Published on International Journal of Agriculture & Agribusiness
Publication Date: June 5, 2019
Nweke, I. A. & Ilo, G. E.
Department of Soil Science Chukwuemeka Odumegwu Ojukwu University, Anambra State
Department of Crop Science University of Nigeria, Nsukka
Soil cultivation events change considerable surface roughness, weaken the soil structure and the carrying capacity of soils. Soil nutrient elements classified as chemical properties for good crop growth and healthy conditions for the crops are mostly sourced from the soil. The essence of cultivation is to break down soil to a fine form to provide an ideal situation for seed germination. It causes gradual reduction in soil fertility and crop productivity. The impacts on soil nutrient concentrations vary from one site to another after cultivation. Every form of cultivation in arable land whether short or long term, intensive or continuous cultivation have effect on soil chemical, biochemical, physical, hydro-physical, biophysical and biological properties of soils. Also land use changes have contributed immensely to the decline in soil nutrients and soil structure. Cultivation using hoe followed by hand weeding with hoe is a common tillage method used by farmers in Nigeria and most of the tropical and semi-arid countries. These types of tillage systems encourage crust, surface sealing and hardpan formation. Cultivation control small annual weeds and diseases.
Keywords: Cultivation, land use, nutrient recycling, soil microbial activity, soil structure modification, Tillage.
Production systems in the most intensified areas of the savannah are characterized by continuous tillage with different tillage implements. Continuous use of these implements results to soil erosion, compaction, and reduced microbiology activity, decline in quality of organic matter and deterioration of other soil physical properties. Ultisoils and oxisoils are susceptible to physical, chemical and biological degradation once brought into cultivation (Amezquita 1998). Land preparation by machinery leads to a constant breakdown and reduction in soil aggregate size the action of rainfall and gravity results in a re-packaging of these aggregate and consequently, the total porosity and pose size are reduced. The resulting changes in macro porosity affect water flow, which in turn affects nutrient availability and this impact negatively on the productive capacity of the soil (Precado et al., 1998). Soil quality is the ability of soils to perform within the ecological system and land use boundaries, in order to maintain biological activity, environmental quality and promote plant and animal health. Therefore it is one of the most important factors according to Nael et al. (2004) in developing sustainable sustaining the global biosphere and land use management. Land use changes especially deforestation of land may rapidly diminish soil quality. Kormali and Shamsi (2009) opined that deforestation destroy soil quality causing permanent degradation of land productivity. Karien et al. (1997) defined soil quality as the capacity of a specific soil function within natural or managed ecosystem boundaries to sustain plant and animal productivity and maintain and enhance air and water quality. From Karien’s point of view, once there is a total decline in soil productivity every other aspect of agriculture is put to a halt since the major source of food is dormant, even, the activities of man will be directly affected.
Edward (1991) and Tisdall (1996) noted that a good soil structure for crop growth should aggregate between 1 to 10mm diameters of which should be stable when wetted, physical break down of aggregates during ploughing and subsequent higher organic carbon mineralization, which may result in N and P losses through leaching and fixation by soil respectively. Continuous cropping and cultivation of many of the world soils that are previously under grass land/forest had led to substantial decrease in soil OM and soil structure. Soil structure ability and soil OC content usually decrease with cultivation (Eynard et al., 2004). Lal et al. (1994) reported that the water stable aggregates where greater in no till compared to the intensive tillage methods. Six et al. (2000) reported that cultivation reduces soil carbon content and changes distribution and stability of soil aggregate. While Nweke and Nnabude (2015a) reported changes in the distribution of silt, clay and exchangeable properties of soil aggregates by cultivation. Soil aggregate stability is a crucial property affecting soil sustainability and crop production. Soil disturbance is a major cause of reduction in number and stability of soil aggregate when nature’s ecosystem is converted to agriculture (Six et al., 2000).
When the soil particles are together and bounded in one accord, the soil structure is perfectly arranged and consistency is sure. This consistency can determine the state of the soil, if the soil carrying capacity is ok to resist deformation with the particles staying together. Though cementing agent play vital function keeping these particles together but the truth is particles will not stay together if they are first bound by attractive force. One of the major components that facilitate the structure and rheological properties in the soil is the absorption of substances that will increase the cohesive force of particles. Thus this review intended to report on various impacts of cultivation and land use changes on soil productivity and agricultural sustainability.
