2.4. Porosity

Aggregation is an indicator of soil structure and results from the rearrangement of particles, flocculation, and cementation [24–26]. Organic matter has been clearly identified as one of the key components of soil structural stability. However, in agricultural soils, it is progressively being depleted by intensive cultivation, without adequate yield of plant biomass. The loss of soil structure is increasingly seen as a form of soil degradation [27] and is related to the activities that are carried out in the soil and by the crop. Maintenance of optimum soil physical conditions is important for sustaining plant growth and other living organisms in soils. Poor soil structure results in poor water and aeration conditions that restrict root growth, thus limiting efficient utilization of nutrients and water by plants [28]. Soil structure also determines

Soils with high organic matter content tend to have larger, stronger, and more stable aggregates that resist compaction, whereas the opposite is true for soils with less organic matter. An improvement in soil aggregate stability has several consequences for an agroecosystem, including reduced risk of soil compaction and erosion [30]. The quality of soil structure greatly depends on the soil organic carbon (SOC) content [31], especially on the fraction of labile SOC (also called the "particulate organic matter" because of this fraction cycles relatively quickly in the soil). Labile organic matter also plays an important role in maintaining soil structure and

Aggregate stability is a keystone factor in questions of soil physical fertility and can be enhanced by means of an appropriate management of organic amendments, which can maintain an appropriate soil structure. This agronomic procedure could improve pore space suitable for gas exchange, water retention, root growth, and microbial activity [9]. Aggregate stability at the soil surface is affected mainly by exposure to rainfall (drop impact and runoff). A bare soil (e.g., a soil from which crop residues have been exported or incorporated into the soil by plowing) is in direct contact with raindrops, which facilitates a breakdown of soil aggregates, increasing soil erodibility. Aggregate degradation can lead to surface sealing and crust formation, which reduces the water infiltration rate and increases the risk of soil erosion and the loss of valuable topsoil [33]. High silt content, together with low organic matter content, results in soils that are more prone to aggregate breakdown and surface crusting [29, 34]. Organic matter applied on the

Soil compaction is a form of physical degradation in which soil biological activity and soil productivity for agricultural and forest cropping are reduced, resulting in environmental consequences. Compaction is a process of densification and distortion in which total and airfilled porosity and permeability are reduced, strength is increased, soil structure are partly destroyed, and many changes are induced in the soil fabric and in various characteristics [35]. Generally, four indicators quantify soil compaction: total porosity, pore size distribution, bulk density, and penetration resistance. Given that root growth is impeded by soil compaction,

topsoil protects to the erosion and favors the aggregation of mineral particles.

the depth that roots can penetrate into the soil [29].

2.2. Aggregate stability

12 Agricultural Waste and Residues

providing soil nutrients [32].

2.3. Soil compaction

Porosity is a main indicator of soil structural quality. Therefore, its characterization is essential for assessing the impact of adding organic matter to a soil system. Reduced porosity results from the loss of larger pores and the increase of finer pores [36].

A soil's porosity and pore size distribution characterize the pore space of the portion of the soil's volume that is not occupied by solid material. The basic character of the pore space governs critical aspects of almost everything that occurs in the soil: the movement of water, air, and other fluids; the transport and the reaction of chemicals; and the residence of roots and other biotas. By convention, the definition of pore space excludes fluid pockets that are totally enclosed within solid material. Thus, porous space is considered a single and a continuous space within the body of soil. In general, it has fluid pathways that are tortuous, variably constricted, and usually highly connected among themselves [37].

The relationship between the storage capacity and the movement of water in soils with porosity is evident and fundamental. However, not only the total number of pores defines the water behavior of the soil but also and in many cases predominantly the shape, size, and distribution of the pores. From the agronomic point of view, the size distribution not only affects the amount of water that can hold the soil but also regulates the energy with which it is retained, the movement toward the plant, toward the atmosphere, and toward other zones of soil.

The use of agricultural wastes as soil amendments facilitates the maintenance of the porosity in two forms: directly, if the agricultural wastes are ligneous matters with high resistance to biodegradation and, indirectly, after the transformation of the initial organic matter into humic substances and forming aggregates and enhancing the soil structure.

