How Dry Soil Can Be Without Crop Yield Loss?

One of the main reasons why some of us put too much water on our plants is that we are afraid of yield loss. This is a legitimate fear. But how dry our soil can be (or how empty our bucket can be) before we lose any yield?

Lesson 1: Soil is like sponge!

Assume you’ve got a fully wet sponge. If you squeeze it, the water comes out of it very easily at the beginning, but after a while to get any water out of it you need to squeeze it harder and harder, and after a while no matter how hard you try no water comes out. And this is what happens to plants if the soil water content drops below Management Allowable Deficit (MAD).

Figure 1. Soil behaves like sponge. The drier the soil the more difficult to get any water out of it.

Lesson 2: Production reduction function

There is a relationship between crop yield and the amount of water that is available in the soil. We call this relationship "production reduction function" (production = growth). The higher the Soil Water Deficit (SWD) (the lower the moisture level), the higher the yield loss. Luckily for us, this relationship is not linear meaning that we've got some screw-up room here!

In the graph below (Fig. 2) you can see that production is reduced quickly at SWD's less than MAD, but before that there is little decrease in yield.

Figure 2. Production reduction function tells us that "the higher the soil water deficit (SWD) (the lower the moisture level), the higher the yield loss". It also tells that if we stay above MAD there will be no yield loss.

As you can see in Fig. 2, at first, as the soil water is depleted from Field Capacity (FC) (100% of available water) down towards Permanent Wilting Point (PWP) (0% of the Available Water) plant production is generally not affected, but there is a point at which production drops off. This point is commonly chosen as MAD. This point and the shape of this curve are different for different plants. Soil water depletion below this MAD point will result in significant yield losses.

Lesson 3: Keep soil moisture between limits!

So what should we do? To maximize production, we need to keep SWD between MAD and FC. Any amount beyond FC will generate deep percolation and any SWD below MAD will result in production reduction. Remember! Soil has a limited capacity for holding water. If we put too much water in it, it’s not going to hold it and it will spill out, lost to deep percolation.

Yield and quality are very strongly correlated with irrigation water management. If you put too little water (anything below MAD), you'll have reduction in yield and quality. Any amount of water beyond FC will result in deep percolation causing fertilizer loss and added cost of labor and pumping. So keep the soil moisture between limits!

Note: Trees are more tolerant compared with many row crops because of their deep rooting system. Therefore, higher MAD may be acceptable with trees.