Insitu assessment of soil nitrate-nitrogen in the pigeon pea-groundnut intercropping-maize rotation system: implications on nitrogen management for increased maize productivity.

Assessment of soil nitrate nitrogen (NO3 - ‒N) was conducted in a pigeon pea-groundnut intercropmaize rotation cropping system at Chitedze Agricultural Research Station (S 130 59’ 23.2”, E 0330 38’ 36.8”) in the 2012/2013 cropping season. In the 2011/2012 cropping season, eight treatments replicated three times in a randomized complete block design, with monocultures and intercrops of either of the two pigeon pea varieties, long (ICEAP 04000) and medium duration (ICEAP 00557) plus groundnut (CG 7) were planted. At harvest, the legume biomass was ploughed into the soil in some of the treatment plots. Maize was then planted in the 2012/2013 cropping season and NO3 - ‒N data was collected from emergence over a period of three weeks. This was done before top dressing with urea. The results of the study seem to suggest that there was high NO3 - ‒N in the soil solution in all the treatment plots over the study period (106.4 mg l-1 to 463.1 mg l-1). It was observed however, that the level of soil NO3 - ‒N in most cases was statistically the same (p>0.05) across the treatment plots. In general, mean soil NO3 - ‒N was higher in the sub than top soil. This was attributable to the soil texture which is predominantly sandy clay loam with low to medium level of SOM, both in the top (mean=0.9-1.6%) and sub soil (mean=1.1-1.6%). Leaching of NO3 - is high under such soil conditions. It is likely that the level of mean soil NO3 - ‒N, into the season, in treatment plots which had no biomass buried under would decline faster than in treatment plots where biomass was incorporated, mostly due to uptake by the maize crop and leaching losses. This may last longer into the season for the latter treatment plots, but may not last until the end of the cropping cycle. For the Malawian smallholder farmers it implies that for this cropping system, N supplement from mineral fertilizer is not optional if reasonably high maize yield is to be realized.