4. Examples of the use of agricultural wastes and the effects on some physical properties

The physical properties of soils condition their quality and, in particular, the porosity which affects different processes related to the transformations of organic matter, gas exchange, the growth of plant roots, and movement of water in the soil, as before it was indicated.

Soil porosity is the property that, due to the effect of compaction, is being altered largely in the European Union (and developing countries), together with the loss of organic matter from soils [77], and, for this reason, our management of the soils should allow maintaining this property at adequate levels.

The use of plant residues as soil amendments is a sustainable alternative to improve the physical properties [28], although we must take into account the characteristics of the waste to ensure its efficiency. Once incorporated into the soil, the waste can be mineralized more or less rapidly, depending on characteristics such as its degree of lignification, its C/N ratio, and environmental conditions [78]. Fresh vegetable residues, such as tomato (C/N = 12) and onion (C/N = 15) residues [79], with high water content, decompose quickly [80] modifying the composition of soil organic matter [9]. However, there are residues with high C/N ratios, such as wheat or rice wastes (C/N = 105), more lignified, which degrade more slowly [81], lasting for more time the modifications they produce on certain physical properties of the soil.

In this second type of waste, we can consider the cereal straw and the palm tree leaves (Figure 1). Both, with high lignin composition and after a conditioning process (drying and crushing), can be used to modify the physical properties of the soil such as bulk density, porosity, and hydraulic conductivity.

These agricultural wastes have a similar total organic matter (determined by loss on ignition) content but a different density, bulk, and particle density (Table 1).

#### Figure 1. Palm tree leaves.

(Spinacia oleracea) [70]. Application of poppy waste at 200 m3

Wheat stalk, cotton stalk, millet stalk, and soybean stalk were used as the main material, and oven-dried lentil straw was used as an additive material in 100:10, 100:15, and 100:20 w:w ratios for 100 g of main material (70% moisture content) to cultivate Pleurotus ostreatus and try

Agricultural wastes can be composted before their application to soil. The composting process, with other residues or alone, facilitates the transformation into a stable organic matter, more complex and more resistant to the biodegradation. However, the control of the process should be undertaken in order to obtain a mature compost [72]. Green tea waste and rice bran were composted, while various parameters such as compost pile temperature, pH, electrical conductivity, nitrate content, and carbon to nitrogen ratio were measured regularly. There was no further change in the state of the compost pile after 90 days indicating that it could be used for agricultural applications [73]. The possible bioconversion of wet olive cake by low-cost biostabilization (vermicomposting process) has been evaluated [74]. Wet olive cake fresh (WOC), pre-composted (WOCP), or mixed with biosolids (WOCB) were vermicomposted for

6 months to obtain organic amendments for agricultural and remediation purposes.

The application of composted organic amendments derived from different crop residues, generally, has a positive impact on the physical, chemical, and biological properties of soils [75].

Crop residues are composed of lignin, cellulose, hemicellulose, micro-, and macronutrients. The degradation of these residues varies depending not only on their lignin and cellulose content and their C/N ratio, which is crop dependent, but also on the environment and soil conditions. Residues with a high C/N level (e.g., wheat straw) decompose slowly, sometimes resulting in the immobilization of soil N. This can be positive in no-tillage systems, creating a mulch that protects the soil from erosion and evaporation, but it also means that there are fewer nutrients available for the next crop. Residues with a low C/N level mineralize quickly, releasing more N and nutrients for the next crop. Only specialized fungi and some microorganisms can degrade lignin. Residues with high lignin content will take longer to decompose

4. Examples of the use of agricultural wastes and the effects on some

growth of plant roots, and movement of water in the soil, as before it was indicated.

The physical properties of soils condition their quality and, in particular, the porosity which affects different processes related to the transformations of organic matter, gas exchange, the

content, soil pH, and soil salinity.

16 Agricultural Waste and Residues

to improve the total harvest amount [71].

than those with low lignin content [29, 76].

physical properties

3.1. Composted agricultural wastes

/ha increased soil organic carbon


Table 1. Density and total organic matter in the wastes.

