**2. Materials and methods**

#### **2.1 Location and sampling**

The Department of Antioquia is located in the following coordinates—top right (8°52'25,748" N; 73°52'51,958" O), lower right (5°25'6,798" N; 73°52'51,958" O), top left (8°52'25,748" N; 77°7'40,239" O), lower left (5°25'6,798" N; 77°7'40,239" O).

Representative soil cartographic units were selected in areas dedicated to the cattle production system. The farms were sampled according to the access *Sustainability of Soil Chemical Properties and Nutrient Relationships in Dairy and Beef Cattle… DOI: http://dx.doi.org/10.5772/intechopen.104647*

roads, and the consent of the producers to sample their farms, amongst other criteria. A total of 440 soil samples were collected from farms dedicated to milk and meat production in different subregions of the Department of Antioquia. Each sample was taken at a depth of 20 cm, making a zigzag path and avoiding taking subsamples at non-representative sites. A square-shaped hole was made and the subsample was taken from the central part of the walls where the soil on the edges was eliminated. About 10–15 subsamples were obtained and placed in a clean container, which was then mixed and homogenised and approximately a one-kilogram sample was taken. Subsequently, they were packed in boxes to prevent the entry of light and taken to the laboratory to perform chemical analysis. The analyses were carried out by wet chemistry using methods certified by the Colombian Corporation for Agricultural Research (AGROSAVIA) in Bogotá, Colombia.

#### **2.2 Statistical model**

Several generalised additive models were built. The models included various effects, such as longitude (from −76.82 to −74.62 W), latitude (from 5.71 to 8.76 N), altitude (from 0 to 2900 m above sea mean level), geoform (wavy, flat, and slope geoform) and also the combinations of the parameters soil chemicals. The analyses were performed with the MGCV library [11] of the statistical software R-project [12]. The Bayesian Information Criterion was used to select the best models.
