THE EFFECT OF CULTIVATION ON THE STRUCTURE AND OTHER PHYSICAL CHARACTERISTICS OF GRASSLAND AND ARABLE SOILS (194–970)

Abstract

SummaryCultivating grassland changes the physical state of the soil. The paper describes observations and measurements made in the field and the laboratory on a wide range of soils (with series names) in England over a period of 25 years. Dry sieving showed that natural weathering of grassland aggregates after three months'exposure was very considerable. The drawbar‐pull on ploughing soil recently out of grass, as compared with old arable, increased with the number of years out of grass.The number of earthworms per hectare on old grassland was 6 to 9 times that on old arable of the same soil series: after three years'cultivation it was reduced by about a half.The possibility of erosion by wind increased with the number of years out of grass. Old arable land on gentle slopes had its resistance to flowing water (sheet erosion) much increased by as little as 2 years under grass. The apparent densities of soils were at a minimum under old grass and increased under cultivation. On some soils arable for many decades, the apparent densities were near those considered limiting for root penetration. The volurne of water per cent in the soil at the sticky point decreased as the years out of grass increased, suggesting a decrease in the number of days the soil could be cultivated without smearing.The water‐stability of soil aggregates decreased as the number of years after ploughing old grassland increased. Whatever the texture, on ploughing old grassland, the water stability of air‐dry aggregates feil sharply in the first – years and then approached the value corresponding to that of old arable soils more slowly, apart from soils of low clay content in which it differed little from old arable land after 2 years. Measurement of changes in water stability and field observations in a 6‐course rotation (3 years ley, 3 years cereal) on a loam which had been in a similar rotation for about a decade after old grassland, suggested that this balance was probably right for maintaining such a soil at a good cropping level. Puddling (poaching) by animals of wet arable loam soils overlying clay resulted in gleying to the surface: ferrous iron was detected chemically. On putting down to grass there was little increase in water stability of the aggregates after 4 years.The reduction in the total pore space caused by harvesting machinery and also as a result of some years'cultivation of grassland was considerable. The ease of penetration of soils when taking cores was compared by counting the number of blows to drive in a corer: the number was much less in a soil recently out of old grass compared with old arable.The root development of cereals was poor in old arable silt soils very unstable to wetting and drying and with few visible biopores (> 10–00 μm). On soils with many visible pores, root development was much greater and yields with similar rates of nitrogen were double.The total N in the soils feil when old grassland was ploughed, sometimes by as much as 75 per cent in about 20 years. There was sometimes a significant corre‐lation between total N and the reduction in water‐stable aggregation during the years following ploughing grassland: the correlation coefficient was much higher on ploughing old grassland rather than leys. The N level dropped much more quickly in the first few years after ploughing old grassland than later.All single‐property measurements give only a limited assessment of the physical state of a soil. Soils can be assessed satisfactorily only by making a range of measurements appropriate to the farming System.