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there were few differences in measured physical properties between tillage treatments. On the Duroc loam, water content, bulk density, hydraulic conductivity, and infiltration in the surface 75-mm of native sod differed significantly from those of soils cropped to wheat. Hydraulic conductivity was greater for no-till vs. plow and subtill between 75 and 150 mm depth and for sod vs. tillage treatment between 225 and 300 mm depth. Infiltration rates were greatest for sod and least for plow, with no-till and subtill treatment between. The decrease in infiltration rate between 15 min and 240 min was 60, 61, and 65% for plow, suhtill, and no-till, respectively, while rates for sod over the same time interval decreased by 30%. Generally, compared to plowed soil, no-tilled soil has greater water content, bulk density, water-filled pore space, root mass, and microbial biomass in the surface 75 mm, with similar trends extending into the 75-150-mm depth. These results indicate that soil physical responses to tillage are largely influenced by previous cropping and soil management practices.

ZERO-TILLAGE FOR EROSION CONTROL AND MAIZE PRODUCTION IN THE HUMID TROPICS

G.E. OSUJI

University of Ibadan, Department of Agricultural Engineering, Ibadan (Nigeria)

The effects of 4 different tillage systems, viz., zero-tillage, manual cultivation, plough only and conventional tillage (ploughing and harrowing), on soll erosion, nutrient losses, and maize yield were studied in the rain forest zone of Nigeria on a sandy loam soil on a 5-% slope. The 4 treatments and a bare control were replicated 4 times on 25 x 4-m runoff plots in a randomized complete block design. The average soll lo~s after 3 years (1982-1984) was 0.5, 13.8, 17.8, 19.6, and 37.9 t ha for zero-tillage, manual, plough, conventional and bare fallow, respectively. For these systems, runoff was 6.8, 59,8, 62.7, 67.8 and 98.9 mm, respectively. Nutrient losses in both runoff and eroded sediment were in the order: bare fallow > plough > conventional > manual > zero-tillage. In the first year, grain yield was significantly lower in the zero-tillage than in the plough and conventional systems. However, from the second year onwards, yields in the zero-tillage treatment exceeded the yields in the other treatments. Apart from the period immediately after seedbed preparation, zero-tillage resulted in better soil physical and chemical properties than other treatments in terms of bulk density, porosity, soil colloidal content, soil infiltrabllity and overall soil fertility.

PLANT-WATER RELATIONSHIPS OF SPRING BARLEY AS AFFECTED BY TILLAGE METHODS

R.B. SHARMA

Sokoine University of Agriculture, Department of Soll Science, P.O. Box 3008, Horogoro (Tanzania)

The influence of tillage methods on plant water status was measured in spring barley on a sandy loam soil. Three tillage methods were used: dlrect-drilllng, tine cultivation to 12.5 cm depth, and moldboard plowing to 20 cm depth. Soll bulk density was higher in the 0-5- and 5-10-cm layers of the dlrect-drilled plots. Soll strength was also higher in the dlrect-drilled plots to 22.8 cm depth compared with plowed plots, and to 15.2 cm depth

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compared with tine-cultivated plots. The only significant effect on soil water content (measured to 90 cm depth) was between 80 and i14 days from planting when water extraction from 0-I0 cm depth was greater in the direct-drilled plots compared with the plowed plots, and at 20 cm depth greater in plowed plots compared with the direct-drilled plots. There were no significant differences between tillage methods in leaf water potential, above ground dry matter production, and grain yield.

INFILTRATION RATE, MEASURED WITH DOUBLE-RING INFILTROMETERS AND A RAINFALL SIMULATOR, AS AFFECTED BY THE AMOUNT OF MULCH AND THE TILLAGE SYSTEM

N. SIDIRAS and C.H. ROTH

University of Hohenheim, Institute of Plant Production, P.O. Box 700562, 7000 Stuttgart 70 (Fed. Rep. of Germany)

Comparltive measurements of infiltration rate were carried out with double-ring infiltrometers and a rainfall simulator to determine the effect of different green manure mulches and conventional, chisel plough and no-tillage techniques. Of the mulches tested, both measuring methods showed the infiltration rate under black oats to be highest, and with both methods significant correlations between the amount of mulch and infiltration rate were obtained. Infiltration rates as measured with the infiltrometers were highest under conventional tillage and lowest under no-tillage, whereas the rainfall simulator produced results of inverse order, no-tillage having the highest infiltration rate in this case. Infiltration rates obtained with the infiltrometers were 5.4, 3.2 and 2.6 times higher than those obtained with the rainfall simulator for conventional, chisel plough and no-tillage systems, respectively.

EFFECT OF CULTIVATION PRACTICES ON INORGANIC AND ORGANIC SOIL PHOSPHORUS FRACTIONS

M.P. VAN DER MAAS and J.L. HAVLIN

University of Guelph, Ontario Agricultural College, Department of Land Resource Science, Guelph, Ont. NIG 2WI (Canada)

Moldboard plow, chisel plow and zero-tillage treatments were applied over 15 years of continuous corn production on a clay loam soil in southern Ontario. Although NaHCO3-extractable P contents were similar, P uptake in the no-till treatment was considerably greater than in the plow treatments. Research objectives were developed to relate differences between tillage to differences in organic, inorganic and microbial P fractions. Soil samples were taken at 0-5, 5-10, 10-20 and 20-30 cm depth in each treatment at 5 different times during the growing season. Resin-extractable P was removed prior to determination of inorganic, organic and microbial P in the NaHCO^ extracts. Subsequently, inorganic and organic P were also determined in Na~H extracts. Generally, microbial P, NaHCO^-organic and inorganic P and resin P were

J greater for no-till than for plowed treatments in the O-5-cm depth. To a lesser extent the differences were also evident in the 5-10-cm depth. Below i0 cm no differences were observed. Differences in P fractions between tillage treatments also diminished as the growing season progressed. These data indicate that P dynamics are much different under no-till and may affect P fertility management.