By Hoorman Soil Health Services
Nitrogen (N) is the fourth most abundant plant nutrient with about 80-85% N sequestered in protein, 10% in genetic components (DNA & RNA), and 5% in amino acids (protein building blocks). Nitrogen makes proteins like enzymes (speed up chemical reactions), hormones (regulate plant functions), and N increases cell growth. Nitrogen strengthens plant cell walls (cellulose), is used in plant energy transfer (ATP), and in photosynthesis (chlorophyll); effecting many plant processes. If a plant has balanced N, it has less disease; but when N is either deficient or in excess, expect more disease and insect problems in the field, garden, with ornamentals, or house plants.
The rate of N application and the form depends on the plant’s life cycle. Nitrogen deficiency or excess N may change the cell wall to become leaky, promoting more diseases. Early on, plants need more nitrates for growth with ammonium sources increasing as the plant matures to increase yield. N stressed deficient plants can’t make full proteins while excess N lowers plant defenses to both disease and insects. Plants typically absorb N in the oxidized form as nitrate (NO3-) or the reduced form as ammonium ( NH4+). Ammonium is 25% more plant efficient than nitrates because it can be easily converted to amino acids but to avoid toxicity, plants need it in small doses and it is easily converted to soil nitrate. Soil health keeps these N forms plant available to optimize plant growth and yield.
Nitrogen interacts with many other plant nutrients. Potassium (K) promotes the increase of nitrates and plant growth, but too much K decreases yield. Adequate phosphorous plus chlorine decreases nitrates and enhances plant ammonium N forms to increase yield. In soybeans, calcium and cobalt are needed for Rhizobium microbes to fix atmospheric N into protein. Supplementing cobalt (a micronutrient) and calcium in soybeans at the right time may increase soybean yields by 3x. Molybdenum, manganese, iron, and magnesium are involved in nitrogen transformations and protein synthesis. As my high school math teacher (Dave Laudick) use to say: It’s as clear as mud. Soil organic matter is a storehouse of many essential micronutrients and allows soil microbes and plants to thrive in a buffered and safe environment. Yes, it’s complicated but worth knowing if yields improve.
Common N related corn diseases are gray leaf spot, stalk rot due to late season N stress (N deficient), and increased aflatoxin due to high nitrates. In soybeans, to much N increases mosaic virus and Rhizoctonia. In wheat, take-all is increased by nitrates, decreased by ammonium; too much N increased powdery mildew; but higher N levels decreases Stagonospora nodurum. Balanced N fertilization is a key to decreasing most diseases.
Time of N fertilization is important. Corn side dressing reduces N leaching and denitrification losses but also decreases Pythium and Rhizoctonia BUT may increase Fusarium and Gibberella stalk rot. Adding a N inhibitor to fertilizer or liquid manure may decrease corn stalk rot by keeping N in the ammonium form late season. In soybeans, avoid over using glyphosate because it chelates or ties up manganese, iron, calcium, and zinc which can affect plant N fixation. In wheat, delaying N fertilizer until spring promotes take-all but avoids excess winter N when its cold and wet, so less Rhizoctonia. Best solution, put on a small amount of N in fall to promote tillering and delay spring N applications until late spring using granular or urea forms of N to reduce foliar leaf stress from liquid N sources.
There are four strategies to reducing diseases associated with nitrogen. The 4 R’s are the right form, right time, right rate, and right place. Use a balanced N fertilizer program with sufficient N in the right form for optimum growth. For corn starter, 25% nitrates and 75% ammonium, is a good mix but placement (2”X2”, 2”X4”) is critical to avoid root stress. Weather, pH, soil conditions (compaction), soil texture, moisture, biological activity, etc. all affect N transformations and plant uptake. Building SOM buffers soils and helps control or moderate these factors. Make timely N applications to avoid N deficiency, excesses, or losses. Modify the soil environment by changing pH (lime), add cover crops to build soil organic matter, reduce soil compaction, add a N inhibitor, avoid over using glyphosate, or supplement with micronutrients to assist in optimal N utilization and less crop disease. Source: Mineral Nutrition and Plant Disease, 2018.