Rock phosphate fertilizer as an effective source of P for agricultural crops and solution to building long term soil P reserves
Phosphorus (P) is an important plant nutrient and is often the least accessible macronutrient and most frequently deficient nutrient in agricultural soils. Phosphorus deficiency is a crucial factor in restricting plant growth and has prompted the wide use of phosphatic fertilizers to maximize yields. Synthetic phosphate fertilizers typically have Low availability after application due to soluble phosphate forming insoluble complexes with cations such as aluminum and iron under acidic soil conditions and with calcium and magnesium under alkaline soil conditions13.Low phosphorus fertilizer recovery can lead to the excessive and repeated application of P fertilizer to cropland.
As crop target yields increase, more phosphorus fertilizer is required to meet plant P requirements. As a source of plant P, rock phosphate has often been shown to be equal or better than synthetic phosphate fertilizer under acid soil conditions and equal or better than synthetic fertilizers under neutral or basic soil conditions when the rock phosphate is blended with acidic agents such as elemental sulphur, humates and/or biologicals. Rock phosphate (RP) fertilizers have long-lasting residual effects4,1. A single application of reactive rock phosphate is often effective for several subsequent crop years, if applied at a heavy enough rate. Rock phosphate also has the advantage of being slow-release fertilizer and does not readily leach or runoff6,33.
Outlined by Rajan et al., 1996, interest in rock phosphates as direct application fertilizer stems from the facts that:
(i) per kilogram of P, rock phosphate fertilizer is generally more inexpensive.
(ii) direct application, with or without amendments, enables use of rock phosphate fertilizers without additional manufacturing
(iii) rock phosphate fertilizers are natural minerals requiring minimum processing; they are environmentally benign31; and
(iv) rock phosphate fertilizers could be more efficient than soluble fertilizers in terms of recovery of phosphate by plants, even for short term crops in soils where soluble P is readily leached, as in sandy soils33 and for long-term crops in other soils28.
Acidic Soils
Nemeth et al., (2002) evaluated the effects of Algerian rock phosphate fertilizer applied annually at a rate of 35 kg P/ha/year and an initial build-up P application of 175 kg P/ha, (applied in autumn) over 5 years. After 5 years they found there were no grain yield differences from annual P fertilization and the initial build-up P dose. The residual effect of the initial build-up P dose was about 2% higher than the effect of annual application. Positive (160 kg/ha) P balances were measured for the initial build-up dose of P in the first year, decreasing with time until 60 kg/ha after 5 years (figure 1). No differences were observed between the rock phosphate and synthetic P sources tested. In contrast, the P balance for the annual application of 35 kg P/ha low (21 kg P/ha) increasing over 5 years (up to 61 kg/ha) (figure 1).
The slow release of P in phosphate rock was beneficial for improving P availability over a prolonged period (e.g., 5 years). Responses to rock phosphate application were dependent not only on the soil acidity conditions, but also on available soil P levels. The positive P-balances on the P fertilization treatments at the end of the first five-year period indicate that significant P residual effect would be expected, and thus soil P utilization can be enhanced with further cropping.
A long-term study found that the agronomic efficiency and phosphorus use over 5 years was as good or greater than triple superphosphate (TSP)14. Rock phosphate showed longer residual P than TSP at the larger amounts of applied P In a first season crop, 10-25% of P was utilised. Rock phosphate supplied steady P to the crop in subsequent years.
The data in figure 3 compares the available soil P from applications of rock phosphate and TSP at 11, 141 and 154 months after application. At each sampling period, available P from rock phosphate fertilizer was on par or greater than available P from TSP applications. Results from this study show that the agronomic effectiveness of rock phosphate fertilizer over the long term, is equivalent to that of synthetic phosphate fertilizer (Tiecher et al., 2013).
Neutral Soils
On a pH neutral, phosphate deficient western Canadian soil, rock phosphate application (applied at a rate of 54 kg P/ha) doubled phosphate availability after 8 weeks (over the check and three time periods) and that the use of elemental sulphur and rock phosphate tripled phosphate availability after 8 weeks (Figure 4). The addition of compost to the rock phosphate and sulphur further increased phosphate availability by 50% after 8 weeks.
Residual P from previous applications of different rock phosphate fertilizers compared to annual TSP applications found that rock P fertilizer, broadcasted and incorporated at a rate of 200 kg P2O5/ha influenced maize grain yields four years after initial application without the addition of annual synthetic P fertilizer. Residual P levels in the soil remained high 1,2, and 3 years after application of rock phosphate (Figure 5).
Calcareous Soils
Conversely, calcareous soils are frequently characterized by low bioavailability of plant nutrients such as P due to a higher pH (7.5 – 8.5) and the presences of carbonate minerals20. In these soils, P availability can be improved by mixing rock P fertilizer with amendments such as, organic matter, manure, compost, sulfur, and P solubilizing bacteria23,24,25,32.
Al-oud (2011) found that the availability of P from rock phosphate increased over time, with the highest concentration of available-P achieved after 60 days (figure 6) of adding rock phosphate regardless of the rate. The solubility and/or availability of rock phosphate in calcareous soil was maximized by increasing the rate of applied elemental sulphur and/or organic manure.
P Scavenging Crops
Crop type can improve P utilization from rock phosphate fertilizer. Legumes are well suited for rock phosphate fertilizers, as good scavengers for P in the soil thanks to the acidifying impact of nitrogen (N) fixation in the soil close to the root zone. This effect can improve the P nutrition of a companion crop (intercropping) or that of the subsequent crop in a rotation27.
Some plant species (e.g., rapeseed, lupines, and pigeon pea) have been studied because of their ability to release organic acids from their roots that result in increased P availability from rock phosphates21. More reactive rock phosphates have potential applications even in alkaline soils with organic acid-secreting crops such as rapeseed (canola).
Rock phosphate fertilizer has been shown to be a valuable source of P for plants and is able to supply P over subsequent growing seasons. In soils where available P is often limited, sedimentary rock phosphate fertilizers can play an important role in building soil P reserves. This in turn reduces the requirement for annual synthetic P fertilizers. Fertify fertilizer contains high quality, natural, locally sourced soft (sedimentary) rock P fertilizer.
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