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Magnesium Recommendations

The optimum magnesium (Mg) level varies with the crop. The optimum Mg is given on the crop sheet for each crop in this handbook.

Look in the left hand column and find the soil test Mg level in ppm as reported on the bar chart in the middle of the soil test report then go across to the appropriate "Optimum Mg" column for that crop to determine the amount of Mg recommended.

Because of potential animal health problems due to an imbalance between potassium (K) and Mg, a higher Mg recommendation is made for some crops when the soil K level is greater than 200 ppm. The need for this adjustment is noted on the appropriate crop sheets in this manual. For crops where this adjustment applies, when the K soil test level as reported on the bar chart in the middle of the soil test report is above 200 ppm, then use the far right hand column in this table to determine the Mg recommendation.

Since the most common source of Mg is magnesium containing limestone, the Mg recommendation is also given on the soil test report as the percent Mg required in the recommended limestone to meet the Mg requirement. To calculate this percentage divide the Mg recommendation determined from this table by the limestone recommendation determined from table 1 and multiply by 100.

Recommendations are given as elemental Mg because limestone is required by PA law to be labeled with the elemental Mg analysis. You can convert the the Mg recommendation to MgO by multiplying the Mg recommendation times 1.6.

Table 2. Magnesium Recommendation (lb Mg/A)
Soil Test
Mg Level
(ppm)
Optimum Mg Level (ppm) Mg recommendation for
grasses when soil test K is
greater than 200 ppm
50 60 100 120
0 100
125
200
250
500
5 95
110
190
240
490
10 90
105
180
230
480
15 85
95
170
220
470
20 80
80
155
205
460
25
75
75
150
200
450
30
70
60
140
190
440
35
60
50
125
175
430
40
60
45
120
170
425
45
50
60
105
155
420
50
0 20
95
140
405
55
0
15
90
140
400
60
0 0 75
125
390
65
0 0 65
105
380
70
0 0 55
105
370
75
0 0 45
95
360
80
0 0 30
80
350
85
0 0 25
75
340
90
0 0 15
60
330
95
0 0 0 50
320
100
0 0 0 45
315
105
0 0 0 30
300
110
0 0 0 20
295
115
0 0 0 15
285
120
0 0 0 0 275
125
0 0 0 0 265
130
0 0 0 0 255
135
0 0 0 0 245
140
0 0 0 0 240
145
0 0 0 0 230
150
0 0 0 0 220
155
0 0 0 0 210
160
0 0 0 0 200
165
0 0 0 0 190
170
0 0 0 0 180
175
0 0 0 0 170
180
0 0 0 0 160
185
0 0 0 0 155
190
0 0 0 0 145
195
0 0 0 0 135
200
0 0 0 0 125
205
0 0 0 0 115
210
0 0 0 0 105
215
0 0 0 0 95
220
0 0 0 0 90
225
0 0 0 0 80
230
0 0 0 0 70
235
0 0 0 0 60
240
0 0 0 0 50
245
0 0 0 0 40
250
0 0 0 0 30
255
0 0 0 0 20
260
0 0 0 0 15
265
0 0 0 0 5
270
0
0
0
0
0