Soil sampling and fertility management

Pawel Wiatrak, Jim Camberato, and Jason Norsworthy

It is very important to perform soil tests and apply lime and fertilizer if needed to avoid yield losses due to nutrient deficiency. More information on these topics can be found at http://www.clemson.edu/agsrvlb/new_page_16.htm.

Plant nutrient applications through fertilizer and/or lime should be based on soil tests. Soil tests help to determine the soil pH and nutrient levels, and thus any need for lime and/or fertilizer applications. These tests can be performed at the Clemson University Agricultural Services Lab, which provides soil analyses and nutrient recommendations on a fee basis. Local County Extension office can help with soil sample bags and submissions forms, and also give advice on taking soil samples. The local office will mail the soil samples to the lab.

Following are steps for obtaining a representative soil sample:

  • Divide field into 2.5 acre areas (also used in the precision ag sampling) based on similar management history and soil characteristics (sample should not represent a field over 10 acres).
  • Take into consideration difference in surface color is the most evident feature separating soil types, indicating possible variability in texture, organic matter content, and drainage.
  • Nutrient removal rates by crops are important in determining residual soil fertility levels. Therefore, cropping history is another important factor to consider when defining a sample area.
  • Construct an accurate map of the field to take samples from the same areas each year, preferably in the fall after harvest to enhance the relevance of annual comparisons.

Taking soil samples:

  • Obtain 10 to 20 soil cores (brush away surface plant residue material) from a 6 - 8 inch depth for tilled and 3 - 4 inch for no-till areas in a zigzag pattern throughout the area to ensure good representation.
  • Place soil cores in a clean plastic bucket and mix thoroughly.
  • Fill a one-pint sample box with the soil subsample.

 Note: In most sandy Coastal Plain soils where deep tillage is practiced to break root restrictive hardpans, a sample of the top 4 inches of subsoil can be used to determine the availability of potassium, sulfur, and magnesium. These nutrients readily leach through the sandy surface layer but are retained in the upper part of the subsoil. The results of subsoil samples can be used to adjust fertilizer recommendations made from the analysis of the A horizon sample.

Soil samples are analyzed for soil pH and the plant-available contents of potassium (K), phosphorus (P), calcium (Ca), magnesium (Mg), zinc (Zn), copper (Cu), boron (B), and manganese (Mn). Based on these soil test results, recommendations are made regarding the correction of soil pH and fertilization requirements to achieve top yields.

These recommendations are based on several years of soil test calibration research conducted in South Carolina and neighboring states. Soil analysis procedures determine the amount of each nutrient available for crop uptake. The new soil fertility ratings and recommendations have been posted at http://www.clemson.edu/agsrvlb/new_page_16.htm. Nutrient levels on the soil test report are indexed into the following categories:

  • Low: soil plant nutrient element level is deficient and an application of this element will result in a significant yield increase. A high application rate is needed to meet the crop requirement, compensate for soil interaction, and build the soil reserves.

 

  • Medium: soil plant nutrient element level is adequate for moderate agronomic crop yields, but a yield response can be expected about 50% of the time from an application of this element. For moderate yield goals, there is probably a sufficient amount of this plant nutrient element without the need to add more than that expected to be removed by the crop; however, for high yield goals, the recommendation should be greater than that needed to compensate for crop removal.

 

  • Sufficient: soil plant nutrient element level is in that range adequate to meet the crop requirement as well as that needed for consistent high crop yield production. A maintenance application rate is recommended to compensate for expected crop removal. Maintaining the surface soil within the "Sufficient" range will ensure that the subsoil essential plant nutrient element level will not be depleted.

 

  • High: this soil plant nutrient element level can adversely affect crop yield and product quality, and a further increase can lead to crop yield decreases as well as plant nutrient element imbalances. Therefore, no addition of this element is recommended, unless needed to compensate for expected high crop removal.

 

  • Excessive: this soil plant nutrient element level will adversely affect plant yield, create nutrient element deficiencies due to imbalances, and can lead to potential ecological damage to the surrounding environment.

