6/14/2021 - By Dr. Jeff Atkinson

Before applying your freshly received delivery of Harrell’s fertilizer, it’s important to understand what’s in the bag and how you can get the most out of it. Looking at the product label on the bag, you’ll notice a series of three numbers, such as 10-10-10. Each of these numbers correspond to an essential element, and depending on the combination of numbers, they may help determine which blend is appropriate for an intended use.

The first number on the bag refers to the percentage of Nitrogen, a 10-10-10 bag consists of 10% elemental Nitrogen. Said in another way, a 50 lb bag of 10-10-10 consists of 5 lbs elemental nitrogen. Nitrogen is one of the 3 macronutrients, the other 2 being Phosphorous and Potassium.

Nitrogen is needed in the plant to help with protein synthesis, DNA construction, formulation of enzymes and as a main constituent of chlorophyll. Nitrogen moves readily through the plant, so when a plant is deficient, the old leaves yellow first and will eventually drop, shortly followed by the new leaves. The plant also slows down its overall growth. So, when we see a “hungry” plant which is uniformly yellow or chlorotic, it is often deficient in Nitrogen.

Nitrogen deficiency in a palm tree

Did you know that our atmosphere is composed of 78% Nitrogen? As good as this sounds for plants, most of them, except legumes, cannot assimilate Nitrogen from the air. The two main sources of nitrogen which plants utilize are ammonium (NH₄+) and nitrate (NO₃-). Nitrogen is often applied in the urea form, which rapidly converts to ammonium and nitrate in the soil through microbial and enzymatic processes. Most ornamental plants prefer something of a balance of urea or ammonium and nitrates, however too much urea or ammonium will make plants “soft and stretchy,” with extended internodes. Ammonium, having a positive ionic charge, will act as a cation and attach to negatively charged soils thus limiting environmental losses by leaching. Nitrates, are negatively charged anions and will not readily absorb to soil, thus are more susceptible to leaching.  Northern greenhouse growers like nitrates, because during cooler months their plants can continue to absorb nitrate despite cool soil temperatures, which is beneficial in the long run. Flowering plants in general love nitrates, because they provide sturdier stems, great bud counts and a good leaf sheen!

Harrell’s NPK+ products are ideal for the previously mentioned growers, with ammonium nitrate and potassium nitrate being key constituents of these mixes. However, keeping a careful balance of nutrients is paramount. For example, excessive Nitrogen can induce deficiency symptoms of Potassium or Calcium, which is called nutrient antagonism. Some fertilizer ratios have Nitrogen in much higher excess over Potassium, however this could result in problems in the future. As a general rule, try to keep your NPK ratios no higher than a 3-1-2 mix, so something like 18-6-12, or high-low-mid.

Some grass type crops might be an exception to this rule of thumb. Foliar Nitrogen levels are often 3-4% in dry weight of the total, so when reading a foliar report, anything below 1.5% is worth looking into. In a soil report, expect 35-80 ppm Nitrogen as good range for control release mixes, higher for quicker release mixes, and 30 ppm or below means you are running out of fuel. Sprays of 20-20-20 or Harrell’sMAX® Ornamental 8-2-4 are helpful in rapidly increasing foliar Nitrogen levels. Spray or drench either with Nitrogen levels at 200-250 ppm.

For more information on which program/product is right for you, or if you have any questions on Nitrogen (or any of the essential nutrients), contact your Harrell’s Representative.