Timing irrigation after a rainy start: Key strategies for farmers

By: Vasudha Sharma, Extension irrigation specialist

After three years of dry summers, Minnesota farmers are now facing the challenges of excess water this season. Recent rains provided abundant water for crops; in fact, more than needed for the time being, leading to saturated fields at certain places. As we approach the critical stages of crop growth, it’s essential to consider the moisture already present in the soil before deciding to irrigate. Here's why waiting to irrigate when there’s ample water in the profile can benefit your crops and your bottom line.

Understanding soil moisture

Frequent spring and early summer rains can saturate the soil profile, filling the soil’s water storage capacity. This moisture is vital for crop growth and development. However, over-irrigation can lead to waterlogging, which can suffocate roots, disrupt nutrient uptake, increase nutrient leaching and ultimately reduce yields. Monitoring soil moisture levels helps ensure your crops receive the right amount of water.

For example, a field with sandy loam soil can hold approximately three inches of water in the top 24 inches of the soil profile. Irrigation is usually recommended to start when the soil profile is 50% depleted. In this case, 1.5 inches of water is still available to plants. If the field had a corn crop planted in early May, the crop water use at this time (late June) is approximately 0.15 in/day. This means that this field will need irrigation in 10 days (1.5/0.15 = 10 days) if there is no rain. If it rains, this period can be longer than 10 days. For potatoes, the approximate crop water use at this time is 0.2 in/day, and for soybeans, it is approximately 0.15 in/day. You can access your field's daily crop water use information using Open ET.

Advantages of delaying irrigation

1. Water conservation: Conserving water by delaying irrigation benefits your farm and contributes to environmental sustainability. It ensures water resources are available for future use and reduces the risk of depleting local supplies.
2. Leaching issues: Over-irrigation can lead to leaching, where essential nutrients are washed away from the root zone. This can deprive your crops of critical nutrients and affect their growth and productivity. By managing irrigation carefully, you can minimize leaching and maintain soil fertility.
3. Root health: Excessive water can damage root systems, making plants more susceptible to diseases and reducing their nutrient absorption. Waiting to irrigate helps maintain a healthy and deeper root environment, promoting stronger, more resilient crops.
4. Insect problems: Excessive moisture can create ideal conditions for pests and diseases. Standing water and damp soil are breeding grounds for insects such as root-feeding nematodes and fungal pathogens. Waiting to irrigate can help reduce the incidence of these pests and protect your crops.
5. Cost savings: Unnecessary irrigation increases operational costs, including water, fuel, and labor. By delaying irrigation until it’s needed, you save these resources for when they are truly essential.

Practical steps for farmers

1. Use soil moisture sensors: These devices provide accurate readings of soil moisture content, helping you determine the best time to irrigate.
2. Use irrigation scheduling tools: Freely available tools like the Irrigation Management Assistant and the Checkbook Spreadsheet help in determining the soil water depletion and manage irrigation efficiently.
3. Weather forecasting: Keeping an eye on weather predictions is essential. If rain is expected, it may be wise to delay irrigation to take advantage of natural precipitation.
4. Regular field checks: Walking your fields regularly allows you to observe crop conditions and soil moisture firsthand. This practical approach complements technological tools.

In conclusion, strategic irrigation management is essential after a wet spring and early summer. By managing irrigation, you can protect your crops, save on costs, and contribute to sustainable farming practices. Make informed decisions, and let the soil’s natural moisture and nutrients work for you.

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