‘Vitamins absent in vegetables’ a headline in a major national newspaper stated last week. What followed was an account that was to make it evident that the quality of vegetables is deteriorating as a result of the mandatory injection of manure into the soil, resulting in a serious threat to public health. A hare-brained story and utter nonsense, according to Professors Oenema, Struik, Van Kooten and Van Boekel of Wageningen University. Vegetables and fruits that are cultivated on Dutch soil are and will always be healthy.
They comment:
“The story in the newspaper claimed that, unlike spreading manure across the soil surface, life in the soil was killed by injecting manure into the soil – in which the newspaper lumped greenhouse vegetables and vegetables grown in the open together for convenience’s sake. As a result, crops could no longer evenly absorb minerals. This would not only mean that they do not contain enough minerals, but also that they do not produce enough vitamins. The consequence for the consumer would be malnutrition, obesity and even an increased risk of developing cancer. We, as scientists, could simply dismiss this ridiculous story, if not for the fact that it is precarious and dangerous for a newspaper to intimidate its readers unnecessarily.
What is the real story behind soil processes, plant growth and the quality of vegetables?
Soil quality
Dutch farming lands are very fertile, but these are substantially fertilised anyway to ensure that high yields are realised. Plants require 14 mineral nutrients to grow well; animals and humans require 18; the four additional minerals are also found in plants. A plant absorbs almost all of its nutrients from the soil through its roots. Fertilisers mainly contain nitrogen, phosphorus and potassium, as these nutrients limit the crop yield. But sulphur, magnesium, sodium, copper, zinc, boron and molybdenum are also applied locally in the form of fertiliser.
Animal manure contains all nutrients and is therefore an ‘ideal fertiliser’, although the ratios between the nutrients in manure do not match the ratios required in order to adequately feed the crop. The ‘buffer’ capacity of the soil and the ‘selective’ absorption of nutrients ensure that this ‘mismatch’ stays within the limit.
The Netherlands is rich in animal manure, as we have many agricultural animals that we, for the most part, feed with imported forage. The agricultural animals utilise only 5 to 30 percent of the minerals in this forage. The remaining 70 to 95 percent ends up in manure and therefore on the Dutch farming lands. Our farming lands are partially nourished with nutrients that were removed from the soil elsewhere.
Too many nutrients in the soil, water and the air are damaging to plants, animals and humans. The policy on manuring aims to limit the loss of nutrients (particularly nitrogen and phosphorus) into the environment and to prevent the undesired accumulation of nutrients in farming lands. The input of nutrients by means of fertilisers and animal manure has decreased considerably as a result of this policy, though the input is on average greater than the output in the harvested crop.
There is no evidence that the fertility of the soil is rapidly deteriorating or that the soil is dying due to this accumulation.
Manure injection and quality of food
Injecting manure into the soil was implemented 15 years ago in order to limit the emissions of ammonia into our air. This measure contributed to reducing ammonia emissions by half, healthier air and less damage to the environment. Injecting manure also has unintentional side effects, such as damage to the nests of meadow birds and to the ‘soil life’. All of the relevant effects of manure injection were recently listed and described in an extensive study. The study does not mention effects on the mineral composition of the crops, as there are no indications of such. The method of fertilisation has a negligible effect on the mineral composition of a crop that has been cultivated on fertile farming land.
There is no causal connection between manure injection and food quality.
Balanced mineral composition
Crops select their nutrients using their root system and are perfectly capable of realising a balanced mineral composition in their products, and this is certainly the case if there are no absolute deficiencies in the soil. Fertilisers do influence the quality of crops somewhat. A lot of nitrogen leads to crops that are rich in water, and this may influence the flavour and quality of the harvested product. Fertilisers also influence the development of crops and therefore their composition. Finally, certain fertilisers can directly influence the quality. More nitrogen, for example, often results in higher protein levels and nitrate levels in leafy vegetables. Effects of this kind always remain within reasonable limits that are determined by the weather, the variety and the cultivation technique.
There is but a slight connection between the supply of minerals and quality and it is nonsense that fertile farming lands yield products that lack minerals or vitamins.
Health
A comparison between a few analysis figures, concerning vitamin C in vegetables for example, does not generate any reliable conclusions regarding public health. Analysis results are never the same. The methods of analysis can change, varieties differ, growth circumstances are variable, the circumstances under which products are stored can have an effect and there is the biological variation. The nutrient levels are never the same and that is not a bad thing, as a normal combination of foodstuffs offers enough of everything. Only people with unbalanced diets encounter problems, but that is through no fault of the vegetables, but rather of the people themselves.
There is no causal connection between manure injection and the life expectancy of humans.
Healthy and tasty
Consumers want food. And fortunately today, food can be produced within the limits described above. The plant-breeding sector is focussing more and more on flavour-stimulating and health-stimulating substances. In addition, farmers and market gardeners are becoming increasingly aware of the effects of their production methods on flavour and health. All this in response to the market. All kinds of products with special features can now be found in shops. Ranging from lycopene-tomatoes to BroccoCress and from a helping of potatoes to allergy-free apples.
Vegetables and fruits grown on Dutch soil are and will always be healthy.”
Oene Oenema (Management of nutrients and soil fertility), Paul Struik (Crop physiology), Olaf van Kooten (Horticulture chains) and Tiny van Boekel (Product designs and quality) are all professors at the University of Wageningen.