Competition for nutrients: getting to the root of the problem
Trees are an important asset on farm. During the last few years an increase in agroforestry practices has seen a rise in intercropping of trees to provide shade to crops and, if leguminous, to improve the fertility of the soil. However, many species have an extensive root system and the minerals and water they take up can deprive other plants, particularly crops planted near to the trees, of the nutrients they need. Deep-rooting trees are preferable to lateral, shallow rooted species as deep roots do not compete for nutrients within the crop root zone but can 'pump' up minerals from lower strata leached from surface layers.
Major leaching can occur in alley cropping systems in the humid
tropics when crop demand is low and decomposing pruning mulches
release nutrients too quickly. To investigate this effect and the
difference
that root depth can make in retrieving lost nutrients, two trees with
differing root depths have been tested with alley cropping systems on
a Ultisol (a mineral soil with high aluminium toxicity in the subsoil)
in N.Lampung, Sumatra by researchers at Wye College (UK),
ICRAF-Indonesia and the University of Brawijaya. Gliricidia sepium,
a predominantly shallow rooted tree, and Peltophorum dasyrrachis,
which has a deeper root system, were used to measure the uptake of
15N placed at specific soil depths: in, between and below
the crop rooting zone.
Strong competition for nitrogen was measured between G.sepium and the intercrops of maize, and groundnut. It was also found to take up little 15N from lower soil depths. In contrast, P. dasyrrachis roots were found to take up relatively high levels of 15N at lower soil depths providing an active 'safety-net' against nutrient leaching. Root activity as well as root length density had to be taken into account when assessing the efficiency of the so called safety-net (retrieval of leached nutrients) but preliminary modelling has suggested that Peltophorum roots in the 40-60 cm soil layer could reduce leaching by 5-10% over the course of a maize crop cycle in the rainy season.
Factors such as intensity and amount of rainfall, timing and quality of pruning and applications of fertilizer will influence results. The challenge for researchers now is to identify tree species with a high safety-net efficiency and to determine the effects of management interventions, such as pruning on root activity, to optimize tree-crop interactions.
