Nitrogen For Non-Bearing Trees

Pacific Nut Producer, February 2016

Mostly good – occasionally harmful

With current high almond prices and an estimated 80,000 acres of non-bearing trees, growers are trying to get these young trees into production as soon as possible by pushing tree growth with high rates of nitrogen (N) fertilizer. More power to them, but growers should be aware that occasional problems can arise from applying more N than small, non-bearing trees can handle at one time.

There is a large family of N compounds. Most growers are familiar with the common N fertilizers: Urea, ammonium nitrate/sulfate, Solution 32 (half urea, half ammonium nitrate), calcium nitrate and calcium-ammonium-nitrate (CAN-17). The N family also contains some crazy relatives, most notably cyanide (CN – found in apricot pits) and nitrous oxide air pollutants (some of them by-products of the incomplete breakdown of N fertilizers). Two outlier N compounds that have been proven to adversely affect plant growth can cause problems for young almond trees: Nitrite and Putrescine. In most years these two compounds are not a concern because rapid shoot growth does not allow them to accumulate to harmful levels in plants or soils. However, when a cool spell follows a warm spell, as can occur during the spring, these two compounds can cause problems. They are discussed separately below.

Nitrite is an intermediate compound in the conversion of ammonium-N to the more usable nitrate form. The conversion is a two-step biological process regulated by two different bacteria species: Nitrosomonas (converts ammounium to nitrite) and Nitrobacter (converts nitrite to nitrate). Nitrobacter activity is suppressed in soils with an alkaline pH (above 7) and by NH3 (which can be released when urea and ammonium materials are applied to soils containing lime). Cool weather also slows the activity of Nitrobacter. When Nitrobacter activity is suppressed, nitrite can accumulate to harmful levels in soils. Several UC tests have demonstrated nitrite toxicity on row crops due to its delayed conversion to nitrate. In poorly aerated soils, nitrate can revert back to nitrite.

It is difficult to diagnose nitrite toxicity with soil and plant analysis since nitrite is transient in both soils and plants; by the time trees show symptoms, nitrite may have already passed through the system and/or been converted to other, less harmful compounds.

Putrescine, another crazy aunt/uncle in the sometimes dysfunctional N family, is an amino acid that can be toxic to plants. Its precursor is the benign amino acid, arginine. Putrescine is eventually converted into useful proteins in plants, but cool weather following a warm spell can delay this conversion, thus allowing putrescine to accumulate to harmful levels.

UC workers have diagnosed putrescine toxicity on grapes where it is referred to as spring fever. On grapes, it is also called false potassium (K) deficiency because the leaf symptoms are similar or identical to symptom of K deficiency. Trees affected by putrescine will show elevated leaf levels of total N and ammonium N. Leaf levels of K in putrescine affected trees may be low, but not deficient; potassium fertilization is not recommended.

The remedy – for both nitrite and putrescine: Do nothing. Non-bearing trees will grow out of the problem when shoot growth resumes during warm weather (bearing trees are rarely affected). N fertilization should be eliminated until leaf analysis shows a need.