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By cultural manipulation we will mean, in a broad sense, everything a farmer might do to increase crop yields per hectare, after he decides what to plant and buys the seed. With this restriction we leave out the agricultural economist and the plant breeder. When we limit our interest to yields per hectare we introduce an element of area. This will include the crop canopy above the surface and the root system below it. Cultural manipulation also includes what the farmer might do to make best use of water, carbon dioxide, nitrogen, and essential mineral nutrients whether they occur naturally or otherwise. We will also include manipulations designed to make the most efficient use of radiant energy during the growing season. Finally, consideration of the length of the growing season introduces the concept of time as one of the objects of cultural manipulation.
This is a vast subject from which we cannot hope to do more than select a few interesting points to discuss. To review the literature we would have to start with the most ancient findings of archaeology when cultural practices were more the concern of priests than of agronomists. Last year a third to a half of the papers published in the area of agronomy would come within our definition so there is still some interest in the subject. Dr. Clements wrote a review in the Annual Review of Plant Physiology recently containing 284 literature citations, one of which I noticed was a review of climatic influences on crop growth that itself contained 10,000 citations. When we speak of cultural practices we are talking about something that is big business in the agricultural field.