Plant nutrition in organic farming

Hunger is still one of the major threats for many people worldwide, even though productivity at farm level has increased substantially during the last 100 years. In Europe for example it took one farmer to feed 2.5 people in 1900, currently the ratio He will feed well over 100.

Crop yields have been multiplied in many regions around the world. However, for European and North American populations the issue is only to get enough food, but quality - how the food has been produced. Some consumers see organic farming as a much better alternative than conventional farming. According to the latest statistics, about 24 million hectares (in total less than 1 % of the agricultural land in the world; Table 1) are under organic management.

Tab. 1: Land area under organic management (extract from SOEL-Survey, 2004)

Development of organic farming
The concept of organic farming developed about 100 years ago in the early decades of the 20th century as a reaction to a series of rapid and profound changes in the way farming was done: introduction of tractors, pesticides, mineral fertilizers and new seeds. Almost “overnight” farming practices changed from low input into an intensive farming with high level of inputs leading to tremendous yield increases. Around 1920 individuals in agriculture in several European countries started to speak out against what they called “industrialization of farming”: they were frightened that the steep increase in farm productivity might lead to products of poor nutritional value, and that such intensive cropping systems would not be sustainable. Could this way of farming be in harmony with “Mother Nature”? In a series of lectures held in 1924, Rudolf Steiner was probably the first to develop a comprehensive organic farming system. This “biodynamic agriculture” is based on the anthroposophic view of soil, seeing the life on and in it as a living, sentient system and the farm as a whole as a living organism.

The term “organic” appears first to have been widely used by Lord Northbourn in the 1940ies in his book “Look to the Land”. He described a holistic, ecologically balanced approach to farming. In the following years, discussions on how to improve soil fertility and soil humus status were driving the development of many different types of organic farming around the world.

Plant nutrition in organic farming
The principles of plant nutrition apply for conventional as well as for organic farming: nutrients are taken up from the soil solution as minerals and plants have a certain nutrient demand for optimal growth. As soon as one essential plant nutrient becomes limited, crop yield will decrease.

One of the problems organic farmers have to face, is the fact that under most circumstances the nutrient balance at farm level is negative, i.e. nutrients like phosphate and potassium are exported from the farm via products sold to customers (Fig. 1).

Fig. 1: Nutrient balance (kg per year) at farm level for an organic farm (Nolte, 1989)

To compensate for these nutrient losses, the regulations for organic farming allow the farmers to purchase certain mineral fertilizers but no nitrogen fertilizers.

Mineral fertilizers allowed in organic farming (extract from EU Regulation 2092/91, 2004):

• Natural rock phosphate
• Aluminium calcium phosphate
• Basic slag
• Crude potassium salts (not undergone chemical treatments to increase solubility)
• Calcium and magnesium carbonate of natural origin
• Magnesium sulphate (e.g. kieserite)
• Calcium chloride/calcium sulphate
• Lime from sugar beet processing
• Mined sodium salts
• Elemental sulphur
• Stone meals

The selection of minerals that are allowed for organic farming has often been criticized because in many cases there is no scientific justification why a product is  on or off the list. Actually, the selection has to be seen in a historic perspective.

Since mineral nitrogen fertilizers are not allowed, the N nutrition of crops in organic farming needs special attention. The only way to apply N is via organic materials. These can be either animal manure, composts or crop residues. It is also possible to grow legume crops (they are able to fix atmospheric nitrogen) and to incorporate them into the soil. The N content of such organic fertilizers is variable, and the N release rate is somewhat unpredictable. Therefore it is difficult to synchronise soil N supply and crop N demand (Fig. 2). As a result, situations may occur when too much N is available in the soil with subsequent risk of nitrate leaching. On the other hand, a fast growing crop might face a period of N deficiency when the mineralization of N from organic sources is too slow, resulting in reduced crop yield.

Fig. 2: No synchrony between soil N release and N uptake during the vegetation period of oil seed rape (Kjellström & Kirchmann, 1995)

Conclusion.
Plants do not distinguish between nutrients supplied via organic or inorganic sources. In organic farming, there is often a lack of nitrogen and this limits the level of crop yield compared to conventional farming.
Scientific studies do not confirm that the nutritional value of the food produced in organic farms is superior per se compared to conventional farming, nor that the influence on the environment is more beneficial.