Grain Legumes and Green Manures for Soil Fertility in ... - cimmyt

INTEGRATING ORGANIC RESOURCE QUALITY AND FARMER MANAGE­
MENT PRACTICES TO SUSTAIN SOIL PRODUCTIVITY IN ZIMBABWE
Abstract
FLORENCE MTAMBANENGWE and PAUL MAPFUMO
Department of Soil Science and Agricultural Engineering, University of Zimbabwe,
P. O. Box MP 167 Mt Pleasant, Harare, Zimbabwe
Maintenance of soil fertility on smallholder farms in Southern Africa is almost entirely dependant on locally available
organic resources, with mineral fertilizers only dominating in exceptional cases for relatively wealthy fc:nners. The exploitation
of nitrogen-fixing legumes for soil fertility purposes, be it in the form of residues from grain or green manure
legumes, is not widespread in Zimbabwe smallholder farming systems .. In a newly initiated study in three agroecological
regions in Zimbabwe, we focus on the differential effects oforganic resource quality and quantity on the manifestation
of soil fertility gradients and subsequently on crop yields as influenced by fanner management practices. Both
Master/Innovator farmers and poor farmers are targeted in the exploration of the within1arm soil fertility gradients
(usually giving rise to high yielding 'rich' and low yielding 'poor' fields) observed under different management regimes.
Preliminary results showed that mineral fertilizer was the most common nutrient source used by over 85% of the fanners
from three identified farmer groups (Class A - Master farmers, Class B -. Innovator fanners and Class C - Resource
poor farmers). Livestock manure and woodland litter featured as common organic nutrient sources in Zimuto, while in
Chinyika and Chikwaka, less than 50% of the farmers used these as additional nutrient sources. As a result of frequent
organic nutri~nt source usage, soil organic carbon contents of the rich and poor fields of Zimuto was significantly
higher than from the other two agro-ecological regions (p < 0.05) with values reaching 11.5 mg C g-l soil (rich field) and
8.5 mg C g.l soil (poor field) . The bulk of the tested soil had very little total nitrogen « 1 mg Ng-l soil). Although the
chances of building soil organic matter (SOM) under granitic sands in Zimbabwe are slim, understanding the influence
of organic resource management on SOM dynamics, short-term N availability and mineral fertilizer use efficiency is
critically important. Target farm sites serve to complement long-term experiments in explaining the interaction between
organic resource management and crop yields. In addition to these preliminary results, the principles and approaches of
the study are also discussed in this paper.
Key words: Organic resource quality, soil organiC matte~, mineral fertilizer
Introduction
Marked differences in soil fertility levels and nutrient
balances can often be observed for the different
fields within one smallholder farm. These withinfarm
soil fertility gradients have a major impact on
overall farm productivity, yet their dynamics are
often poorly understood. Several reasons could be
attributed to this high variability, although inherent
soil properties, micro-climatic conditions and
farmer management practices may be obvious determining
factors (Mapfumo and Giller; ~001; Sanchez
and Jama, 2002; ·Smaling et aI., 1997). Farmer
management of fields varies with time of planting,
type of crop planted, type and quality of added inputs
(including access to cash for purchasing inputs)
and the efficiency with which nutrients are
used. Most smallholder farmers in Zimbabwe recognize
the importance of soil fertility and its conservation.
There is also general knowledge among both
scientists and farmers that the application of organic
residues improves the physical conditions of soil,
although infonnation as to why this happens is still
often very limited (Sanchez etal., 1989; Palm et aI.,
1998).
Agricultural activities and maintenance of soil fertility
in a smallholder farming commUnity is almost
entirely dependant on locally available resources.
However, given the present scenario where traditional
strategies for sustaining soil and crop productivity
have been outpaced by the growing human
population, a diminishing natural resources base,
and the downward spiral of many national economies,
the question of resource availability has become
topical. The big question is: what options are
available to the smallholder fanner for soil amelioration?
Annual woodland litterfall may be as much as 5 t
ha- l and measured annual litter collections in Masvingo,
southern Zimbabwe, by Nyathi and Campbell
(1993), ranged from 0.2 to 1.2 tonnes per household.
In reality, most communal areas are in a severe
~tate of deforestation. Livestock manure, cattle
manure in particular, is a traditional source of plant
nutrients and can be one of the cheapest sources of
organic fertilizer in many smallholder communities
Grain legumes and Green Manures for Soil Fertility in Southern Africa 57