(The rhizosphere is a biological bazaar (market place) where microbes and plants trade nutrients, metabolites, and exudates)
Rhizosphere refers to the area of soil that is directly affected by a plant’s root system, associated root secretions, and microorganisms.
The rhizosphere region around a plant’s roots contains an abundance of bacteria and microorganisms that depend on the plant’s sloughed off cells, sugars, and proteins as a primary food source. In exchange, the bacteria and microorganisms help create a nutrient-rich, aerated place for the plant’s roots.
Within the rhizosphere, the coexistence of bacteria, plants, and micro-organisms create a key symbiotic relationship. The rhizosphere is where a plant’s mycorrhizal fungi are present
The fertiliser provides the nutrients to grow and nourish pastures and crops.
Plants require 17 essential nutrients to thrive. Fertiliser supports plant growth and replenishes nutrients after each harvest.
Nitrogen, phosphate, potassium and sulphur are the four most important nutrients for crop yields and sustainable food production:
Nitrogen (N) makes up about 78 percent of the air we breathe. It is inert and insoluble in this form.
To manufacture nitrogen fertiliser, it must be removed from the air and combined with hydrogen to make ammonia, which is then converted to urea. This is applied directly to crops as a nitrogen fertiliser or used as a building block to make other nitrogen fertiliser products. Phosphorus (P) is present in all living cells and is essential to all forms of life. Found throughout our bodies, it is concentrated in our teeth and bones. The source of phosphorus in fertiliser is phosphate rock, which is typically mined from the earth’s crust then reacted with acid to produce different phosphate products.
Potassium (K) is also found throughout nature and is found in our bodies in muscles, skin and the digestive tract. Good health requires a sufficient intake of potassium. Plants use potassium for functions like photosynthesis and protein formation. Potassium, or potash, is mined from naturally occurring ore bodies that were formed as seawater evaporated. The deposits are a mixture of crystals of potassium chloride and sodium chloride. Sodium chloride is also known as table salt. After it is mined, the potassium chloride is purified into granular fertiliser.
Sulphur (S) is essential to produce amino acids, which are the building blocks of proteins found in all living things. Sulphur also helps give crops like onion, mustard and radish their characteristic flavour. While it can be found naturally in the soil, it is not always in a form plants can use.
Soil texture refers to the relative amounts of inorganic particles i.e. Sand, Silt and Clay. Sand grains are large and coarse, clay particles are vary fine and smooth, and silt particles intermediate.
The way in which soil particles are grouped or bound together to form lumps or aggregates is known as soil structure. There are two main types of soil structure, (10) single grained and (2) compound structure. Soil structure can be modified by adopting various soil management practices including aeration, tillage, crop rotation, irrigation, drainage etc.
The density of soil can be expressed in two ways. (1) The density of solid (particle density), particles of the soil and (2) the density of the whole (Bulk density) soil that is inclusive of pore space. Generally soils with low bulk density have better physical condition than those with higher bulk densities. Texture and structure of a soil, its total pore space and organic matter content are all related to bulk densities. Soil density can be modified with aeration.
Between the soil particles there are empty spaces which are occupied by air and water and are termed as pore spaces. Pore spaces between the aggregates of soil particles are macro pores and those between the individual particles of the aggregates are micro pores. Sandy soils have a higher percentage of macro pores. Typically, sandy soils never become water logged and allow water to percolate downward more rapidly than clay soils. Typically, moisture content in sandy soils is relatively low when compared to clay soils.
Clay soils contain a higher percentage micro pores when compared to sandy soils. Clay soils are more susceptible to water logging which can adversely effect root respiration and microbial activity. A proper balance between the macro and micro pores can be maintain by timely aeration.
Soil color is helpful in determining soil properties. A dark brown or black colored soil indicates its high organic matter content and fertility. A red or yellowish soil shows good aeration and proper drainage. A white color, resulting from the accumulation of salts of alkali indicates deterioration of soil fertility and its unsuitability for normal growth of many crops
Soil temperature and plant growth
Soil micro-organisms show maximum growth and activity at optimum soil temperature range. All crops practically slow down their growth below the temperature of about 90C and above the temperature of about 500 C. The biological processes for nutrient transformations and nutrient availability are controlled by soil temperature and soil moisture. Soil temperature has a profound influence on seed germination, root and shoot growth, and nutrient uptake and crop growth. Seeds do not germinate below or above a certain range of temperature but Micro-organisms functioning in the soil are very active while a certain range of temperature, which is about 270 to 320C. It is necessary to know whether the soil temperature is helpful to the activities of plants and micro-organisms and the temperature could be suitably controlled and modified. The various factors that control the soil temperature are soil moisture, soil colour, slope of the land, vegetative cover and general soil tilth. Aeration can be used to control soil temperature, regulate soil moisture, improve drainage, stimulate microbial activity and improve overall soil tilth.