Laboratory experiments were performed on cylinders similar to those used for the determination of densities of organic materials, according to UNE-EN13040:2008 and the methods of soil analysis of SSSA-ASA [82–84]. These experiments showed that the agricultural residues applied (hay straw and palm tree leaves, air dry and cut with a size of approximately 4 cm in length) modified the density of soils and improved their porosity.

the agricultural waste, it will be more or less efficient. In this sense, straw residues reduce the

Physical Properties of Soils Affected by the Use of Agricultural Waste

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Bulk density decreases in the soils, which means that the porosity, spaces that can be filled with air and water, increases. This is observed in Figure 4, where the changes in the porosity of the two soils were showed. Porosity increased when the amount of agricultural wastes applied

Obviously, the types of waste that improve the porosity of soils also favor the movement of

One of the parameters that gives information on the movement of water in soils is the saturated hydraulic conductivity (Khs), based on Darcy's law, and calculated by using a constanthead permeameter. The texture of the soils determines the quantity and size of the pore, and, therefore, we should expect that more clay soils have lower Khs values than those with a sandy

was greater. Hay straw residue increased the porosity more than palm tree residue.

Figure 5. Saturated hydraulic conductivity (in cm/h) in soils amended with hay straw and palm tree leaves.

water. This fact is very important because it allows a better root growth.

bulk density more than that of palm tree leaves.

Figure 4. Porosity (%) in soils amended with vegetable wastes.

texture.

Figures 2 and 3 show the changes of the particle (PD) and bulk (BD) densities in two soils (soil 1: sandy clay loam; soil 2: clay loam), when these wastes were added in a proportion (waste/ dry soil): 0, 3, and 6% (w/w).

The agricultural residues reduced the densities of the two soils, depending on the dose applied. The apparent densities were clearly affected, which indicates that the addition of the amendments favors that the soils were less compacted. Depending on the physical characteristics of

Figure 2. Evolution of particle density (PD) and bulk density (BD) (in kg/m<sup>3</sup> ) in soils amended with hay straw.

Figure 3. Particle density (PD) and bulk density (BD) (in kg/m<sup>3</sup> ) in soils amended with palm tree leaves.

the agricultural waste, it will be more or less efficient. In this sense, straw residues reduce the bulk density more than that of palm tree leaves.

Bulk density decreases in the soils, which means that the porosity, spaces that can be filled with air and water, increases. This is observed in Figure 4, where the changes in the porosity of the two soils were showed. Porosity increased when the amount of agricultural wastes applied was greater. Hay straw residue increased the porosity more than palm tree residue.

Obviously, the types of waste that improve the porosity of soils also favor the movement of water. This fact is very important because it allows a better root growth.

One of the parameters that gives information on the movement of water in soils is the saturated hydraulic conductivity (Khs), based on Darcy's law, and calculated by using a constanthead permeameter. The texture of the soils determines the quantity and size of the pore, and, therefore, we should expect that more clay soils have lower Khs values than those with a sandy texture.

Figure 4. Porosity (%) in soils amended with vegetable wastes.

Laboratory experiments were performed on cylinders similar to those used for the determination of densities of organic materials, according to UNE-EN13040:2008 and the methods of soil analysis of SSSA-ASA [82–84]. These experiments showed that the agricultural residues applied (hay straw and palm tree leaves, air dry and cut with a size of approximately 4 cm in

Figures 2 and 3 show the changes of the particle (PD) and bulk (BD) densities in two soils (soil 1: sandy clay loam; soil 2: clay loam), when these wastes were added in a proportion (waste/

The agricultural residues reduced the densities of the two soils, depending on the dose applied. The apparent densities were clearly affected, which indicates that the addition of the amendments favors that the soils were less compacted. Depending on the physical characteristics of

) in soils amended with hay straw.

) in soils amended with palm tree leaves.

length) modified the density of soils and improved their porosity.

Figure 2. Evolution of particle density (PD) and bulk density (BD) (in kg/m<sup>3</sup>

Figure 3. Particle density (PD) and bulk density (BD) (in kg/m<sup>3</sup>

dry soil): 0, 3, and 6% (w/w).

18 Agricultural Waste and Residues

Figure 5. Saturated hydraulic conductivity (in cm/h) in soils amended with hay straw and palm tree leaves.

Figure 5 shows how the addition of agricultural wastes affected the saturated hydraulic conductivity of soils. It is observed that, without the addition of residues, the clay loam soil (soil 2) has a lower value of Khs than the sandy clay loam soil (soil 1). The positive effect of the incorporation of the amendments on the hydraulic conductivity of the two soils used was clear. Hay straw produced a greater increment than palm tree residues in both soils.

Regions. Closing the loop - An EU action plan for the Circular Economy. COM 614 final. 2015. 21 p. Available from: http://eur-lex.europa.eu/resource.html?uri=cellar:8a8ef5e8-99a0-

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This example of addition of vegetable wastes to the soil demonstrates the positive influence on some physical properties, and the importance of recycling of agricultural wastes in origin can help the strategy of zero waste of the European Union and, moreover, improve the quality of our soils.