### 2.5. Bulk density

One of the most prominent indicators of soil structure is soil bulk density (dry bulk density (BD)), its determination does not require any specific expertise or expensive equipment, and it is based on sampling undisturbed soil. Bulk density (BD) is calculated as the ratio of the dry mass of solids to soil volume. The values of both bulk and particle density are necessary to calculate soil porosity [38]. Porosity can then be derived from BD, knowing or approximating the particle density value [21].

This physical property is dynamic and varies depending on the edaphic structural conditions. It can also be modified by soil biota, vegetation, and mechanical practices, trampling by livestock, agricultural machinery, weather and season of the year, etc. [39, 40].

Bulk density is an important indicator of soil quality, productivity, compaction, and porosity. BD is mainly considered to be useful to estimate soil compaction. Root length density, root diameter, and root mass were observed to decrease after an increase in BD [41]. However, the interpretation of BD with respect to soil functions depends on soil type, especially soil texture and soil organic matter (SOM) content [21].

3. The use of agricultural wastes in soils

and physical properties, especially in non-flooded soils [57].

tivity of soils; and increase microbial activity [26, 59, 60].

ifying the root system development [29].

Agricultural residues used as soil amendments or fertilizers may represent an excellent recycling strategy [54]. They are important to improve soil physical (e.g., structure, infiltration rate, plant available water capacity), chemical (e.g., nutrient cycling, cation exchange capacity, soil reaction), and biological (e.g., SOC sequestration, microbial biomass C, activity, and species diversity of soil biota) properties as organic soil conditioners [55–58]. Cultivating crops that produce substantial amounts of residues can increase SOC in the soil profile, depending on the tillage practices used [29]. Incorporated residue can beneficially influence soil chemical

Physical Properties of Soils Affected by the Use of Agricultural Waste

http://dx.doi.org/10.5772/intechopen.77993

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Organic residues can contribute to the development of soil structure with a binding agent in the formation of aggregates. The application of organic wastes to soils reduces bulk density; increases total pore space, mineralization, available nutrient elements, and electrical conduc-

Crop residue application offers several environmental and ecological benefits for the soilwater-plant system, including improved soil structural quality, which ensures optimum soil functions. Generally, the incorporation of crop residues increases soil porosity (especially the large pores) and reduces soil bulk density, regardless of tillage operations. Large pores are particularly favored because organic matter is much less dense than mineral particles. The application rate can affect the extent of compaction. The effect of crop residues in a given tillage practice also depends on soil type and depth. When they are mechanically incorporated, crop residues can reduce the bulk density at depth. Conservation tillage with the incorporation of crop residues increases SOC content near the soil surface, whereas in conventional tillage, soil C is distributed throughout the plowed area. Soils with higher organic matter content tend to have higher aggregate stability and therefore less risk of compaction and soil erosion [29]. With regard to soil hydraulic properties, the presence of crop residues on the soil surface tends to increase hydraulic conductivity at the surface, whereas tillage affects soil hydraulic properties both at the soil surface and below it because of the destabilization of soil aggregates [61]. The influence of residue management on crop production is complex and variable and results from direct and indirect effects and interactions. A direct effect is, for example, the presence of residues on the soil surface, which constitutes a direct obstacle to crop emergence. Indirect effects include residue mineralization, which leads to more nutrients available for the plants or the presence of organic matter from residues modifying the soil structure and therefore mod-

Incorporation of vegetable crop residues affects soil quality not only in terms of nutrient supply but also by influencing soil food web organisms and improving soil physicochemical properties, resulting in a better environment for crop growth and improved productivity [62–69]. The application of organic residues on carbon and nitrogen mineralization and biochemical properties in an agricultural soil led to a significant increase in soil microbial biomass size and activity [54]. Poppy waste, a suitable seed-free, inexpensive source of non-animal-based organic carbon, was used to evaluate its effect on soil organic carbon content and production of Bocane spinach
