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Fertilizers are salts. Therefore, when salinity is a concern,
it is critical to pay close attention to how much fertilizer you apply,
what kind of fertilizer you apply, where you apply fertilizer, and when
you apply fertilizer.
Apply only the amount of fertilizer that will achieve maximum
economic yield. Additional fertilizer will increase soil salt content
and cost more, but will not increase yield. Base fertilizer application
rates on annual soil sampling. This practice will help you know how much
fertilizer you actually need and prevent over-application. Use a reputable
laboratory that understands your salinity management goals. Avoid unnecessary
application of potash or micronutrient fertilizers.
Different fertilizer forms have different salt indices.
In other words, you can apply the same amount of nitrogen using different
fertilizers and apply vastly different amounts of salt in the process.
Choose fertilizers that will supply the crop’s needs and have the lowest
salt indices. Anhydrous ammonia has the lowest salt index of all nitrogen
fertilizers (Table 1a). Among phosphorus fertilizers,
triple superphosphate has the lowest salt index, considerably lower than
monoammonium phosphate (MAP) or diammonium phosphate (DAP). If you need
potassium fertilizer, potassium sulfate has a much lower salt content
than potassium chloride (muriate of potash).
The closer that fertilizer is applied to seeds or growing
plants, the greater the risk of salt burn. Do not apply fertilizer with
the seed if the crop yield is already below optimum due to salts. Never
apply urea or ammonium fertilizers directly with the seed. Banding phosphorus
fertilizer doubles its efficiency. Therefore, banding keeps fertilizer
rates down and helps to avoid excess salts. Be careful to avoid banding
too close to the seed. Use Table 2 to determine
how close you can put the fertilizer band without causing a salt burn.
You will need to know the soil texture and the amount of N and K2O you
intend to apply.
Crops are most sensitive to salts when they are in the seedling
stage. Salinity also reduces germination. Therefore, it is wise to avoid
fertilizer application during the early growth stages. If you have salinity
problems, avoid starter fertilizers and apply as much of the N as a sidedress
as possible, eliminating pre-plant N applications. Coated fertilizers
can reduce early season salinity by gradually releasing nutrients into
the soil. Fertigation is a good practice in saline soils because it allows
you to spoon feed the N in small doses through the sprinkler system. However,
do not pump anhydrous ammonia into irrigation furrows because this will
increase the Sodium Adsorption Ratio (SAR) of the irrigation water and
increase the sodicity risk.
Applying fertilizer only at required rates, choosing fertilizers
with low salt indices, banding fertilizer away from the seed, and delaying
fertilizer application until after plants are established will help to
protect your crop from excess salinity due to fertilizer application.
by Jessica G. Davis
Extension Soil Specialist
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Table 1a. Salt indices
of nitrogen fertilizers.
| |
Salt Index
per unit of N
|
| anhydrous ammonia |
0.6 |
| ammonium nitrate |
3.0 |
| ammonium sulfate |
3.3 |
| ammonium thiosulfate |
7.0 |
| urea |
1.6 |
| urea-ammonium-nitrate |
2.3 |
|
Table 1b. Salt indices of phosphorus fertilizers.
| |
Salt Index
per unit of P2O5
|
| diammonium phosphate |
0.6 |
| monoammonium phosphate |
0.5 |
| monocalcium phosphate |
0.3 |
| simple superphosphate |
0.4 |
| triple superphosphate |
0.2 |
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Table 1c. Salt indices of potassium fertilizers.
| Fertilizer |
Salt Index
per unit of K2O
|
| potassium chloride |
1.9 |
| potassium nitrate |
1.6 |
| potassium sulfate |
0.9 |
| potassium thiosulfate |
2.6 |
| sulfate of potash-magnesia |
2.0 |
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