Soil pH and liming

A soil pH too low or too high can adversely affect nutrient availability and hence, soybean growth and yield. The best pH for soybeans grown in most South Carolina soils is between 5.8 and 6.5. If soil tests show a low soil pH, lime can be applied to enhance yield by:

  • Reducing toxicity of soil aluminum and/or manganese.
  • Improving uptake of phosphorus, potassium, and molybdenum.
  • Increasing availability of calcium and magnesium (with dolomitic lime).
  • Improving nitrogen fixation by Bradyrhizobium japonicum bacteria in root nodules.

It is important to perform soil tests and apply lime and fertilizer if needed to avoid yield losses due to nutrient deficiency. The following are some considerations regarding the nutrient management practices to help with maintaining high soybean yields.

The approximate amounts of nutrients removed from the soil by a 60-bushel soybean crop are presented in Table 1. The nutrients are removed from the field in the grain. Many of the nutrients left in the stubble are recycled and available to the next crop.

Table 1. Approximate nutrient utilization for a 60-bushel soybean crop*

Plant Part

Nutrient

N

P2O5

K2O

Mg

S

 

lb/A

lb/A

lb/A

lb/A

lb/A

Grain

240

48

84

17

12

Stubble (leaves, stems, etc.)

84

16

58

10

13

Total

324

64

142

27

25

*Source: IMC Plant Food Utilization Guidelines

Fertilizer recommendations, based on the soil test for maximum soybean yields, are shown in the Tables 2-4.

Table 2. Fertilizer recommendations for soybean yield goal of 30 Bu/acre based on soil test results in South Carolina.*

Phosphorus

Potassium

 

Low

Medium

Sufficient

High

Excessive

 

pounds of N-P2O5-K2O per acre

Low

0-  90-80

0- 90-60

0-  90-40

0-  90-0

0-  90-0

Medium

0-  60-80

0-  60-60

0-  60-40

0-  60-0

0-  60-0

Sufficient

0-  30-80

0-  30-60

0-  30-40

0-  30-0

0-  30-0

High

0-    0-80

0-    0-60

0-    0-40

0-    0-0

0-    0-0

Excessive

0-    0-80

0-    0-60

0-    0-40

0-    0-0

0-    0-0

* Source: http://www.clemson.edu/agsrvlb/new_page_16.htm

Table 3. Fertilizer recommendations for soybean yield goal of 50 Bu/acre based on soil test results in South Carolina.*

Phosphorus

Potassium

 

Low

Medium

Sufficient

High

Excessive

 

pounds of N-P2O5-K2O per acre

Low

0-100-100

0-100-80

0-100-60

0-100-0

0-100-0

Medium

0-  70-100

0-  70-80

0-  70-60

0-  70-0

0-  70-0

Sufficient

0-  40-100

0-  40-80

0-  40-60

0-  40-0

0-  40-0

High

0-    0-100

0-    0-80

0-    0-60

0-    0-0

0-    0-0

Excessive

0-    0-100

0-    0-80

0-    0-60

0-    0-0

0-    0-0

* Source: http://www.clemson.edu/agsrvlb/new_page_16.htm

Table 4. Fertilizer recommendations for soybean yield goal of 70 Bu/acre based on soil test results in South Carolina.*

Phosphorus

Potassium

 

Low

Medium

Sufficient

High

Excessive

 

pounds of N-P2O5-K2O per acre

Low

0-110-120

0-110-100

0-110-80

0-110-60

0-110-0

Medium

0-  80-120

0-  80-100

0-  80-80

0-  80-60

0-  80-0

Sufficient

0-  50-120

0-  50-100

0-  50-80

0-  50-60

0-  50-0

High

0-    0-120

0-    0-100

0-    0-80

0-    0-60

0-    0-0

Excessive

0-    0-120

0-    0-100

0-    0-80

0-    0-60

0-    0-0

* Source: http://www.clemson.edu/agsrvlb/new_page_16.htm

When double cropping with small grains, all fertilizer for both crops can be applied prior to planting small grain in the fall. Note: For both full-season and double-crop soybeans grown in the sandy Coastal Plain soils of South Carolina, apply sulfur in the fertilizer at a rate of 10 pounds per acre.

