Tighten the nitrogen cycle in organic farming


Organic fertilizer applied before planting mineralizes before the crop begins to absorb nitrogen. Levels of nitrate released by microbes in the first six to eight weeks after planting are easily eliminated because the crop has not matured enough to absorb it (courtesy E. Wingate.)

Before Haber and Bosch invented atmospheric nitrogen fixation, farmers used manure, compost, cover crops and crop rotations to keep their soils fertile. Farmers relied on biological processes to feed their crops, and the limited availability of nitrogen often compromised yields. Mineral Fertilizer N revolutionized agriculture, bypassing microbial decomposition to directly feed the crop’s excess nitrogen levels. Virtually unlimited nitrogen availability produced significantly higher yields, but excess nitrogen began to pollute groundwater, rivers, wetlands and oceans.

The green revolution has demonstrated the impressive yield potential of agricultural crops and raised the quality expectations of consumers. Modern organic practices seek to minimize nitrogen pollution while achieving crop yield and quality measurements comparable to conventional crops. Carbon-based organic materials provide initial protection against nitrate leaching on the first application; however, once the microbes convert the organic nitrogen to nitrate, it seeps in just as easily. Excessive application of fast mineralizing organic fertilizers negates any environmental benefit provided by organic amendments.

Keep it tight “

The most efficient and environmentally friendly fields have “tight” nitrogen cycles, where nitrate levels remain low, but crops grow vigorously with optimal nitrogen concentrations in leaves and petioles1. Since nitrogen flows between several biological and mineral basins, traditional soil nitrate tests do not always indicate nitrogen sufficiency or deficiency in organic production. Ideally, active microbial populations regularly release nitrogen into the crop, and healthy root systems rapidly take up nitrate and ammonium before it accumulates in the soil solution1.

Nitrogen requirements are well understood for most agricultural crops, and nitrogen uptake curves generally help guide fertilizer application rates. In conventional production, growers can match the nitrogen uptake curve by adjusting nitrate and ammonium applications to meet demand at different stages of growth. Organic growers don’t have the luxury of matching pound-for-pound crop N requirements with fertilizer. Organic amendments provide nitrogen in carbonaceous forms which must be processed by microbes before it becomes available to plants. Bacteria break down amino acids, proteins, and peptides, mineralizing organic nitrogen to form ammonium. Other microbes then quickly convert ammonium to nitrate by nitrification. The yield and quality of the crop largely depend on the match between the timing of nitrogen mineralization and the crop’s nitrogen uptake curve.

Changes applied

While soil quality and environmental conditions influence mineralization, the type of organic amendment applied impacts nitrogen availability more than any other factor. Meticulous soil incubation studies conducted at UC Davis elucidate patterns of nitrogen mineralization induced by commonly used organic fertilizers. Dr Patricia Lazicki and colleagues identified four main categories of amendments based on the mineralization potential of nitrogen6. Categories of organic amendments include (1) yard waste compost, (2) manure composts, (3) granular fertilizers, and (4) liquids, blood and feather meal, and based fertilizers. of guano6.

Growers can apply material from one category or another to meet different needs throughout the year. Fertilizers with high C: N ratios mineralize slowly compared to materials with less carbon and more nitrogen. Lazicki et al. found that less than 5% of the total nitrogen in garden pruning compost mineralized after 84 days of incubation in the soil at humidity and temperature levels comparable to field conditions in California. Green waste compost has a C: N ratio of about 20: 1 and contains relatively little N relative to carbon. Microbial growth increases in response to carbon, but is limited by low nitrogen supply. Microbes incorporate nitrogen into their biomass, temporarily removing it from the available plant reserve.

Manure and animal amendments contain more nitrogen and have lower C: N ratios, so microbial growth slows down due to the limited carbon supply, leaving excess nitrogen available for plants. Soil incubation demonstrated 15-30% N mineralization for manure-based compost applications and 35-55% mineralization when granulated organic fertilizers were used. Category 4, comprising liquid organic fertilizers, blood and feather meal, and guano-based products, had the highest total mineralization. 60-90% of the nitrogen applied with the amendment was mineralized after 84 days. Most of the mineralization has occurred within the first two to three weeks after application. Several other studies have shown similar mineralization rates for organic fertilizers2,3, ⁴, ⁵.

