There are so many factors that can influence the amount of nutrients found in food, even when naturally grown and before it has been processed in a factory. The first and most important place to look is the soil in which the plant grows. Yet this is rarely a place that many consider when selecting their food. Soil must be living, breathing and nurtured to ensure good nutrition. It is well recognised that much of the agricultural land used in Western culture is depleted and lacking in minerals and micro-organisms. At this time of year when fields have been harvested and are being prepared for next years crops the evidence of depletion is available for all to see. Light coloured, sandy brown soil, dry and hard, just freshly ploughed and often without a bird in site any where across the field! A clear sign of little or no topsoil or humus and even fewer micro-organisms and earthworms for birds to feed on.
The soil is broken down into several layers. The first 2 inches are called the humus where decaying matter is integrating with the soil and micro-organism life should be plentiful. The next 10-12 inches is called the topsoil and is considered to be the most biologically active. It contains micro-organisms and bacteria, worms, fungi and nematodes. The next 16-18 inches are called the subsoil and this tends to be a lighter coloured more sand/clay base with plenty of minerals. The success of plant growth will be greatly dependent of these soil layers functioning correctly to provide the plant with the correct environment for optimal growth. Decaying plant matter, and active micro-organisms, fungi and earthworms all help to keep the top layers of soil light in texture, oxygenated and full of minerals. This provides necessary warmth and slight acidity which helps seeds germinate and plants grow. Good topsoil can hold virtually its own weight in water too which helps to transport minerals and nutrients to the plant roots. High quality soil will determine the quality of the plant that can grow from it and the nutrient density it will likely achieve.
Plants being eaten in season will mean they are eaten at peak ripeness when nutrient content is likely to be at its highest. The amount of time between the plant being harvested and it being eaten is another determining factor for nutrient density. Plants do contain many antioxidants which can serve as a protective mechanism to degradation, but whilst they do their job of preventing spoilage, the nutrient content diminishes. Therefore the longer they remain stored the lower the vitamin and nutrient content bcomes. Fruits that are picked unripe and then slowly allowed to ripen whilst they are imported by boat from other parts of the world will also be lower in nutrition. Even if they were grown in good soil they have not been allowed to develop to full maturity and have been separated from their nutrient source. This is similar to a mother giving birth to a premature baby. Separated from its internal, safe nutrient source early immediately sets the child at a disadvantage. It may take years of care and nourishment to make up the shortfall compared to other children of a similar age. Unfortunately fruit does not have someone to carefully tend and nurture it after it has been prematurely picked and so its nutrient density remains at its lower level even when ripening occurs to make it edible. This lends well to supporting the idea of local, organic, seasonal produce where possible. This will be grown in better soil and will have been allowed maximum time in touch with its nutrient source to generate better nutrient density.
It is also fair to say that the nutrient content of any animal produce eaten can only be as good as the plant materials or animals that they ate. It is also an interesting observation to note that most animals eaten by man are herbivores. Cattle, pigs, sheep, chicken (they do eat insects), and many fish all live almost exclusively on plant materials. These plants must be of the highest quality so that the nutrients we need become concentrated within the animals and can pass them along to humans at the top of the foods chain. It is rare that we eat animal produce that comes from dominantly carnivorous food. Some cultures do eat carnivorous animals, but these will not be the only source of nutrition, plants and herbivorous animals will always be part of the diet.
The quality of the soil determines the quality of the plants, which determine the quality of the animals and as we are at the top of the food chain the quality of the planst and the animals determines the quality and health of human kind! This is Natures way. It has governed health on this planet for thousands of years and it has worked just fine. You know what they say, if it ain't broke...