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Soil pH - Located in the tropic zone with high temperature and high rainfalls (>1,600 mm annually), the soils in Hainan have undergone severe nutrient leaching and a high degree of weathering. As a result, acid soils prevail on the island. According to a soil survey of mango plantations, the extremely acid, strong acid, slightly acid and neutral soils account for 75.4%, 22.3% and 2.3%, respectively. Their pHs range from 4.0 to 6.8 (Table 1).
Table 1. Soil pH in mango orchard in Hainan
Low soil pH usually causes mango root damage by excessive H+ and Al3+, which directly reduces nutrient availability in the soil and limits root extension and penetration. Absorption studies for 7 plant nutrients conducted on soils sampled from pineapple and banana plantations showed that the absorption rate for P, K, S, Cu, Zn, Mn and B was 36.3 to 87.4%, 7.5 to 41.2%, 2.0 to 64.3%, 26.9 to 32.8%, 12.9 to 41.5%, 4.6 to 38.4% and 12.9 to 70.1%, respectively. The sequence for the soil absorption rate for the 7 plant nutrients was P>B>Cu>Zn>K>S>Mn (Table 2)
Table 2 Rate of soil absorption for the 7 plant nutrients (%)
Soil organic matter - In Hainan, the weather is favorable for plant growth year round. Under such circumstance, the estimated falling tree leaves and branches can be up to 10 to 11 t/ha, much higher than people usually think. The turn over of biomass is fast in soil, less favorable for soil organic matter build-up. Table 3 shows rather low organic matter contents of 8.55 g/kg to 28.8 g/kg (an average of 15.94 g/kg) in five fruit plantations.
Table 3 Nutrient status of soil in fruit orchards in Hainan
Soil status of N, P and K - All the soil samples analyzed from the fruit plantations are deficient in N and P, and majority deficient in K. All the soil samples from the fruit plantations are deficient in total N and available N with an average of 0.59 g/mg and 27.0 mg/kg, respectively (Table 3). The total P ranges from 0.24 to 9.2 g/kg (average of 0.37 g/kg), and available P from 2.8 to 19.5 mg/kg (average of 7.18 mg/kg). There was only one soil that had adequate amounts of total K and two having adequate available K. The data is a reflection of low soil fertility on the island, resulting from low nutrient bearing parent materials, strong leaching and inadequate fertilizer management.
Nutrient demand for tropic fruits - According to numerous studies and analyses, nutrient demand for tropical fruits is generalized in Table 4, though different fruits and/or species may differ. The data indicate that K is the number one nutrient for tropical fruits, followed by N, Mg, P and S. Banana in particular has the highest demand on K supply so insufficient supply usually delays its flowering and significantly reduces yield and quality.
Table 4 Nutrient absorption of tropic fruits (kg/ha)
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