Soil acidity is a major problem worldwide as it decreases plant growth by affecting the availability of nutrients and causes various toxicities. Acid soils are commonly deficient in phosphate so that both conditions require correction, which can be carried out by application of a single mineral ameliorant.

Liming is the most common way to ameliorate acid soils. At some locations sources of natural lime are inadequate or expensive. Alkaline industrial byproducts such as metal smelting slag, wood ash and chicken litter ash may be used as replacements for lime under these circumstances. Such byproduct materials are commonly dumped however they may be used to ameliorate soils with consequential environmental benefits. Some byproducts contain phosphate so that both adverse soil conditions can be overcome by a single application of byproduct.

This research is focused on the use of iron-smelting slag, wood ash and chicken litter ash, each of which acts as combined liming agent and phosphate fertilizer. These byproducts were chemically analysed for available plant nutrients using Association of Official Analytical Chemistry (AOAC) standard methods. The plant nutrients present include phosphorus, calcium, magnesium, sodium and potassium (Table 1). The major compounds present were identified by XRD to be apatite, calcite, and quartz in the two ashes and calcium magnesium silicate (akermanite) and calcium aluminium silicate (gehlenite) in slag. The CaCO₃ equivalence for slag is 93% with the ashes having lower values depending on charcoal content.

Table 1. Major Plant Nutrients in Byproducts

Waste Materials	P (%)	Ca (%)	Mg (%)	K (%)	Na (%)
Slag	0.26	27.19	5.28	0.07	0.34
Chicken Litter Ash	12.6	6.48	1.18	2.36	0.59
Wood Ash	0.44	6.60	1.10	2.17	0.74

A glasshouse experiment was carried out to identify the effectiveness of the wastes as phosphate fertilizers on a highly P-deficient acid lateritic soil. Treatments included various types and rates of industrial byproducts and included monocalcium phosphate, dicalcium phosphate and rock phosphate as reference materials. Various levels of phosphate were applied, ryegrass was planted and harvested after 8 weeks and at 4 week intervals thereafter. Dry matter yield ranged from 0.025 g to 2.3 g/pot for the first harvest, from 0.03 g to 2.3 g/pot for the second harvest and many plants died of P deficiency before the third harvest. The agronomic effectiveness of the materials as phosphate fertilizers was calculated by comparing the various amounts of phosphate required to produce the same yield for the various materials. This is the “horizontal comparison” or “substitution value” procedure that gives values of Relative Effectiveness (RE) that are independent of the rate of application of the fertilizers. The RE values for all the materials relative to monocalcium phosphate (100%) for the first harvest are as follows, 50% for dicalcium phosphate, 31% for rock phosphate, 7% for partly burnt chicken litter ash, 7% for totally burnt chicken litter ash and 1% for wood ash and slag. The RE values for the second harvest were 100% for monocalcium phosphate, 80% for dicalcium phosphate, 40% for rock phosphate, 10% for partly burnt chicken litter ash, 8% for totally burnt chicken litter ash and 2% for wood ash and slag. Clearly chicken litter ash has appreciable value as a phosphate fertilizer whereas wood ash and slag are ineffective.

An evaluation of the liming effect of the byproducts indicates that they may be used as a soil amendment on acid soils. Byproducts are also sources of other plant nutrients so they may be regarded as a form of compound fertilizer.