If soil sample results are medium for phosphorus or at any level for potassium, there is little evidence that banding of these nutrients near the soybean crop row is more beneficial to grain yield than broadcast applications. When the soil test shows low phosphorus level, it is recommended to place one-half of the phosphorus rate close to the crop row in a band.

Note: To avoid seedling injury, place the fertilizer band 2 inches below and 2 inches to the side of the soybean row. All recommended fertilizer for no-till soybeans should be broadcast prior to planting.

With soil test levels showing medium P and K levels, it may be possible to reduce recommended broadcast rates slightly when banding, but additional expense in handling a starter material at planting may offset any savings involved.

When soil test levels for P and K are adequate, and no fertilizer is recommended, there is no evidence to support a yield response to banded applications of starter materials. Unlike corn, which is sometimes planted in cool soils which restrict seedling uptake of nutrients like phosphorus, soybeans are planted in May or June when soils are warm enough for good root growth and optimum nutrient uptake.

Manganese (Mn) deficiency may occur when soil pH is above 6.2 on the poorly drained soils in the Atlantic flatwoods of the lower Coastal Plain. When soil conditions are conducive to Mn deficiency, and/or the previous crop has shown deficiency symptoms, Mn can be applied to the soil to prevent deficiency in soybeans. The recommended rates are from 10 to 15 lb of Mn/A broadcast or 4 lb of Mn/A within a band. The less time the Mn is in the soil prior to planting, the more effective the application will be in preventing the deficiency.

During soybean vegetation, symptoms of Mn deficiency include interveinal yellowing on young leaves while the veins remain dark green. If the soybean crop exhibits symptoms of a Mn deficiency, foliar applications of Mn are effective in alleviating the deficiency. The recommended rate of foliar-applied Mn is 0.1 to 0.2 lb of Mn/A (0.4 to 0.8 lb of manganese sulfate). If Mn deficiency symptoms reappear a 2nd or 3rd Mn application may be needed to correct the nutrient deficiency. The lowest effective rate is preferred because of lower cost, less likely leaf burn, and ease in dissolving the fertilizer. The Mn should be applied in at least 20 gal of water/A.

Another way to correct a Mn deficiency is by rotating a high soil pH field to corn and using recommended rates of acid-forming nitrogen fertilizers. This practice may lower soil pH to more optimum level for Mn uptake by soybeans. Corn is more tolerant than soybeans to low soil Mn levels. Wheat should not be planted on high pH soils since wheat is extremely susceptible to Mn deficiency, and foliar applications of Mn are not usually effective in correcting the problem.

Note: Tank-mixing manganese (Mn) with glyphosate is not recommended due to antagonism and therefore reduced glyphosate efficacy in weed control. Manganese applications can be made 7-10 days after glyphosate application with little reduction in weed control or Mn utilization.

Boron is readily leached from soils in environments receiving frequent rainfall; thus, boron deficiency may occur when soybeans are irrigated to optimize productivity. In nonirrigated soybeans, the leaching of boron is minimal. Availability of boron decreases substantially as pH increases above 6.5; hence, alkaline soils are generally low in available boron, inducing boron deficiency. Since soil pH should not exceed 6.5 in production fields, there should be little or no risk of boron deficiency.

Boron applications in irrigated soybeans or at high soil pH should be based on leaf concentrations of boron. Soybean yield will respond positively to foliar applied boron only when concentrations in leaves are 10 ppm or less. In such instances when boron is needed, it should be applied at 0.2 lb/A at the early-pod stage (1/8 to 1/4-inch pods) and can be mixed with insecticides if needed; however, tank mixing boron or other foliar applied nutrients with Roundup may severely reduce weed control.

Boron applied at 0.2 lb/A will cost about $2/acre plus application costs. Applications of boron to plants having boron concentrations greater than 60 ppm may result in yield decreases; therefore, routine use of boron is discouraged.