Do Direct Seeded Peas Benefit
From Starter Fertilizer Applications


Tim Fiez, Extension Soil Fertility Specialist
Department of Crop and Soil Sciences
Washington State University

Introduction and Summary of Past Research Results

Growers do not commonly apply fertilizer to dry pea crops as past research trials have found little yield benefit from N, P, or S fertilizers. The 1972 Washington State University Extension Bulletin, E.M. 3379, Dry Pea and Lentil Fertilization, summarized four fertilizer studies conducted during the late 1960’s. These studies examined: 1) band versus broadcast application of P and K on eroded hilltops, 2) starter fertilizer applications with the seed versus broadcast application, 3) N fertilization, and 4) large plot comparisons of starter fertilizers using a dual boot drill application. The study of P and K applications on eroded hilltops found no K response and a P response only when soil test P levels were in the 2 ppm range (sodium acetate extraction). The starter fertilizer trial applied the fertilizers with the seed. Peas are extremely sensitive to fertilizer salts and all applications of N or P with the seed decreased yields. A starter application of S (8 lb S/acre applied as CaSO4) did not reduce germination but there was no positive effect on yield. The N fertilizer trial conducted at three locations in 1968 found that N applications decreased yields. Increased weed growth associated with the N applications was one of the reasons noted for the lower yields. Finally, the large plot test of starter fertilizers found a slight yield increase (100 lb/acre) from an application of 19 lb N/acre and 22 lb S/acre as compared to the control. However, no statistics are provided in the 1972 bulletin to assess the confidence in the yield differences. As a whole, the 1972 bulletin suggests that pea or lentil fertilization is not warranted except under very low soil P levels. The most recent summary of pea and lentil fertilizer requirements can be found in the University of Idaho Northern Idaho Fertilizer Guide for peas and lentils, CIS 448. This guide includes tables listing fertilizer needs by soil test levels.

This past research indicates that growers should expect little benefit from pea or lentil fertilization. However, changes over the last 30 years such as improved weed control and increasing interest in direct seeding of peas and lentils, has caused us to re-examine the issue of fertilizing peas, specifically, direct seeded peas. Starter fertilizer applications have been found to increase cereal yields in direct seed systems particularly when root diseases limit root interception of immobile soil elements such as P. The 1972 Dry Pea and Lentil Fertilization publication mentions grower and dealer reports of favorable responses to starter fertilizers in cold, wet springs although the research contained in the report found no benefit in one year of trials. Recent work by various agriculture industry personnel has also found some positive responses to pea fertilization. Thus, as part of a systems approach to developing direct seed systems for peas and lentils (Guy et al., 1998), we initiated studies in 1998 to determine if there are yield benefits from applying starter applications of N, P, and S at seeding time with direct seed drills.

New Pea Soil Fertility Experiments

Starter applications of N, P, and S were tested under tilled and notil seeding systems at four locations during the 1998 growing season. All the fertility trials were conducted within grower’s fields as part of a larger on-farm test of tillage system influence on pea yield and post pea harvest soil conservation. The location of the four fertility trials and the cooperating growers are listed in Table 1. Because of the on-farm nature of these trials, the tillage systems are not identical between sites but each site included a pure notil treatment and a fall chisel or plow treatment.

The fertility treatments consisted of starter (20 lb/acre) rates of N, P, and S in all possible combinations. In both the tilled and notil plots, the fertilizers were band applied 2" directly below the seed at planting time. Experimental observations included plant stand, harvest yield, aboveground biomass at harvest (check and the N + P + S fertilizer treatment only), and 1000-seed weight (check and the N + P + S fertilizer treatment only).

Results and Discussion

Because we banded the fertilizers below the seed by approximately 2", we observed no stand reductions from the fertilizer applications, which had occurred in past studies. At all plot locations, the fertilized plots could be visually differentiated from the non-fertilized control plots. The fertilized plots seemed to have greater early growth and vigor. However, at the two sites analyzed so far, the Riggers and Schultheis sites, there were no significant yield responses to any fertilizer treatment, and the results were the same in pure notil plots and in plots which were tilled prior to planting (Table 2).

The N+P+S treatment increased non-seed aboveground biomass ("the vine weight") at the Riggers site by 22% compared to the control without increasing seed yield while there was no increase in non-seed aboveground biomass at the Schultheis location. Weight per 1000 seeds was similar between the N+P+S and non-fertilized treatments at the Riggers and Schultheis sites. The observation that the vine biomass was increased 22% by the N+P+S treatment at the Riggers site yet yield and 1000-seed weight were not affected could be due to several factors. The greater vine growth might not have resulted in an increase in pod number or there was an increase in the number of pods but each pod set fewer peas.


Table 2. Dry pea yield as affected by combinations of N, P, and S fertilizer at 20 lb/ac.

 Fertilizer Treatment

---------------Pea Yield----------------

 Riggers: Craigmont, ID

Schultheis: Colton, WA



Banded Check (no fertilizer)








N + P


N + S


P + S


N + P + S





Results from 1998 confirm previous studies that have found little benefit from starter fertilizer applications on peas. The pea fertility experiments are being repeated at two locations in 1999 to further examine this issue.


Guy, S, D. Thill, R. Veseth, J. Hammel, T. Fiez, and J. Yenish. 1998. Residue Production and Retention in Small Grain Cereal and Legume Rotational Systems with Different Tillage Practices. STEEP III Progress Report.