Field crops soil testing

When to sample

Nutrient levels usually vary only slightly from season to season. Soil sampling just after harvest of a high-yielding corn or soybean crop usually results in a slight depression of exchangeable potassium compared to later fall, winter or spring sampling. The best time to sample is when the acreage is lying idle: for example, summer for winter wheat or later fall and winter for spring-planted crops. Most people take soil samples in spring, just before applying fertilizer. This leaves little planning time for important fertilizer and limestone application decisions. Spring is also the time when soil testing laboratories are overloaded with samples. This will delay the return of results. Fall and winter sampling leaves more time for planning limestone and fertilizer programs for the coming year.

Soil Sample Form for Field Crops

How to take a field crop sample

A well-taken soil sample results in appropriate recommended rates of fertilizer and limestone. It is best to use a soil coring device because it takes an equal amount of soil from the surface through the sampling depth (6 to 7 inches or tillage depth if deeper) and uniform soil cores at each sampling location. Soil probes and augers are available through ag companies or your ag extension specialist may be able to help you locate a supplier. You can use a shovel, however it is not as good as a probe or auger. If you use a shovel, dig a hole to the proper depth, shave a 1-inch slice from the side of the hole and save the vertical 1-inch-wide center portion of this slice.

Collect samples from uniform areas using your field map (obtained from FSA or county soil survey). Each composite soil sample should represent uniform areas of a field and should consist of 15 to 20 separate cores. For each composite sample, avoid sampling areas with obvious differences of soil color and texture, slope, crop rotation or fertilizer, lime and manure applications. Sample differing areas/composites separately. Mix individual samples (15 to 20 separate cores) to obtain one composite sample in a clean plastic pail (metal pails contaminate the soil with micronutrients) and retain 1 pint (fill MU soil sample box). Soil sample boxes may be obtained free from your local University Extension center. A composite sample should not represent more than 20 acres.

Field crops for which recommendations are given

Recommendations are given for the following crops — up to four crops with associated yields can be selected for each sample. For one soil sample submitted you may get recommendations for corn at 80 bu/A, corn at 90 bu/A, corn at 100 bu/A, corn at 110 bu/A OR for one sample you may obtain recommendations for corn at 80 bu/A, soybeans at 40 bu/A, wheat at 40 bu/A, alfalfa at 3 to 7 tons per acre. So you may choose any combination of four of those crops and yields listed in the table below with the restrictions listed on yield.

Code Crop Common yield goals
Forage establishment
1 Alfalfa or alfalfa/grass establishment 0
2 Birdsfoot trefoil/grass establishment 0
3 Clover or clover/grass establishment 0
4 Cool season grass establishment 0
5 Lespedeza/grass establishment 0
6 Overseeding legumes into grass 0
7 Warm season grass establishment 0
10 Alfalfa or alfalfa/grass hay 3 to 7 ton/A
11 Alfalfa or alfalfa/grass pasture 100 to 250 cow days/A
12 Birdsfoot trefoil/grass pasture 100 to 200 cow days/A
13 Bluegrass pasture 100 to 200 cow days/A
14 Bermudagrass hay 2 to 6 tons/A
15 Bermudagrass pasture 100 to 250 cow days/A
16 Clover or clover/grass hay 2 to 5 tons/A
17 Clover or clover/grass pasture 10 to 250 cow days/A
18 Cool season grass hay 2 to 6 tons/A
19 Cool season grass pasture 100 to 250 cow days/A
20 Cool season grass seed/hay or pasture residue 0
21 Cool season grass/stockpile fall growth 0
22 Lespedeza/grass hay 1 to 4 tons/A
23 Lespedeza/grass pasture 100 to 250 cow days/A
24 Sudangrass hay 3 to 5 tons/A
25 Sudangrass pasture 100 to 250 cow days/A
26 Warm season grass hay 2 to 5 tons/A
27 Warm season grass pasture 100 to 250 cow days/A
Row crop and small grains
100 Barley 40 to 80 bu/A
101 Buckwheat 500 to 1000 lbs/A
102 Cotton (lint) 500 to 1500 lbs/A
103 Corn (grain) 80 to 250 bu/A
104 Corn (silage) 10 to 25 tons/A
105 Wheat/Soybean double crop* 30 to 80 bu/A
106 Wheat/Sunflower double crop* 30 to 80 bu/A
107 Wheat/Grain sorghum double crop* 30 to 80 bu/A
108 Wheat/Sorghum silage double crop* 30 to 80 bu/A
109 Oats 50 to 100 bu/A`
110 Popcorn 1500 to 6000 lbs/A
111 Rice 3000 to 7000 lbs/A
112 Rye 30 to 70 bu/A
113 Grain sorghum 4000 to 10000 lbs/A
114 Sorghum silage 12 to 30 tons/A
115 Soybeans 30 to 70 bu/A
116 Sugarbeets 15 to 24 tons/A
117 Sunflowers 1200 to 2500 lbs/A
118 Tobacco 25000 to 4000 lbs/A
119 Wheat 40 to 100 bu/A
201 Southern peas 0
202 Watermelon 0
099 Idle 0