Fertilizers are the most central crop input in a greenhouse, with the possible exception of water in irrigated systems. Fertilizers also comprise the biggest portion of the input costs. Yet, of all inputs, they’re often the one that has the biggest losses.
It’s therefore very important to have a correct understanding of fertilizers and the soil-water-plant-system, and a relevant approach to fertilizer management. This can be achieved by focusing on these three aspects: (1) how to think about fertilizers, (2) which to use and (3) how to execute your fertilization plan.
The principles I’m about to explain are universal for all plant production systems, such as soil-based, soilless or hydroponic systems. However, as the application of the principles have some differences between the systems, I will give examples as I explain.
How to think
Before we do anything, we need to know what we should do, and preferably why. This can be divided up in three sub-categories: what the plant needs, how much it needs and how to make it available to the crop.
What are the plants’ needs?
First of all, all plants need all essential plant nutrients. Always. There seem to be confusion regarding this among some producers and fertilizer suppliers. And the demand for the nutrients come in certain ratios, which are often quite similar for horticultural crops like vegetables and fruit (see table).
How big is the need?
The main difference in fertilizer demand between crops is based on their capacity to assimilate sunlight, i.e. the growth rate. Fast-growing crops need more fertilizers than slow-growing ones. Note that it’s not the size of the plant that matters but the capacity to form new plant tissue. The crop takes up the nutrients from the soil (or the soilless environment) in the form of a flow over time. The rate of this flow must at least be enough to sustain the growth rate and development of the crop. Otherwise, we will see inhibited growth or even deficiency symptoms.
Plant availability of the nutrients?
The plant nutrients are only available to the plant when they’re dissolved in the water accessible in the root zone, regardless of if the system is soil-based, soilless or hydroponic culture. So plant availability is determined by the supply of soluble fertilizers, and in soil-based systems, by the rate by which the microbes decompose the organic material, which in turn frees up nutrients that were locked up in the soil, in a process call mineralization. There must always be enough nutrients dissolved in the soil or growing media water to sustain a sufficient uptake rate by the plant.
Another important parameter for nutrient availability is pH. From the perspective of plant nutrition, an optimal soil pH is around 5.5. It’s preferable to try to enable a pH between 5.0-6.0 in the root zone. Outside of that range, some nutrients become unavailable to the crop. Even though some plants are adapted to soils with higher pH, and are able to acquire required nutrients, the optimal range generally remains the same in all systems.
What fertilizer products should we use?
What kind of product to use is a matter of what kind of production system is in mind, but in the thought process we should start with a complete fertilizer product that supplies all the essential nutrients. Then we scale back the need to supply some of these nutrients depending on what, and how much, the soil or the system can supply them. The irrigation water also contains naturally occurring nutrients that should be accounted for.
This does not imply that you must necessarily fertilize with all nutrients. You must simply ensure that there is access to all of them in the production system. If you have a good and fertile soil there is rarely the need to supply all micronutrients, as they can be attained by the crop from the soil. The strongest contrast to this would be a soilless hydroponics system, in which case you really must supply all nutrients via the fertilizers.
Regardless of system, nitrogen-phosphorus-potassium (NPK) fertilization is virtually always necessary in order to attain optimized yield levels. I would even argue that the other macronutrients (magnesium, calcium and sulfur) need to be supplied via the fertilizers, if high productivity is to be maintained. Even in systems where the soil is very fertile, it’s unlikely that it can sustain a sufficient flow of these nutrients that matches the demand, when aiming for high yield levels via different inputs.
For the micronutrients, it’s usually not as much a matter of their presence in the system as it is an accessibility issue, in which the pH is the most important parameter. The best way to manage the pH in the root zone is via the fertilizer. N is supplied to the plant either in the form of ammonium or nitrate. Ammonium lowers the pH in the root zone in conjunction with nutrient uptake. So by using a fertilizer with an ammonium/nitrate-ratio adjusted for your conditions, you assure that the nutrients present in the root zone can be taken up by the plant (see table).
In order to scale back according to the above reasoning, testing of both soil and water is necessary. However, don’t rely too much on the test results. They serve as a very good starting point, but always look at the crop response, as other factors can affect the outcome.
Note that it can often be worthwhile to use a complete fertilizer, even though the soil might have a high fertility level, or even if you are able to make your own mix for soilless and hydroponics culture. Especially a product that provides the micronutrients in chelated forms that will remain accessible to the crop, and that has the above-mentioned ammonium/nitrate-ratio. Then you guarantee the supply of all nutrients, almost regardless of soil conditions or other parameters in soilless systems.
How should we do it?
Once you have decided which products to use, you need to supply them in the best way possible. This means to supply on demand over time. The only way to do this with precision is via high-frequency fertigation. Optimally, nutrients can be supplied at each irrigation, but if that’s not possible the doses need to be adjusted. Depending on limitations in the production system, you can scale back the precision level of the on-demand supply, in which case you need to supply more nutrients in advance. If you need to scale all the way back to one, or only a few fertilizer applications, for the whole season, I would recommend a controlled-release fertilizer that follows the need of the crop.