In many parts of the world food security is at risk. One of the biophysical root causes of falling per-capita food production is the declining quality and quantity of soils. To reverse this trend and increase soil fertility soil and plant nutrients have to be replenished. This review provides a literature survey of experiences of using multi-nutrient rock fertilizers for soil fertility enhancement from temperate and tropical environments. Advantages and limitations of the application of rock fertilizers are discussed. Examples are provided from two successful nutrient replenishment projects in Africa where locally available rock fertilizers are used on highly leached acid soils. The potential of combining organic materials alongside rock fertilizers in soil fertility replenishment strategies is stressed.
Phosphorus deficiencies are limiting crop production in agricultural soils worldwide. Locally available sources of raw phosphate rock (PR) are being recognized for their potential role in soil fertility improvement. Phosphorus bioavailability is essential for the efficiency of PRs and can be increased by acid treatments. The utilization of organic acid producing micro‐organisms, notably Aspergillus niger, presents a sustainable alternative to the use of strong inorganic acids, but acid production of A. niger strongly depends on the mineral content of the growth media. This study compared the phosphorus mobilization efficiency of two biological treatments, namely addition of acidic cell‐free supernatants from A. niger cultivations to PRs and the direct cultivation of A. niger with PRs. The results show that addition of PR to cultivations leads to significant differences in the profile of organic acids produced by A. niger. Additions of PR, especially igneous rocks containing high amounts of iron and manganese, lead to reduced citric acid concentrations. In spite of these differences, phosphorus mobilization was similar between treatments, suggesting that the simpler direct cultivation method was not inferior. In addition to citric acid, it is suggested that oxalic acid contributes to PR solubilization in direct cultivations with A. niger, which would benefit farmers in developing countries where conventional fertilizers are not adequately accessible.
Free living diazotrophic bacteria are known to enrich nitrogen of organic matter sources. In this paper we report of experiments using rock biofertilizers mixed with two types of organic matter (earthworm compound and ice cream waste) inoculated with free living diazotrophic bacteria. The earthworm compound and P and K biofertilizers were mixed to form substrates S 1 (earthworm compound 3 dm 3 ? PK biofertilizer 1 dm 3 and waste ice cream 1 dm 3 ); S 2 (earthworm compound 2.5 dm 3 ? PK biofertilizer 1.5 dm 3 and waste ice cream 1 dm 3 L) and S 3 earthworm compound 2.0 dm 3 ? PK biofertilizer 2.0 dm 3 and waste ice cream 1 dm 3 ), and subsequently inoculated (100 mL pot -1 ) with 3 free living diazotrophic bacteria isolated from different Brazilian soils. The control was an uninoculated earthworm compound. Samples were collected at various incubation time (0; 15; 30 and 45 days) and analyzed for total N. Total N concentrations were highest in S 1 , S 2 and S 3 substrates at 34, 27 and 29 days, respectively. The isolate NFB 1001 increased total N in all substrates and the best results were obtained at 34 days in S 1 substrate which contained the highest amount of earthworm compound. The isolates promoted a decline in N content after 30 days of growth, indicating the best time to produce the organic biofertilizer. The organic biofertilizer enriched in nitrogen by free living diazotrophic bacteria is of relevance to organic agriculture.
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