Agriculture

Retrieval Strategies for Deep-leached Nitrates

The recommended retrieval strategy depends on the depth in the soil profile at which the nitrates are concentrated.

Introduction

Nitrates that are leached beyond the rooting zone of annual crops are an unnecessary economic loss and a potential environmental concern to the producer. However, there are some basic principles that can be used to recover excess soil nitrates.

Consider the following scenarios:

  • Nitrates have been found to be in excess of 150 lb/ac in the top four feet of the soil profile but not in excess beyond the four-foot depth:
    • This suggests that excess nitrogen fertilizer has been added and has built up over time, but has not moved significantly beyond the root zone of annual crops. Refrain from adding nitrogen fertilizers, except for a starter fertilizer placed with the seed when necessary. Annual crops should still remove the bulk of the nitrates if within four feet of the soil surface, although deep rooted crops may be more effective ( see Table 1).
  • Nitrates may or may not be within 150 lb/ac in the top four feet of the soil profile but are in excess of 20 lb/ac/ft below the four foot depth:
    • This suggests that excess nitrogen fertilizer has been added and has moved beyond the root zone of annual crops. Deep rooted crops are needed to retrieve nitrates below the four foot depth ( see Table 1). Refrain from adding nitrogen fertilizers, except for a starter fertilizer placed with the seed when necessary.

Should legume crops used for nitrate retrieval be inoculated?

When establishing legumes, such as alfalfa, to extract deep leached nitrates, it is recommended to inoculate the seed with N2-fixing rhizobium. This is to ensure that the alfalfa crop will not be deficient in nitrogen before the roots are able to extract deep-leached nitrates. Alfalfa with N2-fixing capability effectively removes deep-leached nitrates because less energy is required by the plant to extract "free" nitrate than the amount required to manufacture nitrogen through N2 fixation. Once the excess nitrate has been used up, the inoculated alfalfa plant will revert to N2 fixation to meet its nitrogen requirements. In this case, only soil testing will determine how effectively the deep-leached nitrates have been retrieved. If the crop has no N2-fixing capability, nitrogen deficiency symptoms will become evident once the deep leached nitrates have been utilized.

Table 1. Crop Selections for Deep Leached Nitrate Retrieval  

Crop Advantages Disadvantages
Alfalfa perennial; high water use; deep rooted (up to 14 ft.) livestock forage - limited markets; legume - fixes N2 and uses NO3; weed control during establishment
Non-nodulating alfalfa same as alfalfa but no N2 fixing; N deficiency indicates NO3 removal limited seed supply; weed control; limited markets
Sweet clover biennial; coarse tap roots coumarin levels; same disadvantages as other legumes
Non-dormant alfalfa similar to perennial alfalfa; easy to establish future crops annual - limited root growth similar disadvantages as perennial alfalfa
Sunflowers cash crop (confectionery) tap roots (up to 6 ft.) annual crop; losses to disease, insects, deer, birds
Perennial grasses (eg., reed canarygrass, crested wheatgrass) perennial limited markets; limited rooting depth
Annual crops (eg., wheat, canola) cash crops, feed markets, etc. (Note: some crops are better than others at extracting NO3 from soil) limited rooting depth
Summerfallow no advantages; more N mineralized and prone to leaching no crop present to extract nitrates (worst-case scenario)

When selecting a crop to retrieve deep-leached nitrates, consider the following factors:

  • the depth at which the nitrates are concentrated
  • what crops grow well in the area of concern
  • soil texture/depth to groundwater/precipitation - how fast do nitrates move through the soil?
  • the time available to remove the nitrates (is a perennial crop feasible?)
  • the marketability of a crop

Monitoring Soil Nitrate

Once a retrieval strategy has been initiated, the soil should be tested every year to monitor the movement/uptake of nitrate. Refer to Appendix F, Monitoring Soil Nitrate, in Manitoba's Farm Practices Guidelines.

Conclusion

Using the above strategies to retrieve deep-leached nitrates will not only preserve ground-water quality but will save the producer money by utilizing fertilizer nitrogen that would have otherwise been lost.

For further information, contact your MAFRD GO Representative.