Written by :
Peter Bolo, Job Kihara, Michael Kinyua and Dr. Fredrick Ayuke; Senior Lecturer, Land Resource Management & Agricultural Technology, University of Nairobi, Kenya.
Over the years, many discussions on the benefits of conservation agriculture vis-à-vis conventional tillage have been held. These have mainly focused on the associated soil physical and chemical benefits. However, on most occasions, little attention has focused on the benefits of practicing conservation agriculture for soil health. Indeed, for the myriad soil benefits and even improvements in crop yield, there is often a direct or indirect association with the activities of soil biological organisms and, generally, soil health.
Erest Omulama, a farmer in Western Kenya, noted that soils that do not receive organic matter inputs are less likely to improve soil health and microbial activities. He stated, “Where farmyard manure has been added, there is living soil. But the soil is dead where there is only mineral fertilizer application.” This statement represents a view shared by many in this region and beyond.
Soil living organisms, including the large-sized (macrofauna) and small-/medium-sized (mesofauna) organisms, among others, play important roles in improving soil conditions and driving ecosystem stability, nutrient availability, and improvement of soil fertility. These organisms can break down (decompose) soil organic matter, make underground tunnels that improve soil water-holding capacity, and increase soil moisture retention. In addition, some soil living organisms, especially the predacious groups, can reduce incidences of pest damage to crops through preying on certain field pests (insects), thus increasing crop yield.
In many instances, conventional tillage systems (the normal practice involving repeated cultivation with limited inputs) have been associated with several disadvantages involving nutrient losses, reduced moisture retention, soil erosion, and a reduction in population abundance of soil biodiversity (soil dwellers) compared with conservation tillage. These disadvantages have been established to be true over the years. Practicing conservation agriculture (a system with minimal or zero tillage, residue retention among other inputs added, and crop rotation) has been shown to solve these problems! Reduced tillage has the potential to increase soil health, improve the population of organisms living in the soil, and increase organic matter accumulation, leading to soil structural stability. In addition, despite the lower input demands than with conventional tillage, conservation agriculture can increase soil health, fertility, and productivity in several ways.
A healthy soil is vibrant in all measures, and equally promotes the life and activities of soil living organisms. “Soil living organisms are the primary soil engineers whose activities determine soil health, ecological processes, and stability, and, in fact, practices that promote soil biodiversity are considered a powerhouse of soil functionality,” says Peter Bolo, a research assistant at the International Center for Tropical Agriculture (CIAT), Kenya.
With funding from CIAT, through the International Maize and Wheat Improvement Center (CIMMYT) and the Federal Ministry for Economic Cooperation and Development (BMZ), and in partnership with the Kenya Agricultural and Livestock Research Organization (KALRO), a study was conducted in eastern (Embu) and western (Kakamega and Nyabeda) parts of Kenya to determine the effects of conservation agriculture and associated management practices on the population of soil living organisms (both large-sized and small-/medium-sized). The study involved taking soil samples from two different soil depths: upper (0‒15 cm) and lower (15‒30 cm). At the three sites, the study assessed the population of soil living organisms (macrofauna and mesofauna) in different tillage systems (conventional tillage, conservation tillage, and farmer practice systems), cropping systems (legume-cereal rotation and legume-cereal intercropping), and residue management (with and without residue retention). The soil living organisms were isolated from the soil samples using different techniques involving handpicking or using a basic apparatus (Berlese or Tullgren funnel).
Image 1. Handpicking macrofauna from soil samples in Western Kenya.
Overall, 58 macrofauna species (large-sized soil organisms) classified into 14 major groups and 18 mesofauna species (small-/medium-sized soil organisms) classified into 7 major groups were found. Four groups of macrofauna species were the most dominant at the three sites: earthworms (Oligochaeta), termites (Isoptera), ants (Hymenoptera), and beetles (Coleoptera). The populations of most of these organisms are often limited by poor soil conditions, low inputs, poor nutrient availability, and increased soil disturbances.
Evidence gathered from this research showed, in some instances, that practicing conservation tillage relative to conventional tillage increased the population of some specific groups of macrofauna and mesofauna. For example, in Kakamega, practicing maize-bean intercropping with residue retention under conventional agriculture increased the abundance of earthworms (Oligochaeta) at the top depth compared with conventional tillage with similar treatment or farmer practice without any inputs. Residues often have a lot of influence on soil microorganisms because they provide food for their growth and activities. Changes in microbes are easily seen where residues are many. However, at the three sites, small changes in soil living organisms might have been caused by the high rate of residue disappearance due to increased termite activity. Previous research established high rates of residue disappearance following increased termite activity in Nyabeda.
In Kakamega, a higher population abundance of springtails (i.e., Collembola group of mesofauna) was observed in conservation tillage systems than in conventional tillage systems. Likewise, in Nyabeda, the population of garden centipedes (i.e., belonging to the class Symphyla) was higher in conservation tillage systems than in conventional tillage systems. This means that residue retention and maize-bean intercropping in the system probably promoted the accumulation of organic matter and modified microclimate that could favor the proliferation of these groups of soil organisms. This points to an increase in their soil beneficial activities. For instance, an increase in earthworm populations could improve nutrient mineralization, soil aeration (through burrowing), and moisture retention, as these are associated with their functions in the soil. However, and interestingly, the population of centipedes (i.e., belonging to the class Chilopoda) was higher in the farmer practice than in both conservation and conventional tillage systems. Chilopoda are predacious, and thus repeated tillage characterizing the farmer practice could have increased the chances of their survival and proliferation through continuous exposure of their prey from the soil. But akin to this, practicing conservation agriculture, to some extent, increased the population of spiders (Araneae), important predacious soil organisms that might reduce the population of crop pests involving insects.
This study shows that practicing conservation tillage with residue retention under maize-bean intercropping, rotation, and sole maize increased the abundance of most groups of soil living organisms compared with systems in which residues/inputs were removed. Therefore, adoption of conservation agriculture practices should be encouraged in order to improve soil health through enhancing the abundance of soil living organisms and optimizing their important soil functions.