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Nitrogen in the Garden

If you are an organic grower and are new to using feathermeal or seed meals you are in for a pleasant surprise. If your garden is small and needs to be very productive you will be delighted with the response. In this photo, you can see how a generous application of nitrogen (along with some boron and gypsum – calcium sulfate, for the sulfur) can change a garden:

Both Veronica Broccoli plants were grown in mineral balanced soil, but the plant on the left received a fall application of feathermeal, gypsum, and boron. ALL the minerals need to be present for great results.

In our own garden we want to grow the best quality (nutrient dense) food we can. Our secondary goal is the production of fresh and preserved vegetables, year-round. So, our gardening is skewed toward quality and efficiency, without too many other constraints. Our limitation is labor — we do all the vegetable gardening ourselves. For us, it makes sense to run the garden with the accelerator pretty much wide open. By adding nitrogen, we can plant closer together and get a larger harvest from a smaller area. This makes mulching and weeding easier. Honestly, we like harvesting quantities of large, great tasting vegetables. But, this strategy is not for all circumstances. If you space your plants further apart you will get fewer but larger vegetables. You won’t need as much nitrogen. Your plants will be more drought tolerant. Your plants can maybe live on less applied water, as they will be drawing moisture from a larger area. Determining nitrogen application rates requires understanding your garden’s limits to growth, and some real thought about what you are trying to accomplish in your garden.

Nitrogen, Life, Fixation and the Atmosphere

Nitrogen is an essential component of all plant life; it is needed for building proteins, DNA and chlorophyll. Plants take it up from the soil, not from the air. In many soils nitrogen can be the limiting nutrient for plant growth.

A plant’s need for available nitrogen is not always convenient for the garden-farmer. There is a delay between planting and peak nitrogen uptake which makes it difficult to know when and whether to fertilize, and how much to fertilize with. To optimize growth, it is important to understand as much as possible about how nitrogen works in the soil and in the plant. (Figure at right is from this reference.)

Even though nitrogen is the most common gas in air, plants are not capable of utilizing it directly. Even leguminous plants which are commonly known as nitrogen fixers rely on a symbiotic relationship with the diazotrophic bacteria that do the actual N fixation.

Corn takes up the most nitrogen when the plant is rapidly growing.

In the atmosphere nitrogen exists almost entirely in the form of N2 – two nitrogen atoms bound together by three covalent bonds. These bonds are quite strong, and it is a good thing that a natural process exists that can break these bonds; our lives depend on it! The process relies in part on a reaction involving an iron molybdenum cofactor. Molybdenum and iron (or -vanadium nitrogenase) must be present in the soil for nitrogen fixation to occur.

Besides nitrogen fixation, nitrogen is naturally available in the soil as a product of the decomposition of once-living beings. Nitrogen cycles from living being to living being via the soil.

According to many university agricultural advisors, the main plant-available forms of nitrogen are nitrate (NO3-), ammonium (NH4+) and urea ( CO(NH2)2 ). These are the plant-available nitrogen sources supplied by non-organic, chemical-industrial fertilizer, and they have been studied extensively. Other plant-available forms of nitrogen exist, such as amino acids, and even more complex forms of nitrogen such as proteins have been shown to be utilized by plants (see this ref). Recently it has been found that roots “eat” entire microbes (see this ref.), strip their nutrients, and send the remains out to gather more! There is more to nitrogen supply than the agro-chem industry would have us think!

Carbon, Nitrogen and Microbes

In the soil, the transformation of once-living materials into plant-available nitrogen happens due to microbial activity. Besides nitrogen, microbes need carbon in large quantities as an energy source and to build their bodies. When we “feed the soil” with cover crops and fertilizers, it’s dinner time for the microbes. Given ample food, moisture and warm conditions, their populations explode.

According to the USDA, microbes need a diet of about 24 parts carbon to 1 part nitrogen. Of that, they use 16 parts carbon for energy and 8 parts to maintain their bodies. Any excess carbon will remain in the soil. Any excess nitrogen will be available to plants.

Since microbes are responsible for converting once-living materials into plant-available nitrogen sources, the carbon-to-nitrogen ratio of the amendment source determines the rate and amount of nitrogen made plant-available during decomposition (see here and here.) If the carbon-to-nitrogen (C:N) ratio of the source is too high and is approaching 20:1, the microbes will use up the plant available nitrogen in the soil for themselves, leaving none for plant growth.  It’s the amendments and composts with a C:N ratio less than 15:1 that leave provide excess nitrogen in the soil for plant growth.

Stable soil organic matter has a C:N ratio between 11:1 and 9:1 (see this ref.) and a large amount of the nitrogen is in the form of amino acids, a plant accessible form of N. These forms are stable in the soil if organic matter levels are maintained or increased.

Nitrogen Release Rates

Few composts and fertilizers release their entire nitrogen component in the first season. Part of the remaining unreleased fraction may be held over from season to season to provide an increasing pool of available nitrogen. The amount retained from year to year depends on several factors. Nitrate nitrogen, the most common source of plant-available nitrogen in the soil, is easily leached, so more is lost in rainy locations. In light soils water moves through the soil more rapidly, so more nitrogen can be lost to leaching than in clay soils. In hot climates, microbial activity is higher and both carbon and nitrogen are released at higher rates.

Predicted N release from various amendments when incorporated into warm, moist soil. From this ref.

Climate plays a big part in how much organic matter can be built up in the soil. Soils have a natural soil organic matter percentage that they tend toward. It is very difficult to maintain high soil organic matter in an irrigated soil in a hot desert climate. And it is relatively easy to build up organic matter in a cool climate with a long winter. In areas where nitrogen is easily leached or lost to the atmosphere, it’s like the meter is running. If nothing is planted, the nitrogen may just go away.

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