Ecologically Sound Nitrogen Management: One Researcher's Educated GuessMark Schonbeck, researcher at New Alchemy Institute
Nitrogen (N) is the most difficult crop nutrient to manage for both organic and conventional growers. Insufficient nitrogen more commonly limits crop yield than any other nutrient deficiency, yet nitrate (NO3-) leaching from agricultural areas is a widespread groundwater pollutant. Soil nitrogen exists in many forms, and the figures for nitrogen on soil test reports are difficult to interpret. Thus managing nitrogen so as to reap the desired yield without causing nitrate pollution is a real challenge. For me, the first step toward meeting this challenge has been to gain a basic understanding of the "eco-chemistry" of nitrogen.
Nitrogen Chemistry 101
Synthetic nitrogen fertilizers generally consist of nitrate and ammonium salts, ammonia and urea manufactured from elemental nitrogen gas by an industrial fixation process. All exist in natural ecosystems, but usually in lower concentration than on a recently fertilized field.
In the soil, soluble nitrogen can be leached out by heavy rainfall, although soil colloids (which comprise the soil's "cation exchange capacity") bind the positively charged forms such as ammonium and amino acids so that they leach less than nitrate. Volatile forms tend to escape into the atmosphere, especially in warm dry weather. Dissolved ammonia (volatile) and ammonium (nonvolatile) exist in an equilibrium regulated by the pH (acidity/alkalinity) of the soil solution. In neutral to acid conditions (pH 7 or below) ammoniacal nitrogen exists almost entirely as ammonium, but as the pH rises to 8 and above, an increasing proportion is converted to ammonia and may vaporize.
The nitrogen cycle
As bacteria, fungi, protozoa, earthworms and other soil organisms feed on organic residues, they convert them into soil organic matter, much of it in the form of humus. Soil organisms "eat" for two purposes: growth which requires both carbon (C) and nitrogen to build proteins and genetic material; and generating energy, which requires carbon (usually carbohydrates) but not nitrogen. When fed nitrogen-rich residues such as dung, the soil life burns some nitrogenous compounds for energy, releasing the nitrogen in inorganic form, primarily ammonium. This is called mineralization, and makes the nitrogen available to plant roots. When served nitrogen-poor materials such as straw, the soil life must take ammonium and/or nitrate from the soil to satisfy its nitrogen requirements. This is called immobilization or nitrogen tie-up.
Humus contains 4-5% nitrogen in a stable form, and is the soil's largest nitrogen reserve. Each year, certain soil organisms decompose about 1-4% of the humus, mineralizing nitrogen and thus completing the cycle. A rich topsoil containing 5% humus that decomposes at a rate of 2% annually will release about 100 lb nitrogen per acre per growing season, sufficient for many vegetable crops. In natural ecosystems, humus is continuously regenerated as soil organisms process plant litter and animal droppings. In agriculture, substantial organic matter and nitrogen are removed from the cycle at harvest, and must be replaced. The organic grower's credo "feed the soil" means returning enough organic matter to the soil each year to replenish the humus.
The nitrogen problem
Certain microbes in the soil generate metabolic energy by converting ammonium to nitrite and then to nitrate, a process called nitrification. In well aerated biologically active soils, any ammoniacal nitrogen released through mineralization is quickly converted to nitrate. Since high concentrations of ammonium and nitrite are toxic to most plants, rapid nitrification is generally favorable to crop growth. However, in sandy, well drained soils nitrate inevitably leaches whenever rainfall exceeds evaporation + crop moisture uptake. When heavy rain falls on more slow-draining soils, air is temporarily driven out of the topsoil. Some microbes cope by using nitrate as a source of oxygen (O2) in a process called denitrification. The nitrate is converted to nitrogen gas (harmless enough) and/or nitrous oxide (N2O), which rises into the stratosphere and contributes to ozone layer destruction. High levels of soil organic matter and nitrate combined with low oxygen levels promote rapid denitrification.
Finally, ammonium ions can become trapped inside certain clay colloid particles (rather than on their surface) making the ammonium relatively unavailable. Normally, this is a two-way street, as trapped ammonium can move slowly back to the surface of the clay colloids.
Ecological impacts of nitrogen fertilizers
Synthetic fertilizers "mainline" inorganic nitrogen into the soil, often accelerating losses and aggravating air and water pollution (Figure 4). In an Iowa State University study, corn fertilized with anhydrous ammonia (a popular fertilizer in the corn belt) took up only 29-45% of this nitrogen, with 49-64% lost via leaching and denitrification. Crops can also volatilize ammonia through their leaves, especially after heavy nitrogen applications. USDA researchers in Georgia reported that wheat given 100 lb nitrogen per acre lost nearly 20 lb of it as ammonia.
