Nitrogen - forms, functions & failings
Nitrogen is needed to grow every crop, but when it is applied can make a big difference in plant health and crop quality. The Neoteric Ag P-N-K Principle states, “Start every crop with phosphorus, grow the crop with nitrogen and finish the crop with potassium.”
Now that the Bloom Stage is finishing and Cell-Division is underway, it is now time to grow the crop. Nitrogen is needed to fortify the canopy and the developing fruit. Supplying the right form is critical for success.
As an independent Certified Crop Adviser, I work with growers and their fertility management. One way I do this is with Nutrient Management Plans (NMP) tailored for their blocks’ needs. In many cases, this begins with a focus on nitrogen fertilizer applications. When starting with a new grower, I often find them applying fertilizers at rates that err on the side of “if a little is good, then a lot is better.” Sadly, these application rates are typically what was recommended by their Adviser and rarely is the application rate less than 40 pounds per acre.
Why do I mention 40 pounds per acre rate? Because application rates at or above 40 pounds per acre suppress beneficial microbial activity, regardless of the soil Cation Exchange Capacity (CEC). The soil holding capacity is a critical detail which should be considered in planning nitrogen application rates in order to improve the productivity of the soil and plants growing in it.
If you do need 40 pounds of nitrogen, I have found split applications of 20 pounds per acre to be more effective than one at 40 pounds, especially when fertigating. Keep in mind with split applications, you need to allow time between applications (2-3 weeks depending upon growth stage). In fact, it’s a good stewardship practice to collect a plant tissue sample after the first fertigation, in order to determine if the second nitrogen application or any additional nitrogen is needed.
The goal for your NMP should be increasing soil health, plant productivity and crop quality. By not over-applying nitrogen fertilizers, you will reduce plant stress (due to nutrient imbalances) and maintain productive development.
I’m not suggesting that you don’t need nitrogen. I think we all agree that nitrogen is vital for growth and production. Rather, selecting the right form of nitrogen is every bit as important as getting the right application rate. Let’s take a more detailed look at each of them.
There is ammonium (NH4+), nitrate (NO3-) and urea ((NH2)2CO). Each one has specific modes of action and as such each stimulates a plant response which can work for you or against you. When choosing forms of nitrogen, it is important to consider what you are trying to do with the nitrogen in the selection process. Are you trying to induce a fruiting response (reproduction) or plant growth (vegetative vigor)?
Ammonium nitrogen promotes fruiting or fruit bud development. In tree fruit and nut crops, fruit bud development happens through photosynthetic production of plant secondary metabolites, during the preceding growing season. The Bloom Stage’s success is a report card as to how well-nourished and photosynthetically productive trees were in the previous growing season in the development of fruiting buds. Having the right proportions of ammoniacal and nitrate nitrogen will stimulate the right amount of reproductive and vegetative development for year-on-year productivity. For crops that grow a seed, kernel or pit applying an adequate amount of ammoniacal-nitrogen is important.
Whereas, if you are growing leafy greens or a lettuce crop, you need more nitrate-nitrogen, because it stimulates vegetative growth, which is the part you harvest and get paid to grow. But, nitrogen over-application can be a challenge when the growing conditions and plant uptake don’t match the application rate(s). Whatever nitrate-nitrogen is not absorbed by the plant moves readily into the soil solution and in the process binds with and drives out calcium, especially in calcareous soils, where free calcium is prevalent in the soil solution. All things in Nature seek equilibrium even in the soil and thereby, this nitrate-nitrogen+calcium reaction continues until all the negative-charges from the excess nitrate (NO3-) are satisfied with positive charges. Keep in mind that when nitrogen fertilizer is applied, regardless of the form its final destination is nitrate because of the nitrification cycle.
Neal Kinsey wrote about overapplication of nitrogen, in Hands-On Agronomy, “Nitrogen drives out calcium. When the soil is open and nitrates leach out and go with the water, it is never a solo journey. It always takes along a passenger. If there is a cache of sodium, nitrogen may take sodium. Otherwise it takes calcium. Nitrogen never takes out magnesium, but as nitrogen leaches downward, the passenger status of calcium is assured…There is a corollary. For every percent calcium taken out by nitrogen, magnesium goes up 1%. Removal of 10% calcium by nitrogen over-supply will increase the magnesium level by 10%.”
The late Jerry Brunetti wrote about nitrogen in, The Farm as Ecosystem,
“Nitrogen is the key element to synthesize protein, averaging about 16 percent of protein in plants, and is therefore a key component of chlorophyll, a molecule consisting of a core element of magnesium surrounded by four atoms of nitrogen, as well as carbon, oxygen, hydrogen, sulfur, and iron. Excess nitrogen in plant tissue is a magnet for insect and disease opportunism, especially when compounded by deficiencies of sulfur, calcium, magnesium, phosphorus and boron.”
“Plants generally require one-third of their nitrogen as nitrate, or NO3-. This is because as NO3 is taken in, a hydroxyl (OH-) radical is released, which is alkaline. This exchange is utilized to maintain cellular pH in the plant. However, when excess nitrogen (from any source) or only nitrate (NO3-) nitrogen is used, this OH- molecule builds up around the root hairs and adversely affects the root ecology because the pH around the root hairs becomes too alkaline, disrupting the desirable populations of microbes and inhibiting the uptake of trace elements.”
In the process of applying your spring nitrogen fertilizers, you could create new conditions that reduce plant productivity, soil health and crop quality without a sound nutrient management plan to guide you. Selecting the right nitrogen formulation for your crop(s) needs and the right rate based on the soil texture (CEC) and stage of plant growth and development are all critical details.
Soil temperature is an important factor for soil nutrient release to begin as it is a microbially-dependent activity. In simple terms, at 68°F soil temperature (8” depth), soil-nutrient release begins.
WSU AgWeatherNet average soil temperature data for the following locations:
Prosser: Between the last week of May and first week of June.
Desert Aire: the second week of May.
E. Wenatchee: the first week of June.
N. Brewster: the first week of June.
Cherry harvest typically begins the first to second week of June, which means there isn’t much nutrient release benefit from the soil for the cherry crop. This year, it looks like soil temperatures are significantly warmer than average “normal” temperatures. Check your soil temperatures to compare with current WSU AgWeatherNet data.
Having an adequate nitrogen supply is crucial for sustained growth and development. It doesn’t matter if you are growing tree fruit, grapevines, hops or row crops, nutrient use efficiency starts with availability at a plant and microbial level.
If you have never used a Nutrient Management Plan or had a complete soil work-up done, contact me to set up an appointment. The cost is minimal and the information is valuable. As spring is here, now is the time to learn how the complete line of Axiom Ag products will work for you and your production goals.
Here’s to your crops’ success!
