In Vietnam, basaltic soil (Rhodic Ferralsols) is rich in total phosphorus (P), but poor in available P, because of fixing by iron and aluminium [1]. Most of crops on the soil has symptom of P deficiency. Sorption of soil P is a major constraint to agricultural production, particularly those in high rainfall areas [2,3]. Deficiency of P is one of the major factors limiting crop production in the soil. Soil P exists in inorganic and organic form. Each form is a continuum of many P compounds, existing in equilibrium with each other and ranging from solution P (taken up by plants) to very stable or unavailable compounds (the most typical). Most of soil, 50% to 75% of P is inorganic [4]. Phosphorus should be added to soil so adequate levels are available for optimum crop growth and productivity. However, P fertilizer can be rapidly fixed (also referred to as sorption) in forms unavailable to plants, depending on soil pH and form of Al, Fe, or Ca content. Conversing P from unavailable form to available one usually occurs too slowly to meet crop requirements.

Corn and groundnut are main annual crops in Daklak province. These crops need high level of P for growth and development [5]. Rock phosphate (RP) is a slow P release fertilizer and has longer residual effect [6,7]. Some studies compared RP’s with single Super Phosphate in their role of increasing crop yield, but a few studies have been conducted with respect to field trials [8,9]. This is to evaluate effect of some RP’s to corn and groundnut on basaltic soil in Vietnam during 2015-2016.

Study site

Researching was implemented in Daklak, about 1,200 km in the south of Hanoi capital of Vietnam. Since the characteristics of the location, Daklak is affected by the tropical monsoon climate and the cool weather of the highlands. Markedly, there are two seasons: rainy season from May to October and dry season from November to April next year. These features give Daklak a special climate from other parts of Vietnam. Also, this weather is good for growing plants such as coffee, rubber trees, corn, groundnut. Basaltic soil in Daklak province has low bulk density and quite high porosity. The soil is acidic, rich in total of organic carbon, nitrogen and phosphorus, but poor in available phosphorus (Tables 1 and 2).

Table 1: Physical characteristics of soil at study site before experiment.

Table 2: Chemical characteristics of soil at study site before experiment.

Materials

The varieties of corn CP888 and groundnut GV10 were used for experiments; with density of 56,000 plants ha^-1 for corn and 266,666 plants ha^-1 for groundnut. The types of nitrogen and potassium fertilizer were urea (46% N) and Muriate of potash (60% K₂O).

Methods

Two parallel experiments with 9 treatments for one were conducted on basaltic soil in Daklak province of Vietnam during 2015-2016. The experiments were designed to RCB (Randomized Complete Block), with 4 replications and plot area of 100 m2. The experimental background consisted 180 kg N - 90 kg K₂O for corn and 45 kg N - 60 kg K₂O for groundnut. Except treatment of control, phosphorus dose of each other were 90 P₂O₅ (Table 3).

Table 3: The treatments of experiment.

The effect of RP’s on corn

The data from Table 4 shows that corn biomass from experimental treatments ranged from 282.6 to 345.2 g tree-1. Applying RP’s increased corn biomass by 20.6-22.2% compared to that of control. The content of N and P in corn leaves was different among experimental treatments, with the lowest found out in control. Applying 90 P₂O₅ increased N and P content in leaves by 0.05-0.8% and 0.06-0.09%, respectively. Table 5 shows that applying P fertilizer increased corn yield by 7.8- 15.9%. Among which, SSP treatment had highest productivity, with 6,983 kg ha^-1. The productivity from RP treatments were 6,538-6,957 kg ha^-1, 7.8-15.5% higher than control. Notably, the difference of productivity among SSP and RP treatments was not much. Even, some RP treatments such as Vietnam RP, Tunisia RP and Togo RP got quite high productivity, equated to SSP. The efficiency of 1 kg P₂O₅ was 10.7 kg grain corn for SSP and 5.2-10.4 kg for RP’s. Thus, applying RP’s increased biomass, leaf nutrient content and productivity of corn.

Table 4: The effect of RP on biomass end nutrient content in corn leaves at flowering.

Table 5: Corn yield and effectiveness of phosphate fertilizer.

The effect of RP’s on groundnut

Under the RP’s applied biomass and root nodulation of groundnut increased by 43.5-48.5% and 109.3 - 115.0%, respectively. Difference of biomass and root nodulation among SSP and RP treatments was insignificance (Table 6). The data from Table 7 presented that groundnut yield increased 15.5-18.3% under 90 kg P₂O₅ applied. In that, SSP increased by 18.3% and RP’s improved by 15.5-16.8%. The efficiency of 1 kg P₂O₅ of RP’s was 2.1-2.3 kg groundnut. There was no difference of productivity among SSP and RP treatments.

Table 6: The effect of rock phosphate on biomass and number of root node.

Table 7: Groundnut yield and effectiveness of phosphate fertilizer.

Applying P fertilizer increased biomass, leaves nutrient content and productivity of corn. The difference of corn biomass and nutrient content in leaves among treatments of SSP and RP’s was insignificant. Improvement in yield and yield components of corn has been recorded earlier with application of P. The yield of corn increased significantly with P level of 90 kg P₂O₅ ha^-1 [10-12]. Application of 90 kg P₂O₅ ha^-1 to corn crop increased grower’s income as compared to the lower rates of P (90>60>30 P₂O₅) [13]. Significant differences could not be established for the P sources, any of the rock phosphate could be used as P source for corn [14].

Applying RP’s improved number of root nodulation of groundnut, which can absorb N2 from atmosphere. Applying P fertilizer as SSP or RP’s increased rhizomes [15]. Rock phosphate stimulated root nodulation, enhanced soil microbiological activity, improved N and P accumulation. It is possible that root diseases, caused e.g., by nematodes may be reduced, so productivity and quality of groundnut were improved [16].

On basaltic soil in Vietnam, applying 90 kg P₂O₅ in RP form improved N and P content in corn leaves, increased biomass by 20.6- 22.0% and corn productivity by 7.8-15.5% in comparison with the control. The output of 1 kg P₂O₅ in RP’s form was 5.2-10.4 kg corn grain. Supplying 90 kg P₂O₅ in RP form increased groundnut biomass by 43.5-48.5%, root nodulation by 109.3-115.0% and productivity by 15.5-16.8%. The efficiency of 1 kg P₂O₅ in RP’s form was 2.1-2.3 kg groundnut bean. The difference of corn and groundnut productivity among SSP and RP treatments was insignificant.