Le compost : base de la fertilisation au sein des prairies pâturées et des systèmes en agriculture biologique

The compost : staple fertilisation in grazed grassland and organic farming systems

Context

In this project, compost valorisation is studied to face two problematic.

The first lies in the obligation, following the application of the Nitrate Directive in the Walloon Area and its limitations in term of organic fertiliser application, to manure the majority of the surface of the farm. So, in grazier farms, valorising mainly their manure on crop or meadow, some solutions have to be found to allow manure spreading on grazed grasslands. Manure composting seems to be an interesting alternative. However, what is its fertilising value ? When is it recommended to apply it ? Do it modify sward palatability ?

The second come from organic farming system to which Walloon Area wants to convert 10 % of its UAS for 2010. Indeed, this system of production recommends the compost as crop fertilisation basis. What performances can be reached in such a system ? Will some fertilisation complements, in term of nitrogen (N), phosphorus (P) or potash (K), be necessary at key crop development stages ?

Objectives

The objective of this project is to answer to these questions.

Concerning compost use in grazed grassland, different trials, among which long lasting trials, have been set up by P. Limbourg since 1979. Main objectives of these trials are to define (1) compost fertilising value in link with its level of maturity and its period of application, (2) the impact of its use on sward palatability and animal performances.

In the context of compost use in organic farming system, a long term trial was initiated in 1998. It aims to perform nutrients (N-P-K) balances in Breeding System typical from the ‘Ardenne Centre with Special Productions’.

Results obtained

Compost valorisation in grazed grasslands

The results obtained by P. Limbourg (2001) underlined compost potentialities in this context. He demonstrate that 15 to 25 % of the nitrogen included in this manure (this ratio being inversely proportional to the level of maturity) is valorised during the spreading year and this, whatever the spreading period. When taking into account the long term effect of the compost, more than 90 % of the total nitrogen amount included in the raw product is valorised within a decade in link to the humus stock increase it induces.So, the spreading of 10T/ha/year of compost covers all the needs in P, K, Ca, Mg and oligo-nutrients from a grazed grassland.Its results also underlined that the use of a mature compost or of a compost having only been mixed once, so with only one temperature increase, has no negative effect on sward palatability or composition : animal performances are maintained (Limbourg et al. 2001). Moreover, compost use sustains clover development.From an environmental point of view, compost use on permanent grazed grasslands has different advantages : bad odour suppression during spreading, acting as a carbon pit with the annual immobilisation of 1 to 1.5 T of Carbon per hectare for spreading of 10 to 15 T/ha/year. Nevertheless, we must take care to the excess in relation to high long term effect of this product. So repeated spreading of 20 T/ha/year of this product leads to nitrate leaching risks as high as the use of mineral nitrogen fertilisation of 150 kg N/ha/year in a grazed grassland (Limbourg et al. 2001).In order to explore this problem more ahead, a nitrogen balance was performed (Stilmant et al. 2004), during 6 years (1995-2000), within two systems grazed by young cattle at an initial stocking rate of 1506 kg of liveweight/ha. Simplified rotational grazing systems, on 3 paddocks, one paddock being mowed once in spring, are used. One of these grazing systems received only composted manure, at the rate of 16.7 T/ha, in mean, on the grazed paddocks and of 35.5 T/ha on the mowed and grazed paddock while the 2^nd grazing system received only mineral fertilisers to ensure similar sward productivity in quantity and quality. Nitrogen dressing were of 0, 13.5, 27, 33, 47 and 75 kg/ha respectively from 1995 to 2000 while P and K dressing were, respectively, of 50 and 60 units/ha on exclusively grazed paddocks and of 100 and 120 units on mowed and grazed paddocks. This experimental scheme was duplicated. In order to perform the N balance at the paddock level, we took into account the available N fraction (Nu) of the compost : 15 and 25 % of the total N amount (Nt) as direct effect, respectively for a young or a mature compost, 10 and 8.6 % of the Nt, annually, as long term effect, during the 7 following years. The other inputs took into account were the N fixation by white clover and the animal supplementation under grazing. Output considered were animal exportation, silage exportation and gaseous losses.

Across the 6 years of trial, organic fertilisation (62.4 kg/ha) bring two times more Nu than mineral fertilisation (31.8 kg/ha). The 2^nd main N input, representing 41% of the Nu input, was linked to the symbiotic fixation by the leguminous. Total inputs were of 135 and 98 kg/ha/year, respectively, in the ‘compost’ and ‘all mineral’ fertilisation schemes. The output are, due to silage exportations, linked to the exploitation mode. Only gaseous losses, always lower than 10 %, are dependant of the fertilisation mode. On this basis we obtained Nu balances influenced both by the fertilisation and exploitation mode :62.9 and 79 kg/ha within grazed paddocks against 5.1 and 36.8 kg/ha in mowed and grazed paddock, respectively for organic and mineral fertilisation schemes. In fine N use efficiency is lower in the ‘all organic’ scheme with 46 % against 65 %. This lower efficiency had to be put in perspective as N balance performed at the farm scale will favour compost use coupled to a lower N fertiliser importation! However, to reach such a target while limiting environmental impact of grazed grassland, compost application rate had to be lower than 10T/ha. That represent the compost available in a system with 1.8 BLU/ha and correspond, at the equilibrium, to the liberation of ± 55 kg d’N/ha.

Compost valorisation in organic farming systems

The system analysed has 45 % of its surface included in a 7 years long cultural rotation : 3 years of temporary grasslands, one year of spelt (crop of economical value), one year of tritical, one year of potato (crop of economical value) and one year with tritical and pea mixture. Compost availability is linked to an animal stocking rate of 1.7 BLU/ha, this means 7.6 T of compost per hectare and per year. This compost is spread at the rate of 15 T/ha all the three years on the permanent grasslands and of 15 T/ha on the 1^st and last years of temporary grasslands and on the tritical, and at the rate of 30T/ha on potato crop. Compost composition is, in mean, of 6 kg of Nitrogen, 4,5 kg of P₂O₅ and 6 kg of K₂O. The evolution of productions quality and quantity is recorded in parallel to soil fertility parameters.Over the 5 years of recording, good yields were observed on temporary grasslands; with productions of 5T/ha the year of sowing and 10 T/ha the years of full exploitation, this after the deduction of 20 % of drying losses for such mixtures rich in leguminous; and on potato crops; with a mean yield of 30 T/ha. However, cereal crops suffered from a nitrogen deficit. This led two low yields of 2.5 to 3.2 T/ha, this last performance was obtained with cereal-peas associations. The normal cereal yield was of 5 to 7 T/ha in this mid-mountain area.

Such performances led to mean nutrient exportations of 22 kg of phosphorus and 127 kg of potash per hectare and per year. These exportations were covered, for the phosphorus, by compost spreading, while a net deficit of 72 kg/ha/an was recorded for the potash. In order to reach an equilibrium and to maintain soil fertility, an annual spreading of potash is necessary, for example under the form of patenkali at the rate of 300 kg/ha/year.

Contribution

The Farming System Section is the project leader. It is occupied of the establishment and the follow-up of the project, as of the analysis of the results.