When synthetic fertilizers flood the soil with inorganic nitrogen, up to 20% of it may be sequestered by biological immobilization and/or clay trapping. This further reduces short-term fertilizer efficiency, but at least it does not pollute.
Anhydrous ammonia is very toxic to soil life and a few farmers have also died in accidents handling the stuff. Ammonium salts and urea (which releases ammoniacal nitrogen) are safer to handle, but can still stress soil organisms. Although these fertilizers are initially alkaline, both crop uptake and microbial nitrification of ammonium are strongly acidifying processes. Therefore, soils fertilized with them need frequent liming. Nitrate fertilizers are gentler to soil life but of course they leach, sometimes taking with them significant amounts of potassium (K), calcium (Ca) and magnesium (Mg). All synthetic nitrogen fertilizers bypass the nitrogen cycle, throwing millions of beneficial soil organisms out of work.
Fertilizer nitrogen recommendations from soil testing laboratories are often far too high because they neglect contributions by soil humus, manure and leguminous green manures. In studies by Rodale Research Center and others, field corn (a very heavy nitrogen feeder) yielded as much grain on less than half the recommended nitrogen fertilizer as it did on the full amount. Overapplication of nitrogen can render crops more prone to pests, diseases, nutrient imbalances and lodging (falling over during storms). Leaf and root vegetables given too much nitrogen often yield watery produce with poor flavor, short shelf life and possibly unhealthful nitrate concentrations. Overfed tomatoes, peppers and squash grow tremendous foliage but set few fruit.
Organic is not always sustainable
Leaving soil idle can also waste nitrogen. The soil life mineralizes humus nitrogen throughout the growing season, and the nitrogen may leach or denitrify if no crop is present to take it up. The name of the game is use it or lose it, especially in humus-rich soils during warm moist weather.
Nitrogen deficiency and the carbon to nitrogen ratio
A given parcel of nitrogen cannot both feed the current crop and build humus. Thus it is very difficult to reap high yields and achieve a net increase in soil humus simultaneously in the same field. An organic soil amendment may mineralize some of its nitrogen promptly, release some over the next year or two, and contribute the rest to the humus pool. The proportions depend largely on the weight ratio of carbon to nitrogen in the amendment. Materials with a carbon to nitrogen ratio of 25-30, (1.5-1.7% nitrogen on a dry weight basis) are "balanced" so that their initial decomposition neither releases nor ties up inorganic nitrogen (Figure 5). Examples include grass-legume hay, and farmyard manure with straw bedding. Organic residues in this range provide the most favorable conditions for soil life and humus formation. Materials with a lower carbon to nitrogen ratio (e.g. unbedded manure, grass clippings, blood meal) generally release inorganic nitrogen whereas residues with a higher ratio (eg. tough weeds, straw, tree leaves) generally tie it up. Because true humus has a carbon to nitrogen ratio of 10-15 its decomposition always mineralizes nitrogen as well as phosphorus (P), sulfur (S) and other crop nutrients.
Healthy, biologically active soil rapidly digests raw organic matter so that even materials with a fairly high carbon to nitrogen ratio may begin to release nitrogen by late summer. However, in exhausted soil, nearly all of the organic matter may be consumed in regenerating humus and soil life, leaving little nitrogen for the crop. This is why some transition fields at first seem chemical-dependent and yield poorly without their "fix." Once organic matter and soil life have been restored, the annual turnover of humus releases enough nitrogen and other nutrients to support good yields.
Closing and integrating the nitrogen cycle
A farm using these strategies in an integrated system can approximate the closed nitrogen cycle found in nature.
Recycling of on-farm organic "wastes" back to the land is an essential component of sustainable agriculture. Some growers import large quantities of manure or other materials from off-farm sources. While this may build soil fertility it is not truly sustainable since it entails withdrawing nutrients from someone else's land. However, using materials from the surrounding community that would otherwise be wasted is appropriate.
One question the farmer faces is whether to compost. Composting concentrates and accelerates the process of humification which the soil life normally carries out on field-applied residues. Applying raw nitrogen-poor residues to a crop is growing may hurt yield by tying up nitrogen, while nitrogen-rich materials may pollute the groundwater or atmosphere, or "burn" the crop by releasing ammoniacal nitrogen too rapidly. Also, some fresh residues release substances that depress crop growth.
