Guano as a nitrogen source for fertigation in organic farming

Abstract

Guano is a natural organic fertilizer used in organic farming management when supplementary nitrogen is needed. In modern irrigated agriculture N top dressing is often applied through the irrigation water. The objective of this research was to evaluate the potential of guano as a source of available N when applied through water, compared with application to soil. The release of mineral and total soluble N from guano to water was monitored, as a function of time, under laboratory conditions, and compared with mineralization of guano in soils. The effect of guano:water ratio, aeration of the mixtures, addition of rock-phosphate or clay minerals, and temperature on the composition of the solutions was determined. Guano released more than 90% of its N content into solution during 10 d at a guano:water ratio of 1:10 at 27°C. The release of soluble N, of which 85% was NH₄-N, was linked with an increase of pH from 7.0 to 8.5 or more. None of the treatments increased the rate or the total N release beyond that. Suboptimal temperatures, 13.6 ± 4.7°C or 50°C, greatly reduced the rate of guano-N release to solution. The mineralization of guano in soil was more rapid than in water, but the total N release was the same, leading to the conclusion that the application of guano through the irrigation water may be as efficient as directly to soil, provided sufficient time for fermentation is allowed.