Rain – Wait – Irrigate?!

Anne Verhallen, Soil Management Specialist, OMAFRA

Even with the cool temperatures and seemingly endless rain showers, you know that someday soon the tap is going to turn off and we will be onto hot and dry. Now is the time to get that irrigation equipment ready to go – the need for irrigation can sneak up on you quickly. This can happen rapidly for a number of reasons:

• The cool, wet spring has delayed planting and plant growth. Root systems will tend to be shallow due to soil conditions.
• Odds are planting conditions were not ideal and there may be areas of the field that have a rough seedbed or may have some compaction – leading to restricted root systems and a greater need for good water management to prevent plant stress.
• We often get very high evapotranspiration rates in late June, July and August, where the soil and the crop are losing 4 to even 8 mm of water per day. On a sandy soil where the crop available moisture may only be 40 mm, this means a very rapid dry down.

Over the last twenty years there has been a number of irrigation scheduling projects across Ontario. One of the chief observations was that growers often delay starting to irrigate to the point of losing some of the benefits of irrigation. With drip irrigation timely water application is even more important. Making the call on the first irrigation is always difficult.

Take a sandy soil for example, where the crop available water is 40 mm, we would set the overhead irrigation point at half that or 20 mm. Once we get past 50 percent crop available moisture, water becomes progressively harder for the plants to extract from the soil, until the soil reaches the permanent wilting point.

This means that on that sandy soil to keep your crop within the optimum moisture levels, you are really only working with 20 mm of easily available water for the crop. Now if you get a week of warm, sunny days with ETs (evapotranspiration) numbers of 4 mm per day, by the end of the week you will be well below the 50 per cent crop available water and will be starting to affect crop efficiency.

You can monitor the ET values in Ontario at the Weather INnovations Incorporated (WIN) website http://www.weatherinnovations.com/ET.cfm.

For more information on scheduling irrigation and making that crucial first irrigation decision see the OMAFRA factsheet Irrigation Scheduling For Tomatoes – An Introduction.