Sintesis dan karakterisasi hidrogel NaCMC-g-Poli (AA-co-AAm) termodifikasi abu sekam padi sebagai superabsorben pupuk lepas lambat makronutrien npk = Synthesis and characterization of hydrogel NaCMC-g-Poli (AA-co-AAm) modified by rice husk ash as superabsorbent slow release macronutrient npk fertilizer