Jamie Stotzka, Consultant Bioagronomist

Healthy soils contain billions of bacteria from thousands of species. A special group of these bacteria, collectively known as Plant Growth Promoting Rhizobacteria or PGPR are known to promote plant health, growth and productivity. PGPR are unlike the nodulating rhizobia only found associating with leguminous plants, as they are free-living in soils residing in the plant root zone whilst also having the ability to enter plant tissue. Nourished by plant exudates, these bacteria provide indispensable services to growing crops.

PGPR’s plant growth promoting effects can be attributed to four main mechanisms: enhanced nutrient delivery, protection of plants from potential pathogens, improved root development and the bioremediation (cleaning) of soils. Probably the most renowned trait of these bacteria is their ability to fix atmospheric nitrogen. Over 78% of the atmosphere is gaseous N2, which is ordinarily unusable by plants. A unique set of enzymes allows PGPR convert this gas into plant useble NH4+. Above each hectare of land, approximately 74,000 tonnes of N2 are within reach to be converted by this process.

Another macro element phosphorus can be present in soils but is often-times ‘locked up’ – i.e. bound to metal elements. PGPR act to solubilise this element by producing organic acids and inconcert with other soil microbes this community has the capability of increasing available soil P by up to 62%. Other essential nutrients are also chelated by PGPR. Working in tandem with other beneficial soil microbes, such as arbuscular mycorrhizal fungi (AMF), these nutrients can be efficiently transported to crops in healthy soils. PGPR can further alter the root architecture and promote plant development via the production of phytohormones such as auxins, cytokinins and gibberellic acid. Plant protection is achieved by regulation of plant internal defences against drought and pathogens.

The positive effects of PGPR have in the past been assumed to be fairly generalist and to apply to a broad range of crops. Commercial inoculants have been available for a number of years to improve plant health and development in arable systems. Most of these products operate under a “more is better” policy and fail to discriminate between the particular effects of different bacterial species on particular crop types. In trials over the past three years, researchers at PlantWorks Ltd have found that this one-size-fits-all approach may have drawbacks and hidden pitfalls.

A range of bacterial species produced by the company were tested in isolation on different crop types, with significant results. Yield results indicated that even within the cereals group, plants did not necessarily share the same ideal bacterial partners. Field trials were established based on outcomes from preliminary glasshouse experiments and in 2017 these produced very encouraging results.

A sugar beet trial run in collaboration with Allpress Farms in Cambridgeshire, which utilised a tailored beet inoculum, produced a statistically significant yield increase of 33% with no reduction in quality of the roots. Potato and field vegetable trials showed similarly positive and significant trends with i.e. 43% heavier leeks produced at Allpress. * Average grain weights on Motts Farm in Essex. Measurements were taken from random quadrat readings taken throughout treated and control areas (PlantWorks, 2018). Previous trial work at Mr Cowell’s farm revealed that through no-till and low input farming systems, as well as rotations including mycorrhiza friendly plants such as Lucerne, the soils were highly biologically active. With wheat on the farm showing very high mycorrhizal colonisation levels of up to 80%. This background fungal network was thought to have further enhanced and supported bacterial function to achieve the positive yield noted. This hypothesis was further illustrated by PGPR trials at GH Dean in Kent.

Here, mycorrhizal and bacterial inoculants were applied in isolation and combination, revealing a positive interaction between the two types of microbes. A tailored selection of bacterial inoculants for a range of cereal crops has now become available within the PlantWorks’ Smart Rotations range. Applied in liquid form these tailored PGPR consortia are easy and straightforward to apply using standard sprayers. Further products to enhance mycorrhizal communities in farm soils can be added within crop rotations to build strong fungal communities to support bacterial function.

It is clear that the inclusion of biological inoculants such as PGPR and mycorrhizal fungi has vast potential to allow for more sustainable crop production with possible savings in mineral fertilisers, the use pesticide products as well as building healthy, balanced soils. It is an exciting time within farming, with a growing database of knowledge, ever more sophisticated advice and well tested products becoming available farmers can now, very practically, intervene within rotations to increase beneficial soil biology to enhance arable yields. For further information contact Ms Jamie Stotzka , Consultant Bioagronomist, PlantWorks Ltd. Office: +44(0)1795 411527 e-mail: