Figure 11 illustrates this concept using a full bucket to show the maximum amount of water that should be applied to the soil. Any more than that is just going to run over the “top of the bucket” and be wasted along with fertilizers, etc. The empty bucket (we don’t want to get near this point) is the soil water content corresponding to a tension of -500 kPa in Fig. 10 where the plants can’t get any more water; the associated water content is 8%. The water envelope would be (water content difference) * (rooting depth) or, in this case (16% – 8%) * 0.15 m = 12 mm of irrigation water.
The 12 mm of water would be the maximum amount of water that should be applied to the turfgrass. But, for optimal performance, we’d only want to fall to -100 kPa. In this particular soil, that would be around 12% VWC (volumetric water content) or a change of 4% VWC and irrigation of 6 mm of water. So, in addition to just seeing the comfort range, we can dial up the amount of water we would need to move it from the bottom of the optimal range to the top.
Measuring water potential together with water content for irrigation management saves you time and money. Why? Because it gives you the right information. To return to the cabin analogy, we didn’t know if we would be comfortable in our cabin just by knowing how many logs we added to the fire. In the same way, we won’t know if the soil is at optimal moisture for plant growth just by knowing how much water is there, or the water content. Soil water potential combined with soil water content gives us the precise full and empty points for optimal performance. As the water potential drops outside the crop comfort zone or optimal range, we know we need to add water. And we know exactly how much water to add. These two measurements combined are powerful tools that can be used to perfect water and nutrient management—without losing time and money to issues caused by over-irrigation.
Watch our irrigation management webinars, or Dr. Colin Campbell and Dr. Bryan Hopkin’s recent turf management webinar, or talk to an irrigation expert. Our soil scientists have decades of experience helping researchers and irrigators measure soil moisture to optimize irrigation.
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