The report, published in February 2009 Journal of HortScience, reviews a highly cited research study titled "The dilution effect in plant nutrition studies" published in 1981 by Jarrell and Beverly in Advances in Agronomy. Jarrell and Beverly found that fertilized plants contained larger absolute amounts of minerals than the unfertilized plants, but these amounts were sufficiently diluted by the increased dry matter that all mineral concentrations declined, except for phosphorus, which is the common fertilizer.
Next, Davis looked at historical food composition data derived from three quantitative reports. While these studies are limited by their ability to be compared due to variation in methods, they found:
apparent median declines of 5% to 40% or more in some minerals in groups of vegetables and perhaps fruits; one study also evaluated vitamins and protein with similar results.Finally, Davis evaluated studies of collections of cultivars of a single food that was grown side by side for purposes of comparing their nutrient content. The foods were broccoli, wheat and maize. The side by side allows the elimination of using historical data in the form of averages (as was done in the previous section), which allows all environmental conditions (soil, fertilization, irrigation, pest control, climate, harvest, sampling, and analytical methods) to be held constant. He found:
plantings of low- and high-yield cultivars of broccoli and grains found consistently negative correlations between yield and concentrations of minerals and protein, a newly recognized genetic dilution effect.In conclusion,
Further studies are needed to assess the generality of dilution effects among foods and to greatly expand the numbers of nutrients and phytochemicals considered. Side-by-side comparisons of multiple cultivars in multiple environments can provide rigorous answers to the many remaining uncertainties. They are also well suited for testing proposed environmental and genetic methods to overcome dilution effects. Specifically, we would like to find ways to decrease the inverse correlation coefficients between yield and nutrient concentration or to decrease the negative slopes in plots of nutrient concentration vs. yield.
Over three billion of the world’s population is malnourished in nutrient elements and vitamin,including in developed countries. Vegetables and fruits are among the richest sources of many nutrients. Thus, declining nutrient concentrations in horticultural products are most unwelcome. Past and ongoing efforts to increase yields, combined with apparent broad tradeoffs between yield and the concentrations of perhaps half of all essential nutrients, work against recent efforts to increase one or a few micronutrients in individual foods.