Soil test calibration and lime research for Eastern Oklahoma - Experiment 801

Eastern Oklahoma soils are naturally acid, and usually become more acidic with intensive cropping. These soils are also deficient in plant available phosphorus (P) and potassium (K). Experiment 801 was established in the fall of 1977. It has been planted to winter wheat, with conventional tillage each fall since then, and was designed to examine wheat response to P and K in an acid soil. Grain yield is measured each year while soil testing and laboratory analysis of the grain are obtained on a less frequent basis.

Initial soil test characteristics and changes with time for selected treatments are shown in Table 1. Without any fertilizer additions (treatment 1) soil pH has remained about the same through the almost 20-year period. The most serious yield limiting soil factors have been soil acidity (pH) and available soil P. Applying N, P, and K nutrients (treatment 8) corrected the P limitation and improved yields, however in the process the soils became more acidic and yield limiting. Addition of ammonium forms of N, such as the ammonium nitrate (34-0-0) used here, contributes to acidity. When ammonium-N is added without P, the yield limiting affects of soil acidity and P deficiencies are combined to result in very low grain yields (6.2 bu/acre, Table 2).

Table 1. Soil test values for selected treatments of experiment 801.
Treatment	Year		pH	N	P	K
(1) 0-0-0	1978		5.3	33	12	128
	1987		5.1	7	20	132
	1996		5.3	3	17	328
(8) 100-120-120	1978		5.1	58	24	161
	1987		4.9	14	123	373
	1996		4.4	129	185	670
(9) 100-0-120	1978		5.2	39	12	171
	1987		4.7	112	21	375
	1996		4.2	623	31	771

Acid Soil. Soil acidity was the dominant yield-limiting factor over the 1978-1996 period (Figure 1). Among the treatments receiving fertilizer, 65 % of the variability in average yield from 1978-98 was a result of differences in soil pH. This simply means that when soil pH is limiting yield (pH below 5.5), applying differing amounts of fertilizers will not compensate for the need to lime the soil, even when soil test values are built up to high levels, such as in treatment 8.

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Soil Test Calibration. The treatments applied in experiment 801 (Table 2) allow periodic evaluation of soil test calibration, as far as identifying the critical soil test level, for both P and K2O.

Table 2. Treatment structure and average yield (1978-98) of long-term wheat experiment 801.
	N	P2O5			K2O	Yield	Soil pH
Treatment	------------ lb/acre ------------					(bu/acre)	(1996)
1	0		0	0		13.5	5.3
2	0		120	120		23.8	5.2
3	100		0	0		6.1	4.2
4	150		0	0		5.5	4.2
5	100		120	0		16.3	4.6
6	100		120	40		20.0	4.8
7	100		120	80		19.2	4.7
8	100		120	120		18.3	4.4
9	100		0	120		6.2	4.2
10	100		40	120		13.3	4.2
11	100		80	120		19.7	4.5
12	150		120	120		14.6	4.2
13	150*		120	120		13.4	4.1
*Split as 100 lb preplant and 50 topdressed in the spring.

Some treatments apply adequate N and P each year while varying the amount of K (treatments 5-8). Another set of treatments assures adequate N and K, and varies the amount of P2O5 applied (treatments 9, 10, 11, and 8). Because of the strong affect of soil acidity, responses to K fertilizer were not significant. Response to P was significant, however, and the relationship of soil test value and yield is shown, as compared to the statewide calibration (Figure 2.). This figure illustrates that as the soil test value approaches 65 there is enough available P supplied from the soil for 100 % of the maximum yield, even though the yield may still be limited by soil acidity. The graph indicates that in this very acid soil the soil test of about 20 is only about 30 % sufficient compared to the state-wide calibration of about 60 % sufficiency. This should not be interpreted simply as soil acidity "tying-up" the phosphorus, but rather a result of aluminum and manganese toxicities preventing normal uptake of available soil-P. Calculations indicated about 13 lb/acre P2O5, in excess of crop removal, is required to raise the soil test P value one unit. This value compares closely to that calculated for other soils in Oklahoma where soil acidity is not a problem.

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Dr. Gordon Johnson,
Dr. Bill Raun,
Plant and Soil Sciences
Oklahoma State University