Fertilizer: All About Nitrogen

N-P-K, Part 1 – Nitrogen by Steve Jones, Master Rosarian



We have all seen those three numbers on all bags and bottles of fertilizers. They are probably the most important numbers a gardener and rosarian should know. These are the amounts of the “big three” elements for plant nutrition. The first number is Nitrogen, expressed as the element N, the second is Phosphorous, expressed as the compound phosphoric acid, P2O5, and the third is Potassium (Kalium), expressed as the compound K2O, or potash.


In the first article of this series, we will concentrate on nitrogen, probably the most important of the plant nutrients.


Nitrogen is the most abundant element in the earth’s atmosphere comprising of 78% of the total atmospheric volume. Normally nitrogen is expressed in its normal state, N2, two nitrogen molecules held together to form a stable compound. Nitrogen is a colorless, odorless gas, and is a vital element in plants and animals alike. When divers come up too fast from a deep dive, they suffer from bends, which is caused by too much nitrogen in the bloodstream. Plants themselves only contain about 2% nitrogen by weight, but it is critical for the plant’s growth and survival.


Nitrogen is important in plants for good strong growth, cell growth, plant respiration, good blooms, and dark green foliage. Few plants can take in nitrogen directly through the air, but generally the element itself is inert in this form. The nitrogen has to be collected and converted to make it available to the plant. This process is called nitrogen fixation. Nitrogen can be collected and converted in several ways: by microorganisms living in the soil (commonly Azotobacter, Nitrosomonas, and Clostridium), by symbiotic rhizobia that live on the roots of plants especially legumes, and atmospheric changes such as lightning and air pollution. In the latter case, nitrogen compounds will dissolve in rain and drop onto the ground. On the East coast, there is a big problem with high nitrates in the air as it forms nitric acid with water, hence the name, acid rain.


For most of history, nitrogen was not available to plants through manmade sources. A common way to get nitrogen into the soil was planting nitrogen fixing plants. Alfalfa, clover, and soybeans collect large amounts of nitrogen in their roots from the air, and it becomes available to new plants when the old crop is tilled under. This is why farmers will let a field go “fallow” or rotate crops, to replenish the nitrogen in the soil. Another added benefit is the farmers can harvest the plants for crops or animal feed. Animal manures are also a good source of nitrogen.


Generally, nitrogen is added to the soil either naturally or by manmade fertilizers. Please refer to the diagram of the nitrogen cycle in this article. Nitrogen is added to the soil from various sources. The materials are changed in the soil by microorganisms to forms that can be used by the plant. Plants need nitrogen to be in a soluble form of nitrate, NO3-. This is why nitrate fertilizers are among the best because they require little change to be available to the plant. Because most nitrogen compounds are very soluble in water, they will quickly pass through the plants feeding zones if the soil is too sandy or porous. This is one reason rosarians need to feed nitrogen to their roses constantly.


Forget the old wives’ tales, plants cannot determine if the nitrogen is from manmade or natural, organic, sources. The nitrogen, may it be in a nitrite (NO2-), ammonia (NH3), ammonium (NH4-), urea, or nitrate, will be converted by the microorganisms, and the plant will never know where it came from. The question is when the nitrogen is added to the soil, what form, and how long will it take to convert it into usable food for the plant.


Most organic materials need to be converted and since it is a multiple step process, the nitrogen may not be available to the plant for up to two months. This is why organic fertilizers as manure and alfalfa are applied soon after pruning, so they have time to breakdown and be available for the plants by springtime. There are some fast and slow acting forms of organic nitrogen. Blood meal and fish emulsion are fast acting nitrogen fertilizers, where manures and compost are slow acting. Organic fertilizers tend to have low amounts of nitrogen. Blood meal is one of the highest at 13% nitrogen, while others are much lower, such as manures and sea kelp (about 0.6%). Humus is an organic material that cannot be broken down to available nitrogen and is a poor source of nutrients. Take care of uncomposted materials such as sawdust and straw, which will actually use up available nitrogen as they decompose.


