The alarming picture of nutrient depletion that emerged from the nutrient balances calculated by Stoorvogel and Smaling at different scales in Sub-Saharan Africa, motivated researchers to carry out multi-scaled studies in order to understand the underlying causes behind this nutrient depletion. Some of these studies have confirmed that nutrient depletion leading to poor soil fertility is largely determined by past and current land use systems and management practices, and pointed at the importance of differences in farming techniques between poor and wealthy households. In some farming systems, one common scenario is reported: positive nutrient balances for wealthiest farmers and negative for poorest farmers.
In Mozambique, the soil nutrient balance studies which were conducted at the national level in different land use types showed annual depletion rates for cultivated fields of 33 kg nitrogen, 7 kg phosphorus and 25 kg of potassium per hectare per year on average. For small-scale farming, the studies showed annual depletion rates of 47.9 kg ha-1 of N, 9.9 kg ha-1 of P and 36.5 kg ha-1 of K for maize, which is the most important crop in Mozambican smallholder cropping systems. These studies did not take into account the wealth status of the farmers despite of the widespread acceptance that land use systems and management practices differ between poor and wealthy households.
It has also been argued that in some farming systems the gradients of nutrient depletion increase with the distance from homestead. This occurs because farmers tend to concentrate manure collected from the ‘kraals’ and other organic matter (ashes, kitchen scraps, garbage) in the fields closer to the homestead, while fields further away often receive no fertilizers amendments.
The main objective of this thesis was to test if the patterns of nutrient depletion reported in many studies conducted in sub-Saharan Africa holds true in Mozambican smallholder farming systems. In order to achieve this objective, soil nutrient stocks and partial nutrients balance analysis were carried out at field level, in Banga
Agriculture and livestock husbandry are the main activities practiced in two different land units: the alto (closer to homestead) and baixo (far-away). Agriculture is practiced without using external inputs and is highly dependent on rainfall. Maize is the main crop in the village and is cultivated in all land units, although those further away are considered to have better yields. Drought is a recurrent phenomenon in the study area and it severely affects the maize yield. A lack of other crops which are tolerant to drought have limited farmers from diversification of crop production. The sale of charcoal, animals, remittances and local brews constitute the main sources of income when crops fail. There are different exchange relationships that underpin their livelihood strategies including kukashela, xicoropa, tsimo and kuwekissa. All these are related to the exchange of labour for land preparation.
Farmers indicated cattle ownership as the main factor in dividing wealth classes. Based on this, four groups were identified: the Very Low Resource Endowment (VLRE), the Low Resource Endowment (LRE), the Medium Resource Endowment (MRE) and High Resource Endowment (HRE).
Soil nutrients stocks and chemical properties (N, P, K, SOC and CEC) were significantly higher (p <0.005) in the far-away fields compared to those close to the homestead. The reason appears to be the inherent properties of the soils. Within the same field type, there were no significant differences in soil nutrient stocks between farmers belonging to different resource endowment groups. The stocks range was: Mananga (SOC 0.43 (%), 0.06 (%) N, 3.84 mg kg-1 P and 1.18 cmol kg-1 K), Gowene (SOC 1.53 (%), 0.13 (%) N, 53.3 mg kg-1 P and 3.09 cmol kg-1 K) and Banhine (SOC 1.1 (%), 0.12 (%) N, 27.5 mg kg-1 P and 2.4 cmol kg-1 K). Despite the lack of external inputs, and with the exception of N and SOC, the levels of the other properties in far-away fields were in adequate levels for crop production, especially maize crop.
Partial nutrients balances (N, P and K) for maize in the most important field type (Banhine) were strongly negative for all resource endowment groups. The partial N balances were: -27.3 kg ha-1, -21.5 kg ha-1, and -18.8 kg ha-1and -30.4 kg ha-1, for the VLRE, LRE, MRE and HRE groups, respectively. In case of P the values were: -2.8 kg ha-1, -2.1 kg ha-1, -1.6 kg ha-1 and -2.9 kg ha-1, for the VLRE, LRE, MRE and HRE groups, respectively, and for the K were: -32.3 kg ha-1, -24.8 kg ha-1, -21.4 kg ha-1 and -35.0 kg ha-1 for the VLRE, LRE, MRE and HRE groups, respectively.
The results of this study contradict earlier research results that indicate a close relationship between wealth status and level of nutrients. The study also refutes other findings that revealed that fertility levels tend to decrease with distance. The land tenure security does not influence the likelihood of soil fertility investment.