Agriculture Reference
In-Depth Information
The lines in Figs 4.17 and 4.20 were fitted using the data points shown to
find values for the constants in the equations, but the line in Fig. 4.19 is a
predicted response using this model.
These equations indicate factors important in determining the rate of
seedling elongation in soil prior to emergence and show how temperature, time,
water potential and mechanical impedance affect this. How the elongation rate
responds to changes in these physical properties as the soil changes in
temperature and moisture content can be predicted. It should be noted that
impedance is not an absolute measure, i.e. independent of the instrumentation
used to measure it, unlike temperature or water potential.
The importance of mechanical impedance to field emergence is illustrated
by a study on onions sown 20 mm deep in a sandy loam soil at a sequence of
dates and then subjected to the press-wheel compaction pressures of 0, 2.8 or
8 N/cm
2
either with or without a subsequent 5 mm irrigation. Percentage
emergence decreased linearly as soil impedance increased (see Fig. 4.21), and
this relationship explained over 80% of the variation in percentage emergence.
Here, impedance was measured as the work done in driving a 2 mm-diameter
flat-ended probe 15 mm into the soil in 20 s. Excavations showed that pre-
emergence losses were almost wholly due to failure of seedlings to emerge
through the soil after germination, rather than failure to germinate.
Seeds sown below the soil surface must inevitably overcome some mechanical
impedance to emerge, but the degree of impedance can vary greatly with soil and
weather conditions. Impedance can increase because of soil compaction, the
slumping of poorly structured soil or because of the formation of a surface crust
caused by rain and subsequent drying.
Fig. 4.21.
The relationship between percentage onion seedling emergence and the
integral impedance of the soil to penetration (equivalent to the work done (mJ)) by a
2 mm-diameter flat-ended penetrometer driven to depth of 15 mm at a constant rate
for 20 s (from Hegarty and Royle, 1978. Courtesy of
Journal of Applied Ecology
).