Once the nitrate is absorbed by the plant, it is converted to ammonium using the energy from photosynthesis. The resultant compound forms the skeleton for glutamic acid, which is then converted to amino acids, which are the building stones for proteins and the genetic makeup of the plant, including DNA and RNA. In turn, most amino acids end up as enzymes, and controls the many plant functions such as: metabolism, photosynthesis, and respiration. Nitrogen is also a vital element in chlorophyll, the “green” in our plants. It is interesting that the microorganisms will convert the ammonium ion to a nitrate, then once in the plant it is converted back.


Manmade or chemical fertilizers come in several forms. Most are water soluble, which means they readily dissolve in water. Urea has some of the highest amount of nitrogen available, 45%, but is not readily available to the plants. In pure form, urea can burn the plant, so a common form is urea-formaldehyde (often called ureaform), which is a slow release fertilizer. Most slow release fertilizers take two to six months to dissolve. As water washes away each layer, it releases small mounts of fertilizer. Osmocote is a common slow release fertilizer. They are excellent for use in potted plants. Water insoluble fertilizers are usually labeled as W.I.N., and require breakdown by microorganisms. This generally applies to most organics.


Manmade fertilizers are generally manufactured. Common manmade fertilizers rich in nitrogen include: urea, ammonia, and the nitrate and nitrite forms of ammonia, calcium, potassium, and sodium. Many of these are too expensive to use in fertilizers, so the cheaper and often slower acting fertilizers are what we see in most store bought fertilizers.


Any fertilizer than is high in nitrogen can burn the plant. The fertilizer will damage the fine root hairs and stop all nutrient uptake. Too high of a nitrogen fertilizer may even kill a plant. A rosarian once added straight fish emulsion to his potted roses; they all died. You have to be very careful with strong fertilizers. Pure urea and uncomposted animal manures can also burn. Steer manures often have high salt concentrations, so it is best to not use them.


The key to whether roses need or have too much nitrogen can be determined by looking at the leaves or the plant. Deficiencies of nitrogen are first noticed on the older leaves; they turn a pale green color then to yellow, lose their luster, yet remain on the plant, overall plant growth is inhibited, the stems become weak and spindly, and bloom size is reduced. Luckily the deficiencies can be quickly reversed. Oxygen deficiency is similar, but adding nitrogen will not correct it.


Excess nitrogen can look like a deficiency. The new leaves are chlorotic, which is a sign of iron deficiency. However, iron will not correct it. Also, foliage tends to be large and excessive in size. For some sensitive varieties, high nitrogen will produce vegetative centers, where the bloom looks as it swallowed a bloom of Green Rose. Lady Sunblaze, The Temptations, and Marchesa Boccella are examples of nitrogen sensitive roses. Exhibitors need to be concerned about an excess of nitrogen. Also, too much nitrogen late in the rose growing season will cause lots of new growth that will not have time to harden and will winter kill in colder regions. Constant use of nitrate fertilizers will in time acidify your soil. So if you use a lot of nitrogen based fertilizer, make sure you test your soil pH. Nitrogen uptake by the plant is also affected by pH of the soil, but since the acceptable range is so large (5.5 – 10), this is rarely a problem.


Most general chemical fertilizers come as 8-8-8 or 10-10-10. Fertilizers with very high nitrogen (the first number), are normally used for lawns. Miracle-Gro and Peters are two water soluble fertilizers with a 20-20-20 rating. These are applied either by mixing in a bucket or with a hose end applicator. Both use urea, ammonium phosphate, and potassium nitrate as the sources of nitrogen. The latter two compounds have about 13% available nitrogen. Bandini Rose Food, a common fertilizer, is rated at 6-12-6, and uses ammonium sulfate and ureaform for nitrogen.


In the next series, we will look at the second number of the “big three,” Phosphorous.