Table 3. Percent error of ash-free estimates for a variety of substances under hypothetical conditions. Typical concentrations of substances in plant and soil were taken from Giese (1973), Anderson (1987), Anderson et al. (1981), Tiessen et al. (1982) and Morgan et al. (1998). The ash content of the contaminating material was varied between 0.93 and 0.96 based on table 2. Plant ash was fixed at 0.03.

        Concentration 
         (g per 100g)     Percent ash in sample
       _______________   ______________________
Sub-     In       In *
stance  plant    C. M.   10        20        80 
__________________________________________________
                       . . . .  % error  . . . . .

N     .5 - 4.      5   .1 - 5   .2 - 14    3 - 260

P     .08 - .3    .5   .2 - 3   .6 -  8   10 - 150

S     .2 -  .5    .7   .1 - 2   .3 -  4    6 -  72

Ca      .08       .3     30        80       1500

Mg      .08       .1     10        27        480 

K       .08       .07     7        18        330

Na      .08       .01     2         2         30
   #
TNC      25        0     -.3       -1        -17
__________________________________________________

* C. M. = contaminating material. For N, P and S, concentrations in C. M. are given as a percent of organic matter. For exchangeable cations of Ca, Mg, Na, and K, concentrations in C. M. are given as a percent of total mass of C. M. For all substances, we assumed that concentration data from soil applied to C. M., and that the analytical method employed for plant analysis is equally efficient for C. M.

# TNC is total nonstructural carbohydrates, assumed zero in soil.