Increasing rock phosphate efficient use through soil amendment with waste biomasses

Loss of soil organic carbon content can limit the soil's ability to provide goods and services. In agricultural soil this may lead to lower yields and affect food security. In this context, the proper use of waste biomasses as soil amendment is a valuable alternative to disposal with numerous benefits to soil fertility with a direct effect on soil organic matter content. Moreover, beside this direct effect waste biomasses can have a beneficial result on nutrients.

In modern agriculture the use of rock phosphate as fertilizer is crucial but abused. Although this non-renewable resource reserves may be depleted in 50-100 years, many farmers still use overabundant amount of rock phosphate-based fertilizers with an additional environmental burden. From a chemical point of view the efficient use of rock phosphate can be increased by some agricultural practices and amending soil with waste biomasses is one of them.

Here we propose the use of 3 different waste biomasses on phosphate rock dissolution and subsequent phosphorus availability. The 3 waste biomasses, citrus pomace, olive oil mill waste and barley spent grains, were selected mainly for their potential direct or indirect effect on pH. This experiment was composed by two steps a bench and a pot trial. In the bench trial the waste biomasses and phosphate rock were mixed and transferred in litterbags. In these litterbags pH, water soluble P, matter loss and total P were destructively analyzed each 10 days for a month. In the pot trial the same combination of waste biomasses and phosphate rock were tested in a soil plant system; some agronomic parameters were measured on rocket salad and pH, soil-P availability, acid phosphatase activity were analyzed in soil.

In bench trial, barley spent grain plus phosphate rock shows the highest water soluble P, citrus pomace plus phosphate rock showed a significant correlation between water soluble P and pH while olive oil mill waste plus phosphate rock has high correlation between water soluble P and matter loss. These two treatments were also the best performing in the pot trial in terms of rocket salad yield and soil available P. Even though the investigation was conducted on a short lived experiment, some results are encouraging and displays good agronomic performances of waste biomasses plus phosphate rock. Nevertheless, next studies should consider other waste biomasses within longer experiments. Additionally, scaling up the experiment to a field application can provide more thorough information about the effects on soil organic carbon and P dynamics.