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Soil moisture and plant roots
Dr. Victor Martin

This column was written before the drought monitor and forecasts were available. Suffice is to say, much of the state needs rain. Last week’s column discussed planting decisions with the variable soil moisture conditions in our area. Today, let’s explore how plant roots grow and react to soil moisture conditions.

Seeds imbibe water, activate, and start to germinate. The first structure to emerge is the seed root (radicle) and it grows downward in response to gravity. For dicots such as soybeans and alfalfa, the radicle develops into the primary plant root, the taproot. In grasses like corn, wheat, and soybeans, the plant develops seminal roots along which along with the radicle supply the plant with water and nutrients initially. However, the primary root system for these grasses soon starts to form above where the seed was planted. This is why grasses have a minimum planting depth so there is room between the seed and soil surface for the root system to form. Dicots on the other hand, since the taproot forms from the radicle simply should be deep enough in the soil for adequate moisture to germinate and grow.  

The result of the previous paragraph is the deeper taproot/lateral root system of dicots (broadleaves). Grasses though have a shallower fibrous root system closer to the soil surface. The benefit for the taproot system is the ability to extract water from deeper in the soil profile, if it’s there. The benefit to the grasses shallow fibrous root system is the ability to more efficiently use water in the upper soil profile. This about it. While we remember the intense rainfall and storms, the vast majority of precipitation events here are 0.25 inches or less. This rainfall doesn’t penetrate deeply into the soil. This, along with regular fires, are the reason the prairie is a grassland. Grasses are better equipped to thrive under prairie conditions.The taproot versus the shallow fibrous root system is also why crops like soybeans, alfalfa, and canola are much more sensitive to saturated soils than plants like corn and grain sorghum.  

Next, where do roots take in water and nutrients? Primarily from the actively growing parts of the root where root cells are dividing and elongating, behind the root cap. Little is taken in by the mature root cells. That is why older parts or the root system produce root hairs to aid in water and nutrient uptake. So, for a healthy root system, it must constantly be elongating and producing new cells. As you notice the above ground growth occurring, the root system is doing the same. 

Now for the important point. Roots don’t grow through dry soil to wet soil. Dry soil is like a barrier. Root growth simply increases in response to moisture conditions.

Dr. Victor L. Martin is the agriculture instructor/coordinator for Barton Community College. He can be reached at 620-792-9207, ext. 207,