Autumn and spring tillage for spring cereals

367 in wheat yield and especially in soybean yield. Wheat yields have averaged 3.0 Mg ha i" Over 5 years, mean soybean yields here ranged from 1.2 to 3.2 Mg ha- . In no year have either wheat or soybean yield been significantly affected by tillage treatments. Soybean root distribution and morphology have been only slightly influenced by tllage. As with yield, there has been greater variability of root growth from year to year than between tillage treatments within a given year. Significant differences among soil physical properties (soil moisture, bulk density, penetrometer resistance) and chemical properties attributable to tillage practices, have been measured both before and after primary tillage. That differences in physical properties have not translated into significant effects on crop growth and yield suggest that compensation occurs and testifies to the complexity of the mechanisms at work.

AUTUMN AND SPRING TILLAGE FOR SPRING CEREALS E. EKEBERG Kise Agricultural Research Station, 2350 Nes near Hedmark (Norway) Effects of the number of harrowings in autumn and spring on ploughed and unploughed loam soll were investigated in 5 trials with in total 21 harvests. The rate of soil drying in spring was similar on autumn-ploughed, autumn-ploughed and harrowed, and unploughed autumn-harrowed land. Unploughed, unharrowed soll was 2-3 days later in drying. Autumn harrowing of ploughed land reduced grain yields by 4% (w/w). In economic terms the reduction equalled 8% of the crop value. Spring-harrowing had no effect on yields of autumn-tilled land. On previously untilled land, spring harrowing was necessary to avoid yield reductions asociated with couch grass infestation. Autumn-harrowlng of ploughed soll led to earlier ripening, as did increased harrowing intensity in spring, both on ploughed and unploughed soil. Sowing depths were reduced by autumn-harrowing of ploughed soil, but increased with harrowing intensity in spring. After 5 years without ploughing, soil reaction had declined and contents of extractable P, K and Mg had increased in the upper soil horlzion. Autumn-harrowing of ploughed soil led to a slightly higher bulk density, and slight reductions in air capacity and air permeability. Springharrowing on such land caused considerably greater changes in these parameters. Spring-harrowlng of autumn-ploughed and harrowed land gave a reduction in soil organic matter content.

WORKABILITY OF SOME KENYAN SOILS H. ELLEN State Agricultural College of Groningen, Prof. H.C. van Hallstraat I, 9726 AG Groningen (The Netherlands) Air permeability tests were carried out under laboratory conditions for 4 types of Kenyan soils, at moisture contents which were set at levels around the Lower Plastic Limit (LPL), and after compressing the soil with loads ranging from 0.I to 0.5 MPa. The Wet Workability Limit (WWL) was assessed and it proved to be correlated with the air-filled porosity. The WWI was slightly lower than the LPL and the moisture content at -I0 kPa matric potential of the soils tested. The relationship between the air permeability and the air-filled porosity can be used to predict the WWL by assessing the