Thank you all for your interesting contributions to date.
I would like to look at some more fundamental issues which I feel have not been sufficiently covered.
To improve plant nutrition, I would suggest the examination of two major aspects of the problem, firstly the availability and uptake of nutrients by plants, making nutrients bio-available to humans, and secondly, examining and education around the problem of plant anti-nutrients inherent to several major crops.
As far as making nutrients available I would tend to disagree with the proposal in posting number 6 that artificial fertilisers provide any sustainable or truly productive solution to nutrient availability.
The reality is that the availability of macro and micro nutrients is largely dependent on the health and diversity of the soil biota. If there is a healthy soil biota (the sum of bacteria, fungi, protozoa etc in the soil) then there is a good chance that the available nutrients both within the mineral matrix of the soil and within the portion of humus in the soil can be made bio-available to the root systems and thus to the plant itself. In a barren soil, i.e. one that has been overused, poisoned or damaged by over fertilisation by chemical based fertilisers plant nutrients are either not bio-available or are totally absent. If they are not available the biota needs to be supported and built up. If they are absent then more fundamental steps need to be taken to introduce nutritive sources into the soil, starting with the humic (compost and plant matter) portion and building from there.
It is generally counter productive to rely on totally external supply of plant nutrients for sustainable agricultural nutritional requirements as these primarily artificial sources of nutrition are often not readily bio-available to plants, or are not present in correct ratios. Most importantly the cost of supplying macro and micro nutrients on an ongoing basis is a significant cost to most food insecure and nutritionally challenged communities that they can ill afford.
So, to sum up this point - research needs to be undertaken on local conditions in order that plants can take advantage of what nutrients are available in the soil and secondly, in building up the source of nutrients, so that they can be taken up by the plants and thus made available to those consuming the plants.
Secondly I would like to deal with the issue of plant nutrition looking at the matter of human manuring. While this is a distasteful subject to many communities and cultures it is one of the elephants in the living room that few wish to deal with. The vast amounts of suitable plant nutrition which is wasted in this manner is one of the major breaks in the link of the nitrogen and phosphorus cycles.
Humanure, or manure made from human faecal matter is rich in base nutrients such as phosphorus, nitrogen and so on. It also is rich in micro-nutrients and supportive of a healthy soil biota if properly treated. Accordingly sanitary practices in an increasingly urbanised world need to take cogniscance of the huge losses of nutrient through not using human waste sources of nutrient.
Of course this matter needs to be treated properly. Proper composting toilets are essential. Urine diversion toilets are also very useful as the urine is a very rich source of plant nutrient.
To sum up - we simply cannot waste this important source of plant nutrients but they must be free of dangerous pathogens when used in agriculture.
If these two issues of plant and soil nutrition are not holistically dealt with in traditional farming communities then steps need to be taken to ensure that they are. Further the use of agricultural chemicals and pesticides not only impacts the health of humans but also of the soil and hence of plants. Therefore chemical use must be stopped or at the very least minimised to its lowest possible levels. This reduces the requirement for outside (bought) inputs and increases the chances of local innovation and solutions.
Other methods such as inter-cropping, layering, permaculture practices along with other agro-ecological systems provide hardy, robust methods of farming which shift away from a reliance on a limited number of crops and 'weeds' - many of which are foods, medicines and otherwise useful, and instead provides a more resilient system. This is supported by the IAASTD 2009 report "Agriculture at a Crossroads" and reinforced by the views of the UN Rapporteur on food security in various studies.
Further research also needs to be made into anti-nutrients in crops, especially from local or regional perspectives where a few major crops may provide significant portions of the diet. In this way natural compounds found in plants such as trypsin inhibitors such as those common in many legumes - themselves important sources of plant based protein - may prevent or reduce the absorption of essential nutrients such as zinc, calcium or iron.
Obviously it is worthless to increase nutrient sources if they are not being actively absorbed. This problem can be dealt with through two primary avenues - one using traditional methods such as fermentation, and secondly by addition of admixtures such as lime or clays to break down certain antinutrients. Soy is traditionally fermented and not usually eaten in its unchanged state because of anti-nutrients being high in this legume, which is a major problem with the increasing growing and inclusion of soy in supplementary foods. In some cases the apparent benefits may be outweighed by the actual costs. This is a very important matter given the modernisation of diets, adoption of different crops in traditional farming practices and changes in lifestyle. The issue is too complex to deal with in such a short contribution but several important papers have been written on this issue; Holz and Gibson (2007) provide some useful starting suggestions, as did the Cornell GEO-PIE project on plant toxins and antinutrients.
Finally I note that genetic engineering of food crops has been touted as a way to deal with nutritional shortcomings. I, together with several other far more authoritative researchers, feel that this is a dangerous dead end wherein we are attempting to fix a symptom but not the cause. A major reason for nutritional deficiencies is a decreasing dietary diversity. Attempting to provide single solutions in single crops such as the infamous case of yellow rice is shortsighted and misses the entire reason why the problem exists in the first place.
Glenn Ashton