R - Modify values in a column subject to condition
===

I know that this is a very silly question but I cannot work out how to do it. 
I want to subtract 300 from the values on each row on the trial1 column if they are larger than 299. 

I tried: 

    sums[sums$trial1 > 299, ][,"trial1"] -= 300

but didn't work. SO far the only way I managed to get it to work is by splitting the data.frame using `subset` and then modifying it with : 

    sums$trial1 = sums$trial1 - 300

and then using `rbind()`. I am pretty sure that using subset and rbind is overkill, but I haven't been able to find a direct way yet...

I used dput to get a sample of my data.frame.

    structure(list(part_no = c(10L, 10L, 10L, 10L, 10L, 10L), trial1 = c(294L, 
    296L, 298L, 300L, 302L, 304L), trial2 = c(295L, 297L, 299L, 301L, 
    303L, 305L), id1 = c(1.5, 1.5, 1.5, 2, 2, 2), id2 = c(1.5, 1.5, 
    1.5, 2, 2, 2), dist1 = c(141L, 141L, 115L, 126L, 177L, 141L), 
        width1 = c(77L, 77L, 63L, 42L, 59L, 47L), dist2 = c(143L, 
        135L, 146L, 255L, 327L, 369L), width2 = c(78L, 74L, 80L, 
        85L, 109L, 123L), ttime1 = c(1752L, 1681L, 1664L, 1798L, 
        1664L, 1697L), ttime2 = c(2563L, 1849L, 2067L, 1933L, 2118L, 
        2245L), no_clicks1 = c(8L, 8L, 8L, 8L, 8L, 8L), no_clicks2 = c(8L, 
        8L, 8L, 8L, 8L, 8L), no_ontarget1 = c(7L, 8L, 8L, 8L, 8L, 
        8L), no_ontarget2 = c(8L, 8L, 8L, 4L, 7L, 8L), e1 = c(1L, 
        0L, 0L, 0L, 0L, 0L), e2 = c(0L, 0L, 0L, 4L, 1L, 0L), rating = c(252, 
        252, 252, 252, 252, 252), prat = c(0.8, 0.8, 0.8, 0.8, 0.8, 
        0.8), ptim = c(-46.2899543378995, -9.9940511600238, -24.21875, 
        -7.5083426028921, -27.2836538461538, -32.2922804949912), 
        ptdiff = c(-47.0899543378995, -10.7940511600238, -25.01875, 
        -8.3083426028921, -28.0836538461538, -33.0922804949912), 
        pdist = c(-1.41843971631206, 4.25531914893617, -26.9565217391304, 
        -102.380952380952, -84.7457627118644, -161.702127659574), 
        pddiff = c(-2.21843971631206, 3.45531914893617, -27.7565217391304, 
        -103.180952380952, -85.5457627118644, -162.502127659574), 
        perr = c(100, NaN, NaN, -Inf, -Inf, NaN), pediff = c(99.2, 
        NaN, NaN, -Inf, -Inf, NaN)), .Names = c("part_no", "trial1", 
    "trial2", "id1", "id2", "dist1", "width1", "dist2", "width2", 
    "ttime1", "ttime2", "no_clicks1", "no_clicks2", "no_ontarget1", 
    "no_ontarget2", "e1", "e2", "rating", "prat", "ptim", "ptdiff", 
    "pdist", "pddiff", "perr", "pediff"), row.names = 148:153, class = "data.frame")

Thanks!