2. Effect of Cultivation on Structural and Rheological Properties of Soil
The effect of intensive cultivation on the rheological properties of the soil depends greatly on the clay mineral contents of the soil. The higher the level of the clay mineral contents the lesser the effects of intensive cultivation on the rheological properties of the soil, and vice versa (Neaman and Singer 2000), Clay dispersion occurs when the electrolyte concentration in the percolating solution in the soil is below the flocculation value of the clay. Under this situation, the flow of water in the soil changes from flow of solution to flow of clay suspension (Neaman 2000). The presence of clay particles in the percolating solution may increase significantly the viscosity of the flowing suspension and therefore, may decrease its fluidity. This, in other words, would reduce soil hydraulic conductivity. When particle-particles interaction occur causing non-Newtonian flow, soil hydraulic conductivity decrease the more (Neaman and Singer 2000). Knowledge of the factors affecting rheological properties of soil clay suspension is essential for understanding mechanisms of water and solid particle transfer through the profile. The rheological parameters of clay suspension can be used to evaluate particle–particle interaction. Small montmorillonite addition under intensive cultivation increased the rheological parameters such as plastic viscosity and Bingham yield value.
Khademi and Mermut (1999) emphasized the various effects of intensive cultivation on the rheological properties of the soil to include earthquake, tsunami, ragging billow, vibration. They said that intensive cultivation decreases the level of the soil carrying capacity. This resulted as a result of continuous agricultural service which weakens the soil structure, thereby making the soil very weak to carry loads. The continuous agricultural practices on a soil that has low clay mineral particles reduced the binding forces between the soil aggregates, thereby causing high leaching and erosion that washes away the soils binding agents and renders the soil very weak to carry loads. Dixion and Golden (1990), discovered that intensive agricultural practice on the soil destroys the soil properties thereby affecting agricultural practices adversely. Buhman et al. (1996) said that an intensive agricultural practice on the soil reduces the soil elasticity. This, he said may occur as a result of soil degradation due to intensive agricultural practices. This reduction in soil elasticity can lead to low soil carrying capacity. When the soil loses its elastic nature, a tractor working on the land can easily sink into the soil, or the tractor can easily crack the soil on its passage on the soil. The rheological properties of the soil are affected in many ways by intensive agricultural practices. The effect of intensive agricultural practices on the rheological properties of the soil can affect both agricultural productivities and human life at large causing loss of lives and properties. Manure supplies OM and improves the water retention capacity of soils (Arriaga and Lowery 2000).
Atterberg constants were highly sensitive to management; they could be used as indicators of soil physical quality for long-term land use and management systems for soils. The consistency limits are important parameters related to soil structural stability. The PL in particular is a useful index of soil physical quality. When there are large differences in LL, PL, and PI it suggests that the optimum water content for tillage varies significantly among the selected cultivated watersheds or sites. When the soil is too wet tillage operations can have detrimental effects on soil structure (Mueller et al., 2003). The knowledge of differences in soil consistency limits can allow a better scheduling of tillage operations and traffic. Mueller et al. (2003) identified the PL as one of the most sensitive parameters to estimate the highest water content for optimum tillage across a broad range of soils. Soil macro-and micro structural properties can be significantly altered by untimely tillage ignoring the dynamics of soil consistency (Barzegar et al., 2004). The soil rheological properties have to do with the carrying capacity of the soils it is the way particles are arranged in their various sizes and how they cluster together that determine the soil rheological property. The rheological properties of the soil are what create the cohesive force binding the soil particles together thereby creating a strong structure that will not bend easily to any deformation. Due to this, the soil have the ability to resist deformation and stress by carrying heavy duty materials that may move across the surface of the soil and that is why it is mostly referred to as the carrying capacity of the soil.
Intensive cultivation is a major obstacle as par its effect to the soil rheological properties. Any factor that will forcefully, separate the soil particles, from each other affect the soil rheological system as the possibility is to come back to its original possession is very low due to the rate by which farmers are venturing into intensive cultivation with the intension to have continuous supply of food crops have degraded the soil’s physical properties which have to do with rheological properties, and due to the increasing rate, research is now made to control this problem, Gregorich et al. (1994) reported that the recent concerns regarding soil degradation and agricultural sustainability have kindled interest in assessment of soil quality. It is when these quality are observed that the degrading nature of the rheological properties due to mans activities in the soil is noticed.
