Concepts of Soil Fertility

As animals require specific amounts of proteins, carbohydrates, vitamins, and minerals for proper growth, plants require nutrients in the form of ions derived from the soil. Insufficient quantities of protein, or even specific amino acids of proteins, can limit growth or specific biological processes in animals. Similarly, each nutrient plays a specific role in the structure of plant cells and the enzyme and energy transfer molecules that carry out the metabolic processes of plants. Because of the unique chemical characteristics of these nutrients, other elements cannot be used as substitutes for them. 

The amounts of nutrients needed by crops varies both with the crop and the yield attainable. Table 2.1 presents the approximate amounts of nutrients that various crops actually take up to achieve the yields indicated. These yields are not exceptionally high, but are normally attainable in areas well suited for growtli.otthese crops. The amounts of nutrients listed do not mean that these are the amounts of nutrients that must be added as fertilizers. In many cases, much of the nutrients come from the soil when adequate reserves of the nutrients are present. In other cases, because of very low fertility, much more than the listed amount of fertilizer nutrients 

Source Better Crops Internationai December 1988, Potash and Phosphate Institute, Atlanta, Georgia, U.S.A., 51 pp. 

Illustrated by Barrel Staves of Varying Lengths Representing Growth Factors. 

The analogy of the barrel illustrates some useful concepts  

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The concept of soil fertility through a living soil which has the capacity to influence and transmit health through the food chain to plants, animals and [humans] and that this can be enhanced over time. 

These observations later stimulated the general belief that prevailed through the earlier decades of the 19th century—namely, that humus is the only or the major soil product supplying nutrients to plants. The direct utilization of humus by plants was fully developed by Thaer (1808,1846), who stated that humus comprises a more or less considerable portion of soil fertility of the soil depends largely upon it since, besides water, humus is the only material that supplies nutrients to plants. This concept was referred to as the humus theory. 

Livestock husbandry is not explicitly addressed in the historical concepts of organic farming. Livestock provided manure and was therefore an important link in the crop and soil fertility chain. Livestock played a major role on its own only in biodynamic farming, especially cattle whose organs are described as important catalysts of processes and transmitters of cosmic energies (Steiner, 1929). Nonetheless, even the pioneers of biodynamic farming did not consider livestock husbandry from an ethical or animal welfare point of view. 

For ecosystems, there is a concept of environmental services (e.g., soil function). Soil conservation provides a range of obvious benefits, including the preservation of land use and fertility, supporting the production of fibers, food, or biofuels. These services have widened over time to encapsulate benefits of carbon sequestration or biodiversity protection. It is relatively easy to place values on the productive capacity in the former examples. It is somewhat harder to find the value of the latter uses, but as the Stern report (Stem 2006) illustrated, it is by no means impossible to calculate the value of, say, an upland peat bog. 

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