Biofertiliser: From big data to big life

Biofertiliser works, but the circumstances under which it can be optimally utilised are not yet an exact science.

Biofertiliser refers to a group of biological products that use microorganisms to increase soil fertility. Biological products are the umbrella term for a collection of natural products that farmers can use to improve soil fertility, stimulate growth in their crops and control diseases and pests. But what are the options with biofertiliser?

Using organisms to increase soil fertility is nothing new – the first product was introduced as far back as 1896. It was with the use of nitrogen-fixing bacteria for treating legumes. It is common today to use these products on legumes, and there are various products on the market.

Likewise, biofertiliser use on non-legumes is not new because the first product was registered in the late 1940s. However, what is new is that the interest in biofertiliser is currently increasing rapidly. It is estimated that the global biofertiliser market amounts to approximately R34.5 billion and is expected to grow at 18% annually from 2021 to 2026. This growth is understandable since fertiliser is the single biggest expense for crop farmers. What is it, and what are the problems around it?

Nitrogen Fixers

Concerning soil fertility, nitrogen and phosphate are the two most critical plant nutrients. These plant nutrients are, ironically enough, common in most soils but are not readily available to plants. Here the microorganisms are employed to mobilise the plant nutrients.

The most common example of biofertiliser is Rhizobium nodule bacteria that fix nitrogen from the atmosphere in symbiosis with legumes. Although Rhizobium bacteria can be found naturally in soil, the species cultivated in the laboratory and applied as a seed treatment is more efficient than its free-living peers.

It is estimated that the bacteria are applied to approximately 250 million hectares worldwide, and collectively they fix a total of 90 million tons of nitrogen each year. The big question with this technology has historically not been if it works or not, but rather to ensure that the applied organisms are still viable. Decades of research has enabled a substantial improvement in the shelf stability and viability of the inoculants.

However, there are also nitrogen-fixing bacteria that can be used with non-legumes. These include members of the genus Azospirillum and Mazospirf families such as Azotobacter and Gluconacetobacter. The latter also forms nodules on the crops’ roots and can potentially transmit nitrogen to the plant.

A review of the research on Azospirillum bacteria shows that between 65% of studies showed that these bacteria increase yields between 5% to 30%. The studies in question spanned various crops such as wheat, maise, sunflower, yellow carrots, sugar cane, tomatoes, eggplant and cotton. Interestingly enough, the studies also found that non-legume bacteria can have other beneficial effects, like improving yields in the soil with heavy metals and under drought conditions.

Phosphate mobilisers

Phosphate is one of the most important plant nutrients and the one trapped most easily in the soil. Although academics still argue, some believe that only 10–15% of the applied phosphate is available to crops. This property is alarming as phosphate stock is limited worldwide, and 70% of the reserves are in China, Russia, Morocco and America. Since this mineral is embedded so easily, large quantities of it are available in soil built up over the years. However, the question remains how to mobilise it, and this is where bacteria and fungi come up.

Studies have found that phosphate can be mobilised successfully by bacteria in the genera Bacillus and Pseudomonas and fungi in the genera Aspergillus, Trichoderma and Penicillium – with fungi generally doing better. A long-term study in America about the various locations of the product JumpStart (Penicillium bilaiae) found that yields increased substantially in 72% of the small and 80% of large trial plots. Crop yield in small and large plots were 1.8% and 3.5% higher, respectively. The study found no substantial pH effect. There was also no difference between seed treatment or band placement; however, it was found that the impact of fungi was bigger in soil with higher phosphorus levels.

In addition to bacteria and fungi, some studies show that mycorrhizal fungi (SMF) make phosphates available to crops. SMF forms a network of fungal threads that interact with most of the plant roots. It was also found that it helps plants with, among others, the absorption of plant nutrients, defence against pathogens and handling abiotic stress. Research about the efficacy of mycorrhizal treatment is still limited.

Does it work?

From the research, it is clear that biofertiliser can work, but more importantly, it does not always work. According to Mr Casper Brink from Sporatec at Stellenbosch University, it can be attributed to various factors.

One of them is the carbon dioxide levels and microbial activity of the soil before it is applied. It is high already; the applied organisms will struggle to compete with the existing organisms in the soil. The opposite will also be true for soil with very high carbon levels within which the applied organisms will struggle to survive.

Furthermore, the formulation and viability of the microbes in the product play a role because many lessons should still be learnt to utilise it optimally. Using Rhizobia as an example took more than a century from its discovery to perfecting the cultivation, storing, and application. One should also take into account

Since the microbes’ operation is crop and even strain-specific, huge differences exist between the respective strains (cultivar or race in stock terms).To use Rhizobium as an example again: Soygro increases and distributes a specific Rhizobium strain as WB74/USDA122. For this reason, it is vital to make use of tested and registered products.

According to Brink, it is difficult to identify specific species or strains in the soil. They are currently using a DNA fingerprint method to determine the diversity and activity of bacterial and fungal species. The species can be compared before and after treatment to determine if it impacted the diversity and activity.

What to do now?

Before conclusions are drawn about using biofertiliser, you have to remind yourself of your work scale. It is estimated that in 1 g of soil, there are more than 10 000 000 000 bacterial and fungal organisms, which means one cubic metre of soil will contain 1.5 million times the number of organisms, assuming that the bulk density of soil is 1.5.

Just think about the amount of diversity in 1 cubic metre of soil, among species and within species adapted for a specific area, crop or tillage practice. Producers will, for example, tell you that they farm with Bonsmaras, but these are Bonsmaras that have been adapted for their area.

That biofertiliser can and will have an impact is inevitable. However, much research must still be done to understand better which organisms from which strain, under which soil conditions, crops and tillage practices will be best suited for a farm in a specific area.