Biological properties of soil
A variety of organisms inhabit the soil. They decompose organic matter, fix atmospheric nitrogen, cause denitrification etc. Specific groups of organisms are responsible for specific activities in the soil. Such activities may be beneficial or harmful to the crop or its yield potential.
Bacteria are generally confined to the 20 to 30 cm. layer and work best when there is (1) good aeration, a neutral reaction, soil moisture content at about half of the soil’s water holding capacity and temperature between 250 c and 380 c.
These organisms produce microscopic threads called mycelia and are found in the organic matter of plant roots. Fungi help in breaking down the somewhat resistant parts of the organic matter like cellulose, lignin, gums etc. A large part of slowly decomposing soil humus is made up of the dead remains of fungi.
They can grow in deeper layers even under dry conditions. Their main function lies in decomposing the resistant parts of organic matter like cellulose.
They are microscopic or very minute sized plants having chlorophyll and are usually found on the surface of wet soils. They help in adding organic matter to soil, improving the soil aeration and fixing atmospheric nitrogen.
Texture and other soil properties and plant growth
Many of the important soil properties are related to texture. Clayey soils show high water holding capacity, high plasticity, and stickiness and swelling whereas sandy soils are conspicuous by the absence of these properties. The most important way in which soil texture affects plant growth is water and with it the nutrient supply. The available water holding capacity of soil is related to soil texture. Timely aeration can improve Soil texture improved water holding capacity.
Soil structure and plant growth
Soil structure influences plant growth rather indirectly. The pores are the controlling factors governing water, air and temperature in soil, which in turn, govern plant growth. One of the best e.g. of the effect of soil structure on plant growth is the emergence of seedlings in the seedbed. The seedlings are very sensitive to soil physical condition so that there should not be any hindrance to the emergence of tender seedlings and there should be optimum soil water and soil aeration. The soil in the seedbed should have a crumb structure so that the roots of the seedling can penetrate it easily. The hard compact layer impedes root growth.
Water is essential for plant growth. Soil is capable of being a storehouse of water and becoming the main source of water for land plants. Soil water plays a significant role in several natural processes- evaporation, infiltration and drainage of water, diffusion of gases, conduction of heat, and movement of salts and nutrients are all dependent upon the amount of water present in soil. Plants meet their water requirement from water stored in soil. Soil moisture can be improved with aeration.
Soil Aeration and plant growth
Oxygen is required by microbe and plants for respiration. Oxygen taken up and carbon dioxide evolved are stoichiometric. Under anaerobic conditions, gaseous carbon compounds other than carbon dioxide are evolved. Root elongation is particularly sensitive to aeration. Oxygen deficiency disturbs metabolic processes in plants, resulting in the accumulation of toxic substances in plants and low uptake of nutrients.
Soil compaction is the process of increasing dry bulk density of soil and reducing pore space by expulsion of air through applied pressure on a soil body. Soil compaction is a limiting factor in seed germination, water transmission and aeration. Timely aeration and the incorporation of biologicals can prevent soil compaction.
Soil tilth is a physical condition of soil, especially in relation to its suitability for planting or growing a crop. Factors that determine tilth include the formation and stability of aggregated soil particles, moisture content, degree of aeration, rate of water infiltration and drainage. Tilth can change rapidly, depending on environmental factors such as changes in moisture, tillage and soil amendments. The objective of tillage (mechanical manipulation of the soil) is to improve tilth, thereby increasing crop production; in the long term, however, conventional tillage, especially plowing, often has the opposite effect, causing the soil to break down and become compacted. 
Soil with good tilth has large pore spaces for air infiltration and water movement. Roots only grow where the soil tilth allows for adequate levels of soil oxygen. Such soil also holds a reasonable supply of water and nutrients.
Tillage, organic matter amendments, fertilization and irrigation can each improve tilth, but when used excessively, can have the opposite effect. Crop rotation and cover crops can positively impact tilth. A combined approach can produce the greatest improvement.
Aggregation is positively associated with tilth. With finer-textured soils, aggregates may in turn be made up of smaller aggregates. Aggregation implies substantial pores between individual aggregates.
Aggregation is important in the subsoil, the layer below tillage. Such aggregates involve larger (2- to 6-inch) blocks of soil that are more angular and not as distinctive. These aggregates are less impacted by biological activity than the tillage layer. Subsurface aggregates are important for root growth deep into the profile. Deep roots allow greater access to moisture, which helps in drought periods. Subsoil aggregates can also be compacted, mainly by heavy equipment on wet soil. Another significant source of subsoil compaction is the practice of plowing with tractor wheels in the open furrow.
