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Agronomy Day 2007

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Taking a Look at Our Roots

tour b
Fabián G. Fernández Fabián G. Fernández
Assistant Professor Soil Fertility
and Plant Nutrition
Department of Crop Sciences

While our soils are capable of storing much of the water needed for corn and soybean production, additional inputs by rainfall or irrigation are needed to sustain crop growth and yield. Dryer- and warmer-than-normal conditions for most of the spring have made water management and root growth issues a “hot topic”. The ability of a crop to take up available water and nutrients depends largely on its root system. The question then becomes: What can I do to manage roots in order to benefit from my soil fertility program even in dry years?

Crop nutrients can reach the root surface of a crop by three main ways: 1) Diffusion: ions moving along a concentration gradient from high to low concentration points (most important for P and K), 2) Mass flow: ions being carried to the root in the convective flow of soil water originated by plant transpiration (most important for N), and 3) Root interception: ions are encountered by the roots as they grow into the soil. Regardless of whether we are talking about root growth or uptake of N, P, or K; soil water is necessary to make it happen.

While we cannot control water supply in rainfed systems, we can manage to improve:

fig 1
A quarter of the root system from a soybean plant at R1 before quantification analysis.

1) The rooting medium. Field operations in the spring can result in substantial compaction when soils are still too wet. Tillage operations under this condition can result in clods that will dry out quickly on the surface due to increased contact with air. This will prevent utilization of water held inside the clod since roots will not grow into the dry soil surface. Compaction can also create a hard pan layer that will restrict root penetration and reduce the total potential volume of soil that can be exploited for water and nutrients by the root system. During planting, if the soil is too wet, the operation can cause smearing of the soil surface near the seed creating a barrier (much like a wall) for the emerging root to penetrate the surrounding soil. 2) The conservation of water already stored in the soil. Water stored in the soil can be quickly evaporated as soils are disturbed and moist soil is exposed to air on the soil surface. Also, maintaining crop residue cover on the soil surface can reduce water evaporation before crop canopy closure. 3) The infiltration capacity of the soil to intercept rain-water. Minimizing disruption of pore conductivity can increase the infiltration rate of rain water. Leaving crop residue on the soil surface can also prevent the soil from crusting. Crusted surfaces can reduce water infiltration and increase surface water runoff. And 4) The nutrient supply through strategic timing and placement techniques. Localized placement of nitrogen and to a lesser extent phosphorous, typically promote root proliferation in the area of high nutrient concentration. While localized placement of nutrients can potentially increase their availability, root proliferation in that area can diminish soil water content as water is taken up along with nutrients. Once soil water is depleted the root might not be able to continue to take up nutrients from this soil volume unless precipitation replenishes water content. Unfortunately, the chance for water replenishment decreases with soil depth and as the crop evapotranspiration (ET) increases. Nutrient placement is also an important consideration for early uptake. Most crops need to come in contact with nutrients soon after germination. A delay in coming in contact with nutrients can have important negative effects on adequate crop growth. Early nutrient availability is also important because for most nutrients and crops, nutrient uptake typically occurs early in the growing season while the root system is actively growing. Once the crop enters the reproductive stage, the root system rapidly declines its expansion. So, the chance that the root will come in contact with “new” nutrients is very small. In fact, if the soil surrounding the root is dry, likely there will not be much uptake of nutrients; but instead we might see nutrient remobilization in the shoot as the seed becomes the most important sink for nutrients and resources.