Estudio químico de los pastos leoneses (fertilidad del suelo y composición mineral de la hierba)

Abstract

1.—The natural chemical fertility of pasture soils in León is very diverse owing to marked lithological differences which cause the formation of types of soils having different rate of evolution and of composition, the main of which are: wet brown soils, Rendsinas, brown open plains and gleyed soils.

2.—The organic fraction is the factor which gives a definite characteristic to the soils we have studied either because of the high levels reached (the most frequent are between 8 % and 12 % ) or because of the rate of evolution (C/N between 9 and 15) ; that is to say, the predominating soils are those containinJ a great quantity of active humus of type “mull”.
3.—The assay on total nitrogen is relatively high (between 0.3% and 1 % ; 0.5 % and 0.6% being the most frequent values). However, some deficiencies may occur as nitrates in Spring when the climatic evolution is quick.
4.—The assay in assimilable phosphorus determined by six methods of extraction having different characteristics is relatively low. The ratio between the various methods used is variable and the coefficients of correlation between them have been determined.
5.—The study on assimilable potassium accomplished by five different methods of extraction is of médium type. The correlations between the various methods are generally higher than those with phosphorus.
6.—In the b›otanic classification of 40 samples we have found 22 varieties of gramineous plants, 8 varieties of leguminous plants and 24 other plants. However, after having studied their assay we may deduce that are six species of gramineous plants: Bromus mollis, Trisetum flavescens, Cynosurus cristatus, Poa feratiana, Lolium perenne, Holcus lanatus, and one leguminous plant: Trifolium repens.

7.—Under the point of view of mineral nutrition of plants we have found that the ratio N-P K regulates the yields and an optimum area or zone for this index which corresponds to 30-35 % of N, 5-15 % of P₂O₅, and 50-65 % of K₂O is obtained. The ratio N-K occurs as the most influencing in the production which decreases on increasing the relative deficiency of K.
8.—The factors which influence on the ratio N-P K in the plants are the pH, the N and the K contained in the soil, extracted with ammonium acetate and nitrate for a pH lower than 6.7.
9.—The ratio phosphorus soil/phosphorus gives coefficients of correlation which vary according to the method of extraction used, type of soils, and to the climatology. For this reason one cannot obtain concluding results. The same thing occurs with potassium.