Salt content is one of the most important characteristics of soil and should be a primary concern when growing any plants. The reason is that salt can dramatically affect a plant's growth in all its life stages, from seed to maturity to decline and death. You can avoid problems associated with salt content in soil by learning some essential information about the role of salt in soils.
Salt in Soil
When fertilizer is applied to soil, the fertilizer dissolves and makes the soil saltier near the point of application. When the salinity of a soil increases, it causes a change in the soil's osmotic pressure, which in turn forces water in the soil to move from a lower to a higher salt concentration. Nitrogen and potassium fertilizers have a higher salt index than phosphorus fertilizers, so salt damage is more likely when using these fertilizer formulations.
Salt and Seed Germination
Specifically in the case of seeds that are germinated directly in soil, salt content is also an important consideration. When a fertilizer that causes a change in the soil's salt content comes into direct contact with a seed, a condition known as salt effect or salt burning can occur. When water moves to soil with a higher salt concentration, less moisture is available for the seed to soak up and germination rates are dramatically diminished.
Avoiding Germination Problems
Use ammonium nitrate fertilizer whenever possible. Ammonium nitrate fertilizer is less likely to impact seed germination because it delivers nitrogen to plants in both an ammonium and a nitrate form. The nitrate form of nitrogen is far less likely to cause seed germination problems, so ammonium nitrate is less likely to cause seed germination problems than fertilizers that do not include nitrate. Having your soil tested for salinity content before cultivating any seeds can also help determine how vulnerable your seeds are to germination problems.
As noted, a seed's failure to germinate as a result of salt injury almost always occurs when fertilizer is added. But another way in which fertilizer can prevent seed germination is by fertilizer toxicity. Seeds can actually suffer a toxic reaction when they make contact with a fertilizer to which they are particularly sensitive. If your seeds do not germinate and you do not suspect salt injury as the cause, your seeds may be suffering ammonia toxicity, and adjusting the type or amount of fertilizer you use may be necessary for successful seed germination.