Of course there is more to plant nutrition than N-P-K, the three letters required to be on every bag of fertilizer, but sometimes it is easy to forget that in the hubbub of commercial garden chatter. This backgrounder covers the basics of the big three nutrients plus some others you ought to know about.
Nitrogen—the N of N-P-K—is the most important plant nutrient, because it is an essential building block of chlorophyll (the green pigment in leaves, without which photosynthesis, and therefore plant growth, cannot occur) as well as a number of other enzymes and hormones central to the plant’s growth processes. It is also the most likely nutrient to be deficient. Its source is the air we breathe, and it exists in the soil only as a byproduct of the soil’s teeming microbial life, rather than as part of the earth’s mineral store of nutrients. When nitrogen is deficient, plants concentrate it in their youngest leaves, so the older, larger leaves turn pale, and in severe cases may wither and fall. Ninety-nine percent of total soil nitrogen is in the organic matter, both living and dead, that the organic gardener is primarily concerned with, and only about one percent is in the soluble inorganic forms found in most quick-fix fertilizers.
Because nitrogen is so essential to plant health, and its supply so fleeting, most plants will consume it far beyond their needs in an orgy of greed, throwing off their own metabolism in the process. Delayed maturity, uneven ripening, and overly succulent growth are all signs of a nitrogen excess. An excess of nitrogen will lead to harvest problems as well. The harvest taken from plants grown with too much nitrogen will not store well, and is likely to be low in vitamins A and C, as well as high in accumulated nitrates, which are toxic.
In the garden environment, nitrogen is in a constant state of change and movement, cycling perpetually through the air, soil, water, and the bodies of plants and animals. It enters the soil in rainwater, or through bacterial extraction from the air, and in the form of applied fertilizers, either organic or chemical. It can be lost back to the air through volatilization, or washed away by rain and irrigation water; otherwise it is available for use by soil bacteria, plants, and the animals that eat them, and then released again upon their death and decomposition.
Phosphorus, represented by the letter P in discussions of chemistry, is also critical to plant growth. While nitrogen fuels the plant, phosphorus is essential to the distribution and storage of that energy in the form of sugars and starches. Without sufficient phosphorus, plants again will be stunted, though the leaves instead of being pale will be purplish from the accumulated sugars created by photosynthesis that cannot be utilized in the absence of sufficient phosphorus. This often occurs in early spring, as phosphorus uptake and utilization is retarded at low temperatures. Phosphorus moves through the garden environment primarily by being used by plants and animals and then recycled in the form of compost or manure, though most manures are relatively low in phosphorus.
Phosphorus is usually added in the form of mined phosphate rock, bone meal, or phosphate fertilizers made from them by treatment with sulfuric acid. Excessive phosphorus is rarely a problem, since nitrogen is more easily absorbed by plants. Where uptake is excessive, though, it can affect a plant’s ability to take up other necessary elements that the plant needs in trace amounts. Once applied, phosphorus will stay put, as it bonds easily with many other soil minerals like aluminum (in acid soils) or calcium (in alkaline soils). Because it becomes “tied up” at high or low pH levels (which we will discuss shortly), phosphorus is much more easily available to plants if the soil pH is kept close to neutral. Organic gardeners prefer to use phosphate rock for building this important soil component, since the nutrients are released slowly over time by the action of soil microbes breaking down its compound forms.
Potassium, listed as K (for its Latin name Kalium). Potassium regulates the processes of plant food creation, transportation, and storage that are fueled by nitrogen and facilitated by phosphorus. Unlike nitrogen and phosphorus, however, potassium is not a constituent of plant cells themselves; rather, it is part of the fluid that fills plant tissues, contributing to the ability of stems and leaves to hold themselves upright. Biennial root crops like carrots and beets are also dependent on potassium to complete conversion of sugar to starch, which makes overwintering of the root and subsequent seed production the following season possible.
Potassium’s cycle in the environment is similar to that of phosphorus, though it is not long held in soil organic matter; it is, however, more available in most manures and in compost made from fresh green materials. While it is mobile within the plant, potassium does not leach, or wash out of the soil, readily; when applied in the form of mined rock power (such as greensand or granite dust) it is available for long-term use by plants. Other good organic sources of potassium are green manure crops such as ryegrass and buckwheat (discussed below) and wood ash, which also contributes a small amount of phosphorus. It is possible to apply too much potassium—especially in small gardens—if you use large quantities of wood ash; this may then lead to a phosphorus or magnesium deficiency.
Three so-called secondary nutrients are now given more credit for the health of growing plants than at first thought: calcium, magnesium, and sulfur. Sulfur never used to be a problem, and it still isn’t for organic gardeners. Even old-time synthetic fertilizers contained sufficient sulfur (as an impurity) for most plant needs. But newer, more refined fertilizers are “purer,” and so sulfur deficiencies have become more of a problem. Deficiency symptoms are similar to those of nitrogen, with which it works in the synthesis of amino acids and proteins that the plant needs. In most manures, composts, and cover crops, sulfur is present in the proper proportion to nitrogen.
Magnesium is the central element in chlorophyll, to which the nitrogen is chemically bound. It performs a function similar to that of hemoglobin in human blood; without it the plant will be anemic. It also relates to phosphorus the way sulfur does to nitrogen. It is sufficiently important to humans that even when a deficiency causes no appreciable damage to plants, their value as food will be lower due to its lack. Unfortunately, potassium, magnesium, and calcium all compete for uptake by plants, and unless care is paid to keep them in balance, deficiencies can result. Fortunately, most composts supply ample magnesium. The simplest solution when starting a new garden in areas with acid soils is to use a high-magnesium, or dolomitic, limestone, thus adding both magnesium and calcium at the same time.
Calcium is important because of the critical role it plays in the structure of cell walls, especially at the growing tips of both roots and tops. But calcium deficiency problems with maturing plants, such as tipburn in lettuce and cabbage, or blossom-end rot in tomatoes, can exist despite relatively abundant calcium in the soil, due to problems with its extraction from the soil by the plant. Specific solutions to these kinds of problems are discussed in the individual plant entries of this site.
A number of other nutrients are needed by the plants in your garden in very small amounts, and are thus called micronutrients. A balanced program of soil enrichment and maintenance pretty much guarantees that none will be seriously deficient, but if in doubt, kelp meal can be used as a micronutrient fertilizer to establish starter amounts of a wide range of different elements. Made from dried seaweed, which has drawn its substance from the diverse ingredients of the world’s oceans, kelp meal is rich in minor nutrients. Just a partial list of the elements contained in dry seaweed includes, beyond nitrogen, phosphorus, and potassium: boron, copper, iron, manganese, molybdenum, zinc, calcium, iodine, a range of sulfates, and—perhaps most important of all—up to 25 percent alginic acid, which stimulates biological activity in the soil and improves soil structure.
That is the key to avoiding nutrient deficiencies in an organic program of soil-building: balance. As we’ve seen, the excessive buildup of one essential nutrient often leads to the displacement of another. Blind feeding of the plants in the garden often leads to impoverishment of the garden itself, and, in the long run, poorer plants.