EFFECTS OF INTENSIVE VEGETABLE PRODUCTION ON CHEMICAL PROPERTIES AND NUTRIENT MINERALIZATION OF A BRITISH COLUMBIA HUMISOL

Abstract

A study was conducted to determine the effects of long-term vegetable production on some organic matter (OM) chemical properties in an organic soil located near Cloverdale, British Columbia. Producers in this area are concerned about yield reduction and some feel this is due to changes in the soil quality. Paired soil samples were collected from two transects: one in a 40-yr-old vegetable field, and the other in an adjacent fence row. The transects were positioned in order to collect samples from a relatively deep organic deposit and from a shallow deposit where a ridge of mineral subsoil had been exposed. Initial analysis showed that cultivation had changed some chemical properties only in the plow layer. The exception was pH which was higher throughout the sample depth of the cultivated transect. Organic matter loss from the cultivated field was estimated 28% by weight, based on ash accumulation. In the cultivated deep organic section (west of the ridge) OM and C contents were reduced by 7.1 and 5.6% respectively, whereas on the cultivated ridge OM, C, N and S contents and the amount of C, N and S mineralized were reduced by 43.3, 41.6, 43.4, 25.9, 42.8, 60.8 and 52.5%, respectively. On an ash-free basis there were no changes in the deep organic section. In the ridge, the OM sulphur content had increased and the amount of N and S mineralized were decreased compared with the uncultivated transect. In the cultivated surface layer, increased S content in the OM was caused by incorporation of a S-rich subsurface layer during tillage operations. Increases in P content in the cultivated transect were attributed to P fertilization. Within the cultivated transect, C, N and S contents in the exposed mineral ridge were 48, 48, and 44%, respectively, lower than in the deep organic surface layer on the west side of the ridge. The C:N ratios were unchanged at about 16. The chemical stability of the OM was attributed to its well-decomposed nature. At this particular site, the degradation due to excessive cultivation was manifested mainly in OM loss, rather than in changes in OM quality. The main concern of producers is to reduce or prevent further loss of soil OM. Key words: Humisol, organic soil, cultivation, mineralizable C, N and S, incubation