No abstract

A classification based on experimental results is proposed for the suitability of land in Britain for repeated direct drilling of cereals. Chalk and limestone soils and other well-drained loams are classified as equally suited for direct drilling of spring and winter crops; with good management most clay soils are suited for winter crops and may benefit from timeliness of sowing. But direct drilling in spring on some soils can lead to lower yields than those from conventional cultivations.

SUMMARYThe effects of overwinter cover cropping, delayed ploughing and method of straw disposal on the quantities of nitrate leached (averaged over three winters during 1989–93) from a chalk loam in Eastern England were examined. The recovery of ‘retained’ nitrogen (retained through cover crop uptake, delayed ploughing and immobilization by straw) in a following spring crop was also assessed. In the first two winters, the rye cover crop decreased nitrate leaching by > 90% (28 kg N/ha per year), as compared with bare fallow treatments. In 1992/93 this decrease was only 23% (10 kg/ha), due to the early onset of drainage before cover was well established. Delayed ploughing on bare treatments, to decrease autumn N mineralization and subsequent nitrate leaching, was ineffectual in 1989/90 but had substantial effects in 1990/91 and 1992/93; N mineralization, inferred from soil mineral nitrogen content, and nitrate leaching were decreased by 31 and 35% in 1990/91 and by 36 and 61% in 1992/93, respectively. Nitrate leaching (averaged over three winters) was unaffected by straw incorporation. There was no evidence of recovery of cover crop N in the spring sown test crops (barley or sugarbeet). In the low soil N input situation encountered in this experiment, it was unnecessary to sow cover crops before early September in years of average or below average rainfall to ensure that the average soil solution concentrations remained below the EU drinking water limit of 11 mg NO3-N/1. However, in wetter seasons substantial N leaching occurred before cover had taken up much N. In 1992/93 N retained against leaching by a rye cover crop in previous years was apparently being remobilized and lost through leaching, although if cover was grown again there was less leaching than from bare land. In the future, an increase in the extent of cover cropping might increase transpiration rates and therefore lead to a decrease in aquifer recharge.

The results presented are from field studies in which soil compaction was measured under tractors working with different wheel loads and levels of wheelslip. Wheelslip proved to be more important in causing compaction than additional wheel loading, and this effect was more pronounced for more powerful tractors. In most agricultural circumstances the aim is to secure the best compromise between weight and slip as alternative means of achieving greater work output from tractors. The results indicate that in situations where there is a need to avoid compaction, particular attention should be given to providing sufficient wheel loading and the possibility of moving faster to take up more power at lower slip.For the range of soil moistures encountered sinkage was the most convenient and satisfactory method of assessing total reduction in porosity, and differentiated between treatments better than any of the other methods (see below) apart from water-entry rates.Measurement of soil density at three levels to 15 cm depth accounted for less than 50 per cent of the loss of porosity estimated from sinkage and the method did not detect differences due to treatment.Shear strength measured at one depth (3.5 cm) differentiated between treatments, but penetration resistance at 7.5 cm did not. Water-entry rates sensitively detected treatment differences but the method was too time-consuming for comparison of all treatments.The efficiency of traction of four-and two-wheel-drive tractors was measured at intervals during the winter on a clay loam and the results related to surface moisture contents. After a steep reduction in efficiency, when the soil returned to field capacity, subsequent changes in efficiency were small.

Evidence relating to nitrate leaching was taken from series of extensive field experiments conducted to support guidance on fertiliser use. Over the last 50 years, it is estimated that increased fertiliser N use on intensive wheat in the UK, has resulted in an increase of 36 kg N ha-' year-' leachable nitrate. Probably more than one-third of this change is due to larger yields resulting in a gradual build up in soil organic matter, the remainder to annual effects of fertiliser application. This justifies the association generally made between fertiliser used and nitrate leached and supports the value of some control of fertiliser use in order to restrict nitrate concentrations in drinking water.

Abstract. Water erosion was recorded between mid‐October and mid‐December 1989 in 31 out of 73 erosion‐susceptible arable fields being monitored in England and Wales. Most fields were drilled to winter cereals. Tramlines and wheelings were the factors most commonly linked with initiation of erosion, particularly where runoff was concentrated on valley floors or headlands. Lack of crop cover (< 15%) was also an important factor at a number of sites; 25–30% ground cover was generally sufficient to protect the soils from erosion. Erosion was initiated by rainfall events of 15 mm or more in a 24 h period, with a maximum intensity greater than 4 mm/h. A large erosion event in south‐west England was associated with 33 mm of rainfall in 4.25 h, with a maximum intensity of 22 mm/h.

A comparison was made of the physical properties of pairs of silt soils differing only in organic matter content. Within the textural group studied, the member of the pair with more organic matter had better physical properties relating to both plant growth and soil management. Increased organic matter give higher water holding capacities and porosities, and decreased compaction, breaking strength and bulk densities. Organic matter content alone was not sufficient to explain differences in aggregate stability to water.

The recovery of nitrogen ‘retained’ through cover crop uptake, delayed ploughing and immobilization by straw was assessed in a spring cropping rotation on a chalk loam in Eastern England (1989–96). The effect of annual cover cropping on yield of the subsequent spring crops and on the soil N balance was also investigated. The recovery of retained N was in part dependent upon cover crop management. Late August-sown cover crops which were incorporated in February/March tended to reduce spring crop yields and crop N offtake. Adverse effects on soil N supply, seedbed conditions and soil water reserves were not in evidence and so an allelopathic effect from the decomposition of the rye cover crop, previously reported by others, may be responsible for the reduction in yield of spring crops. When the cover crops were drilled later and their early destruction was followed by a short fallow period, spring crop yields and N offtake were increased. The soil N balance indicated that over the course of the experiment there was a positive N input to the system due to continuous cover cropping. This input may be held as immobilized organic N, in which case it could be made available to subsequent crops over a number of years or lost via other routes. Nitrate concentrations in drainage water increased with the number of years under cover cropping.