Phosphate Fertilizer in Planting Trials

Planting trials with ryegrass (Lollium) and alfalfa (Medicago sativa)

In cooperation with the company HGoTech we investigate the plant growth when fertilizing with recycled phosphorus using PHOS4green compared to conventionally fertilized plants: Convince yourself of the quality of PHOS4green fertilizers and compare the results, measured in biomass (fresh mass, dry mass).

Three different phosphorus recyclates were investigated in a planting trial. The fertilizers were produced at the Glatt Technology Center using the ash-based PHOS4green process. Rock phosphate, commercial triple superphosphate, PHOS4green-P38 (double superphosphate), PHOS4green-P46 (triple superphosphate) and PHOS4green Special (phosphate compound fertilizer) were used (Figure 2). The experimental control was a standard unfertilized substrate.

The study answers the following questions:

• Does plant growth – when fertilized with recycled phosphorus – differ from that of conventionally fertilized plants?
• Are there measurable differences in biomass (fresh mass, dry mass)?

Ryegrass (Lollium) and alfalfa (Medicago sativa) were used as experimental plants to represent agricultural crops with different levels of phosphorus uptake. Ryegrass is a model cereal, while alfalfa represents a large number of dicotyledonous crops (potato, beet, canola, etc.). This makes it possible to harvest the above-ground biomass multiple times and thus characterize the medium-term dynamics of the fertilizers applied. In the experiment, there were five harvests at intervals of about four weeks (adjusted to the growth rate of the plants). The reason for this is that phosphorus compounds that are initially highly soluble in water can change to poorly soluble ones and then become unavailable to plants (also known as phosphate aging). In the same way, phosphorus can be released from compounds that are initially poorly soluble.

Root exudates are an important criterion for the release of acids. Dicotyledonous plants such as alfalfa generally release significantly more acids than monocotyledonous ryegrass. There are also differences in phosphorus uptake capacity within dicotyledonous plants. In particular, high uptake values are observed in canola even with fertilizers that release phosphorus at low levels. Alfalfa seems to be more the “average” plant.

The trials were made in plant pots. These are to some extent artificial because of the limited soil volume, but provide a valuable – and unbiased – assessment of the fertilization effect. This standard method for (initial) scientific evaluation allowed fertilization effects to be isolated and assessed under otherwise constant conditions. For studies such as this, it is essential that the soil not contain significant phosphorus reserves in order to directly assess the effect of fertilizers. In addition, the environmental conditions and substrate are identical for all plants. Similarly, naturally occurring mineralization and transformation processes that release phosphorus from the soil could also be eliminated. A commercial triple superphosphate fertilizer was used as a benchmark, allowing an unrestricted comparative assessment.

During the experiment, the fertilizers were first ground and mixed into the soil. This was necessary because of the relatively small amount of phosphorus per container (0.3 g). If applied as a pellet, this would have resulted in very different distribution in the root zone, causing the roots of the germinating plants to come into contact with the fertilizer at different times. This could have distorted the results. The application in solid form corresponds to the common practice.

Evaluation parameters of the fertilizer effect

The biomass formed (dry matter) and the phosphorus concentration in the above-ground plant parts were evaluated to assess the fertilizer effect. The fresh mass harvested (quantitative measurement) gives an indication of the fertilization effect and can be statistically evaluated. The dry matter (biomass) is the relevant parameter. However, this is only available at a later stage, when the plant material has dried out completely. The product of both factors is the so-called phosphorus decomposition parameter, which is used to evaluate the absolute amount of phosphorus absorbed in the shoot. There is usually a close correlation between fresh and dry matter, since the water content of the plant parts is the same for all varieties. There may be differences if the plants are irrigated differently or absorb the water differently. This is not normally observed in trials with phosphorus fertilizers.

Results of planting trials with fertilizers made from phosphorus recyclates

Fertilizers produced with the PHOS4green process resulted in the formation of a similar amount of fresh mass as fertilizers treated with triple superphosphate. Total dry matter produced after four harvests was not significantly affected by the positive control. Plants produced the same amount of biomass as those fertilized with triple superphosphate.