The foundation of optimised plant nutrition In open land vegetable production
The key to optimised plant nutrition is to apply nutrients on demand. Demand-driven fertilisation could mean several things, depending on who you ask.
Therefore, I want to be very clear about what I mean. When I talk about fertilising according to demand, I mean to meet the ongoing nutritional need of a crop in order to maintain the desired growth rate and sought-after composition of compounds in the crop.
The concept is not new, but it’s remarkably unnoticed, considering how effective and well substantiated it is. The theory behind the method was developed by Torsten Ingestad and his co-workers at the Swedish University of Agricultural Sciences several decades ago. Ingestad was an intuitive genius who made several groundbreaking discoveries in nutritional science. He was recognised for his work with the Marcus Wallenberg Prize, presented by the King of Sweden in 1989. The Wallenberg Prize corresponds to a Nobel Prize in Biology, and is given for pioneering research in forestry.
The concept has many significant attributes: increased utilisation rate of fertilisers and water resources, plant nutrition immediately available according to crop need, and minimised risk of nutrient leaching and adverse environmental impacts. Demand-driven fertilisation also enables increased nutritional control. An additional advantage is that you can quickly and easily add all nutrients to the crop, at virtually any quantities. The crop can access all nutrients, including the micro nutrients. The concept prevents salt damage and inhibited growth, which can occur at high salinities in the soil solution from large single doses of solid fertilisers. In addition, the method rationalises labour in-put, the use of machinery, and the use of expensive fertilisers. Ingestad realized the importance of starting from the vantage point of the nutrient flow from the environment into the plant. Just like humans who want food when we're hungry, plants constantly need doses of nutrition in accordance with immediate demand. We cannot eat food for a whole season in one sitting and meet the nutritional need for several months. It’s the same for plants. The natural growth pattern of a plant is exponential, meaning its weight increases by a
certain percentage each day. Therefore, nutrition should be administered in doses that increase exponentially to accurately meet the need of the crop.
The second basic principle is about the proportions between the nutrients; plants need all 14 essential nutrients in specific ratios. In other words, the second principle deals with what the composition of a fertiliser should look like in order for it to promote maximum growth. With high precision, Ingstad’s team could determine how these proportions look for many types of crops. These proportions are listed in the table. They stay constant at exponential growth. Remarkably, this nutritional composition is almost universally applicable to all crops. I say almost, as there are minor adjustment for species and what part of the crop is harvested.
When plant nutrition is delivered in accordance with the momentary need of the plant in this way, fertilisation becomes the controlling factor for growth rate, the distribution between root and shoot, and a range of quality aspects. Ingestad observed that if nutrition is supplied to the crop in the above proportions and in accordance with the ongoing need of the plant, there will be no symptoms of deficiency.
At maximum growth, all resources go to growth. Above all, the part of the plant above ground will grow rapidly, as the crop has access to everything it needs. Growth rate decreases when resource shortage occurs, for example due to lack of a nutrient.
This, in turn, often leads to an increase in resource allocation to the roots to compensate for the nutritional deficiencies.
Furthermore, at maximum growth, all energy from photosynthesis goes to growth. So, by slowing down growth, resources from the photosynthesis are allocated to other things, such as sugar content, antioxidants, chemical defence, compensation for mycorrhiza, and flowering and fruiting. This principle applies to all quality aspects.
So, by adjusting the nutritional input you can control both the quantity and quality of the plant part that you want to harvest. If you grow potatoes, you need to grow at maximum rate, because the potato is a stem botanically speaking, so maximum biomass equals maximum yield. If you grow carrots, the nutritional supply must benefit the roots, because this is what you want to harvest. If you grow tomatoes, you should slow down growth only slightly, to increase sugar and antioxidant levels, which affects the taste positively and increases the quality of the product.
By following the principles of demand-driven fertilisation, the nutritional utilisation rate of the crop is maximized and nothing is wasted. This is positive, not only for the crop production but also for the environment. If all nutrients added through fertilisation are absorbed by the plant and maximally used, this will minimise the risk of nutrient leaching during the growth season. Furthermore, you need to buy less fertiliser per kg of harvested crop, which helps your finances and means the responsible management of limited natural resources.
I have often asked myself why this concept hasn’t received more attention. I have come up with three main reasons. First, Ingestad undertook most of his research within forestry, and therefore the concept has not spread to other disciplines. Secondly, I have observed a conservative attitude among agronomists and horticulturists, in terms of incorporating new ideas, especially from disciplines other than their own. Third, Ingestad's fertilisation model requires continuous nutrient irrigation (fertigation) of the crop, sometimes until harvest. Thus the concept hasn’t been actively spread to all growers.>
My last point touches on the limitations of the concept’s applicability. Without a system that supports fertigation it is more complicated to fertilise on demand. Still, for farmers who do not use irrigation and who have to limit the number of fertiliser doses, the principles still apply more or less, depending on logistic and financial considerations. Control release fertilisers can be of good use in such circumstances.
It would advantage growers who already have an irrigation system in place to apply by fertigating using a demand-driven approach.
Ramp based irrigation systems are the primary mobile alternative with potential for effective application in open land horticulture, because the system spreads the water evenly with small losses to wind, which is necessary for the fertilisers to be distributed sufficiently evenly across the cultivation area.
However, I’m certain that demanddriven fertilisation can further bring drip irrigation to the fore in open field vegetable production. Of course, drip irrigation is often the most expensive irrigation system in such production, which is the main reason for the limitation in its use. At the same time, demand-driven fertilisation through drip irrigation is the most optimal way to grow crops. There is no production method that is so resource-saving and at the same time allows for such high precision. Here it's not just a matter of the system allowing nutrient supply on demand, it’s about giving the crop exactly what it needs exactly when it needs it. This solution is more viable than ever, now that there are special machines that lay out and collect the hoses. For those who are used to it, the process is relatively quick and easy. In a relatively dry year, such a system could provide a maximum harvest. So in summary, demand-driven is to give the crop what it needs, when it needs it, and in correct doses. In other words, how to supply nutrients to a crop to optimise quality and quantity at harvest.
▴ The table illustrates the proportions of nutrients needed at maximum growth for most crops. The ratios are expressed in percentage of nitrogen. Note that certain proportions are expressed in intervals. These differences are, in part, due to differences in species, but in particular to which parts of the plants that are harvested.
▴ The curve shows growth response to nutrient supply. When the nutrient supply reaches a certain level, it can start to generate growth in a crop. As the nutrient supply increase, growth will increase to the exact same degree, until the crop reaches maximum growth rate. Thereafter, increase if the nutrient supply will not give any response, until it becomes so high that toxicity will adversely affect the crop. Demand-driven fertilization entails keeping the nutrient in supply on a constant optimum.
▴ The curve illustrates how much nutrition is added to the crop from inception to any given point during season, in an accumulated fashion, when fertilized on demand. The dose increases exponentially as the crop grows exponentially. Growth will reach a maximum rate, then start to decline due to self-shading, aging and external factors, which reduces fertilizer need over time. The S-curve provides a simplified and generalized picture of how the plant nutrients should be applied during the growing season. Depending on the crop, harvest can occur before growth is as low as sowing, which means a shortened curve.