Soil that is well aggregated has a range of pore sizes. Each pore size plays a role in soil’s physical functioning. Large pores drain rapidly and are needed for good air exchange during wet periods, preventing oxygen deficiency that can drown plants and increase pest problems. Denitrification by conversion of nitrogen to gaseous forms is increased in oxygen-deficient wet soil. In degraded soil large pores are compressed into small ones.
Small pores are critical for water retention and help a crop endure dry periods with minimal yield loss.
Soil tilth can be obtained through mechanical and biological manipulation.
Mechanical soil cultivation practices, including primary tillage (mold-board or chisel plowing) followed by secondary tillage (disking, harrowing, etc.), break up and aerate soil. When soils become degraded and compacted, such tillage practices are often deemed necessary. The tilth created by tillage, however, tends to be unstable, because the aggregation is obtained through the physical manipulation of the soil, which is short lived, especially after years of intensive tillage. Aggregates in such soils readily dissolve during subsequent rains, causing the soil to settle and become dense and hard, requiring further tillage.(mechanical manipulation of soil)
The preferred scenario for good tilth is as the result of natural soil-building processes, provided by the activity of plant roots, microorganisms, earthworms and other beneficial organisms. Such stable aggregates break apart during tillage/planting and readily provide good tilth. Stable aggregates are held together by organic bonds that resist breakdown during soil saturation. These organic materials are themselves subject to biological degradation, requiring active amendments with organic material, and minimal mechanical tillage.
Crop rotation can help restore tilth in compacted soils. Two processes contribute to this gain. First, accelerated organic matter decomposition from tillage ends under the sod crop. Another way to achieve this is via no-till farming. Second, grass and legume sods develop extensive root systems that continually grow and die off. The dead roots supply a source of active organic matter, which feeds soil organisms that create aggregation. Beneficial organisms need continual supplies of organic matter to sustain themselves and they deposit the digested materials on soil aggregates and thereby stabilize them. Also, the living roots and symbiotic microorganisms (for example, mycorrhizal fungi) can exude organic materials that nourish soil organisms and help with aggregation. Grass and legume sod crops therefore return more organic matter to the soil than most other crops.
Some annual rotation crops such as buckwheat also have dense, fibrous, root systems and can improve tilth. Crop mixtures with different rooting systems can be beneficial. For example, red clover seeded into winter wheat provide additional roots and a more protein-rich organic matter.
Other rotation crops are more valuable for improving subsoils. Perennial crops such as alfalfa have strong, deep, penetrating tap roots that can push through hard layers, especially during wet periods when the soil is soft. These deep roots establish pathways for water and future plant roots, and produce organic matter.
Crops rotation can extend the period of active growth compared to conventional row crops, leaving more organic material behind. For example, in a corn-soybean rotation, active growth occurs 32 percent of the time, while a dry bean–winter wheat–corn rotation is active 72 percent. Crops such as rye, wheat, oat, barley, pea and cool-season grasses grow actively in the late fall and early spring when other crops are inactive. They are beneficial both as rotation and cover crops, although intensive tillage can negate their effects.
The soil management practices required to maintain soil tilth are a function of the type of soil. Sandy and gravelly soils are naturally deficient in small pores and are therefore drought prone, whereas loams and clays can retain and thus supply crops with more water.
Sandy soil has lower capacity to hold water and nutrients. Water is applied more frequently in smaller amounts to avoid it leaching and carrying nutrients below the root zone. Routine application of organic matter increases sandy soil’s ability to hold water and nutrients by 10 times or more.
Clay soils lack large pores, restricting both water and air movement. During irrigation or rain events, the limited large pore space in fine-textured soils quickly fills with water, reducing soil oxygen levels. In addition to routine application of organic matter, microorganisms and earthworms perform a crucial assist to soil tilth. As microorganisms decompose the organic matter, soil particles bind together into larger aggregates, increasing large pore space. Clay soils are more subject to soil compaction, which reduces large pore spaces.
Such soils natively have little tilth, especially once they have been disturbed. Adding organic matter up to 25% by volume can help compensate. For example, if tilling to a depth of eight inches, add two inches of organic materials.
