Tissue Testing For Nitrogen And What Lab Tests To Ask For
Nitrogen is an essential plant nutrient required by all crops. Tissue tests are conducted routinely on crops to know if adequate nitrogen is available to meet the crop’s needs and to produce high yields. However, in testing plant tissues there is no test to measure for “nitrogen”. There are different forms of nitrogen and each form has different consequences on the growth and yield of crop. Below are some forms reported in tissue analytical results and what they mean in regards to understanding the nitrogen nutritional status of crops.
Total N
Total N is the sum of all forms of nitrogen inside the plant. This includes amino acids, proteins, and other organic forms of nitrogen. It also includes nitrate, ammonium, and other inorganic forms of nitrogen. Most tissue tests include Total N as an indicator of the overall nitrogen status of the crop. This is useful information, but does not give enough information to know if the crop has enough “usable” nitrogen or if the crop is nitrogen deficient.
Ammonium
Ammonium (NH4+) is the form of nitrogen that plants use to make proteins and other essential compounds inside the plant. However, ammonium is a free radical inside plant cells and is highly toxic. Crops seldom have more than 5 ppm (mg/kg) of free ammonium inside plant cells. To detoxify the ammonium the plant quickly converts it into protein, using available carbohydrates as the energy to do so. High levels of ammonium taken up by a crop, especially young plants with low carbohydrate reserves, can temporarily stunt the growth of the plants. In small seeded crops, like tomatoes, high does of ammonium fertilizer can actually kill seedlings. However, for most crops the amount of ammonium inside the plant tissues is so small that testing for ammonium in tissues is not needed.
Nitrate
Nitrate (NO3-) is the predominant form of nitrogen in the soil when soil temperatures are warm. Nitrate is taken up by the plant just as easily as ammonium. However, nitrate is not toxic inside the plant, and the plant can accumulate high levels of nitrate in the tissues without harm. Unlike ammonium, nitrate cannot be converted directly into protein and other organic forms inside the plant. Nitrate must first be converted into ammonium using a process called nitrate reduction. Enzymes inside the plant convert nitrate into ammonium. These enzymes contain molybdenum. Without molybdenum, nitrate remains as nitrate in the tissues and cannot be used by the plant for growth.
Organic Nitrogen
Organic nitrogen is nitrogen that is held in proteins and other organic compounds. To calculate the quantity of organic nitrogen in tissue subtract the nitrate levels from Total N.
(Total N – Nitrate = Organic N).
Molybdenum
Molybdenum is a metal and obviously is not a form of nitrogen. However, it is an essential plant nutrient used in the enzymes to convert nitrate to ammonium. The more molybdenum in the plant tissues, the more of these enzymes are formed. With more nitrate reducing enzymes formed, the faster nitrate will convert to ammonium for use in plant growth. When molybdenum is non-detectible in corn tissues, nitrate levels as high as 4,000 ppm have been found (with the plant not being able to use this nitrate for growth). Once molybdenum is applied, nitrate quickly converts to ammonium for plant use. Molybdenum is the most common micronutrient deficiency in agricultural soils worldwide, partly because it leaches from the soil just like nitrate and is poorly available in the soils with pH < 5.5.
What to Test for in Crops for Nitrogen Status
When trying to understand the nitrogen status inside a crop, include: Total N, nitrate and molybdenum in the laboratory tests. Most laboratories will test for molybdenum in tissues and soils, but it must be requested.
What to Test for in Soils for Nitrogen Status
In soils, test for: ammonium, nitrate, Total N and molybdenum. In soil the organic form of nitrogen can be calculated by subtracting nitrate and ammonium from Total N
(Total N – (Nitrate + Ammonium) = Organic N)
AgriGuardian™ markets all essential plant nutrients, including all individual micronutrients such as AgriGuardian Moly™ (molybdenum). Molybdenum is an essential plant nutrient required by all crops. As indicated above, if molybdenum levels are low in crop tissues, nitrate may be accumulating in the tissues and are unusable. AgriGuardian Moly™ can help to get the full benefit of the available and applied nitrogen in both the plant and in the soil, especially if nitrate levels are high. Applying AgriGuardian Moly™ in such cases is much more economical than applying additional nitrogen. Have crop tissues tested to know the nitrogen and molybdenum status of your crops.
For more information on using tissue testing to improve nutrition in over 1,400 crops, get a copy of Plant Analysis Handbook III, available from your AgriGuardian distributor, or available online through Amazon and eBay. Also, contact your local AgriGuardian™ distributor for more information on AgriGuardian Moly™ and other AgriGuardian™ products, or visit AgriGuardian.com.
