international fertilizer correspondent
No 8

Feed the soil to feed the people, the IPI jubilee


Feed the soil to feed the people, the IPI jubilee
- Session 1: Policy issues related to food supply and the environment
- Session 2a: Economic and social issues
- Session 2b: Economic constraints
- Session 3a: Plant nutrients for sustainable agriculture
- Session 3b: Imbalance in nutrient supply
- Session 4: Potash in agriculture
- Acquiring and putting knowledge into practice - the role of IPI

- The law of the minimum
- Soil fertility in Czech Republic in decline

IPI Publications

Publications from other sources

Other editions of IFC


Session 1: Policy issues related to food supply and the environment
Session 2: Economic and social issues
Session 2: Economic constraints
Session 3: Plant nutrients for sustainable agriculture
Session 3: Imbalance in nutrient supply
Session 4: Potash in agriculture
Acquiring and putting knowledge into practice - the role of IPI

Session 3: Plant nutrients for sustainable agriculture

Crop yield and quality are subject to a complex system of uncertainties as Paul Fixen from the Potash & Phosphate Institute (PPI) explained. Light, temperature, moisture, and soil are mostly out of our control whereas fertilizers, lime, pesticides, irrigation, tillage and cultural practices can be modified to suit the needs of the crop. Both groups of factors influence the plant's metabolism, its composition and, ultimately, its yield and quality. When considering fertilizers, the basic question is to decide how much to use. How can the demand of the crop be met without overshooting the mark with too high a rate? Too much fertilizer wastes money and may spoil the environment. Too little means that a nutrient hungry crop cannot yield properly and money is again wasted. A crucial point in fertilizer recommendations is nitrogen. N uptake by the crop may range between less than 20% of the applied N in unfavourable weather conditions to as much as 70% as long-term experiments with wheat in China have shown, provided that N is balanced with P and K.

A complex system involving uncertainty

A complex system
Modified from Beaufils, 1973

Paul Fixen described a range of decision support items that can be used to manage nitrogen and other nutrients. The crop's appearance can provide qualitative, semi-quantitative or precise information on the needs of the plant depending on whether a low tech approach with coloured charts is used or a more high tech approach with sophisticated sensors. An old but still under-used concept is crop nutrient removal. This should be combined with soil test information because although the balance between offtake and input can be calculated, this does not indicate the level of depletion from the soil. Soil testing itself is a powerful decision aid but reliable laboratories that use methods appropriate to local soils and crops are essential and not available in all regions. Economic analysis links input decisions with yields, forecasted profits and costs of production. Environmental risk assessment may be important for assessing at what level of N or P use water quality problems may arise. A new approach to determining the indigenous supply of a specific nutrient, when others are nonlimiting, is the use of omission plots to measure nutrient response. Source integration tools assist farmers and advisors to develop manure management plans by keeping manure application records and determining supplemental fertilizer needs. In his conclusion, Paul Fixen said that during the last half century, most of the scientific fundamentals of nutrient management have remained unchanged but that the technological and social context has changed markedly. Nutrient management decision aids exist for both large scale enterprises in developed countries and for small landholders in developing countries and their importance will increase as the demand for improved efficiency and productivity increase. Finally, the fertilizer industry's involvement in development, refinement and adoption is critical to the sustainability of the industry and of agriculture.

Often need to verify with
plant analysis

Biotechnology in concert with nutrient management was the title of a paper presented by Marc van Montagu from the Institute of Plant Biotechnology of Developing Countries (IPBO). He reminded the audience that the intensive use of fertilizers and pesticides in agriculture today goes back to the early '50s. Since then, crop breeding has centred upon the selection of elite cultivars for growing under high input conditions, such as those that formed the basis of the Green Revolution. However, in Marc van Montagu's view, this high input/high performance form of agriculture is not sustainable. On the one hand there are environmental problems and, on the other, farmers in developing countries simply cannot afford the high inputs required. The plant of the future, for both developed and developing countries, should perform under low input conditions. Biotechnology could achieve this and meet many of the needs of developing countries including better food security, added nutritional value or biofortification in the form of better amino acid balance, vitamins or other micronutrients. Biotechnology could also lead to new bulk products such as sugars, fatty acids, or waxes for industry. With better disease resistance and/or nutrient use efficiencies, biotechnology could also help to reduce agricultural pollution and help to preserve water supplies.

What is the potential, for instance, for improving nutrient uptake and assimilation? van Montagu says that because of the availability of fertilizers, plant breeding in the past mostly focused on yield and quality traits, whereas mineral assimilation efficiency was neglected. It is indeed a known fact that only 30-50% of applied N and around 45% of P is taken up by plants. The remaining N and P are potentially polluting to the environment. Some answers are emerging: molecular cloning of membrane transporters to manipulate uptake of individual nutrients, multiple transporters to mediate uptake and movement of K in plants, manipulating organic anion exudation from plants to enhance P acquisition, creating sinks and pathways in order to establish substrate fluxes from inorganic ions to organic high-value compounds and to avoid effluxes.

