Leaching – Another Nitrogen Loss Mechanism

Last time we discussed denitrification as one way for nitrogen to be lost from the soil during saturated conditions.  This usually occurs when the soil is poorly-drained and water doesn’t move, creating anaerobic conditions for bacteria to convert nitrate into atmospheric nitrogen and nitrous oxide gas.

However, we must also recognize that nitrate-nitrogen can also be lost through leaching; that is, movement with the flow of water downward and away from the rooting zone.

Unlike ammonium-nitrogen (NH4+) that can bind to soil particles, nitrate-nitrogen (-NO3) is carried in the water solution and is not bound to soil particles (at least to the extent of ammonium).

When is nitrate at risk of leaching?  Primarily, any time the soil moisture is above Field Capacity and below Saturated.  Field Capacity is best described as a scenario where the soil is moist but not dripping wet; water forms a thin film around each soil particle but allows for air between the particles.  This water is easily extractable by plants.  Think of it as the “Goldilocks” level of soil moisture – not too wet, not too dry, and where useful bacteria can perform mineralization of organic matter and nitrification of ammonium to nitrate.

Saturated, as the name suggests, is when all available air space is vacated and filled by water.  Water will naturally flow downward by gravity; it is dripping wet.  This condition is where denitrification plays well.

Between Field Capacity and Saturated, water will naturally flow downward with gravity, and with it nitrate.  The highest flow rates of water will be closer to the Saturated level and then taper off as Field Capacity is achieved.

Technically, nitrate movement can also occur when soil moisture is lower than Field Capacity.  This will be through capillary action and the draw of roots.  Water in the soil is always attempting to find equilibrium and thus will bring nitrates along with that search.  This means that nitrates in sub-Field Capacity conditions can move downward, but in some cases they can move upwards as water is evaporated from the surface or taken up by the plants.

What soil conditions are most conducive to leaching?  Generally, soils with lower water holding capacity and higher saturated hydraulic conductivity, or KSat, will tend to leach nitrate more than soils with higher water holding capacity and lower KSat.  Sands, sandy loams, loamy sands, and even loams will more likely leach than silt loams, clay loams, silty clays, and silty clay loams.

Consider the difference in Available Water Capacity and KSat between the Valentine fine sand found in the Nebraska Sandhills, the Hastings silt loam located in the central valley of Nebraska, and the Luton silty clay found in the Missouri River bottoms, with data sourced from the NRCS over a 180 cm profile.

In the Valentine fine sand, water is rated to move at about 13 inches per hour.  In Hastings silt loam, that value is around 1 inch per hour, and at 0.007 inches per hour in the Luton.

Leaching of nitrate is highly likely in the Valentine fine sand.  This is why smaller and more frequent applications of nitrogen are recommended in these soils.  On the other hand, nitrate is unlikely to leach very far in Luton silty clays, with the more likely fate of nitrate loss to be denitrification.  In the Hastings silt loam, it is quite possible to experience both leaching and denitrification losses, but likely not in as great of magnitude as either Valentine or Luton.

In areas with tile drainage, leaching can especially be an issue, with excess nitrate nitrogen finding its way into the tile, then downstream to streams and rivers, affecting drinking water and wildlife habitat. 

So what can be done to reduce nitrogen losses due to leaching?  First, knowing your soil is key; building strong management plans that build in the water holding capacity and saturated drainage rate is important.  Apply nitrogen timely when the crop needs it.  Using either urease (above ground) or nitrification (below ground) inhibitors to maintain the nitrogen in the ammonium form longer helps to slow down leaching losses.  We cannot control the rain or when the river floods the bottom, but we can control our management practices to minimize losses.