Section 3 (Crop Management)

Rajbhandari
Extensively cited data of Dalal (1974) indicated that total P uptake (6 and 16 weeks after planting) by
sole crop maize was greater than a maize/pigeonpea intercrop. The intercrop pigeonpea P uptake was less
than that of the sole crop at 6 and 16 weeks after planting, but at 24 weeks there was no difference
between sole and intercrop P uptake. The soil P status remained unaffected 16 weeks after planting.
Srinivasan and Ahlawat (1983) also recorded no difference in soil P status between sole crop pigeonpea
and intercrop with green gram or sorghum. Wahua (1983) observed no difference between P uptake (up to
50 DAP) of sole and intercrop maize; however, at higher fertility levels uptake by the sole crop was
greater than intercrop and the reverse was true at lower fertility levels. The uptake by intercrop cowpea
was much lower than the sole crop 40 days after planting. CIAT (1980) reported lower P concentration in
leaves of intercropped cassava and cowpea compared to sole crops. Mason et al (1986) observed that
intercropping cassava with cowpea reduced the P concentration in leaf, stem and storage roots of cassava
at early growth stages (till 80 days after planting) but not after. Most of the experiments reviewed were
conducted with adequate P fertilization, and none of them dealt with intercrop response to P across a
range of P levels in different environments.
Intercrop systems have been advocated to be more efficient in land use compared to sole crop
systems, and therefore suitable and desirable in sustainable agriculture. It is unclear whether the increased
efficiency is due to only more resource extraction or to more efficient conversion of resources to dry
matter and marketable yield.
The experiment under different environments were conducted with the following objectives:
• To compare the growth in dry matter and grain yield maize and soybean in maize/soybean and
maize/rice intercrops with sole crops under different environments.
• To quantify the effect of interspecific competition on P uptake and conversion efficiency of
intercrops compared to sole crops under different P availability.
Materials and Methods
Two sites in Hawaii, having contrasting soil and climatic conditions, were utilized to conduct an
experiment in three environments. Permanent plots were utilized for the intercropping experiments, where
ten target phosphorus concentration in soil solution have been established since 1971. The field trial at
Wailua Experiment Station, Kauai, was planted on a highly weathered clayey, sesquic, isothermic,
Anionic Acrudox. Two field trials were conducted during the summers of 1988 and 1989 at the
University of Hawaii Poamoho Experiment Station on a silty clay of the Wahiawa series classified as
clayey, kaolinitic, isohyperthermic, Rhodic Eutrustox.
Main plots of ten target P levels (0.003, 0.006, 0.012, 0.025, 0.05, 0.1, 0.2, 0.4, 0.8, 1.6 mg P/L in soil
solution) were laid out in an augmented block design (Federer 1956). The target P levels in soil solution
were achieved using phosphorus sorption techniques (Fox and Kamprath 1970). Appropriate amounts of
P fertilizer in each treatment were applied as triple superphosphate before the last tillage operation to
achieve the targeted levels of P in soil solution. In Kauai the four middle P levels (0.025, 0.05, 0.1 and 0.2
mg P/L) were replicated three times, the four extreme levels (0.003, 0.006, 0.8 and 1.6 mg P/L) occurred
only once and the remaining two (0.012 and 0.4 mg P/L) were replicated twice. Each of 20 main plots
(12.19 m by 9.14 m) contained each of the three sole crops (maize, rice and soybean) along with
maize/rice and maize/soybean intercrops. In Poamoho, four P levels, 0.012, 0.025, 0.050 and 0.100 mg
P/L, were replicated three times and the rest of the unreplicated treatments constituted the 18 main plots
(15.24 m by 5.49 m). Each main plot in 1988 contained one of the sole crops (maize, rice or soybean) and
the two intercrop patterns. In 1989, main plots contained the maize/soybean intercrop with both maize
and soybean sole crops. Maize spacing was 0.90 m by 0.25 m for both sole crop and intercrop. Sole crop
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