Nitrates in relation to composting and use of farmyard manures. (SA Spring 1994 (v6n2))

Spring 1994 (v6n2)

Nitrates in relation to composting and use of farmyard manures.

P. Ott, S. Hansen, and H. Vogtmann

Lockeretz, W. (ed.). 1983. Environmentally Sound Agriculture; Selected papers from the 4th International Conf. of the IFOAM, Cambridge, Mass., 8/18-20/92. Praeger Publishers, NY. pp. 145-154.

This chapter presents an analysis of several experiments conducted in Europe. The research focused on the effects of organic fertilizers on nitrate leaching, and includes data from composting, lysimeter, and field plot experiments.

Manure Use on Organic and Conventional Farms

The authors make a clear distinction between the underlying reasons for using manures on farms in Europe. In the conventional system, manure is viewed as a waste which must be disposed of in the cheapest and most convenient way possible. With organic farming methods, on the other hand, manures are a valuable means for transferring nutrients from one part of the farm or region to another. According to the authors, many nitrate leaching studies have been conducted using systems based primarily on the disposal of manures. As a result, the negative aspects of such disposal are emphasized, such as runoff and nutrient accumulation within a watershed; these aspects are then attributed to the organic system as well. The organic system is viewed by many people as relying solely upon manures to supply nutrients and is therefore considered by these people to be more polluting than conventional farms.

Table 1. Changes in organic matter and nitrogen (N),; phosphorus (PO), and potassium (K20) content of farmyard manure during composting (average of 16 different composting windrows).

Length of composting period

(months)

Calculation basis¹ Nutrient 0 2 4 12

Organic matter as percent of initial organic matter

100 68 46 38

Percent of current dry matter N 2.1 2.9 3.0 2.6

P ₂O₅ 1.1 2.0 2.1 2.3

K ₂O 1.6 1.6 2.4 2.5

Percent of initial amount N 100 95 86 76

P₂O₅ 100 109 100 100

K₂ O 100 90 90 72

¹figures are average of values from 16 different composting windows

Nutrient Losses During Composting

This experiment utilized four compost windrows which served as the control treatments in a larger mineral additive study. The material was composted for 8 months. The composting area was designed to allow collection of the leachates from the individual compost piles. The authors present data from a previous composting experiment showing the changes in nutrients after composting (Table 1). The organic matter content decreased substantially over the composting period; the loss of carbon led to increased concentration of most nutrients. In absolute amounts, phosphorus was very stable, while nitrogen and potassium were lost at a similar rate.

Results from the present experiment show that nitrate loss in the leachate was relatively low (Table 2). On a quantitative basis, the loss of nitrate was less than 0.1 pound of nitrate per ton of fresh manure over the composting period. Much of the nitrogen was likely lost to volatilization. Conversely, potassium escapes mainly, if not exclusively, in a water soluble form.

Lysimeter Study

Three different fertilizers, all applied on a nitrogen-equivalent basis, were tested for nitrate leaching over 2 1/2 years: stockpiled farmyard manure (FYM), composted FYM, and "NPK fertilization." The fertilizers were applied to soil in Iysimeters (metal cylinders) 38 cm in diameter and 60 cm tall. The initial application of chemical nitrogen to a crop in the winter led to a much more rapid migration of nitrate in the soil than did the organic fertilizers. During the following winter fallow (after no fertilizers were applied in the fall), this trend reversed. This pattern demonstrated the slow release effect of manures and the need for cover cropping during the winter to conserve nitrate. Nitrate leaching (and availability) was usually less with composted FYM than with stockpiled FYM.

Field Study

The field experiment was designed to evaluate nitrate leaching after fertilization with an "NPK fertilizer," FYM, and composted FYM. The materials were applied at two nitrogen rates (50 to 80 kg/ha or 100 to 160 kg/ha) to plots of corn and wheat crops over three years.

Composted FYM led to the greatest increase in both soil organic matter and soil nitrate content in the top 30 to 40 cm. However, at the 120 cm depth (below the root zone), the chemical fertilizer plots had two to three times more nitrate than the organically fertilized plots. Leaching at this depth in the organically fertilized plots did not differ from each other nor from the control plots. The authors hypothesized that the high nitrate values obtained in the upper soil levels with compost were due to mineralization occurring at a time when the crop did not take it up anymore. Nitrate has the same leaching potential from mineral or organic fertilizers, however, the presence of lignin degradation products (from compost) may regulate mineralization or nitrate mobility. While nitrate levels in the root zone were highest in the composted plots, corn yields with compost were lower than yields with manure, probably because of the reduced availability of nitrate.

For more information write to: Research Institute for Biological Farming, Oberewil, Switzerland.

Table 2. Composition of leachates from composting farmyard manure (average of four replicates).

Fresh Weight basis (ppm) Dry Weight basis ¹ (%)

Total Nitrogen 49 4.5

Nitrate 36 3.3

Phosphorus (P₂0₅) 21 1.9

Potassium (K) 280 25.5

¹Dry weight is 0.11 percent of fresh weight.

(Cl-SWN.122) Contributed by Chuck Ingels

[ Back | Search | Feedback ]