Interaction Among Water Nutrients and Roots

There is no doubt that water is a scarce resource in the SAT, but this does not mean that water always limits growth and production of crops. It should be more accurate to say that the natural supply of water does not match the crop demand in time and space. Water cannot be properly supplied to crops when and where they require it, because of erratic rainfall pattern, loss from soil surface through pan evaporation and loss into deep soil layers through leaching. Water is a resource beyond management for most of the SAT

Fig. 8.2. Proportion and amount of N derived from atmosphere (Ndfa) by pigeonpea when intercropped with sorghum, pearl millet, groundnut and cowpea.

farmers who have no access to reservoir or irrigation system.

On the other hand, nutrients, particularly nitrogen, are manageable by the farmers to some extent, though the natural supply is almost identical with the case of water. A considerable amount of N, mainly NO3 (roughly 100 to 200 kg ha-1) is usually found in the soil solution at the time of planting (Fig. 8.3). The entire amount of N disappears from the soil solution within 50 to 100 days, suggesting active dynamics of N in the soil solution during an initial cropping period. Since N accumulation in the crops is much less, and slow, as shown in Fig. 8.3, most of the N disappears from the system without being utilized by crops.8 This finding raises a question for the traditional farming practice of basal application of N fertilizer. It leads to the expectation that delayed N application will enhance the dependency of sorghum on native soil N, thereby increasing N use efficiency of the system.

The soil solution is the aqueous liquid phase of the soil, which provides the immediate source of nutrients for plants and microorganisms and acts as a temporary sink for some of their products. Nutrient levels in soil solution have been related to plant growth

Inorganic N in soil solution

■ I

Sorghum harvested i

\ Total N in intercrops

Days after sowing

Fig.8.3. Changes in soil solution N (50 cm depth) and plant accumulated N in pigeonpea/sorghum intercropping.

in many studies. Nitrogen from soil and fertilizer is finally released into the soil solution and becomes available to the crops. In soil solution, mineral N is subjected to a dynamic state, which means that there is always turnover of N in and out of the soil solution, even though its concentration may remain constant. The major inflow processes into this pool include fertilization, mineralization, rainfall and flow from neighboring layer of soils. The outflow of NO3 from it is mainly due to plant uptake, immobilization, volatilization, denitrification and leaching. Although we can only observe the balance of those complex processes, plant uptake seems to affect its pool size most intensively,9 especially near the rhizosphere. Nitrate is a major form of N under upland conditions, and its fluctuation in soil solution could more clearly reflect the root development and nutrient uptake activity of roots than NO3 extracted with KCl, which is commonly used to assess available N to plants.

Under resource-limiting conditions ubiquitous in the semi-arid tropical environments, crops with deep root systems may have an advantage in exploiting soil resources. To understand root distribution within the soil profile, roots are collected with various methods such as auger, monolith, trench wall and minirhizotron, and expressed in terms of length or weight of a unit soil volume against soil depth. This normally gives an exponential pattern skewing heavily toward deep soil layers.9 The cereals develop more roots near the soil surface and less in deep layers than the legumes. Even within the cereals, this is more evident in rice than in corn. To achieve more efficient exploitation of limited soil resources, it would be ideal to combine two crops that display different developmental patterns of root systems. In addition to root system architecture expressed by root distribution along the soil profile, root function is also important.10,11 Even though the crop has a well developed root system, it would be rather a waste of carbon allocation if it does not function properly for uptake of nutrients and water. Germ plasm improvement and crop management should pay attention to both architecture and function of root systems.

There is a significant interaction between root development and nutrient status in soils. The application of N and/or P as fertilizer enhances root development in terms of length in almost all layers of soil profile. Through this interaction, it can be expected that fertilizer application may increase water uptake, resulting in less volumetric soil water content in the soil profile compared with unfertilized soils. This observation suggests that the proper management of nutrients will increase crop production not only through direct effects on crop growth but also through enhancement of water utilization due to improved root development.

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