By gathering organic residues into a compost heap and providing the right balance of air and moisture, the farmer can effectively convert them into soil organic matter within 3 to 24 months. Hot composting (140 degrees F maintained for a week) with turning to get all parts heated, kills most weed seeds and crop pathogens. However excessive heating (over 160 degrees) drives off nitrogen as ammonia, and should be corrected by turning and watering the pile. As the material composts, its carbon to nitrogen ratio decreases toward that of humus (10-15), at which point some of its nitrogen becomes readily available to crops. Well ripened compost will not "burn" crops, tie up nitrogen or release allelopathic compounds.
However, composting can be very labor-intensive, and has other limitations. Carbon and nitrogen in the starting materials must be fairly well balanced. Mixtures with carbon to nitrogen ratio less than 25 will lose substantial nitrogen as ammonia during initial heating. Mixtures above 40 may decompose slowly, fail to heat up, and cannot yield as much true humus (which contains 4-5% nitrogen). Mixtures at 25-35, containing nitrogen rich materials (grass clippings, garbage, manure, succulent young weeds) and carbon-rich materials (leaves, straw, stemmy weeds and crop residues) work best. Sawdust and wood shavings decompose very slowly and may give poor results if they comprise a large portion of the mixture.
In fully ripened compost, some nitrate may be released through the same processes that occur in the soil. Since this nitrate is subject to leaching and denitrification, Robert Parnes of Woods End Agricultural Institute has recommended that compost be used when it is only partially humified, with a carbon to nitrogen ratio of 17 to 20. This might better nourish the soil life, since fully ripened compost is essentially "soil organic matter" and bypasses the "residue" phase of the nitrogen cycle (Figure 6). Also, in situ decomposition of coarse residues may build soil structure more effectively than adding finished compost. During crop production, nitrogen-poor materials like straw can be used as a surface mulch, which will not tie up much N in the root zone.
Manure presents a particular challenge because an enzyme in the feces rapidly converts urea in the urine into ammonia. Fresh manure applied to fields and left on the surface can lose half its nitrogen within a few days during warm dry weather. Injecting liquid manure a few inches beneath the soil surface, or tilling solid manure in immediately after spreading can minimize nitrogen losses. However, field-applied raw manure may "burn" sensitive crops, and can pollute groundwater with nitrate especially when spread in winter or on idle ground. Many organic certification programs forbid raw manure on leaf or root vegetables because of the dangers of human pathogens and of nitrate buildup in the produce.
When manure must be spread raw, it should be applied to vigorously growing grass forages or cover crops at moderate rates (5-15 tons horse or cattle manure, 3-10 tons sheep or pig manure, or 2-5 tons poultry manure per acre). Pasturing livestock at reasonable stocking densities may be the most ecologically sound way to spread raw manure, since the amount deposited is well within the ecosystem's ability to process it.
Manure without bedding stored in an open pit or lagoon may lose up to 80% of its nitrogen; thus unbedded manure should be fermented in a closed cistern, bunker or biogas generator. Organic acids produced during fermentation lower the pH and convert ammonia to nonvolatile ammonium ion. Because the manure also contains calcium, magnesium and potassium, it will not acidify the soil like ammoniacal fertilizers do, and can be beneficial when used in moderation.
Composting manure with carbon-rich materials such as straw, tree leaves, waste paper or certain food wastes can stabilize nitrogen and produce a high quality amendment that will not "burn" crops. Successful examples include poultry manure + cranberry waste (Mass Natural Compost) and dairy manure + newspaper or cardboard (pilot project conducted in 1986 by Bruce Fulford at Seaside Dairy in Massachusetts). Providing livestock with ample straw bedding can accomplish the needed carbon-nitrogen balance. Sawdust and wood shavings are less effective because they resist microbial breakdown, whereas the newly-available shredded newspaper bedding controls odors better than sawdust and composts well with manure.
Even bedded manure may release ammonia faster than microbes can immobilize it, especially during hot composting. Peat moss, colloidal phosphate, zeolite clay have been used successfully to bind this ammonia. Mixing soil (about 10% by weight) into the compost can also be effective if the soil has a fair amount of clay and/or humus.
Growing and conserving nitrogen in place
Young, succulent legumes decompose rapidly after they are tilled in, giving a high fertilizer value, but not building much humus. Again, the "use it or lose it" rule applies. More mature legumes or legume-grass mixtures, having a higher carbon to nitrogen ratio, yield a lower fertilizer equivalent but contribute more humus. Rotating to a perennial legume-grass sod for two years or longer builds soil structure, humus and organic nitrogen. However, breaking the sod causes a burst of mineralization, and should be done just prior to planting a heavy feeder like corn. Leguminous vegetables from which a harvest is taken (e.g. peas, beans) often add little nitrogen to the soil, since much of the nitrogen is in the pods and seeds.
