Nitrogen is Too Often Overused in Gardens and Agriculture
Nitrogen is a macronutrient meaning it is needed in large quantities by plants. It is used in plant growth because it is integral in the making of new cells and energy (photosynthates from photosynthesis). Nitrogen plays an important role in amino acids (precursors to proteins which do just about everything in a plant cell much like human cells) and in DNA and RNA replication.
Nitrogen comes from two places for plants – the soil and the atmosphere. It’s impossible for plants to directly access N2 from the atmosphere but, some of their surface microbial partners can. More about this below. Though soil contains a lot more nitrogen (N), plants can access it. It must be modified (processed by microbes) into ammonium NH4+ or nitrate NO3-. Plant microbes make these using atmospheric N moving into soils or after recycling N from plants and other organisms in organic matter. Therefore keeping the litter from plants on your soils leads to healthier soils. Microbes are relying on larger creatures e.g., arthropods, to break down plant tissues into smaller pieces they can then break-down further. Thus, a healthy soil includes a lot of bugs as well as fungi and bacteria.
The great thing about clay soils, they have a lot of surface area upon which things like positively charge ammonium ions can bind. Which means they can hold the ammonium over long periods of time so roots can access as needed. How is this? Clay soils are negatively charged thus positively charged ammonium is attracted. Nitrate on the other hand moves fast through the soil; there’s nothing to hold it. Movement can be as a gas with the help of bacteria (volatilizing) causing the N to move back to the atmosphere. This is a serious source of greenhouse gas emissions from agriculture. The other loss of N from soils is via leaching as water (rainfall or irrigation) moves it through to deeper soils layers or with soil erosion off into waterways. This is a serious pollutant leading to algal blooms killing fish in rivers and bays.
Why in the world would I suggest it is overused in gardening if it is such an important nutrient to cellular function, making of cells and thus plant growth and health? Three reasons, 1) too much nitrogen means plants growing too fast and for perennials there is a trade-off, and 2) too much fertilizer can break down the beneficial relationships between plants and their microbes and in effect killing those microbes. The third reason is because the damage done to other soil fauna; the soil should contain and ocean of diversity, but high nitrogen inputs reduce the diversity of soil organisms leading to sterilized soils with poor diversity and abundances. All of this leads to a feedback in which more N fertilization is needed because the soil habitat is breaking down. Fast growing plants are weak wooded, or attractive to pests because of the C:N, and have poor immune systems.
Figure 2. Ammonium ion
Over–used N fertilization leads to more pest pressures. How can this be? There is a thing called the C:N ratio, the fraction of carbon relative to the fraction of N in plant tissues. From the perspective of plant eaters, lower carbon means easier to digest (less energy). Research repeatedly demonstrates the lower C:N, the more insects and pathogens will attack that plant. Over fertilization makes your plants attractive to pests! To make the situation even worse, N-fertilization speeds up plant growth resulting in lower plant immune function. The fertilization is basically telling plants, “Grow fast die young". Instead of making chemistry to fight pests or building tissues as barriers, plants focus on increased numbers of cells.
What is the alternative? First, get a soil test to determine how much nitrogen your soils need and in what form to apply it. Second or alternatively manage your soils using ecology. The following list of plants (from ruralsprout.com) will fertilize (fix nitrogen) your soils. They will also feed the organisms in your soil as well as neighboring plants. Keep dead plant tissues on the soil for organisms to consume. Both a diversity of plant rooting types and organisms in the soil make the soil structure more hospitable. Structure refers to the physical properties of soil which are either friendly to living things, or not. Compacted clay is a great example. It has terrible soil structure; it is too dense for roots or other organisms to locate pockets of air and water. Plant roots need air, plants are aerobic organisms not anaerobic. Poor structure of soils leads to low water holding capacity. It will increase erosion of topsoil and it if water-logged can favor pathogenic fungi.
PLEASE, be sure to double check – are any of these species considered weedy or invasive where you live?
Short Table of N-fixing Plants
All beans and peas