Updated: Mar 8, 2021
So far this season, November was relatively dry, but October, December and January have been wet. Consequently we see in the popular press statements such as “nitrate leaching has been higher than usual this winter.”
Crops which were established early have taken up some of the 2020 available Nitrogen, but later drilled crops may not have. Hence the press statement about the risk that some of this Nitrogen has been lost (leached.)
Lets look at this in a little more detail.
Nitrate leaching is a naturally occurring process, the amount of nitrate leached is governed by two major factors:
1) Nitrate levels in the soil
2) Water movement through the soil profile
If the water “keeps going” it ends up either in drainage pipes and then ditches or alternatively in ground water reserves.
This is not a black and white “fact” there are other factors which influence the outcome.
The soil in your field has a variety of retentive properties depending on it texture, organic matter content and cation exchange capacity. The crop type, its growth and development also play a part.
Working with “bare soil” researchers compared “leaching” within different textural classes of soil. A sand, 85:15 sand:peat, a loamy sand and sandy clay loam soils were used. The study was to determine the effect of soil texture on Nitrate retention and the results clearly showed that soil texture affected the soils retention ability. (In reality we want to know how much we have, what is lost (leached) is a complete waste but what is there we can use.)
As you may expect the sand adsorbed the least amount of Nitrate then the loamy sand and finally the sandy clay loam. Soils with more silt, clay, and organic matter retained more Nitrate than the straight sand.
Nitrate leaching is very important but you need to put it into context and to do this you need your soil analysis to show soil texture and organic matter content. With this basic knowledge you can be more realistic about Nitrate leaching and how it may impact on your Nitrogen fertiliser policy for this season.