What organic fertilizers mean to plants and soil

Improved soil structure, a season-long supply of nutrients, and an increased water-holding capacity are just some of the benefits of using organic fertilizers.

How does a corn or tomato plant react to organic versus synthetic inorganic fertilizers? Are organic tomatoes higher in lycopene and vitamin content than conventional tomatoes? These are some interesting questions for both consumers and producers.

The fundamental process of nutrient absorption by plants is well established. Irrespective of whether nutrients originate from an organic or inorganic sources, plants are only capable of absorbing nutrients in certain forms. For example, nitrogen is only absorbed as nitrate (NO3-) ions or ammonium (NH4+) ions and potassium only as K+ ions. Thus, plants cannot distinguish between nutrients derived from organic and inorganic fertilizer sources.

Inorganic fertilizers readily provide nutrients in these forms. However, when organic forms such as manure or compost are applied to the soil, the organic matter has to be first mineralized, or broken down, by microorganisms and its nutrients released to the soil as ions. This enables plant roots to absorb them. Thus, nutrients derived from organic fertilizer sources are not as readily available to plants compared to nutrients from soluble synthetic fertilizers. Organic sources have a lower N-P-K grade, which requires the application of large quantities per acre. Most organic fertilizers also have nutrient levels closer to 1-1-1 grade. Therefore, supplying all the nitrogen needs of plants using organic fertilizers may lead to an over application of phosphorus.

Let’s not overlook the multiple benefits of organic fertilizers. The gradual release of nutrients would ensure a season-long supply. Being less soluble means minimum nutrient losses to the environment and less salt injury to plants. They add organic matter contributing to soil health increasing both the water-holding capacity and cation exchange capacity. They stimulate microbial activity and improve soil structure. Organic fertilizers also are valuable sources of micronutrients to plants.

As to the nutritional value of the product, there is no consistent scientific data to prove that organic sources are superior to non-organic fertilizer sources, despite some consumer perceptions. The lycopene content of tomatoes is not significantly different between organic and non-organic fertilizer treatments. Research indicates that factors such as the variety, season and stage of maturity have far greater influence on some of these chemical characteristics. This debate, however, will continue.

For more information on how consumers perceive organic foods, read MSU Extension’s Jeannine Schweihofer article, Consumers associate food labeled organic as being healthier.

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