Soil Moisture & Irrigation Calculator

Applying water to a field is not a guessing game; it is a complex physical science. If you over-irrigate, you waste expensive pumped water, wash away vital nitrogen fertilizer, and cause the plant roots to rot from oxygen deprivation. If you under-irrigate, the plants undergo severe drought stress and yield drops drastically. The Soil Moisture & Irrigation Calculator utilizes precise agronomic data—including the specific water-holding capacity of your soil and the depth of your crop's roots—to calculate the exact millimeters or inches of water required to perfectly refill the soil profile.

Understanding Field Capacity

Different soils hold completely different amounts of water. Sandy soil is like a sieve; water drains right through it. It has a very low "Water Holding Capacity" (WHC). You must water sandy soil frequently, but in very small amounts.

Clay soil is like a dense sponge. It holds massive amounts of water and takes a very long time to dry out. If you apply the massive amount of water required to fill a clay profile onto a sandy field, 80% of that water will simply drain past the roots into the groundwater, totally wasted. The calculator adjusts the math based entirely on the specific soil type you select.

Management Allowed Depletion (MAD)

You never let a field dry out completely to 0% moisture, or the plants will permanently wilt and die. Conversely, you don't keep the soil at 100% moisture, or the roots will rot.

Farmers use a metric called Management Allowed Depletion (MAD). This dictates exactly how dry you let the soil get before turning the pumps on. For highly sensitive crops like strawberries, the MAD might be 30% (meaning when the soil drops to 70% full, you water it). For hardy crops like established wheat, the MAD might be 50%. This tool uses the MAD threshold to calculate exactly how much water is needed to bring the soil back up to 100% Field Capacity.

How to Use the Calculator

Select your Soil Type (e.g., Loamy Sand, Silty Clay). Next, input the Root Depth of your specific crop (a shallow-rooted onion requires vastly different math than a deep-rooted corn plant).

Finally, input your current soil moisture reading (often obtained from a digital tensiometer probe in the field). The tool will calculate the Net Irrigation Requirement, outputting the exact inches or millimeters of water you need to apply to the field. To calculate the nutrient requirements for that same field, use our NPK Fertilizer Calculator.

The Role of Evapotranspiration (ET)

Once you fill the soil profile, how long will it take to dry out again? This is dictated by Evapotranspiration (ET)—the combination of water evaporating from the hot soil and water physically sweating out of the plant leaves.

On a cool, cloudy day, the ET rate is very low. On a 100-degree, windy summer afternoon, a mature corn field can transpire over 1/3 of an inch of water in a single day. Advanced farmers track the daily ET rate (provided by local weather stations) and subtract it from the soil capacity calculated by this tool to perfectly predict the exact day the pumps must be turned on again.