Crop yield

It is not possible to predict crop yields without detailed knowledge of meteorological conditions, soil type, farming practice, fertiliser use, irrigation, etc. Comparison between different crops is made even more difficult by differences in growing seasons and harvesting methods. Some arable crops are planted annually, e.g. cereal grains, and may be cropped more than once, e.g. grasses. Others are planted every few years and harvested annually, e.g. sugarcane, or may grow for long periods before harvesting, e.g. more than ten months for some varieties of cassava. Trees may grow for many years and be totally harvested (timber logging); other tree crops may grow from the continuing roots and be harvested as coppice every few years, e.g. willow, hazel and some eucalyptus. Table 11.4 is a summary of data to estimate the maximum biofuel potential of crops in terms of heat of combustion and continuing energy supply. The data for aquatic crops assume abundant nutrients. Grasses are assumed to have frequent harvesting in the growing season. We emphasise the considerable uncertainty of such data and the rule that such generalisations should never be applied for actual developments without site-specific testing.

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