In the twentieth century numerous studies were made on the role of organic matter in defining soil fertility. Experimental fields were established in Rothamstead England (1843), Morrow, the U.S. (1876) Askov, Denmark (1894), Halle / Saale, Germany, Groningen, Netherlands, Dehéreim, France, Fundulea, Podu Iloaiei Suceava, Romania. The long-term experiments made in these fields contributed importantly to the knowledge of the effect of organic and mineral substances on improving soil fertility.
These long-term researches conducted worldwide established the utility of organic fertilizers for maintaining or increasing the organic component of the soil. The introduction of organic residues in soil means turning to good account the energy included in these livestock excreta. About 49% of the chemical energy contained in the organic compounds of the food consumed by animals is excreted as manure, where significant percentages of macro and micro- elements are to be found.
Consumption of organic products is a growing process, so agriculture must keep up and produce ever more. Obtaining products by producingno harmful effects to nature is almost impossible. One thing is sure, that farmers try to minimize these negative effects as much as possible.
Soil, which is the focus of organic farming, is considered a complex living environment, closely interacting with plants and animals. By its specific techniques, organic farming aims to increase the microbiological activity of the soil, to maintain and increase its fertility.
The organic substance used as fertilizer is an important component in order to maintain or restore the soil fertility. Collection, storage and fermentation of vegetal wastes so as to decrease their volume and improve their physicochemical properties are a requirement of organic farming.
For many considerations, the organic fertilizers are preferred in organic farming as poorly soluble nutrients are mobilized with the help of soil microorganisms.
Fertilization is an important means of increasing the amount of organic products and the methods of fertilization used vary from one farm to another. For fertilization, the natural fertilizers represented by animal or vegetal remains are used in organic farms.
The fertility and biological activity of the soil must be maintained and improved by the cultivation of legumes, green manure crops or deep-rooting plants in an appropriate rotation. Also, the fertility must be maintained by incorporating organic substances in the soil as compost or from the production units, which respect specific production rules.
Besides the use of legumes in rotations, the role of animals in the organic system facilitates nutrient recycling. The potential for recycling the nutrients through fertilizer application is high. Thus, both the nutrients from the grazing period and the nutrients from the stall period are concentrated in solid manure and urine which are available for redistribution. By grazing, the animals retain only 5-10% of the nitrogen existing in the grass consumed. Together with the manure, they remove about 70% of nitrogen in the urine and 30% in the solid manure.
Not all initial nitrogen in manure is used by herbs in the production of dry matter in the crop. Much of the nitrogen may be retained in roots, immobilized in organic matter in the soil or lost by leaching or denitrification. Also, the loss of nutrients during storage may occur due to leaching and volatilization, which depend largely on how these fertilizers are managed. The nitrogen losses as ammonia or nitrogen gas in the fertilizer can be of 10% of the total weight when it is tamped in the pile and reach 40% when the pile is loose and turned. The gaseous losses of urine can be of 10-20% and even higher when it is shaken. Because of this, the application in spring is more efficient because the leaching losses are lower than in the case of application in autumn or winter.
The organic fertilizers positively contribute to the modification of physical conditions in the soil by increasing the field capacity for water, aeration, porosity and brittleness, and the black colour of organic matter will lead to easier and faster heating of these soils.
It should be mentioned that, when using organic fertilizers it is very easy to overcome the nutrient dose that needs to be applied. Therefore, the amount applied for a complete rotation of the cultures should be limited to the equivalent of nutrient from the manure produced by maximum 2.5 to 3 units of cattle / ha.