The role of conservation agriculture in organic matter deposition and carbon sequestration
Principles of conservation agriculture
Conservation agriculture makes use of soil biological activity and cropping systems to reduce the excessive disturbance of the soil and to maintain the crop residues on the soil surface in order to minimize damage to the environment and provide organic matter and nutrients. It is based on four principles:
Although the principles are not new (except for that of minimal disturbance to the soil), it is the fact that they are applied together in conservation agriculture that generates positive outcomes. All the practices (minimal tillage, soil cover and crop rotation) are combined for synergy and added value. In the past, farmers may have tried but abandoned the use of cover crops or zero tillage because of weed problems or yield declines. There is also a need for improved weed control and rotations for biocontrol of pests and diseases and nutrient uptake. Integration of the conservation agriculture principles provides a win-win situation for both people and the environment, which has catalyzed successful expansion of the area under conservation agriculture worldwide.
Conservation agriculture aims to:
Organic matter deposition
The reduction of soil disturbance through zero-tillage, the use of cover crops and the preservation of crop residues on the soil surface result in increased activity of the soil and in the accumulation of organic matter, mainly in the topsoil.
An argument often heard in the discussion on conservation agriculture is that it is only feasible in the humid and subhumid tropics and that the generation of sufficient biomass in semi-arid regions is the limiting factor to start implementing conservation agriculture.
However, recent research has shown that even in semi-arid areas of Morocco the application of the principles of conservation agriculture bears its fruits. Mrabet (2000) reports higher yields through better water use and improved soil quality; the latter caused by an increase in soil organic C and N and a slight pH decline in the seedzone (Bessam and Mrabet, 2003; Mrabet et al., 2001a, 2001b).
Increased carbon sequestration
World soils are important reservoirs of active C and play a major role in the global carbon cycle. As such, soil can be either a source or sink for atmospheric CO2 depending on land use and the management of soil and vegetation (Lal, 2005) (Figure 23). The conversion of native ecosystems (e.g. forests, grasslands and wetlands) to agricultural uses, and the continuous harvesting of plant materials, has led to significant losses of plant biomass and C (Davidson and Ackerman, 1993), thereby increasing the CO2 level in the atmosphere.
In particular, the practice of burning agricultural fields before cultivation has a disastrous effect on soil organic carbon content. Figure 24 shows the reduction in soil organic carbon in agricultural fields after 100 years of burning crop residues and weeds compared with an area that was not burned or ploughed during the same period. The topsoil layer (0-5 cm) represented the greatest carbon loss (36 percent) compared with the area that was not burned. Soil N stock in the same layer was reduced by 16 percent. The carbon stock was reduced not only through burning, but because of the whole land-use management, especially a drastic reduction in diversity of species as monocropping was practised (Amado et al., 2005).
Table 7 lists general practices that determine whether soil will be a sink or a source of atmospheric CO2.
As shown in Table 7, soil can play a part in mitigating CO2 levels (Paustian, 2002). This removal process is achieved naturally, and quite effectively, through photosynthesis. Living plants take CO2 from the air in the presence of sunlight and water, convert it into seeds, leaves, stems and roots. Part of the CO2 is retained or “sequestered”, or stored as C in the soil when decomposed.
In particular, systems based on high crop-residue addition and no tillage tend to accumulate more C in the soil than is lost to the atmosphere. Carbon sequestration in managed soils occurs when there is a net removal of atmospheric CO2 because C inputs (crop residues, litter, etc.) exceed C outputs (harvested materials, soil respiration, C emissions from fuel and the manufacture of fertilizers, etc.) (Izaurralde and Cerri, 2002). Management practices that increase soil C comply with a number of principles of sustainable agriculture: reduced tillage, erosion control, diversified cropping system, balanced fertilization, etc.
In the early years of no-tillage systems, the organic matter content of the soil is increased through the decomposition of roots and the contribution of vegetative residues on the surface. This organic material decomposes slowly, and thus the liberation of C to the atmosphere also occurs slowly. In the total balance, net fixation or sequestration of C takes place; the soil is a net sink of C.
Land use and land management determining whether soil will be a sink or source of atmospheric CO2
|Soil as a source of CO2||Soil as a sink of CO2|
|Soil properties: coarse textured soil, excessive drainage, high susceptibility to erosion||Soil properties: clayey soil, poorly drained ecosystems, depositional sites, including footslopes|
|Land use: seasonal crops, simple ecosystem, shallow roots and low root-shoot ratio||Land use: perrenial crops, diverse ecosystem, deep roots and high root-shoot ratio|
|Soil management: intensive tillage based on plough, negative nutrient balance, residue removal and/or burning, continuous cropping, loss of soil and water by runoff and erosion||Soil management: no tillage, positive nutrient balance, mulch farming cover crops in rotation, cycle, soil and water conservation|
Source: adapted from Lal, 2005.