Body fluids and animal products help meet the crop’s nitrogen requirements during rapid growth when the roots take up more nitrogen than basic mineralization can provide. Adding moderate amounts of liquid organic fertilizer improves yield and crop quality, but overuse can cause nitrate build-up and leaching. On the other end of the spectrum, applying green waste compost too close to the plantation can cause nitrogen immobilization and subsequent crop deficiency.

Categories of organic amendment based on the mineralization potential of nitrogen. Results show that microbial growth increases in response to carbon, but is limited by low nitrogen supply (Courtesy E. Wingate.)

Manage germs

Growers can tighten the nitrogen cycle by increasing organic matter, increasing microbial abundance and diversity, and maintaining optimal soil moisture. Soils with low baseline microbial activity are prone to nitrate peaks and drops when fed with organic amendments. The influx of carbon and nitrogen can cause a temporary spike in microbial growth, immobilizing the nitrogen when it is incorporated into microbial bodies. Later, when microbes run out of food, their populations collapse, releasing excess nitrogen into the crop. The increase in biomass and microbial diversity buffers the nitrogen supply. Large, robust soil ecosystems contain growing and endangered populations. Continuous microbial renewal provides a constant nitrogen supply, reducing the likelihood of nitrogen spikes and accidents.

Providing plenty of carbon and labile nitrogen can increase microbial biomass and stabilize nitrogen dynamics1. Growers can apply green waste compost before planting or in the fall before spring planting. The high C: N ratio will likely result in initial nitrogen immobilization followed by slow mineralization. Green waste compost contributes very little to the nitrogen available to plants in the first year after application, but it does increase the microbial biomass needed for a regular nitrogen cycle. Cover crops and reduced tillage also increase organic matter and microbial growth.

After feeding the microbes, growers can feed the crop with granular and liquid organic fertilizers that mineralize quickly. Materials with a low C: N ratio should be applied early in the season and during rapid growth. Apply liquid organic fertilizers at moderate rates every 7-14 days during peak vegetative growth. Including a wide range of amendments in organic systems will increase microbial biomass and soil organic matter while providing plenty of nitrogen available to plants when crops need it most. By carefully managing the use of organic fertilizers and fostering a robust microbial community, growers can deliver all of the environmental benefits offered by organic farming without compromising crop yield or quality.

Eryn Wingate is an agronomist with Tri-Tech Ag Products Inc. in Ventura County, California. Eryn creates nutrient management plans for fruit and vegetable growers to improve crop yield and quality while promoting soil health and environmental protection.

Sources

  1. BowlesTM, Hollander AD, Steenwerth K, Jackson LE (2015) Tightly Coupled Plant-Soil Nitrogen Cycle: Comparison of Organic Farms in an Agricultural Landscape. PLoS ONE 10 (6): e0131888. https://doi.org/10.1371/journal.pone.0131888
  2. Gaskell, M., & Smith, R. (2007). Sources of nitrogen for organic vegetable crops, HortTechnology hortte, 17 (4), 431-441. https://journals.ashs.org/horttech/view/journals/horttech/17/4/article-p431.xml
  3. Hadas, A. and L. Kautsky. (1994) Feather meal, a semi-slow release nitrogen fertilizer for organic farming. Fert. Res. 38: 165-170.
  4. Hartz, TK, & Johnstone, PR (2006). Availability of nitrogen from nitrogen-rich organic fertilizers. HortTechnology, 16, 39-42. https://doi.org/10.21273/HORTTECH.16.1.0039
  5. Hartz, TK, Smith, R. & Gaskell, M. (2010). Availability of nitrogen from liquid organic fertilizers. HortTechnology, 20, 169-172. https://doi.org/10.21273/HORTTECH.20.1.169
  6. Lazicki, P, Geisseler, D, Lloyd, M. (2020) Nitrogen mineralization from organic amendments is variable but predictable. J. Approx. Qual. 49: 483-495. https://doi.org/10.1002/jeq2.20030


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