Improvment of hydromorphic rice cultivation. Five years research in North Cameroon

abonos verdes arroz de secano bottomland soils cameroon cameroun camerun cinc deficiencia de oligoelementos engrais npk engrais vert fertilizantes npk green manures herbicidas herbicide herbicides hydromorphic soils malezas mauvaise herbe npk fertilizers riz pluvial rotation culturale rotational cropping sol de bas-fond sol hydromorphe suelo de hondonada trace element deficiencies upland rice variedades varieties weeds zinc