By Job Kihara, Peter Bolo, Michael Kinyua, John Mukalama, Rolf Sommer, and Andrew Margenot.

 

“Where farmyard manure has been added, there is living soil. But the soil is dead where there is only mineral fertilizer application”. This statement by Erest Omulama, a farmer in western Kenya, represents a view shared by many in this region and beyond. What Erest is actually implying is the lack, or reduced activity, of soil microbes in soils that do not receive organic matter inputs.

Yet, these are two extremes in crop production: organic farming on the one hand, and industrial fertilizers on the other. Organic farming is viewed as promoting proliferation of soil life and diversity, while industrial fertilizer is seen as an ‘oppressor’ of soil life, simply supplying nutrients.

A group of scientists, farmers and agricultural practitioners visiting CIAT’s long-term trials in western Kenya where the study on living soils has been undertaken. Credit: John Mukalama.

We argue that except for small land plots, organic resources within the reach of many households are inadequate to supply all needed nutrients for boosting crop productivity, especially on smallholder farms. For several years, we have called for a combination of both organic resources and industrial fertilizers in crop production. But the question for farmers has always remained: “How do the applications of industrial fertilizers affect soil life”?

Indeed, a lot of research in the last two decades has focused on the best agricultural practices to improve chemical and physical soil quality and productivity. But little attention has been directed towards the changes in the soil microbial diversities. These play a key role in decomposing soil organic matter, and thus keeping more carbon in the soil. Soil microbes also help recycle nutrients and supply plants with vital food, while keeping the soil structure stable. The lack of research into how such soil health functions, has led to a wide gap of information and knowledge on maintaining healthy and sustainable wider ecosystems.

Image of an unhealthy, poor, soil taken from Western Kenya. Credit: Peter Bolo.

Soils are full of microbes. “A handful of soil contains more microbes than the total human population on earth,” says Dr. Gerrit Gerdes, Country Programme Manager of Soil Protection and Rehabilitation for Food Security Initiative of the Deutsche Gesellschaft für Internationale Zusammenarbeit (GIZ) working in western Kenya.

With funding from GIZ, and in partnership with the Kenya Agricultural and Livestock Research Organization (KALRO), we took on the challenging task to study the changes in soil bacteria using an advanced DNA sequencing technique, called Illumina. We also traced their activity related to different key soil functions, such as nitrogen or phosphorus mineralization, by studying related enzyme activities.

Using our long-term trials spanning 13 years under consistent management, we demonstrated techniques which range: from conventional to reduced tillage; residue removal

to high organic input; Tephrosia candida systems; no fertilizer to liming and phosphate rock applications; rotation and inter-cropping systems. The results are fascinating!

Our research has found that microbial populations, diversity and activities are higher in reduced tillage systems where there is combined application of fertilizes and retention of crop residues. Integrated application of fertilizers and residue retention in maize-soybean rotation systems favored a high diversity of various functional groups of bacteria.

These include high diversity of nitrogen fixing symbionts, phosphorus solubilizing micro-organisms and organic matter decomposers – actinobacteria, cyanobacteria, proteobacteria – under reduced tillage compared to conventional tillage systems. What this means is that these microbes therefore play essential roles in nutrient cycling – nutrients that come from both manure and synthetic fertilizers.

Removing either residues from a farmer’s field – or application of industrial fertilizers – as the only input on the farm, decreases microbial diversity and functions. In other words, a combination of both can have a more beneficial impact than either technique by itself.

Application of nitrogen fertilizers alone for example reduced the population of some

Image of a healthy (living) soil taken in Western Kenya. Credit: Peter Bolo.

soil microbial communities like cyanobacteria and Kribbella – another microbial species involved in nitrogen cycling in the soil. Among the cropping systems, inter-cropping, which is commonly practiced by farmers, promotes higher microbial populations, such as for nitrospir – a species involved in the nitrification process in the soil – compared to rotation systems.

Also, combined application of lime and Minjingu phosphate-rock, similar to treatments under poor management – no residues or manure applied – reduced the activities of enzymes involved in phosphorus mineralisation; an indication that such practices in isolation do not increase soil health, fertility and productivity.

Besides microbial populations and enzyme activities, soil aggregate stability is higher in the reduced tillage systems where residues were retained compared to where the residues are removed from the field.

Therefore, our results show that in order to improve soil health and realize increased productivity, there is need for a combination of fertilizer techniques: integrated application of mineral fertilizer, retention of residues in the farm, employing proper – as little as possible – tillage and cropping systems. This will favor high soil microbial population, diversity and abundances, resulting in improved ecosystem health and sustainable agricultural productivity.

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