Filosofía y Ciencia ante el rompecabezas llamado “Realidad”
3. Realidad Posmoderna: La Teoría del Caos y La Hiperrealidad
In New Zealand, the use of PRs as alternatives to water-soluble P fertilizers has been evaluated mainly for pastures. Following an extensive review of research on the use of
PR for direct application, Bolan et aL ( 1990) proposed the following criteria for a PR
to be effective fertilizer:
- The PR must be classed as reactive
- The soil has a pH (in water) of 6.0 or less - The site has a mean annual rainfall >800 mm
These criteria were derived mainly based on information from pasture field trials. The following discussion summarizes the evidence to suppon the adoption of the above criteria.
With respect to chemical propenies of PR, it has been shown that reactive PRs (RPRs) such as Sechura, Nonh Carolina, Gafsa and Arad PRs generally perform better in New Zealand's acidic pasture soils than unreactive PRs such as Nauru, Christmas Island and Florida (Figure 2.4). Chatham rise PR has also been identified as a reactive PR (Mackay
el
al., 1984a, 1984b; Syersel
al., 1986). Results of a glasshouse trial by Mackayel
al. ( 1984a), for example, demonstrated that Sechura reactive PR was 4 to 20 times more effective than unreactive Tennessee PR for ryegrass grown on six New Zealand soils. Several long-term field studies have shown that RPRs were equally or more effective than water-soluble P fenilizers (e.g. Greggel
aI., 1988; Mackayet
aI.,1984b; Rajan and Gillingham, 1986, Mackay and Wewala, 1 990; Rajan et al., 199 1 a). Provided soil pH and climatic conditions are met, the effectiveness of RPRs generally increased with time and, after a lag period of 1 to 2 years RPRs, were as effective or slightly more effective than soluble P fenilizers (Sinclair and Dyson, 1988).
Although fine PRs « 150 �m) have higher dissolution rates than the coarser materials, a number of field studies (e.g. Mackay and Wewala, 1990; Rajan
el
al., 1 991a), have shown that the unground "as received" PRs are still suitable for direct application. Under field conditions, granulated RPRs are also equally effective as the finely ground materials (Mackayel
al., 1984b, Officer, 1989). It appears that under New Zealand pasture conditions chemical reactivity of PR may be more important than the particle size.High effectiveness of RPRs in New Zealand pasture soils is generally obtained in soils with pHs (in water) less than 6.0. It has been discussed in Section 2.6. 1 that soil acidity plays an imponant role in controlling PR dissolution. In most New Zealand soils with pH <6.0, the amount of soil acidity, as measured by titration, is not likely to be limiting PR dissolution (Bolan et aI., 1 990). These workers estimated that the amount of acid in these soils is sufficient to dissolve between 294 to 998 kg PR-P ha-1 provided that moisture (Le. transport of H+ to PR) is not limiting. The size of Ca sinks, which also
Re'lative
N 0 0 Christmas Island C t> Duchess Christmas n .., Island A 0 (1) \0 Nauru .., (1) 0 n Tennessee '< 0 - florida "U ;r 0 CJ) Chatom "U Rise :::T 0 ,.... (1) North .., 0 Carolina 0 7'" CJ) Gafsa Sechuraagronomic
� m 0 0 � t> t> t> t>t> t>effectiveness
(70)
CD 0 t> t> t>t>t> t> � � 0 N 0 0 � �f
I>t»� t>t>t> ���� � � t> t>Figure 2.4 Relative agronomic effectiveness of various phosphate rocks for
pastures in New Zealand soils under glasshouse (�) and field (�) conditions (Compiled from Bolan et al., 1990).
controls PR dissolution, is relatively large in most New Zealand acid soils (Bolan
er
al.,1990) thereby not limiting PR dissolution under field conditions.
Because of the low concentration of phosphate maintained by PR in soil solution, perennial crops and others with permanent extensive root systems are most likely to use PR-P most effectively. In New Zealand agriculture, PR is considered an effective direct� application fertilizer for perennial crops such as grass-clover pastures but not suitable for annual crops such as cereals (wheat, barley, maize) that require high P uptake over a short season with limited P cycling during the season. For this reason, RPRs are mainly used on grass-white clover based pasture. The presence of perennial legume (white clover) in pasture has a beneficial effect on the effectiveness of RPR due to its acidifying effect in the clover rhizosphere which can increase PR dissolution (Section 6.2. 1 ) and the availability of P to the plant. The accumulation of high quality organic matter in permanent pasture soils due to plant (and animal) residues may also supply extra acid during decomposition (Williams, 1 980) which can be important for PR dissolution.
Provided that soil acidity is not limiting, the dissolution of PR is controlled by moisture since it controls the diffusion of base and dissolution products (Section 2.6. 1 ). Additionally, it also affects the rate of nutrient uptake by plants. Most field trials showing high effectiveness of RPR have been conducted in areas with evenly distributed annual rainfall of at least 800 mm. In areas with soil pH of 6.2 and a mean annual rainfall of 1000 mm, RPR was as effective as water-soluble P fertilizer (Mackay
er
al . • 1 984b). On the other hand, in areas with annual rainfall intensity less than 800 mm. the effectiveness of RPRs is low even if the soil is acidic. It is apparent that the sink size for Ca in soil is likely to increase with increasing rainfall intensity due to increased Ca leaching, increased plant growth and higher soil organic matter contents.Based on the tentative criteria listed above. Bolan