The Challenge

The phosphorus (P) from plant-based poultry diets is difficult for poultry to utilize, so poultry diets are supplemented with inorganic P. As a result, the manure produced by poultry can contain up to four times more P than other types of manure. This increases the potential for P to escape into the environment and eventually into water courses. This in turn leads to the growth of oxygen-depleting plant life, which causes harm to other organisms - a process known as eutrophication. The application of P-rich manure therefore creates an increased environmental risk.

Dr Valtcho Jeliazkov of Mississipi State University, and adjunct professor at the Nova Scotia Agricultural College, has been investigating agronomically, economically and environmentally viable methods of reducing P availability in manures. Research has shown that solubility of P in manure can be reduced by incorporating aluminium-based compounds.

The Research

Field, container and incubation studies were conducted. The specific objectives were 1) to evaluate several amendments foe their capacity to reduce available P in dairy and poultry manures, 2) to estimate optimal rates for amendments as P availability reducing agents and 3) to field-test pre-treated manures to estimate crop response as productivity and nutrient uptake using three different soil types.

An incubation experiment was conducted to evaluate the ability of aluminium sulphate, ferric chloride or calcium hydroxide at 0, 100 and 200 g kg^-1 of manure, to reduce phytoavailable (available to plants, Mehlich-3 extractable) P in liquid dairy, laying hen and broiler chicken manure. Mehlich-3 extractable P was reduced from 59 per cent to 97 per cent for all manures treated with aluminium sulphate and ferric chloride. Calcium hydroxide treatment resulted in a Mehlich-3 extractable P reduction ranging from minus 17 per cent to 51 per cent. A container experiment was then carried out to study timothy growth and soil P levels as affected by the addition of the pre-treated manure to soil. Timothy yields in all dairy manure treatments were 45-57 per cent lower than an N-P-K control but were differemt from an untreated manure control. Dairy manure plus aluminium sulphate (200 g kg^-1) reduced water-extractable P by 82 per cent compared to the N-P-K control. All other manure and amendment treatment combinations were not statistically different from the N-P-K or the untreated soil controls in terms of extractable P or grass yield. Significant reductions in Mehlich-3 extractable P were seen for the aluminium sulphate or ferric chloride amendments with varied results from the calcium hydroxide amendment.

Two field experiments were conducted to verify the findings from the incubation and the container studies. This study demonstrated that the mineral pre-treatment of manure could reduce the extractable P content of the manure and soil without negative effects on plant growth.

The Bottom Line

In field experiments, mineral pre-treatments reduced extractable P content of manure and soil without negative effects on plant growth. Aluminium sulphate and ferric chloride compounds produced promising results, with all other manure and compound combinations producing results no different from controls. Pre-treated manures did not reduce timothy yield. Timothy yield is largely dependent on the previous year fertility and soil type. The results from this project contribute to the development of a feasible management tool for reducing P availability in dairy and poultry manure.

April 2008