The organo-mineral fertilizers have created as a result of necessity to ameliorate the sandy soils and sands poor in humus under the conditions of the intensive agriculture development.
This category of fertilizers is represented by L-120 and L-110 types, complex binary fertilizers with nitrogen, phosphorus and humic substances produced in Romania. They are obtained by activation lignite powder with phosphoric acid followed by the ammonization and granulation, respectively, with urea-formaldehyde (Dorneanu et all., 1971; Rogoz et all., 1972). The organo mineral fertilizers obtained by this procedure contain the nitrogen under several forms connected ionitic as ammonium humate, the ammonium phosphate resulted from the reaction of ammonium with phosphoric acid used in excess to activate the coal and as urea-aldehydic compounds. The phosphorus is as ammonium phosphates, calcium phosphates resulted during the coal activation, both dispersedly crystallized forms in the coal mixture and urea-formaldehydic polymers, and as humo-phosphates too (Dorneanu and Preda, 1972).
The main characteristics of these fertilizers consist in the fact the nitrogen content is 9.13 - 11.3 per cent, the total phosphorus content is 11.6 - 21.5 per cent of which water soluble phosphorus is 42.2 - 62.8 per cent and ammonium citrate soluble phosphorus is n 87 - 88.4 percent depending on the fertilizer type. Also the organic carbon content is 19 - 23.8 per cent, and the cation exchange capacity is 102.3 - 112.6 me/g physical fertilizers. The spectral (IR) analyses regarding the quality of humic substances reveal that their cation exchange capacity is 2-3 times than that of lignite, and, at the same time, reactions of humic molecules with phosphoric acid take place forming humo-phosphates.
Research carried out applying the method of percolation with water in sandy columns (Dorneanu and Popa, 1974) reveals that the nitrogen in urea incorporated in sand is leached in a proportion of 47.4 per cent on the depth of 500 mm and 35.16 per cent on the depth of 1500 mm, and the nitrogen in organo-mineral fertilizers is leached only in a proportion of 7.26 - 11.7 per cent. 1.03 - 1.43 per cent, respectively.
The slow leaching of nitrogen in these fertilizers, reflecting the leaching decrease, has a particular importance in high efficient use of nitrogen and prevention of ground water pollution caused by nitrates in the areas of sands and sandy soils which are characterized by a high permeability.
