2. Importance of the physical properties of the soil

The physical properties of the soil are very important for agricultural production and the sustainable use of soil. The amount and rate of water, oxygen, and nutrient absorption by plants depend on the ability of the roots to absorb the soil solution as well as the ability of the soil to supply it to the roots. Some soil properties, such as low hydraulic conductivity, can limit the free supply of water and oxygen to the roots and affect negatively to the agricultural yield.

#### 2.1. Soil structure

In the EU plan action for the circular economy, we can find targeted actions for various types of waste. Agricultural wastes can be reflected in two aspects of this plan: recycling of nutrients

Recycled nutrients are a distinct and important category of secondary raw materials, for which the development of quality standards is necessary. They are present in organic waste and can be returned to soils as fertilizers. Their sustainable use in agriculture reduces the need for mineral-based fertilizers, the production of which has negative environmental impacts, and

Bio-based materials, e.g., those based on biological resources (such as wood, crops, or fibers), can be used for a wide range of products (construction, furniture, paper, food, textile, chemicals, etc.) and energy uses (e.g., biofuels). The bioeconomy hence provides alternatives to fossil-based products and energy and can contribute to the circular economy. Bio-based materials can also present advantages linked to their renewability, biodegradability, or compostability. On the other hand, using biological resources requires attention to their life cycle environmental impacts and sustainable sourcing. The multiple possibilities for their use can also generate competition for

Agriculture is one of the major activities that produces wastes and consumes space, the agricultural soils. It is important to find a synergy between this activity and the soil. In this sense and following the considerations of the EU, crop residues are an important source of plant nutrients and organic matter [2]. Reuse of organic materials is desirable in order to reduce waste streams and to take advantage of the soil benefits associated with added organic

Nowadays, it is well known that the application to the soil of organic amendments derived from urban, agricultural, industrial, or municipal activity has several agronomic and environmental effects [4]. This addition can be a good strategy to maintain or even increase the levels of organic carbon in the soil [5]; to improve physical properties such as stability of aggregates and soil porosity [6–8]; to incorporate nutrients such as N, P, and K, thus avoiding the high fossil energy costs and therefore the impact on global warming due to the production and the use of synthetic fertilizers [9]; and to help cushion climate change through the sequestration of

Considering the physical properties and the soil organic carbon (SOC), organic matter amendments can increase water holding capacity, soil porosity, water infiltration, and percolation while decreasing soil crusting and bulk density [11–13]. One of the main measurable effects of the repeated application in the soil of organic wastes is the increase of soil porosity and, therefore, the decrease in the bulk density of the soil [8, 14]. It is also expected to be beneficial for the work of tilling the soil, thus reducing the draft force and, consequently, a possible decrease in tractor fuel [15]. The energy saved due to the lower resistance that the soil offers when being worked if we apply waste is being ignored from the waste treatments that imply the application to the soil of this in the environmental evaluations. However, reducing green-

depends on imports, e.g., phosphate rock, a limited resource [1].

them and create pressure on land use [1].

matter and associated plant nutrients [3].

house gas emissions can be important [15].

atmospheric CO2 by the organic compounds of the soil [10].

and biomaterials.

10 Agricultural Waste and Residues

Soil structure is one of the most important soil's physical factors controlling or modulating the flow and retention of water, solutes, gases, and biota in agricultural and natural ecosystems [17, 18]. Soil structure is very important in soil productivity and is a limiting factor of crop yield [19, 20]. Soil structure controls many processes in soils. It regulates water retention and infiltration, gaseous exchanges, soil organic matter (SOM) and nutrient dynamics, root penetration, and susceptibility to erosion [21]. For these reasons, soil structure stands out among the physical properties of the soil, since it exerts an important influence on the edaphic conditions and the environment.

The term "structure" of a granular medium refers to the spatial arrangement of solid particles (texture) and void spaces. Most soils tend to exhibit a hierarchical structure. That is, primary mineral particles, usually in association with organic materials, form small clusters or "firstorder aggregates." These form larger clusters or "second-order aggregates" [22]. Aggregate hierarchy in soils is reflected in increasing aggregate size with each successive level. However, the term "structure" in soil cience generally carries a connotation of bonding mechanisms in addition to geometrical configuration of particles [22]. Organic matter acts as a cement that can help the formation of aggregates and, therefore, the soil structure.

Without hierarchical structure, medium- and fine-textured soils such as loams and clays would be nearly impermeable to fluids and gases [22]. Moreover, the soil organic carbon has a greater effect on aggregation especially in coarse-textured soils [23]. Thus, structure plays a crucial role in the transport of water, gases, and solutes in the environment and in transforming soil into a suitable growth medium for plants and other biological organisms [22].

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

these indicators are probably negatively correlated with root growth and rooting depth [29]. Even more, these properties are closely related to water movement, water availability for

Physical Properties of Soils Affected by the Use of Agricultural Waste

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

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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

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

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

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

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

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

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

livestock, agricultural machinery, weather and season of the year, etc. [39, 40].

from the loss of larger pores and the increase of finer pores [36].

constricted, and usually highly connected among themselves [37].

substances and forming aggregates and enhancing the soil structure.

plants, and soil gas exchange.

2.4. Porosity

soil.

2.5. Bulk density

the particle density value [21].
