**3.2. Methods**

of sediment samples and infiltrated water was registered. Double aluminium potassium sulphate (20 ml at 10%) was added to the sediment samples to precipitate suspended solids and separate the water by decantation. The solids in the bottle were dried for 24 h at 105°C and the soil loss was measured by gravimetry, expressed in grams. Once the rainfall was stopped, the soil boxes were left for 24 h to allow the drainage ceased from them, the soil was randomly sampled, allow to dry slowly until it had a moisture equivalent to 40% WHC. Before the rainfall system started, the same soils with the same treatments were sampled and were

The decanted water was transferred to a plastic bottle and weighed. The volume was expressed

( ) *<sup>V</sup>* <sup>10</sup> *Runoff mm <sup>A</sup>*

( ) *<sup>V</sup>* <sup>10</sup> *Infiltration mm <sup>A</sup>*

The results of all these measurements were the sum of the 12 samples per treatment collected

Applications of tannery sludge are restricted due to high Cr content, even when Cr(III) content in the sludge is considered to be unavailable. Nevertheless, the use of tannery sludge for reforesting the north of Guanajuato (a natural reserve in Dolores Hidalgo, Guanajuato) is appealing because, it can avoid soil degradation due to the increasing erosion in the region, and at the same time, it can reduce contamination by tannery sludge dumping to open air in

The influence of Cr on nitrogen transformation in soils has been studied and reports show somewhat mixed results. James and Bartlett [53] found no inhibition in treatments containing Cr(III) in sewage sludge or tannery effluent but that nitrification was inhibited by Cr(VI) at a concentration of 10 μg g−1 in soil suspensions. Chang and Broadbent [54] observed that nitrogen

´ <sup>=</sup>

The infiltration was calculated with a similar equation and expressed in mm [50–52].

(considering the water density of 1 g cm−3). The runoff was calculated by the equation:

´ <sup>=</sup>

), *A* is the area exposed to rainfall (1500 cm2

), and10 is the

adjusted to 40% WHC before the soil microbial activities determinations.

in cm3

in one run.

**3. Case study**

**3.1. Introduction**

Leon, Guanajuato.

where *V* is the volume of runoff (cm3

250 Organic Fertilizers - From Basic Concepts to Applied Outcomes

*3.1.1. Semi-arid soils and tannery sludge*

factor to convert cm to mm.
