Phosphorus is needed by plants in relatively large amounts; this is why it is classified as a primary plant nutrient. It plays a key role in photosynthesis, carbon metabolism and nitrogen fixation. It is also a key component of nucleic acids-DNA and RNA.
When phosphorus is deficient in a plant the following symptoms show:
– Dark-green colouration in leaves- the dark green colour is as a result of high carbohydrate accumulation but low carbohydrate utilization.
– Phosphorus deficiency slows down carbohydrate utilization (8).
– Other symptoms include stunted growth, delayed maturity and increased susceptibility to diseases.
Rock Phosphate: an organic source of phosphorus
Rock phosphate is found in all continents of the world (figure 1). It is used as a raw material in the manufacture of inorganic phosphorus fertilizers and can be used as organic phosphorus fertilizers when applied directly to the soil. Direct application of rock phosphate increases crop yield and soil phosphorus levels (1, 2,3).
Other benefits to the direct use of rock phosphate are:
1. It is relatively cheap compared to inorganic/chemical phosphorus fertilizers.
2. It is environmentally friendly. As a natural source of phosphorus, it avoids the use of inorganic fertilizers which contribute to green house gas emission during its manufacturing process.
3. In addition to increasing soil phosphorus, it adds other nutrients to the soil. Studies show rock phosphate increases the soils exchangeable calcium and magnesium cations; it also increases carbon accumulation which in turn improves soil quality (1,3).
However, rock phosphate is not without its challenges.
Though it is a natural source, it is classified as a non-renewable resource. Continuous use will lead to depletion if no alternative is found. It is predicted that reserves will be depleted in about 500-600 years from now (4).
Research has shown rock phosphate to be more beneficial in acidic soils than alkaline soils; this is because rock phosphate is more soluble in acidic soils (pH <5.5) (4,5,6). Phosphorus in a soluble form is available for uptake by plant roots. To make phosphorus available in alkaline soils rock phosphate is partially acidified with inorganic acids before application (2).
This approach is not applied in organic agriculture as it involves the use of inorganic compounds. An approach more welcomed in organic farming is the integration of rock phosphate with compost or the integration of rock phosphate, compost and phosphate solubilising micro organisms. This combination has proven to improve the availability of phosphorus to plants (4,5,7).
Phosphate solubilising micro-organisms, composts and rock phosphate
Phosphate solubilising micro-organisms (PSMs) have the ability to convert organic or inorganic phosphorus into forms available for plant uptake, through the process of solubilisation and mineralization (4, 5). They do this by producing organic acids which acidify their surrounding soil and lower the soil pH therefore, creating a favourable condition for phosphate solubilisation (5). Examples of common PSMs among the bacterial community are Pseudomonas, Bacillius, Rhizobium , Enterobacter. While among the fungal group Aspergilli and Penicillum are well known.
A study showed that PSMs integrated with rock phosphates, and compost increased crop yield and biological nitrogen fixation for two legume crops (chickpea and lentil) compared to inorganic/chemical phosphorus fertilizers (6).
1. Hu et al (1996) The effect of direct application of phosphate rock on increasing crop yield and improving properties of red soil. Nutrient Cycling in Agroecosystems 46: 235-239.
2. Ditta A and Khalid A (2016). Bio-organo-phos: A sustainable approach for managing phosphorus deficiency in agricultural soils, organic Fertilizers – From Basic Concepts to Applied Outcomes, Larramendy, M; Soloneski, S (Eds). Available from-
3. Use of phosphate rocks for sustainable agriculture-
4. Sharma et al (2013) Phosphate solubilizing microbes: sustainable approach for managing phosphorus deficiency in agricultural soils. SpringerPlus 2:587.
5. Khan et al (2009) Phosphorus solubilizing bacteria: occurrence, mechanisms and their role in crop production. J Agric Biol Sci.1:48–58.
6. Ditta, A (2014) Impact of rock phosphate enriched compost (RP-EC) and phosphate solubilizing microorganisms (PSM) on nodulation, growth and yield of chickpea and lentil. PhD thesis, University Of Agriculture, Faisalabad.
7. Nisha et al (2014) Role of phosphorous solubilizing microorganisms to eradicate P- deficiency in plants: A review. International Journal of Scientific and Research Publications-http://www.ijsrp.org/research-paper-0714/ijsrp-p3177.pdf.
8. Qiu J and Isreal DW (1992) Diurnal starch accumulation and utilization in phosphorus-deficient soybean plants. Plant Physiology. 98: 316-323.