Want to improve yields under sub-surface drip? Then reduce deep drainage losses.

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In Australia, processing tomatoes are grown in the irrigation districts of the central Murray valley using sub-surface drip irrigation. The industry wishes to lift yields but believes low soil pH and poor soil structure are major impediments to achieving their 200 t ha-1 target. However, past efforts to ameliorate soils have not led to yield increases.

Nine commercial processing tomato crops were monitored during the 2019-20 season to assess yield limiting factors, with the hypothesis that water stress is the major constraint. A highly significant correlation (R2 = 78%) was found between red fruit yield and the number of days the soil in the root zone was either waterlogged or too dry during January and February, supporting the hypothesis.

This was predominantly an issue in the texture contrast soils (i.e. loams over clays). It was recommended that further research be done to improve physical properties in this soil type – mainly by finding ways to increase soil organic carbon contents.

The main issue, however, was excessive deep drainage in all soil types under current irrigation regimes (leaching fractions of 34-52% and 11-38% for the texture contrast soils and uniform clays respectively). Excessive deep drainage reduces irrigation efficiency.

It also affects productivity because fertigation water that moves below the root zone takes fertiliser with it. As a result, crops may not receive adequate nutrition and soil acidification rates are increased. It was recommended that sensor-based irrigation scheduling be adopted in all sub-surface drip systems to turn valves off and avoid over-watering.

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