The effect of cultivation on the rheological properties of the soil is not easily noticed by mere looking at the soil but a degraded soil can be noticed via the kind of product gotten from that soil, when productivity is decreasing, that way decline in the soil properties can easily be noticed. Unless the soil is allowed to regain its lost structure and texture to improve its quality and capacity to function, the physical properties which also have to do with the rheological properties, the chemical and biological properties as well will remain dormant and the soils capacity to function at this point will be extremely low. Most times this continuous cropping does have a short term effect, on properties of the soil, still scientist believe it has an unprofitable long term effect but due to farmer’s interest for profitable yield the effects are usually ignored. Nevertheless the irony is that the effect is still noticed as well as soils deterioration. When the soil is cultivated at a continuous interval, the rheological properties goes weak gradually creating an avenue where the particles of the soil will end up standing individually. When it has come to this stage the soil carrying capacity is extremely low, this of course is dangerous to the soil as foreign materials can come into the soil and change the soil chemical and biological component since there is no network of soil particles restraining their movement and this can change the soil pH over a long period of time. Liming application on the soil at this stage for rapid change is not advisable too for the soil pH. Intensive cultivation is a gradual process that destroys the soil rheological properties and when a particle is displaced from its functional position, the probability that the particle will be going back to its original position is zero since the destructive nature of continuous cropping on the soil properties is not easily reversible. Farmers are therefore advised to go back to their normal culture practice which is bush fallowing to allow the soil to rest as it awaits its next planting season. But this again is location dependent, land tenure system, and population increase this present days.
Number of factors influences rheological behaviour of soils. One of the most and significant factor is the water or moisture content. If one is to be able to either maintain soil condition or to change it to a more suitable condition, he must first have an understanding of soil behaviour; this behaviour must eventually be properly described. Soil conditions and properties, widely varying types of forces and widely varying types of behaviour must all be included in any description before the description can be satisfactory. Therefore, through testing the physical / mechanical properties of some fields, the analysis of water / moisture content will help to deeply understand the rheological behaviour of the soil. Vanden Bygaat et al. (1991), observed a reduction of total pore volume and in total number of pores in the top 0 and 0.25 m of soil after 11 years of no cultivation they stated that increase in root penetration and biological activity in non cultivated fields might facilitate aeration and water entry due to formation of bio pores and decrease the bulk density in the long term. Li et al. (2004) found that tillage affected soil bulk density differently at different slope 0-2 m depth of the cultivated soil was lower than corresponding values for the forest and pasture soils. Comparative losses of soil organic component and TN due to cultivation of forest or pasture have been reported (Brown and Lugo 1990; Spaccini et al., 2001, Wu and Tiessen 2002). Poor water infiltration caused by the pan can lead to water logging of the top soil layers. The effect of the compaction slows down water infiltration in the soil, and also reduces the movement of air through wet soil and disturbs root growth (Charman and Murphy 1998). The amount of dispersible clay is a function of total clay content and the percentage of clay dispersed was controlled by factors such as clay mineralogy, CaCO3 and organic matter content of soil. The tendency of different soil for hard- setting and crushing as a result of structural collapse, was reflected intensive in the modulus of rupture. The soils under intensive agricultural practices had significantly higher modulus of rupture than their non-tillage counterparts as a result of the agricultural practices carried out on them (Busscher 1990). When the soil is lacking air and water, it becomes inhabitable for soil microbes, which makes the soil a living entity. The effect of inadequate air and water in the soil is as a result of the effect of agricultural practices on the soil structural properties.
Surface sealing leads to low infiltration, increased runoff, erosion and transport of sediments, nutrients and pollutants from the land even at low slope gradients i.e. soil aggregate breakdown and dispensability under rain depends on clay mineralogy. Soil aggregate stability is an important factor especially in cultivated areas with high rain fall erosivity. Breakdown of aggregates produces micro aggregates and primary soil particles from original structural units and thus, results in pore collapse which reduces infiltration, increases runoff, erosion and subsequently may cause further soil degradation. Aggregate stability depends on soil internal (texture, organic matter, clay mineralogy, CaCO3, Fe and A1 oxides) as well as external, time dependent properties (climate, tillage, biological activity, wetting, and drying). The difficulties in quantifying the role of soil properties and breakdown mechanism to soil aggregate stability, which differs with climatic zones and soils, has been widely recognized (Amezketa 1999). The effects of clay mineralogy on hydraulic properties and aggregate stability yielded conflicting results as soil usually contain a mixture of different clay minerals and differ in chemical dispersion, texture, organic matter and cultivation history (Reichert and Norton 1994). The effect of tillage on aggregate stability is not clear, as tillage may disrupt dense or compacted soil layer, hence increase infiltration, conversely tillage may cause poor structure, smoothing of the surface and loss of organic matter and increase in dispersed clay. The use of different measurement methods could also result in different ranking of soil aggregate stability.