Iron and zinc are the main targets for biofortification. These essential nutrients are stored, almost exclusively, in the husks of cereal grains and are therefore lost in milling and polishing. It is little wonder that 63% of pre-school children and 76% of pregnant women in South East Asia are anaemic because they rely on polished rice for their main diet. By transferring, for example, soybean ferritin genes into rice, the transgenic rice becomes a source of iron. Genetic engineering can be a valuable tool for manipulating input traits, for example creating plants with resistance to biotic stress, such as pests and diseases, or to abiotic stress, for example salinity or drought. It can also be used to control physical traits such as maturity, plant architecture, pod shattering or shelf life, and output traits such as the content and quality of starch, protein, oil and other nutritional elements.

Clik to enlarge
click to enlarge

The benefits of having plants that are more resistant to pests and diseases cannot be over estimated. Not only do susceptible plants have higher production costs but the agrochemicals used to protect them are of concern to consumers and environmentalists. Resistance to salinity or to drought would expand the cropping area into salt or drought affected land and would also contribute to less variable yields when plants are subjected to these climatic and soil-borne stresses. Researchers have identified a whole array of functional and regulatory proteins that could be engaged in drought resistance (see figure).

The potential for crop improvement has never been greater thanks to the recent advances in plant genome data. What is needed now, says van Montagu, is a better integration of fundamental and applied research, start up companies for developing challenging prototypes, and industries developed for turning these prototypes into products and commercializing them. However, there is still considerable public concern on GMOs, not least that control and regulation mechanisms are inadequate and the effects on biodiversity should GM crops escape' into the environment. According to van Montagu, however, GM crops are as safe as conventional crops. Turning to the economic issue, the argument is that big life-science industries will sooner or later control the food chain and that this would be to the detriment of developing countries and to small and medium sized family farms. On the other hand, a transition towards sustainable development is inconceivable without science, engineering and technology. van Montagu closed with the statement that 21st century plants will be GM-plants.

"We need nature more than nature needs us" was the opening message of Caroline Drummond from LEAF (Linking Environment And Farming), Stoneleigh, UK. Her concern is that inappropriate use of inorganic fertilizers and, in particular, organic manure can lead to excessive nutrient losses from soil to air and water with consequences for the environment. The philosophy of LEAF is to establish a whole farm policy which provides efficient and profitable crop and animal production that is economically viable and environmentally responsible by a mix of the best of traditional practices and the best of modern technology. Their concept is "Integrated Farm Management", IFM, which combines aspects such as organisation and planning, soil management and fertility, animal husbandry with pollution control, and wildlife and landscape management. It is an approach designed for better profit, for biodiversity, for market opportunities, for building public confidence and for resource efficiency.

The LEAF wheel

Under an Integrated Farm Management system, forward planning is critical. Farmers review the accuracy and suitability of previous applications of nutrients and calculate all inputs. On a field by field basis the aim is to maintain soil N, P and K levels through the use of nutrient balance sheets and regular soil testing while meeting the nutrient requirement of the current crop. Also to be taken into account are contributions from grazing stock, soil type, nutrient reserves and previous crop residues, and relating them to the nutrient need and removal by the harvested crop.

Yield and quality, food and product safety, environmental protection and the sustainability of the soil and ecosystem are the goals covered by the paper of Avi Shaviv from Technion, Haifa on Optimizing Fertilizer Use Efficiency. The approach depends on farm size and on the system - whether rainfed, irrigated, protected or in high tech greenhouses. Decision support tools are needed to evaluate yield and quality, to monitor nutrient availability and environmental impact, and to integrate and analyse the information by applying models, GIS etc.

The main task of an efficient fertilizer application is to synchronize the nutrient demand by plants with supply, including the ability to apply optimal nutrient compositions. Modified fertilizers and improved application techniques are important if this goal is to be achieved. Modified fertilizers include bio-inhibitors, which interfere in the conversion of N, and control solubility, for example by coating. Nitrification of ammonium in soil is the process that eventually can lead to the loss of N from fertilizers to the environment through leaching and/or volatilisation. It can be controlled by nitrification inhibitors such as N-serve, DCD, or DMPP (ENTEC) and allows for ammonium/nitrate nutrition to be scheduled. The resulting acidification of the rhizosphere improves availability of P and micronutrients. Shortcomings of nitrification inhibitors are that some sorption capacity is needed to avoid leaching, there is still poor control over the reaction rates, and the effectiveness strongly depends on the crop.

Nitrous oxide emission, field plots

from Zerulla et al., 2000

Controlling urea hydrolysis e.g. with NBPT is another step toward managing the nutrient supply. Uncontrolled hydrolysis increases soil pH, which in turn affects the availability of P and micronutrients and leads to losses of ammonia.

Bio amendments, i.e. the use of nitrification and urease inhibitors can be combined with a particular application technique such as split or depot application to combine controlled release and spatial availability of fertilizer nutrients.

Precision application techniques, fertigation and, ultimately, fully automated and controlled glasshouses with recycling of water and nutrients, are also used to synchronize demand with supply. The variable rate application in precision fertilization based on soil, yield mapping, GIS processing and GPS positioning is without doubt an important step to improve nutrient use efficiency. Fertigation, that is the injection of nutrients into the irrigation water, provides very good control over timing, concentration and composition of nutrient application. The direct delivery to the roots assures high nutrient use efficiency combined with low nutrient losses. And last but not least, the recycling system adds value by protecting the environment.