Soil Nutrient Variability in Southern Piedmont Soils

Crop and Soil Environmental News, October 1996

Stephen J. Donohue,
Extension Specialist, Soil Testing and Plant Analysis

When sampling soil, it is usually recommended that one collect a small amount of soil from a number of locations within a field, then mix the soil together in a pail for a good, representative sample. The reasoning behind this approach is that soil nutrient level tends to vary across a field due to the heterogenous nature of soil. This natural variation is further compounded when fertilizer and lime are applied, because of occasional missed streaks or overlaps with normal application equipment and methods. Sampling instructions typically call for collecting 2-5 cores or subsamples per acre within the field. Questions sometimes arise as to whether one has to collect so many subsamples. More specifically, what is the minimum number of subsamples needed for a representative sample. A study was conducted at the Southern Piedmont Agricultural Research and Extension Center in July, 1996 to address this question, i.e., to evaluate natural nutrient variability in a field and the need for collecting a composite sample for reliable, realistic nutrient assessment.

The experimental area chosen was approximately one acre in size, was in a long-term tall fescue sod, and had received minimal maintenance over the years. No fertilizer was ever applied to the area and one lime application, 2 T/A, was made in the early 1980s. The soil was a Chesterfield-Mayadan-Bourne complex with a sandy loam surface and increasing clay with depth. The field was sampled on a 40' x 40' grid with a total of 25 sampling location points. Sampling depth was 4". All samples were analyzed in duplicate by the Virginia Tech Soil Testing Laboratory for pH, phosphorus (P), and potassium.

Soil pH data are presented in Figure 1. The pH ranged from 4.9 - 6.0 (average = 5.6) in the one acre field.

Figure 1. Soil pH.

Figure 2 illustrates the variation in limestone recommendations for the field (target pH = 6.2). This information is also presented in tabular form (Table 1).

Figure 2. Lime Recommendations, Tons/A

Table 1. Soil pH and lime recommendations.

pH Range*	No. of Samples	Lime Recomm., Tons/A
4.5 - 4.9	2	3
5.0 - 5.4	7	2
5.5 - 5.9	12	1
6.0+	4	0

*Average pH = 5.6.

Even though the field had not been limed for 15 years, the variation in pH and resultant lime recommendations was rather significant. The average pH, 5.6, called for a lime recommendation of approximately 1 Ton/A. However, if only one point or location was selected for the sample, as little as "0" or as much as 3 Tons/A of lime would have been recommended.

Soil P results are presented in Figure 3. The P level ranged from 2 - 38 lb/A with an average of 7.1 lb/A for the field.

Figure 3. Soil test P, lb/A.

Table 2 illustrates the variation in P recommendations for the field. Most of the samples tested in the Low category where 110 lb/A of P₂O₅ would be recommended. However, two locations in the upper right corner of the field tested in the High category where no fertilizer would have been recommended. Had this been the only area sampled in the field, the entire field would have been under-fertilized.

Table 2. Soil test P and P₂O₅ recommendations.

Soil Test P*		No. of Samples	P2O5 Recomm., lbs/A
Level	Range, lb/A
Low-	0 - 3	6	120
Low	4 - 8	16	110
Low+	9 - 11	1	100
Med	12 - 35	0	70
High	36 - 110	2	0

*Average = 7.1 lb/A (L)

Soil K results are presented in Figure 4. The K level ranged from 24 - 128 lb/A with an average of 60.5 lb/A for the field.

Figure 4. Soil test K, lb/A.

Table 3 illustrates the variation in K recommendations for the field. Most of the samples tested in the Low or Low+ categories where 100-110 lb/A of K₂O would be recommended. However, four locations tested in the Medium category, with two of these locations calling for significantly less fertilizer (i.e., 70 lb/A). Again, this points to the need for sampling multiple locations for a representative composite sample.

Table 3. Soil test K and K₂O recommendations.

Soil Test K*		No. of Samples	K2O Recomm., lbs/A
Level	Range, lb/A
Low	16 - 55	14	110
Low+	56 - 75	7	100
Med-	76 - 100	2	90
Med	101 - 150	2	70

*Average = 60.5 lb/A (L+)

The above data illustrates typical variation in soil nutrient level one might expect in an area receiving minimal fertilizer and lime applications. Greater variation would be expected if fertilizer and lime had been applied on a regular basis. It is important that standard soil sampling instructions calling for 2-5 cores/A collected in a random fashion be followed for a good representative sample and meaningful fertilizer and lime recommendations.

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