This erosion box is a teaching aid, which demonstrates, in miniature, the erosive effect of water on different soil contours. - - Soil Conservation in the Andes. Terraced agriculture was practiced on the slopes of the Andes by the Incas, but with the passage of time these terraces fell into disrepair as ploughing was introduced. Farmers found it was easier to plough up and down a slope rather than following the lateral contours of the hillside, but in doing so they encourage erosion. The rains washed down precious farmland into the coastal valleys, flooding them. Now, the Peruvian Government, with the assistance of USAID, is teaching the Indios, the indigenous population, farming methods, which control erosion, produce better crops, and will restore forest and pastureland to the mountain slopes and steep places.