How to use pig slurry in fertigation for fruit trees

Until a few years ago, talking about fertigation was the mixture of water with chemical fertilisers for application in different types of crops. Over time, the need to improve the management of pig slurry and the move towards a more sustainable production model, the well-known circular economy, has led to the study of alternatives. Among them is fertigation with organic fertilisers from slurry. Although its use is already daily in extensive crops such as cereals, IRTA is trialling its application in fruit trees. We spoke about this study and the characteristics of organic fertigation with IRTA researcher Josep Rufat and technicians Xavier Vallverdú and Maribel Abadia.

Equipment performing fertigation tests on fruit trees. Photo: Rotecna.

There is increasing talk about reusing slurry as organic fertilisers. What exactly is this?
We propose replacing chemical products with organic fertilisers from liquid pig slurry. To analyse its performance and guarantee its use, we have a trial underway in which we test how organic fertigation works on fruit trees. We examine the fertiliser's characteristics, application, and production results, considering legislative limitations, environmental effects, and the product's safety for the end consumer. First, the slurry is separated into solid and liquid on the farm to understand the process. We keep the liquid part, which we then mix with water and apply with a drip irrigation system, although sprinklers can also be used.

What are the characteristics of liquid slurry?
One of the problems with this organic product is that its composition varies over time, from one farm to another, from the production phase (sows, fattening or weaning), and also depending on the time of application;... So, to know the composition of the organic fertiliser that we will apply, you must always conduct an analysis and see what richness and composition NPK (nitrogen, phosphorus and potassium) it has, which is the most important thing. In this sense, it is not a product with a balanced composition; it is usually low in potassium, so in some cases, it has to be compensated. However, in the time that we have been carrying out this trial, about 5 years, we have not seen that the NPK balance has dropped from the point of view of potassium when applying this organic fertiliser. Because fruit trees are not excessively demanding of nutrients or because the soil already has potassium, we have not had problems.

How should organic fertigation be applied?
According to the Global Gap guidelines, which set out the requirements to ensure the food safety of agricultural products, we can apply these organic fertilisers up to two months before harvest. For example, in the case of the trees with which we do the test, we collect the fruit at the beginning of September, but we can apply it until the beginning of July. From there, we cannot use this liquid slurry to avoid problems arising from its application. The application is healthily safe, but indirectly, you can have contamination, which does not have to be a problem with the application of this product, but that is attributed to it and could be a grave issue, so it is essential to comply with the recommended application periods.

What are the benefits compared to the application of other chemical fertilisers?
To begin with, we are reusing a mislabeled by-product, such as slurry, at practically zero cost. In many cases, the farm owner needs to get rid of this product and even pay for it so that the farmer can obtain nutrients at zero cost; otherwise, if we compare it with mineral fertilisers, he has to pay for them. If it is economically advantageous due to different factors, it is a product with a tremendous fertilising capacity. It can replace chemical products in part or all. Throughout the trial, no productive difference was detected concerning mineral fertilisers. On the other hand, the IRTA efficient water use program technician, Xavier Vallverdú, explains that some analyses have also been carried out to examine soil microbiology. These have shown an increase between 25 and 30% in microbiological activity. Now, we have to analyse this activity and see if it helps the productivity of fruit trees.

What are the obstacles to take into account in organic fertigation?
The main drawback is transport, so the distance between the farm and the farms where the liquid slurry is applied must be minimised. Ideally, you should look for a farm close to the farms where the slurry is produced to reduce transport costs as much as possible since the liquid phase occupies a large volume and to guarantee the economic viability of its use as fertiliser. On the other hand, at a financial level, an investment must be made to have the irrigation system prepared with a sturdy filtration system to prevent any remaining solids from the slurry from accessing the irrigation system and obstructing or breaking the drip. Finally, another drawback would be that, depending on the fruit, and whether it is for fresh consumption or export, a series of regulations must be complied with that Maribel Abadia, a technician in IRTA's post-harvest programme, is well aware of, who is helping us to assess the safety of using the liquid phase of slurry at a microbiological level for fertigation. As Maribel points out, by using an organic fraction of pig origin, there is a risk that there may be foodborne pathogens such as E.coli or salmonella. That is why, throughout the test, analyses of samples of leaves, fruits, irrigation water, and soil were carried out, with all the results being negative. It must be remembered that getting a positive result when consumed abroad could seriously affect the market, so we must be cautious.

Does the agricultural and livestock sector see the reusing of liquid slurry as a solution for the future?
The farmers who are doing it and others who are considering it are mainly those who also have farms and have them near their crop farms. In these cases, it is when the system is most profitable. It is a solution for the future to reuse the slurry produced by the farm, although you have to be very careful with all the technical issues to avoid problems of seepage into the soil or obstructions in the drip system. It is, therefore, a system that may work in the future, but it will not completely replace chemical fertilisers.

What are the challenges for fertigation to be consolidated as a fertilisation alternative?
This type of fertiliser will be used increasingly, especially in those territories where the producing area (farms) is close to the consuming area (also farms). The greatest challenge is to be very careful with the health aspects of its application. In this sense, chemical fertiliser is very convenient because you pay for and apply it compared to organic fertilisers, which have certain limitations and obstacles to their implementation. Still, some farmers already see its benefits as a solution to the problems with slurry in general.