diff --git a/test/parameters.jl b/test/parameters.jl
index 6bcaf355..04b5b76c 100644
--- a/test/parameters.jl
+++ b/test/parameters.jl
@@ -1,19 +1,41 @@
 import AIBECS: units, @units
 import AIBECS: prior, @prior
+import AIBECS: limits, @limits
 import AIBECS: initial_value, @initial_value
 import AIBECS: flattenable, @flattenable
 import AIBECS: description, @description
-@initial_value @units @description @prior @flattenable struct TestParameters{T} <: AbstractParameters{T}
-    xgeo::T | 2.17 | u"mmol/m^3" | "Geological mean P concentration"             | LogNormal(0,1) | true
-    τgeo::T |  1.0 | u"Myr"      | "Geological restoring timescale"              | LogNormal(0,1) | false
-    k::T    | 10.0 | u"μmol/m^3" | "Half-saturation constant (Michaelis-Menten)" | LogNormal(0,1) | true
-    w₀::T   |  1.0 | u"m/d"      | "Sinking velocity at surface"                 | LogNormal(0,1) | true
-    w′::T   | 0.22 | u"m/d/m"    | "Vertical gradient of sinking velocity"       |    Normal(0,1) | true
-    τDOP::T | 0.25 | u"yr"       | "Remineralization timescale (DOP to DIP)"     | LogNormal(0,1) | true
-    τPOP::T | 5.25 | u"d"        | "Dissolution timescale (POP to DOP)"          | LogNormal(0,1) | true
-    σ::T    |  1/3 | NoUnits     | "Fraction of quick local uptake recycling"    |   Uniform(0,1) | true
-    τDIP::T | 30.0 | u"d"        | "Uptake maximum timescale (DIP to POP)"       | LogNormal(0,1) | true
+const ∞ = Inf
+@initial_value @units @description @limits @flattenable struct TestParameters{T} <: AbstractParameters{T}
+    xgeo::T | 2.17 | u"mmol/m^3" | "Geological mean P concentration"             |  (0,∞) | true
+    τgeo::T |  1.0 | u"Myr"      | "Geological restoring timescale"              |  (0,∞) | false
+    k::T    | 10.0 | u"μmol/m^3" | "Half-saturation constant (Michaelis-Menten)" |  (0,∞) | true
+    w₀::T   |  1.0 | u"m/d"      | "Sinking velocity at surface"                 |  (0,∞) | true
+    w′::T   | 0.22 | u"m/d/m"    | "Vertical gradient of sinking velocity"       | (-∞,∞) | true
+    τDOP::T | 0.25 | u"yr"       | "Remineralization timescale (DOP to DIP)"     |  (0,∞) | true
+    τPOP::T | 5.25 | u"d"        | "Dissolution timescale (POP to DOP)"          |  (0,∞) | true
+    σ::T    |  1/3 | NoUnits     | "Fraction of quick local uptake recycling"    |  (0,1) | true
+    τDIP::T | 30.0 | u"d"        | "Uptake maximum timescale (DIP to POP)"       |  (0,∞) | true
 end
+function prior(::Type{T}, s::Symbol) where {T<:AbstractParameters}
+    if flattenable(T, s)
+        U = units(T, s)
+        if limits(T, s) == (0,∞)
+            μ = log(ustrip(upreferred(initial_value(T, s) * U)))
+            return LogNormal(μ, 1.0)
+        elseif limits(T, s) == (-∞,∞)
+            μ = ustrip(upreferred(initial_value(T, s) * U))
+            σ = ustrip(upreferred(10.0U)) # Assumes that a sensible unit is chosen!
+            return Normal(μ, σ)
+        elseif limits(T, s) == (0,1)
+            return Uniform(0,1)
+        end
+    else
+        return nothing
+    end
+end
+prior(::T, s::Symbol) where {T<:AbstractParameters} = prior(T,s)
+prior(::Type{T}) where {T<:AbstractParameters} = Tuple(prior(T,s) for s in AIBECS.symbols(T))
+prior(::T) where {T<:AbstractParameters} = prior(T)
 Distributions.gradlogpdf(::Uniform, x::T) where {T<:Real} = zero(T) # Required for these tests because undefined in Distributions...
 
 p = TestParameters()