Source: Penn State Extension

By Sjoerd Duiker 

Your soil is a marvelous medium to store and draw water from. We have been reminded how critical that is during these past weeks when we were approaching a drought in much of Pennsylvania. Fortunately much needed precipitation has come in most parts over the past couple of days.

But do you ever wonder how much moisture you really can store for your crop? Everything depends on your soil. This information is available through the USDA soil survey.

There is now an app called SoilWeb that can be loaded on your smartphone and can tell you which soil type you are standing on. A lot of detail about that soil is available through that resource. 

Last year we did an exercise at the Penn State Field Diagnostic Clinic to find out information about some common soils in the valleys of the Ridge and Valley system of central Pennsylvania.

The app allowed us to check the texture and plant available water of each layer of a typical soil profile of the different soils. It gives a lot of information, among others the plant available water.

This is the water available between field capacity (24-48 hours after soil has been thoroughly soaked) and permanent wilting point (when plants start to die from drought). We typically wish that the moisture content does not decrease below 50% of that range to avoid any moisture stress to our crops.

So how much water does a typical soil profile hold?

We looked at a Hagerstown silt loam, an Opequon silt loam and an Andover silt loam. The Hagerstown is a deep limestone soil. It can hold 6.85 inches of plant available water.

  Texture PAW (inch/inch depth) PAW (inch for layer) 50% of PAW (inch for layer)
0-8 Silt Loam 0.18 1.44 0.72
8-45 Silty Clay Loam 0.13 4.81 2.4
45-50 Clay Loam 0.12 0.6 0.3
Total     6.85 3.43
Table 1. Example calculation of Plant Available Water for Hagerstown silt loam (assuming 50” rooting depth)
PAW=plant available water

The Opequon is a shallow limestone soil. It can hold 2.48 inches of plant available water.

  Texture PAW (inch/inch depth) PAW (inch for layer) 50% of PAW (inch for layer)
0-9 Silty Loam Clay 0.18 1.44 0.72
17-Sep Silty Clay 0.13 1.04 0.52
45-50 Bedrock      
Total     2.48 1.24
Table 2. Example calculation of Plant Available Water for Opequon silt loam

The Andover is a poorly drained soil due to a fragipan at shallow depth that doesn’t let water percolate. Andover soils are usually found on the side of the mountain, where seeps come out of the hillside. Interestingly, they only hold 2.75 inches of plant available water.

  Texture PAW (inch/inch depth) PAW (inch for layer) 50% of PAW (inch for layer)
0-9 Channery Loam 0.15 1.35 0.73
9-23 Channery Loam 0.10 1.40 0.70
23-60 Gravelly Clay Loam Saturated much of the year    
Total     2.75 1.43
Table 3. Example calculation of Plant Available Water for Andover silt loam (assuming rooting depth to fragipan at 23 inches)

What can we learn from these numbers? First, it takes almost 7 inches of water to replenish the Hagerstown profile if it is bone dry, while it takes 2-3 inches to replenish the Opequon and Andover. Typically the soil will not get that dry so it takes perhaps half that to completely refill the soil profiles.

The next question is, 'How long does this provide my crop with sufficient moisture?’ Let’s take corn as an example. Just after emergence, corn uses about 0.4 inches per week, but after silking it uses about 1.4 inches per week. The difference is due to the canopy coverage and the temperatures of the season. On the Hagerstown silt loam it takes about 2.5 weeks to start experiencing some drought stress after the entire soil profile has been recharged in the peak of the corn season, but on the Opequon and Andover it takes only 1 week.

It becomes really questionable if it makes sense to plant corn on these droughty soils because everybody knows that it does not rain every week in Pennsylvania. Perhaps you have to think of other crops. The Opequon can grow winter cereals such as wheat or barley. The Andover is not suited for those crops because it sits so wet in the winter and early spring. Perhaps sudangrass or sorghum is a better choice, being more drought resistant.

Practices that can deplete moisture are actively growing cover crops (this has been an issue this spring) and tillage. Practices that conserve moisture are dead residues from cover crops or main crops. It is also important to get the water into the soil. It is very frustrating when water runs off the soil while you know underneath it is still dry. This happens when the soil surface is unprotected and poorly aggregated. Keeping the soil covered at all times is a way to stimulate water to infiltrate instead of running off.

Building soil health with continuous no-tillage, cover crops, manure and compost, and returning crop residue to the soil, are ways to help improve aggregation. Over the years these practices will also improve the water holding capacity of your soil by building its organic matter content.