I’ve been reading in both popular and scientific press that for soybean fields with low fertility, early planting substitutes for addition of needed soil-based nutrition (Allen, 2025, Loman et al., 2024). The implication was that low fertility fields intended for soybean production and that remain unfertilized are less vulnerable to nutrient deficiencies if these fields are planted early. This observation flies in the face of other work, both earlier and ongoing, that I’m aware of.

In 1983, Peaslee and co-workers published field research on the interaction of soil potassium (K) nutrition and soybean planting date (Peaslee et al., 1983). Three levels of soil test K had been established in these long-term plots; 144, 204 and 429 lb/acre – considered low, medium and very high, respectively, by the ammonium acetate extraction test used in Kentucky at that time. Table 1, below, shows the soybean yield responses that were found. The soil test level designations are like those we use today. The soybean planting date range is later than those now used by Kentucky soybean producers. That said, please note the strong positive interaction between earlier planting and soil K nutrition. The total yield spread is 53 - 31 = 22 bu/acre, and though early planting contributes most to that yield spread, soil K nutrition isn’t far behind.

I’ve also been following the sulfur (S) work of Dr. Shaun Casteel, at Purdue (Casteel, 2023). He’s been looking at the soybean-S nutrition yield response as related to planting date. Between 2018 and 2022, Dr. Casteel compared the yield response to S nutrition between mid-May and early June planting dates, about 3 weeks apart (Table 2). The soybean yield response to added S was 8.2 bu/acre with mid-May planting dates and only 2.3 bu/acre with early June planting dates. In 2023, Dr. Casteel expanded the number of planting dates to three, 18 April, 12 May, and 7 June. The response to S addition was 20.2, 14.6, and 3.0 bu/acre, respectively (Table 3).

Loman et al. (2024) measured soybean yield in 133 fields between 2014 and 2021, trying to find “which and how well soil test values predict yield of unfertilized soybean”. The relationship between yield and planting date was also reasonably good and they then ‘binned’ the data by planting date. The planting dates were first divided into Early (2 April-23 May) and Late (24 May-11 June) groups. Then, a Very Early (2 April-9 May) data subset was split off the Early dataset and a Very Late (5 June-11 June) subset was split off the Late set of data.

For each planting date bin, they looked at relationships between soil fertility parameters and soybean yield and reported that soil fertility was more important to yield in later planted soybean than earlier planted. This caused them to recommend later planted soybean in high fertility fields and early planted soybean in fields with lower fertility to reduce overall fertilizer needs.
At first glance, it was clear that the Late/Very Late yield data were indeed much more affected by one or more of the soil test parameters, while the Early/Very Early yield data were much less influenced (Loman et al., 2024). But when I dug into the supplemental material (Loman et al., 2024, suppmat) provided with this paper I found some indications that might explain why these authors report results very different from those of Peaslee et al. (1983) and Casteel (2023).

Table 4 gives soil test parameter (soil organic matter, soil pH, soil test phosphorus and potassium) for the Very Early (n=38) and Very Late fields (n=34), as taken from Loman et al. (2024. suppmat). The first indication that some of the Very Late fields might be less fertile than the Very Early fields is in the soil organic matter (SOM) data; both minimum and first quartile values are considerably lower in the Very Late subset. There was also a lower minimum pH value in the Very Late subset, but this might not make much of a difference to the overall analysis.

The more important indications that the population of Very Early fields were generally more fertile than the Very Late fields were in the Table 4 soil test phosphorus (STP) and potassium (STK) data. In AGR-1 (Ritchey and McGrath, 2022), the minimum and first quartile values for Very Early field STP, 44 and 56 lb/acre, are considered medium-high to high. Those same values for Very Early field STK, 182 and 230 lb/acre, are considered medium-low to medium for soil K supply to the crop. Very Late fields exhibited much lower minimum and first quartile values for STP and STK (Table 4). The values here are comparable to AGR-1 values because the UK soil test labs also use the Mehlich 3 extraction procedure to measure STP and STK.

The implications of these differences in the two data subsets are evident when one considers the general relationship between grain crop yield and soil test nutrient levels (Figure 1). Crop yield responsiveness falls dramatically as soil test values rise - the Law of Diminishing Returns (Figure 1). As the Very Early planting date data subset contains no very low or low STP or STK values, then there would be much less chance, with that data subset, that yields from these unfertilized Very Early planting date fields would show a relationship with soil fertility – all the fields in this data population were already fertile enough – no low fertility fields in the data subset. The Very Late data subset included fields with very low to low soil fertility levels, and not surprisingly that data subset exhibited a relationship between soybean yield and soil fertility. The conclusions of Loman et al. (2024) are not well supported when you consider that these two data subsets are not equally ‘representative’ as regards having an equivalent range in soil fertility levels for the sampled fields.

The more obvious conclusion is that soybean field soil fertility matters, regardless of soybean planting date. Fields that are low in soil fertility need lime and fertilizer, not earlier planting – which is not a substitute for good soybean nutrition. And there are soil nutrients (K, S) which the existing data suggest are especially important when planting soybean early.

Allen, M. 2025. Timing matters: Early planting benefits soybean in unfertilized, low-fertility fields. MidAmerica Farmer Grower. Feb. 14.

Casteel, S.N. 2023. Sulfur Solutions in Soybean. https://ag.purdue.edu/department/agry/faculty-pages/soybean-station/_docs/2023_1221w_s-fertility-in-soy_short.pdf

Loman, M.H., C.N. Sible and F.E. Below. 2024. Soybean planting date affects the relationship between soil test values and grain yield. Soil Sci. Soc. Am J. 88:2194-2210. https://doi.org/10.1002/saj2.20753

Loman, M.H., C.N. Sible and F.E. Below (suppmat). 2024. Soybean planting date affects the relationship between soil test values and grain yield. Soil Sci. Soc. Am J. 88:2194-2210. saj220753-sup-0001-suppmat

Peaslee, D., T. Hicks and D. Egli. 1983. High residual potassium level, early planting give top soybean yields. Better Crops 67 (Summer): 8-10.

Ritchey, E., and J. McGrath. 2022. 2020-2021 Lime and Nutrient Recommendations, AGR-1. Univ. Kentucky Coop. Extn. Svc., Lexington, KY.

Citation: Grove J., Lee, C., 2025. Early Planted Soybean Does Benefit from Good Soil Fertility. Kentucky Field Crops News, Vol 1, Issue 3. University of Kentucky, March 14, 2025.

Dr. John Grove, UK Soil Research & Extension
(270) 365-7541 
jgrove@uky.edu