1. Introduction
Organic farming, with an emphasis on a holistic approach to agriculture, avoids the use of synthetic fertilizers and pesticides [
1]. More specifically, the United State Department of Agriculture (USDA) also made the following recommendations: “The producer may manage crop nutrients and soil fertility by applying (1) A mined substance of low solubility; (2) Ash obtained from the burning of a plant or animal material, except burning as a means of disposal for crop residues produced on the operation; (3) Sewage sludge (biosolids) must not be used.” [
1]. With no chemical inputs, organic farming must be skillfully managed. In fact, soil nutrient management poses a serious challenge to organic practices, especially if the farmed soil is acidic and phosphorus (P) deficient as is often the case of Oxisols [
2,
3].
Along with nitrogen (N) and potassium (K), P is a major (needed in large amounts) and essential nutrient to all crops [
4]. More specifically, P is a component of genetic molecules (DNA, RNA) as well as adenosine triphosphate (ATP), which transfers energy during photosynthesis and respiration [
2,
3,
5]. Phosphorus-deficient plants grow slowly, appear dark green or blue-green as a result of stunted growth with concentrated green pigment in leaves. In advanced stage of P deficiency, leaves often turn purple [
2,
6,
7].
Although well-matured compost, green and animal manures are allowed in organic farming, their uses are often intended for supplying N rather than P for crops. That is because P concentrations in these materials are relatively low. For example, total P ranges: 0.04–2.3% in compost [
8], 0.2–0.4% in green manure [
9], and 0.4–2.1% in animal manure [
9]. With relatively high total P concentration (10–18% [
3]), rock phosphate could provide significant P to crops if its P releasing potential can be properly managed.
Chemical formula of rock phosphate is Ca
10(OH)
2(PO
4)
6 with some minor substitutions of OH
− by fluoride (F
−) or carbonate (CO
32−). In the US, rock phosphate is often mined from sedimentary rocks in Florida and North Carolina [
3,
10]. Unlike chemical P fertilizers, such as ammonium phosphate (NH
4H
2PO
4) or triple superphosphate (Ca(H
2PO
4)
2·H
2O), rock phosphate is sparingly soluble in water [
3,
11]. Thus its availability to crops is very low, unless the conditions are strongly acidic (pH < 5.0) as predicted by the following reaction.
However, in acid soils the concentrations of aluminum (Al) and/or manganese (Mn) are so high that any P that is dissolved from the rock is precipitated (as shown below) before it can move far enough in the soil for plant roots to absorb it. Adsorption of P on solid Al and iron (Fe) oxides, which are abundant in Oxisols further reduces P availability.
Moreover, Al
3+ and H
+ in acid soils can damage the root systems such that no uptake of water and nutrients, including P, would be feasible [
12]. For this reason, mined lime stone (CaCO
3), crushed coral lime, wood ash, or biochar is often applied to alleviate soil acidity, and improve crop growth [
12].
Both P nutrient and soil acidity also markedly affect the growth of young legumes, especially their nodulation and N
2 fixing capability [
13]. For example, soybean plants need between 12 and 15 mg/kg P in soil as extracted by the Olsen method for adequate yield [
14], and most legumes perform best at neutral pH [
3].
Given such an apparent conundrum, our objective was to identify an optimal condition in which the soil pH is low enough so that rock phosphate would be adequately soluble, yet is high enough so that soil Al (and perhaps Mn or H) does not adversely affect soybean seedlings, which served as a test crop.
2. Materials and Methods
2.1. The Soil and Organic Amendments
A strongly acidic, Al toxic Oxisol (Humic Rhodic Kandiudox, Paaloa Series) from a former sugarcane plantation in Oahu, Hawaii, was used in this experiment. In the non-amended state, soil pH (1:1 in water) was 4.5, KCl-extractable Al = 0.85 cmol
c/kg, NaHCO
3-extractable (Olsen) P = 5.8 mg/kg, and total organic carbon = 3.02%. The soil had a silty clay texture with 14% sand, 42% silt, and 44% clay. The organic inputs consisted of rock phosphate from central Florida, coral lime from Western Samoa, and ground cowpea leaf to provide N and K and other micronutrients. Their nutrient composition is listed in
Table 1. The organic amendments were mixed thoroughly with the finely ground (<5 mm diameter) soil, which was stored in plastic pots containing 2 kg of air-dried soil each.
2.2. The Experimental Design and Chemical Analysis of Plant Tissue
A factorial setup consisted of 3 levels of coral lime (0, 1, and 2 g/kg equivalent to 0, 2, and 4 tons/ha if 1 ha is assumed to weigh 2 × 106 kg), 3 levels of rock phosphate (0, 75, and 150 mg P/kg) and 3 levels of cowpea green manure (0, 5, and 10 g/kg). The pots were arranged in a randomized complete block (RCB) design with 3 replications.
The treated soil was moistened with deionized water to field water holding capacity (28%) and incubated in a greenhouse (at the Magoon facility, University of Hawaii) for two weeks. Thereafter, one 10-day old seedling of soybean cultivar Kahala was transferred to each pot and grew for one month. Dry matter weight and nutrient content of the above-ground plant tissue were measured as follows. Plant tissue was thoroughly washed first with tap water then with de-ionized water, and oven-dried at 70 °C for 3 days. Subsequently the dried materials were weighed and ground to pass a 0.5 mm sieve. The finely ground material (0.50 g) was ashed at 400 °C in a muffle furnace for 4 h. The ash was dissolved in 20 mL of 1 M HCl, filtered through a Whatman No. 42 paper, and stored for elemental analysis with an inductively coupled plasma spectrometer.
2.3. Soil-Solution Extraction
Soil samples were collected at transplanting and a centrifugation method was used to extract the soil solution [
15,
16]. Briefly, approximately 200 g of moist soil were placed into a modified Buchner funnel and centrifuged at 500×
g for 30 min. The solution collected (approximately 10 mL) was stored at 4 °C for subsequent chemical analyses (Al, pH, P, etc.)
2.4. Chemical and Statistical Analyses
Soil pH was measured in water as follows. Twenty grams of soil were mixed with 20 mL of deionized water, stirred for 2 min, equilibrated for 30 min. Thereafter, pH of the slurry (and of the soil solution) was measured with a combination pH electrode and a digital pH meter.
KCl-extractable Al was obtained as follows. Five grams of soil were shaken in 25 mL of 1 M KCl for 30 min, then centrifuged at 12,000×
g for 10 min. The extraction procedure was repeated once and the combined extract was brought to 50 mL with 1 M KCl. Soluble Al was determined by the eriochrome cyanine R colorimetric method [
17].
Soil P was extracted with 0.5 M NaHCO
3 solution as follows. One gram of soil was placed in a centrifuge tube with 20 mL of 0.5 M NaHCO
3 solution. The tube was shaken for 30 min and centrifuged at 12,000×
g for 10 min. Soluble P was measured by the molybdate-ascorbic acid method [
18]. Soil-solution organic carbon was determined by the Mn(III)-pyrophosphate method [
19].
For statistical analysis, Minitab v. 21 (Minitab LLC, State College, PA, USA) was used to detect treatment effects on soil properties and plant growth parameters via a 3-way factorial ANOVA. Fisher Least Significant Difference (LSD) test with the significant level set at 95% was performed on all 27 treatments (when there were significant interactions among the lime, rock phosphate, and green manure amendments). Pearson correlation was also performed on some parameters, such as Ca and P in plant tissue vs. those in soil solution. Sigmaplot v. 14.5 (Systat Software Inc., Chicago, IL, USA) was used to construct graphs and build regression equations.