Jules Lavalle published an influential book, Histoire et Statistique de la Vigne de Grands Vins de la Côte-d'Or, which included an unofficial classification of the Burgundy vineyards in five classes and which built on Morelot's book. In decreasing order, Lavalle's five classes were hors ligne, tête de cuvée, 1ère cuvée, 2me cuvée and 3me cuvée. Lavalle's classification was formalized in modified form by the Beaune Committee of Agriculture in 1861, and then consisted of three classes. Most of the "first class" vineyards of the 1861 classification were made into Grand Cru appellations d'origine contrôlées when the national AOC legislation was implemented in 1936. Burgundy wine has experienced much change over the past 75 years. Economic depression during the 1930s was followed by the devastation caused by World War II. After the War, the vignerons returned home to their unkempt vineyards. The soils and vines had suffered and were sorely in need of nurturing. The growers began to fertilize, bringing their vineyards back to health. Those who could afford it added potassium, a mineral fertilizer that contributes to vigorous growth. By the mid-1950s, the soils were balanced, yields were reasonably low and the vineyards produced some of the most stunning wines in the 20th century. For the next 30 years, they followed the advice of renowned viticultural experts, who advised them to keep spraying their vineyards with chemical fertilizers, including potassium. While a certain amount of potassium is natural in the soil and beneficial for healthy growth, too much is harmful because it leads to low acidity levels, which adversely affect the quality of the wine. As the concentration of chemicals in the soil increased, so did the yields. In the past 30 years, yields have risen by two-thirds in the appellations contrôlées vineyards of the Côte d'Or, from 29 hectoliters per hectare (hl/ha) (yearly average from 1951 to 1960) to almost 48 hl/ha (1982–91), according to a study by the Institut National des Appellations d'Origine. With higher yields came wines of less flavor and concentration. Within 30 years, the soils had been significantly depleted of their natural nutrients. The period between 1985 and 1995 was a turning point in Burgundy. During this time, many Burgundian renewed efforts in the vineyards and gradually set a new course in winemaking, producing deeper, more complex wines. Today, the Burgundy wine industry is reaping the rewards of those efforts. Wine characteristics and classification Burgundy