Leaching Characteristics of Organic Fertilizers

What is an Organic Fertilizer?

An organic fertilizer is derived from natural materials. From a biological perspective, this means that the fertilizer is comprised of animal or vegetable matter, or by-products of living organisms. These can include bone meal, blood meal, feather meal, meat meal, fish meal, seaweed meal, or extracts of any of these meals. Similarly, amino acids, which are the water-soluble building blocks of proteins, can also serve as organic fertilizers.

Do Organic Fertilizers Leach?

Leaching refers to the event whereby a material (such as a fertilizer) dissolves in the soil water and moves away, ultimately into groundwater or surface water. All soluble materials, including fertilizers, can leach. The real question is whether a fertilizer leaches rapidly or slowly. If leaching is very slow, then the chances are good that most of the fertilizer will eventually be consumed by the local plants and animals, leaving very little to can escape into the ground water. If leaching is very fast, then there is little time for the fertilizer to be consumed locally, thereby allowing a large percentage of it to rinse through the soil and into ground water.

What Determines Whether a Fertilizer will Leach Fast or Slowly?

Leaching Rate is determined by the water-solubility of a fertilizer combined with the amount of soil water at any time. Soil type and compaction also affect the leaching rate of fertilizers. If a fertilizer is completely water-soluble, it will remain in the soil only as long as the soil is reasonably dry. But if it rains, the water will dissolve the fertilizer quickly, and some will flow away with the water, especially if the soil is porous. If it rains heavily, a lot of the soluble fertilizer can be lost rapidly. Ultimately, it will end up in the ground water or surface water, where it can contribute to pollution.

If a fertilizer is insoluble in water, it will not leach. However, an insoluble fertilizer cannot be absorbed by plants. Plants can only absorb materials when they are dissolved in water. Therefore, insoluble fertilizers are not immediately available to plants. Such a fertilizer only becomes available when it changes into a water-soluble form. Organic fertilizers are changed by soil microbes like bacteria and fungi. These microbes release enzymes to dissolve the organic material so it can be absorbed into the bacterial or fungal cells. However, the bacteria and fungi do not manage to absorb all or the dissolved organic material. Some is stolen by nearby plant roots, and some can leach away, especially when it rains.

Do Organic Fertilizers Leach Fast or Slowly?

Most, but not all organic fertilizers are insoluble. Insoluble materials do not leach. What actually leach away are the soluble by-products (nitrogen forms and phosphorus, potassium, calcium, magnesium, iron, etc.) from microbial digestion (called “biodegradation”). Since biodegradation occurs slowly, these soluble by-products tend to leach slowly. The rate of leaching of an organic fertilizer’s by-products is determined by the rate at which it is converted to water-soluble forms by soil microbes, by the amount of soil water, and by the porosity of the soil. Fortunately, microbes change organic material into water-soluble forms rather slowly and gradually, so bacteria, fungi, and plants have plenty of time to absorb what they need before it leaches away. Plants do not need organic materials from these fertilizers. Instead, plants only need the nitrogen and minerals present in organic fertilizers. Since organic materials tend to biodegrade slowly, only tiny amounts of their nitrogen and minerals happen to be soluble and unabsorbed at any one time. So when it rains, very little tends to leach away.

What are Some Examples of Organic Fertilizers?

Bone meal, blood meal, feather meal, fish meal, and kelp meal are examples of organic fertilizers that are not soluble in water. They change to soluble forms by the digestive activities of soil microbes. Amino acids are examples of organic materials that are soluble in water. When proteins are biodegraded by microbes, amino acids are released. These can leach away with rainfall, but they tend to be very rapidly consumed by soil bacteria, fungi, and roots. Amino acids are rich in nitrogen, an important plant nutrient. Once processed through microbes, amino acids eventually yield soluble nitrate or ammonia. These are the standard forms of nitrogen that plants readily absorb and utilize.

Summary

1. Leaching refers to the event whereby a material (such as a fertilizer) dissolves in the soil water and moves away, ultimately into groundwater or surface water.
2. Leaching Rate is determined by:
   1. the degree of water-solubility of a fertilizer combined with
   2. the amount of soil water at any time, and
   3. soil type, porosity, and compaction
3. Most organic fertilizers are insoluble, so they tend to solubilize and leach slowly. Their rate of leaching is largely determined by the rate at which they are digested and converted to water-soluble forms by soil microbes.
4. Bone meal, blood meal, feather meal, and fish meal are examples of organic fertilizers that are not soluble in water, so they leach slowly as they are digested.
5. Amino acids are examples of organic materials that are soluble in water, so they can leach rapidly.