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The effect of application of rock dust at different rates alone as well as in combination with FYM and chemical fertilizers and the resultant effect on growth and yield of Coleus (Solenostemon rotundifolius) var. Sreedhara was evaluated by conducting a laboratory incubation study and field experiment at College of Agriculture , Vellayani during 2006- 2007.
The results of the incubation study revealed that increasing the rate of application of rock dust resulted in an increase in the available nutrient contents of soil. Application of rock dust in conjunction with an equal quantity of FYM also enhanced the availability of all the major as well as minor nutrients. The pattern of release of available N was found maximum during 30th day of incubation and for P, the highest value was recorded during 120th day of incubation. The release of K was found to increase over time reaching the maximum during the later part of the study. The pattern of solubilisation of micronutrients viz. Fe, Mn and Zn revealed that there was a gradual increase in their concentration from the start of experiment, reaching the highest values during the later stages of incubation. Application of rock dust at a higher rate i.e.12 t ha -1 along with an equal quantity of FYM resulted in the maximum release of almost all the nutrients viz. N, P, K, Fe, Mn, and Zn through out the incubation period. Addition of rock dust along with an equal quantity of FYM resulted the percentage increases from 7 to 17.5, 19.50 to 29.22, 22.95 to 45.38, 10.47 to 14.14 15.58 to 26.36 and 1.3 to 8.6 respectively for available N, P, K, Fe, Mn and Zn when compared to the application of rock dust alone.
The results from the field experiment conducted to evaluate the efficiency of rock dust revealed that the plant growth characters like number of branches per plant at 90 DAP and plant spread at 60 DAP showed significant variation due to the application of rock dust @ 10 t ha -1 mixed with equal quantity of FYM and kept for 15 days before the field application. Leaf chlorophyll concentration was increased by 47.36 per cent due to the application of rock dust when compared to POP recommendation. Yield component like number of tubers per plant was also increased by 14.81 per cent due to the application of rock dust. Application of rock dust @ 10 t ha -1 along with equal quantity of FYM and 50 per cent of the chemical fertilizers NPK @ 30:30:50 kg ha -1 also produced yield (17.26 t ha -1, B. C ratio 2.63) equivalent to POP (19.55 t ha -1, B. C ratio 2.39). This treatment also produced the highest number of tubers per plant, dry matter content of plant parts and total dry matter production. The highest yield of 19.55 t ha -1 was obtained for POP recommendation. But application of rock dust along with half the recommended dose of NPK and FYM also produced the similar yield as (17.26 t ha -1) that of POP recommendation. This shows that partial substitution (50 per cent) of chemical fertilizers with rock dust can be recommended to the farmers where ever it is locally available.
The starch content and cooking quality of the tubers were also favoured by the application of rock dust @ 10 t ha -1. Starch content increased by 35.02 per cent due to the application of rock dust. Priming rock dust with FYM two weeks prior to field application resulted in the maximum release of P, K, Ca, Mg, Fe, Mn and Zn from rock dust. Rock dust application resulted in the percentage increases of 34.55 to 43.12, 47.96 to 58.49, 20, 17.55 to 55.00, 74.50 to 78.69 and 20 for P, K, Mg, Fe, Mn and Zn respectively when compared to the application of inorganic fertilizers for coleus. Rock dust @ 10 t ha-1 along with equal quantity of FYM resulted in the highest returns per rupee invested (B.C ratio 2.89).
It can be concluded from the results of the study that the present recommended dose of in organic fertilizers for coleus can be reduced to half provided it is applied along with rock dust @ 10 t ha -1 . 100 per cent substitution of chemical fertilizers with rock dust 10 t ha -1 and FYM 10 t ha -1 can be recommended for coleus wherever rock dust is locally available.