A plant’s nutrition centers around its macronutrients: nitrogen (N), phosphorus (P), and potassium (K), which a plant uses in the largest quantities. Because they are so important to the plant, they should therefore be important to the gardener.
Fertilizers list the NPK values for these three macronutrients on the front label. The first of the three numbers indicate the percentage of nitrogen. Ammonium nitrate (NH4)(NO3) has an NPK rating of 34-0-0. Therefore 34 percent of the weight of the fertilizer is nitrogen, and 66 percent of the weight is something else (hydrogen and oxygen in this case).
The second number is the amount of phosphorus by weight as if the phosphorus was expressed as phosphorus pentoxide (P2O5). This is true even if the phosphorus is in another form. Phosphate Rock with a NPK rating of 0-30-0 indicates that it contains enough phosphorus to create enough phosphorus pentoxide (P2O5) to equal 30 percent of the total weight.
The third number is potassium content by weight as if the potassium was expressed as potassium oxide (K2O). An NPK rating of 0-0-60 for potassium chloride denotes a potassium content equal to 60-percent potassium oxide. NPK ratings are proportions, so a fertilizer with an NPK rating of 2-1-2 has the same relative proportion as a fertilizer made from the same ingredients at 4-2-4. The difference would only be in the percentage of fillers or in the concentration.
While all three are used by plants for most of their life, the proportion needed can change based on the developmental stage. An increase in nitrogen promotes leaf and structure growth, important for strong early development. Higher phosphorus levels promote vigorous flowering and fruiting.
Nitrogen is one of the most important nutrients for plant growth. As a bacteria, it both prepares it for uptake by plants and helps return it to the soil after the plant expires.
In gardens, nitrogen deficiency is the most common nutrient deficiency. Nitrogen is important for the proper development of chlorophyll (the green in leaves) used in photosynthesis. Nitrogen compounds comprise from 40 to 50 percent of the dry matter of plant cells. It promotes large healthy foliage, absorption by roots, and proper plant development and is used in chlorophyll, amino acids, proteins, and nucleic acids production. Trees and shrubs absorb nitrogen directly from ammonium (NH4) well, but flowers and vegetables respond better to nitrogen further processed by bacteria into nitrate (NO3).
Nitrogen-deficient leaves will contain relatively little chlorophyll, tending to be pale green or yellow in color, while the plants’ growth will have slowed. Nitrogen is very mobile in plants, enabling them to readily move supplies where they are most required. Such transference is common from old growth to young growing tips when supplies are short. This mobility and re-utilization of nitrogen explains why deficiency symptoms appear first in the older parts of plants, working their way up to the grow tips. This same type of symptom creep from bottom to top is common to all mobile nutrients.
The Nitrogen Cycle
The air we breathe is mostly nitrogen gas (N2), a fact often overlooked in favor of an emphasis on the amounts of oxygen (O2), carbon dioxide (CO2), or pollutant content. The reason for this is simple; the nitrogen gas in the air is so stable that, for most practical purposes, it can be considered inert. Our bodies can’t extract or make use of it directly. This is because when two nitrogen atoms are attached in a nitrogen gas molecule, they form a triple bond, which makes them less available to interact with other molecules or atoms. Nitrogen is very “vain” in the sense that once it pairs with itself, it tends to shut most of the rest of the world out.
In nature, nitrogen can be collected from the atmosphere by certain types of nitrogen-fixing bacteria such as Azotobacter. These bacteria fixate best in neutral to alkaline, nitrogen-deficient soils. Some of these bacteria form symbiotic relationships with the roots of plants, especially legumes such as clover. Gardeners can use this to their advantage by purposefully planting legumes, and then using the resulting plant material as a nitrogen source for other plants. Lightning strikes can also fixate atmospheric nitrogen, but are harder to cultivate in a garden.
A synthetic method of nitrogen fixation involves the Haber-Bosch process, which uses high pressure, high temperature, and a catalyst to convert nitrogen and hydrogen gases into ammonia (NH3). When ammonia is exposed to acidic conditions, it can pick up an additional hydrogen atom to form ammonium (NH4+). While the Haber-Bosch process has allowed for an increase in the amount of ammonia-based fertilizers available to humans, it does so by imbalancing the natural system and increasing nitrogen pollution.
Nitrogen is returned to the atmosphere by denitrifying bacteria that releases nitrogen gas from nitrates not taken up by plants.
