Salinity and Leaching Requirement

Question 1

Explain how to control salinity in an irrigation scheme: what is the problem when salts accumulate, why do they accumulate, and what can be done about it

Answer 1

• Problem: similar affect to drought in soils - creates water stress, roots have to overcome..
• Why they accumulate: irrigation water containing salts, capillary rise brings in salts
• Controlling salinity: leaching -> adding excess water to crop (more than ETc) to wash out salts and make them go into deep percolation, provide drainage, use irrigation water with low EC, plant salt tolerant crops

Question 2

Explain the factors to consider when calculating the leaching requirement

Answer 2

• Leaching requirement: minimum amount of irrigation water that must percolate below the root zone in order to maintain soil salinity at given level – to a salinity level to which the particular crop would not suffer, or in other words within the tolerance (ECe) of the crop
  > ECe is dependent on crop, its growing state and relative yield expression due to salinity
• To calculate, need to know ECe and ECw which is the salinity of the applied irrigation

Question 3

Explain how net and gross irrigation water requirements and peak irrigation requirements are calculated, including the calculation for paddy rice

Answer 3

• FIGURE
• Inet = ETc - gains
• Igross = Inet/losses (equation above)
• Ipeak = Inet of the month with the highest ETc value. Also include correction factor to ETc to be more conservative - figure 4.1 on page 189 (need RAW)
• Paddy rice:
  > Inet = ETc + deep percolation losses + soaking requirement - gains (rainfall)
  > Soaking: water to saturate the soil and water layer (~100mm)
  > Why flooding? For weed control and avoid nitrogen leaching out
  > Ipeak is calculated at the beginning of the period of planting - takes into account saturation, water layer and maintenance

Question 4

Explain how to measure the EC of a soil saturation extract (and why such a procedure is used)

Answer 4

• ECe is measured using electrical conductivity sensors
  > Collect soil sample
  > Add distilled water soil is completely saturated -> this is a way of standardising the dilution (because dilution affects EC)
  > Extract all salt by solving into water, filter out the soil particles
  > Use an conductivity probe to measure the EC of the extract
• By measuring the EC of a soil saturation extract, you can assess the salinity status of the soil and make informed decisions regarding soil management practices. It helps determine if corrective measures are required, such as leaching excess salts through irrigation or implementing soil amendments to improve soil quality. The electrical conductivity measurement is typically expressed in units of deciSiemens per meter (dS/m) or milliSiemens per centimeter (mS/cm)