Abstract
Summary Use of isotope methods to measure the availability of phosphorus (P) in soils that are well supplied with P is well established. We have evaluated such methods for acid tropical soils with very small P contents, which are less well studied. The isotopically exchangeable P in soil suspensions ( E value) and that in plant growth experiments ( L value) were measured in soils that had received varying amounts of P fertilizer in two field experiments in Colombia. We determined the E values over 4–5 weeks of equilibration allowing for the kinetics of isotope exchange. The decrease in radioactivity in the soil solution at a particular time, t , divided by that at the start ( r t / R ) was described by three parameters ( r 1 / R , r ∞ / R , and a coefficient n ) derived from the time course of isotopic exchange over 100 minutes. Values of E t were calculated either from measured values of r t / R or those extrapolated until 12 weeks. Agrostis capillaris was grown on the same soils labelled with carrier‐free 33 P‐orthophosphate ions to obtain L values. Agreement between E values derived from measured and extrapolated values of r t / R was satisfactory, but errors in n and r ∞ / R limited the precision with which we could estimate E values. For most soils, the P concentrations in the soil solution were greater than the detection limit of the malachite green method (0.9 µg l −1 ) but smaller than its quantification limit (3.6 µg l −1 ). In the soils with the least available P, the P content of the seed limited the determination of the L value. The E values were strongly correlated, but not identical, with the L values measured for the same time of isotopic exchange. We conclude that these approaches are not precise enough to detect in these soils the ability of a plant to access slowly exchangeable forms of P or to quantify the mineralization of organic P. However, these isotope techniques can be used to estimate the total fraction of added fertilizer P that remains available to the plants.
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Claims 7
Claim · model-written summary
E values were strongly correlated, but not identical, with L values for the same time of isotopic exchange.
- Taxon
- Agrostis capillaris
Agrostis capillarise0 - Outcome
- E valuese1
- Qualifier
- strongly correlated, but not identicale1
- Comparator
- L valuese1
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e1“The E values were strongly correlated, but not identical, with the L values measured for the same time of isotopic exchange”
e0“Agrostis capillaris was grown on the same soils labelled with carrier‐free 33 P‐orthophosphate ions to obtain L values”
Claim · model-written summary
The isotope approaches are not precise enough to detect the ability of a plant to access slowly exchangeable forms of P in these soils.
- Taxon
- Agrostis capillarisAgrostis capillarise0
- Outcome
- the ability of a plant to access slowly exchangeable forms of Pe2
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e2“We conclude that these approaches are not precise enough to detect in these soils the ability of a plant to access slowly exchangeable forms of P or to quantify the mineralization of organic P”
e0“Agrostis capillaris was grown on the same soils labelled with carrier‐free 33 P‐orthophosphate ions to obtain L values”
Claim · model-written summary
The isotope techniques can estimate the total fraction of added fertilizer P that remains available to plants.
- Taxon
- Agrostis capillarisAgrostis capillarise0
- Outcome
- total fraction of added fertilizer P that remains available to the plantse3
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e3“However, these isotope techniques can be used to estimate the total fraction of added fertilizer P that remains available to the plants”
e0“Agrostis capillaris was grown on the same soils labelled with carrier‐free 33 P‐orthophosphate ions to obtain L values”
Claim · model-written summary
E values from measured and extrapolated values of r t / R agreed satisfactorily.
- Taxon
- Agrostis capillarisAgrostis capillarise0
- Outcome
- Agreement between E values derived from measured and extrapolated valuese4
- Comparator
- extrapolated values of r t / Re4
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e4“Agreement between E values derived from measured and extrapolated values of r t / R was satisfactory”
e0“Agrostis capillaris was grown on the same soils labelled with carrier‐free 33 P‐orthophosphate ions to obtain L values”
Claim · model-written summary
Errors in n and r∞/R limited the precision of the E value estimates.
- Taxon
- Agrostis capillarisAgrostis capillarise0
- Outcome
- precision with which we could estimate E valuese5
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e5“but errors in n and r ∞ / R limited the precision with which we could estimate E values”
e0“Agrostis capillaris was grown on the same soils labelled with carrier‐free 33 P‐orthophosphate ions to obtain L values”
Claim · model-written summary
In soils with the least available P, the P content of the seed limited determination of the L value.
- Taxon
- Agrostis capillarisAgrostis capillarise0
- Treatment
- soils with the least available Pe6
- Outcome
- determination of the L valuee6
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e6“In the soils with the least available P, the P content of the seed limited the determination of the L value”
e0“Agrostis capillaris was grown on the same soils labelled with carrier‐free 33 P‐orthophosphate ions to obtain L values”
Claim · model-written summary
For most soils, P concentrations in the soil solution were above the detection limit but below the quantification limit.
- Taxon
- Agrostis capillarisAgrostis capillarise0
- Outcome
- P concentrations in the soil solutione7
- Value
- 0.9 µg l −1e7
- Comparator
- detection limit of the malachite green methode7
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e7“For most soils, the P concentrations in the soil solution were greater than the detection limit of the malachite green method (0.9 µg l −1 ) but smaller than its quantification limit (3.6 µg l −1 )”
e0“Agrostis capillaris was grown on the same soils labelled with carrier‐free 33 P‐orthophosphate ions to obtain L values”