Nonlegume green manures can serve as "catch crops" planted after harvest to absorb "leftover" inorganic nitrogen, thus minimizing losses (Figure 6). Winter rye grows anytime the temperature is above freezing, absorbing up to 60 lb soluble nitrogen during late fall and early spring. Annual ryegrass, while not as hardy as rye, is an excellent nitrogen scavenger for southern New England if planted by September 15. Oats planted in August will "catch" nitrogen in the fall, then winterkill leaving an easy-to-till residue for early spring planting. Overseeding a catch crop into vegetables prior to harvest keeps the soil continuously covered by live plants, thus further conserving nitrogen. Land that comes out of production during summer can be planted to a warm-season crop like sudangrass or buckwheat. Since buckwheat is a light feeder, sudangrass may be the crop of choice for a really rich soil.
Getting just enough nitrogen at the right time
In healthy, biologically active soils, exact nitrogen accounting is less important than regulating mineralization of humus nitrogen to parallel crop needs and replenishing the humus consumed. Conditions which accelerate organic matter decomposition and nitrogen mineralization include tillage, annual crops, sandy soil, warm, moist, well aerated soil, and alternating wet/dry cycles. Conditions which retard these processes include no or reduced tillage, perennial crops, clay soil, cold, dry or waterlogged soil and the presence of nitrogen-poor residues. Tillage is an especially potent stimulant of mineralization, typically releasing 25-30 lb nitrogen per acre per operation, and up to 100 lb or more when breaking a sod.
Careful balancing of factors which speed and slow mineralization is an essential part of nitrogen management. Thus a wet clayey soil might be cover cropped in sod to improve structure, drainage and humus content, then tilled thoroughly to release nitrogen before planting a vegetable. In contrast, sandy soil should be tilled as little as possible, especially during summer, to avoid wasting humus and nitrogen. Summer weeds might be controlled by heavy mulching rather than cultivation. On sandy soil at New Alchemy, we have experimented successfully with mowing a rye + vetch cover crop in late May and planting broccoli through the resulting mulch without tillage. Nitrogen release appeared adequate and the mulch suppressed weeds.
On a newly-organic farm in Nova Scotia, David Patriquin of Dalhousie University observed nitrogen deficiency and poor yields in oats, and traced the problem to inadequate drainage. Ridge tillage helped drain and warm the soil in spring, releasing more nitrogen and considerably enhancing oat yields. Nitrogen fertilizer might have elicited a similar yield increase, but would only have masked and possibly aggravated the underlying problem.
Mineralization rates increase with soil temperature, from very slow at 45 degrees F to rapid at 80 to 100 degrees F. Thus the release of nitrogen from humus roughly parallels the nitrogen demand of a warm season vegetable crop like summer squash or sweet corn (Figure 7). The nitrogen demand of a fall vegetable peaks when mineralization rates are declining, but nitrogen left over from summer may suffice. Early spring vegetable crops, often an important source of farm income, present the greatest challenge because their growth peaks while the soil life is just beginning to wake up. The result is nitrogen deficiency, and a moderate application (30-50 lb nitrogen per acre) of fast-release nitrogen such as blood meal may be justified. Growing a legume the previous fall that winterkills (e.g. fava bean) may provide available nitrogen for early spring crops, but may also cause some nitrate leaching. Other strategies include planting on a well-drained south-facing slope, ridge tillage, row covers, generous application of well ripened compost (which is dark-colored and contains some nitrate), and foliar feeding. Fish fertilizers, manure compost tea or other natural liquid fertilizers applied directly to leaves give a much larger growth boost per pound of nitrogen than soil-applied fertilizers.
With a combination of these strategies, and probably dozens more conceived by innovative growers, we can chart a course between the twin rocks of yellow stunted crops and polluted wells to a sustainable, biologically based soil fertility.
I would like to thank Ralph DeGregorio of New Alchemy Institute, William Brinton and Robert Parnes of Woods End Agricultural Institute, Marianne Sarrantonio of Rodale Research Center and Win Way of University of Vermont Extension for sharing in person their wisdom on nitrogen in agricultural systems, without which I could not have integrated the myriad bits of book knowledge into a coherent conceptual picture.
This page was last modified on February 25, 2004 at 9:30:15 PM.
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