Improvements of maize based cropping systems in semi-arid Tanzania by the use of cowpea, water conservation and phosphorus application [5 manuscripts included]

absorption de substances nutritives alomado azote billonnage caupi clima semiarido conservation de l'eau cowpeas cultivo intercalado culture intercalaire fosforo intercropping nitrogen nutrient uptake phosphore phosphorus ridging rotation culturale rotational cropping semiarid climate tanzania tanzanie vigna unguiculata water conservation zea mays