 2016 to increase robustness of results.
  • To provide advice to growers on ways to maximise nitrogen fixation in the pea crop - This aim remains unchanged. The extension will increase robustness of conclusions drawn and increase confidence in the advice provided to growers.


 a)   To quantify in relative terms the number of soil dwelling rhizobia in soils with differing levels of soil phosphate.

This objective relies on a method to enumerate rhizobia numbers in soil samples with different P indices. In the original proposal it was suggested to use the Most Probable Number (MPN) method for enumerating viable free living rhizobia in soil. Reliable establishment of the MPN method at PGRO has failed and reasons behind the failure are described in section 10 of this proposal. Therefore, this objective has not been achieved yet and remains unchanged for the proposed extension. A new method to quantify rhizobia populations in soils has been developed by PGRO and tested on a small set of samples. Successful nodulation was achieved and this new pot test will replace the MPN method. Soil samples from project FV 380 from seasons 2013 and 2014 sampled over a range of P indices from 1 to 3 are still available at PGRO and rhizobia will be quantified using the newly developed pot test.

b)   To quantify in relative terms the number of soil dwelling rhizobia before the pea crop is sown and at harvest.

This objective relates to the field trials undertaken in 2014 and 2015 where the effect of starter  fertilisers containing phosphorus with and without the addition of nitrogen on rhizobia numbers in soil was supposed to be investigated. As described in objective a, the MPN method was unreliable and has been replaced with a new pot test to quantify rhizobia. For the project extension, the objective is extended to include the 2016 field season. A minor change to the original proposal lies in a different timing of soil sampling. Rhizobia are most active during plant growth so instead of quantifying numbers of rhizobia at harvest, numbers are quantified before sowing and at first flower. This ensures to detect even minor effects of fertiliser applications on rhizobia numbers.

c)   To quantify in relative terms the effect of applying starter fertilisers (containing phosphorus with and without nitrogen) on nodule formation and yield.

Results obtained in 2014 and 2015 showed that, in both seasons, both starter fertilisers with and without nitrogen led to yield increases. In 2015, nodulation under field conditions was assessed and results indicated that neither fertiliser affected nodulation. During the proposed  project extension in 2016, yield data and data on field nodulation will be collected, and this will increase robustness of results obtained in the previous two seasons.

d)   To include this information in dissemination activities of project FV 380. This includes advice directly to growers at PGRO open days and Road shows.

Dissemination of results of project FV 380 has been completed and a final report has been published. Results obtained in project FV 428 will be disseminated to growers at PGRO open days, road shows and in form of reports. On successful completion of the project, an AHDB factsheet will be produced.