de Bie, Kees 
  
which translates to an actual yield of 351 (‘000 Bath/ha). This amount is used to 
re-calculate the relative contribution of each variable to the overall yield gap in 
non-logarithmic terms. A review of identified yield constraints follows hereafter. 
The loglinear behavior of the yield data results in exponential yield increments for 
each partial yield gap closed. Thus, constraints cannot be ranked (Table 4, Figure 
12). A consequence is also that management of the studied mango orchards must 
strive for the highest level of technology available. Interaction effects between 
technology aspects implemented outweigh each individual contribution. The 
present management is at a level that is not yet restrained by the law of 
“diminishing returns’. 
LOCATION AND WATER MANAGEMENT 
There is clearly a relation between terrain specifications and the water holding 
capacity (WHC) of soils. Mango requires a drought (dormancy) period for flower 
initiation. Subhadrabandhu (1986) noted that vegetative growth must have ceased 
and newly developed shoots must have reached "maturity" before flower buds are 
initiated. A shallow ground water table and/or a sufficient water storage (roots go 
down to six meters depth; Purseglove 1977) voids the impacts of a dry season. 
Such soils are clearly not suitable for mango orchards. Soils in hills with a poor 
WHC are less affected by this problem. 
Young orchards and fully-grown crops that have a growth flush require proper 
water management. Supplementary irrigation facilities and specialized skills to 
assess by orchard when to apply water are a precondition for good growth. The 
average precipitation in Phrao is 1171 mm/year; the optimum mean annual rainfall 
for mango is indicated as 1500-2000mm (FAO 1992^). Orchards on terraces with 
soils of poor WHC clearly suffered from water stress during flushes and fruit 
bearing. The lack of water at periods when it is most needed make these soils less 
fit for orchard establishment. 
PH OF THE TOP-SOIL 
Literature suggests optimum soil pH ranges for mango of 5.0-7.0 (FAO 19925), 
5.5-7.5 (Purseglove 1977), and 5.5-6.5 (DLD 1989). This study identifies a positive 
relation between mango yields and soil pH, suggesting that near-neutral (up to 
8.0) pH levels are most suitable for mango. 
YEAR EFFECT, PRUNING AND WEEDING BY TRACTOR 
Purseglove (1977) reports that climate influences biennial bearing and that a high 
soil C/N ratio is conductive to flower initiation, as is the abundant production of 
new growth during a proceeding 'off year. Use of fertilizers during flower initiation 
should be avoided (Sauco 1989). Use of growth regulators may induce flowering. 
Tongumpai et al. (1989) reported successful use of ‘Cultar’ (paclobutrazol; inhibits 
gibberellin biosynthesis), applied as ‘collar drench’ to several mango varieties in 
Thailand. All cv's flowered intensely 3-5 months after the treatment, whereas 
untreated trees did not flower at all. Cv's that flower with difficulty may also need a 
  
International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B7. Amsterdam 2000. 
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