Fortunately for gardeners, there are nitrogen forms that are already fixed that can be used as nitrogen sources. Plant material, animals, and animal waste all contain organic nitrogen, which decomposes into ammonium (NH4+) in a process known as ammonification. In organic gardens, manures and plant meals are commonly added to gardens as source materials for this process.
Nitrification in nature takes two steps. First, ammonia-oxidizing bacteria (AOB) oxidize ammonium (NH4+) or ammonia (NH3) to produce nitrite (NO2). Second, nitrite-oxidizing bacteria (NOB) oxidize those nitrites into nitrates (NO3-).
Plants absorb nitrates (and ammonium to a lesser extent) through their root hairs and convert them to organic nitrogen for use in developing plant material. Animals eat plants (or other animals) to make use of this organic nitrogen for their own cells before returning it as part of the ammonification process above.
Organic vs. synthetic Fertilizers
Organic fertilizers frequently make use of organic nitrogen and are added to source material for the ammonification process. This means that for the nitrogen to become available to plants, the organic nitrogen first decomposes to ammonium and then converts to nitrites (with an “i”), and then finally nitrates (with an “a”). This takes time and tends to complete over a matter of weeks or months. Organic fertilizers tend to be slower acting, and longer lasting than synthetic fertilizers.
In contrast, synthetic fertilizers can simply include nitrates. This allows for a shortcut that eliminates the time needed for decomposition and nitrification. Since nitrates are soluble in water, they can reach the root system quickly. Synthetic fertilizers also tend to be faster acting than organic fertilizers. and should be reapplied more often as they also wash away faster.
In synthetic or organic gardening, the washing away and wastewater aspects are cause for pollution concern, since high levels of nitrates in the waterways can (along with phosphorous pollution) lead to damaging algae blooms and other health risks. Eutrophication occurs when a body of water becomes nutrient rich and the resulting algae bloom and plant growth overloads the water. As the plants and algae die and decompose, oxygen is absorbed at an unsustainable level causing stagnation and a poisoning of the water.
Understanding the natural nitrogen cycle can help a gardener coax nitrogen from where it is to where it is wanted, with a minimum of waste and pollution. Nitrogen in the waterways is not only a pollutant, but a waste of potential resources. The nitrogen cycle is an example of the importance of bacteria in life, and if it weren’t for bacteria, then there wouldn’t be nitrogen for plants, which we need so we can eat, or a way to return the nitrogen in our bodies to the world for reuse.
Phosphorus is required for photosynthesis, blooming, and root development, and is also used to form nucleic acid which is an essential part of living cells. Compounds of phosphorus are used in respiration and the efficient use of nitrogen. It’s important throughout the life cycle of the plant, but use is elevated during flowering.
Phosphorus deficiencies usually manifest as a generalized underperformance of the plant. Leaves may develop a bluish tint. Phosphorus assists in nitrogen uptake, so symptoms of phosphorus deficiency are often similar to those of nitrogen deficiency. An overdose of phosphorus may cause iron and zinc deficiencies.
Rock phosphate is available in two forms: “soft rock” phosphate and “hard rock” phosphate. Soft rock phosphate contains a higher amount of immediately available phosphorus, and is usually the choice for container soil enhancement. Hard rock phosphate is better suited to improve a field where plants are to be grown for several years.
Potassium, also known as potash, is important for photosynthesis, carbohydrate and protein creation, and disease resistance. It’s used in the “plumbing” of the plant: liquid movement within the plant, stems, roots, etc. Many enzymic reactions require potassium, and it assists in silica uptake.
Potassium deficiency often shows as a yellowing/browning/dying of the leaf edges, curled over leaves, followed by yellowing spots in the interior of the leaf face. Discolored spots may also appear on the undersides of leaves. Deficiency symptoms show first on lower leaves as flecking or mottling on the leaf margins. Prolonged deficiency results in cell death along the leaf margins and the plants can show signs of wilt. These symptoms first display in older leaves, and continue to work up through to the newer leaves if not corrected. Growth, root development, disease resistance, and bud size are reduced. Overdosing potassium can result in calcium and magnesium deficiencies.
The NPK rating will state the proportions of nitrogen, phosphorus, and potassium of a fertilizer. To compare fertilizers, read the labels for the NPK rating and listed ingredients. The ingredient list should declare the type of nitrogen used. Considering these factors can help with making an informed decision about choosing a fertilizer that meets a garden’s needs.