Soil supply with phosphorus in Satsuma mandarin plantations in the Neretva Valley

Citrus growing is an important activity in the Neretva valley. There are 1,816 ha of Satsuma mandarin plantations (Citrus unshiu Marcovitch) registered in the Register of Farmers (PAAFRD, 2018). Soils in these plantations have alkaline reactions, which makes it difficult for plants to access phosphorus. The average area of a plantation is small, which complicates the implementation of agrotechnical measures. Unfortunately, the fertilization itself is often not based on a chemical analysis of the soil (and/or plant material), which results in inappropriate fertilization, and can have unfavorable ecological, physiological and economic consequences. To gain insight into the state of supply of physiologically available phosphorus, the soil of the top-layer (0-30 cm) and sub-layer (30-60 cm) was taken and analyzed from 55 Satsuma mandarin plantations, with an average plantation area of 1.03 ha. The obtained results of soil chemical analyses are classified into five generally accepted classes of phosphorus supply – mg P₂O₅ 100 g^‑1 of soil: i) very poorly supplied: <5 mg (6 orchards), ii) poorly supplied: 5-12 mg (18 orchards), iii) well supplied: 12-20 mg (16 orchards), iv) highly supplied: 20-30 mg (8 orchards), and v) very highly supplied: >30 mg (7 orchards). Very significant differences were found in the values of physiologically available phosphorus, from very poorly supplied to very highly supplied soils. The results show that less than 50% of plantations require regular annual application of phosphorus fertilizers. Also, with a certain number of plantations, one can talk about “luxury fertilization” (soil highly supplied with phosphorus), where values exceeding 40 mg P₂O₅ 100 g^‑1 of soil have been determined. For a complete insight into the state of nutrition of an individual plantation (and thus further fertilization), it is necessary to take samples of plant material (leaves) whose analysis results, together with soil analyses, will give a complete picture of the state of supply and determine further targeted fertilization.