Avoiding salt injury from nitrogen fertilizers in orchards

Nitrogen fertilizers can injure fruit trees if used incorrectly. Here are a few tips to consider in preventing salt injury from N fertilizers.

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 (Table 1). 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 oz N per tree. This would require 4 to 8 oz calcium nitrate, but just 1 to 2 oz urea.

Table 1. Salt index* values for some common N fertilizers

Fertilizer

% N

Salt index per unit fertilizer

Salt index per unit N

Ammonium nitrate

33

105

300

Ammonium sulfate

21

69

328

Calcium nitrate

12

53

442

Di-ammonium phosphate

18

29

161

Mono-ammonium phosphate

11

27

245

Natural organic

13

3.5

70

Urea

46

75

162

* Salt index is the increase in osmotic pressure resulting from addition of fertilizer to a solution, relative to effect of the same amount of NaNO3 (SI = 100)

Young or small trees with limited root systems are at greatest risk. A common mistake is fertilizing newly planted trees before the soil has settled around the roots. If dry fertilizer is placed beneath trees before rain has settled the soil, the next heavy rain can move the fertilizer directly into the root zone and damage trees. It is best to wait until new trees have leafed out and soils have settled before fertilizing new trees. Older, larger trees have more widespread roots and are more tolerant of salts.

Spread fertilizer over the rooting area to disperse the salts. Trees can be injured if fertilizer is piled in a small area close to the trunk because subsequent rains may move high amounts of salt directly into a small region of the soil. If you are fertilizing new trees, spread the material loosely in a 2- to 3-foot wide circle around each tree or place it in a 3- to 4-foot wide band down the row.

Consider split applications. Applying half the amount in two applications several weeks apart is particularly beneficial on sandier, leachable soils. This approach tends to maintain needed amounts of N in the root zone for a longer time and usually increases the amount used by the trees. Split applications also disperse salts over time and reduce the risk of salt injury.

Dr. Hanson’s work is funded in part by MSU’s AgBioResearch.

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