Conventional nitrogen (N) fertilizers are salts that can injure trees if used improperly. Fertilizers can increase the salt levels of the soil solution. High soil salt levels can prevent roots from absorbing adequate water so trees grow poorly or are sometimes killed. There are a few rules of thumb about salt injury from N fertilizers:
Fertilizers vary in their effect on soil salts, as measured by the salt index (SI). Materials with high SI’s increase soil salts the most. The SI for fertilizers is usually based on a unit of fertilizer. Per unit of fertilizer, ammonium nitrate has the highest SI value, and calcium nitrate has a low value. However, based on the amount of N, calcium nitrate is one of the highest and urea and ammonium nitrate are relatively low. This is important to know because fertilizer rates are given in units of N. A typical rate for new trees is 0.5 to 1 ounce N per tree. This would require 4 to 8 ounce calcium nitrate, but just 1 to 2 ounce urea.
For more on this story, click here.