**3. Soil**

*Microorganisms*

**2. Tropical agriculture**

the temperature drastically [7].

equator and limited by the tropics of cancer (23.5o

the parts of the earth where the sun reaches at an altitude of 90o

(AMF) were found to symbiotically improve plants' productivity by colonizing their roots [3]. In the 1920s and 30s, *Trichoderma,* the commonly known soil-inhabiting fungi were found to biologically control pathogenic fungi, thus having the potential in protecting agricultural crops [4]. In addition to protecting plants against pathogenic fungi, some strains of *Trichoderma* have been found to induce multiple benefits to plants when they colonize their roots [5]. Another group of fungi known as *Piriformospora indica* that beneficially colonizes and inhabits plant roots was discovered in the 1990s [6]. Once these microorganisms colonize and inhabit plant roots, they induce physiological changes and modify the expression of genes in the

Geographically, the tropical region is the region of the earth that centers in the

the two tropics during the average length of the year are contained in the tropical region. The sun's position makes this region not to experience notable changes in temperature (seasons), and during the wet seasons, water evaporation produces abundant rainfall in this "torrid" region due to constant daily radiation. A dry season which ranges from a month to over 6 months also occurs at different times and regions depending on the sun's position during the year and the region. A bimodal or unimodal distribution may be presented by these dry and wet (raining) seasons during the year. However, tropical ecosystems vary considerably from deserts to rainforests, and the concept of vertical geography which ranges from hot lowlands to snow-capped mountains within a few hundred kilometers can change

In terms of crop, tropical agriculture is usually described as those crops that were either introduced or adopted by European conquerors in the tropical nations that are under their dominion. Tropical agriculture is often dominated by crops [8]. A large number of plants use in agriculture today were originated and domesticated in the tropics, mountainous area where ecoclines often overlap [9–11]. This is due to the wide range of microclimate, temperatures and rainfalls in the mountainous tropics thereby increasing genetic diversification through selection, mutation and adaptation [12, 13]. A major reservoir of plant and animal biodiversity is the tropical ecosystems which play essential roles in global climate regulation and biogeochemical cycling [14, 15]. The exact yield potential for almost all tropical fruit crops still remains unclear. Though some industrialized crops such as banana, oil palm and citrus have very high production efficiencies, this is exceptional to the norm. For most tropical crops, the maximum recorded yields are much higher than the average yields over large area. Poor soil and water management, pests and diseases, low commodity prices, shortage of skilled and productive labor and failure of the market to provide incentive to growers are some of the reasons for the yield gap [16]. Crop production in the tropical regions is highly diversified compared to the large acre crop farming system in the temperate regions. In the tropics, food and industrial crops may be cultivated either in small holdings, plantations or in mixed gardens. Plantation crops may be large and continuous but they may be owned either by major corporations with uniform cultural practices or by a number of contiguous small farmers with varying practices [17]. Large number of pathogenic bacteria, fungi, viruses and other pests, especially insects often destroy tropical crops. The harsh winter conditions of the tropical regions do not reduce the pathogens population as in the temperate regions. The availability of plants, which serve

N) and capricorn (23.5o

S). All

and move between

plant they reside in, thus improving plants' productivity and resilience.

**64**

Soil is gradually formed from various parent materials, which is modified by climate, time, micro- and macro-organisms, vegetation and topography. It is a complex mixture of minerals, organic compounds and living organisms that interact continuously in response to natural, biologically, chemically and physically imposed changes [19]. In addition to root anchorage, the soil serves as a natural medium for plant growth and habitat for a wide range of microbes [20]. The growth of soil microbes and plants may be directly and indirectly influenced through a variety of interaction. These interactions may result in positive growth through mutual benefits, negative growth through antagonism, or no growth stimulation through neutral effects [21].

Cropland soil can be classified into three types and these include spermosphere, rhizosphere and bulk soil. Spermosphere is the portion of soil that surrounds germinating seeds. It has been described as the short-lived, rapidly changing and microbiologically dynamic zone of soil that surround germinating seed [22]. Rhizosphere has been described as a narrow soil zone that surrounds the roots of leguminous plant which stimulates intense bacterial activities [23]. It is a huge reservoir of microbial diversity. The release of exudates from the roots of plant into the rhizoenvironment initiates the establishment of rhizosphere. The exudates include plant mucilage, mucigel, root secretions and lysates [24]. Thus, exudates are the most vital factors that contribute towards the dynamics of rhizosphere. The rhizosphere is the most important niche that affects diverse aspects of plant life. Bulk soil is composed of soil outside the spermosphere and rhizosphere. With regard to microbial activities, it is considered to be the least dynamic. Out of the three groups of cropland soils, it occupies the largest portion of cropland soil. A large population of micro- and meso-organisms that include surviving propagules of soil inhabiting plant pathogens are harbored in the bulk soil [25].
