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.
REFER TO FARM PRACTICES GUIDELINES FOR
PRODUCERS IN MANITOBA:
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