Agriculture Reference
In-Depth Information
defined, representing a major research challenge. Smallholder agriculture represents
a very wide range of conditions in different countries. Defining general guidelines
for “adequacy” is therefore unrealistic and undesirable. Attention should be focused
on local conditions, as presented in this chapter.
2.2.2.2 Selection of Case Studies Focusing on “Use of Fertilizers”
In the limited context of this chapter, a number of case studies have been selected
to illustrate the use of fertilizers as mentioned by IFAD (2012) as one of three soil-
related means to overcome problems in small-scale agriculture, achieving or main-
taining soil security as expressed by the ecosystem services provided. Attention is
focused on particular soil types, to allow extrapolation of data obtained, if possible.
Case studies will not be restricted to developing countries in Africa but will also
include two examples from the Netherlands, demonstrating the universal and unify-
ing power of the soil-related ecosystem services concept.
“Use of fertilizers” is a very broad statement that needs to be specified to become
operational both in terms of defining optimal application rates and institutional set-
tings, which will now be discussed.
Determining optimal fertilizer needs of crops is, after pedology, one of the oldest
activities in soil science research, and procedures have evolved over time during the
past century, summarized as follows:
1. A large number of relatively small plots on experimental stations are fer-
tilized with increasing quantities of chemical fertilizer (N, P, and K), and
yields are determined. Soil samples are only taken of the surface soil (e.g.,
0-20 cm). Statistical procedures are used to relate application rates to yields,
thereby defining optimal rates for farmers. Experiments over many years
allow expressions of different weather conditions; however, much scatter is
obtained in graphs relating yields to fertilization rates and the soils being
used are often not or only broadly characterized in terms of sand, clay, silt,
and peat. Nutrient analyses are restricted to surface soil. Still, such graphs
are the basis for most current fertilizer recommendations and are the source
of the spectacular development of production agriculture in the 20th cen-
tury (see the two Dutch case studies to be discussed later).
2. In a variant of 1, fertilization rates are adjusted to what is locally feasible (see
later examples of Kenya). This relates to the proposal by Tittonell and Giller
(2012) to focus on the difference between the actual yields (Ya) and the locally
attainable yield (Yl), rather than on the often used difference between Ya and
Yw, where Yw is the theoretical water-limited yield where the assumption
applies that all growth factors are optimized, except for available water that
corresponds to local availability. Yw is beyond reach when soil fertility is as
limiting as it often is in smallholder agriculture in developing countries.
3. Considering the major differences between conditions on farm fields and
experimental plots on experimental stations, experiments are focused on
farmer's fields as part of Farming Systems Research. This involves active
participation by farmers and joint learning in interaction with research-
ers (e.g., Adjei-Nsiah et al. 2008; Sonneveld et al. 2008). Results obtained
