Phosphate fertilisation in soil-based cultivation is still often based on insights gained during the 1970s. More recent research indicates that you can use less phosphate, especially that given via the watering pipelines. World stocks of phosphate are slowly shrinking; every reason to take a critical look at its use.
Exactly when it will happen is not clear but certainly at some point the supply of phosphate (P) in the world will be completely used up. The mineral is mined in North African countries such as Morocco but the largest stocks lie in politically unstable regions such as the Western Sahara. Unlike fossil fuels there are no alternatives: gone is gone. Its recovery, such as from sewage sludge, offers the only solution.
On a global scale this is a large problem: once it’s all gone agricultural production will drastically decrease. Horticulture will also feel the tremors approaching: the price of phosphate will rocket sky high long before there is nothing left to excavate. That will automatically lead to more efficient use. But before it gets that far there are several reasons to look more critically at the amount of P supplied during soil cultivation. The environmental argument – phosphate can leach out – plays less of a role in glasshouse production than in open field production but why would you give too much anyway?
High PAL-figure
In practise P-fertilisation in soil crops is still often based on old information. However, in recent years Wageningen University & Research Greenhouse Horticulture has carried out trials in the Netherlands that could lead to new ways. This relates to both the amount and the method of fertilisation. “Trials with lettuce and radish covered 25 crops over three to four years without phosphate fertilisation and we saw no loss in production.” The trials were on soils with a high PAL-figure (a measure of the supply in the ground, see box). “At such a high figure, in principle, you don’t need any fertiliser. Enough is available in the soil,” says fertiliser specialist Wim Voogt.
A trial with chrysanthemums (seven crops next to each other) gave the same picture. Here too there was enough phosphate in the soil. The only exception in chrysanthemums is the first two to three weeks of production. Then the plant does not have enough roots to sufficiently draw on the stocks. “We didn’t provide any pre-fertilisation in the soil in the chrysanthemum trial but we did fertigate (fertilisation via the watering system) for two weeks. That produced the same result as when we fertilised during the entire cultivation period via the watering system. After two to three weeks the plant has enough roots to independently take phosphate out of the soil,” he says.
Stock soil with fertiliser
Based on these results Voogt calls for a turnaround in P-fertilisation; concentrate on stocking the soil with fertiliser based on sampling (see box) and use little or no fertigation. “I believe fertilising via the watering system is unnecessary in almost all cases. The exception is at the start of the cultivation cycle for crops that are propagated in a plug or (pressed) pot such as chrysanthemum and lisianthus. Steering the plant based on the supply in the soil is more effective and cheaper,” he says.
Monopotassium and monoammonium phosphate, which are given via the water pipelines are relatively expensive compared with triple super phosphate for the soil. “During fertigation all the phosphate completely dissolves. If you apply that to a soil of pH 6.5-7.5 it immediately settles in the upper layer (it reacts with calcium and magnesium, for example). Therefore this is not an effective way to get the fertiliser to the roots.”
Awkward message
As already mentioned the exception is a young plant in a pot. This is due to the uptake capacity of the roots. “Because the P-concentration in the soil is very low, adequate uptake requires a well developed root system. This happens automatically in a plant grown from seed because the root development keeps pace with the need. This is not so at first in plants grown in a pot because at the moment they are planted they are already fully occupied with growth,” says the fertiliser specialist. “Therefore, for example for chrysanthemums, ideally you should stock the soil with fertiliser once per year to compensate for that extracted and fertigate for two weeks per crop.”
Economical use of phosphate is a somewhat awkward message, he realises. Growers don’t want to take any risks even if research shows that a different method of fertilisation is possible. As well as being cheaper the environmental argument also plays a role but unfortunately governments, water authorities or consumers do not reward such efficient use.
Organic production
Organic producers fertilise their crops with organic fertiliser and composted vegetative waste. In this case the main focus is on providing sufficient nitrogen. But with this method of fertilising the ratio of the P-contribution is always too high. This is because the N:P-ratio in the fertiliser is higher than that required by the crop. On the other hand the amount of potassium in the fertiliser is often too low.
A shortage of potassium is not a problem; that can be supplemented with fertilisers that are permitted in organic production. But a rise in the phosphate level can become a problem although Voogt says he has never seen problems in a crop due to an excess. However, leaching can increase. “Actually an organic producer should be able to separate the application of nitrogen and phosphate,” he says. “The difficult thing is that there are too few separate N-sources available. You then arrive at fertilisers such as blood meal but that also contains phosphate. It is always a case of trying to find the right balance between N-shortage and P-excess based on a good mineral balance.”
The researcher is fine-tuning the fertiliser strategy further. One of the research projects concerns the optimal combination of phosphate fertiliser, water and substrate using pot chrysanthemums as the trial crop.
Difficult aspects
Phosphate is a nutritional element with some difficult issues. Part of it disappears each time fertilisation occurs as it becomes tied up in the soil and is thus hardly available to the plant. This has to be fully taken into account when using virgin ground (grass or farmland that is converted into greenhouse soil).
The amount of available stocks in the soil can be determined, for example, using a PAL-extraction. In this way you measure the amount that is immediately available plus some of the tied up phosphate. To fully understand this Wim Voogt compares the P-stock in the soil with warehouse shelving. The most easily available part is, as it were, on the lowest shelf. Then comes a part on a higher shelf that with just a little effort is available. Then there is a part that is difficult to reach as it is on a very high shelf; it is only available in very exceptional circumstances. In the end everything ends up at the back on the high shelf.
Annual extraction
Based on new data the PAL-figure required for glasshouses has been adjusted to 80. The rating has to be built up when using virgin soil. When the stocks are at the right level a grower can maintain this by replacing the amount used up annually with a reserve supply of fertiliser. For chrysanthemums this is around 200 to 250 kilo P/ha, for example.
If too much builds up, at a certain moment, the soil will become saturated. Then the phosphate will leach out in the drain water to the same degree as nitrogen. This will occur first on sandy soil, but not so quickly on clay.
Summary
Based on research, Wageningen University & Research, the Netherlands, is calling for a change in phosphate fertilisation for soil-based crops. Concentrate on building up a reserve supply of phosphate based on sampling and use little or none in the watering system, except for the first couple of weeks if plants are in small pots. Phosphate is a difficult element that is quickly bound up in the soil.
Text: Tijs Kierkels. Photos: Wilma Slegers
