A Natural Fertilizer with Phytotoxic Activity

Melissa Ann Pline Brown, Graduate Student, and Jeanine Davis, Ph.D., Advisor
Department of Horticultural Science
NC State University

Introduction

Bag of soybean meal

Soybean meal is commonly used as a nitrogen source in organic production. Occasionally, plants are burned or germination is reduced. These studies were designed to examine that.

Germination Inhibition

Greenhouse studies showed that soybean meal (SBM) can inhibit the germination of small seeds. Collards (Southern Georgia), lettuce (Black Seeded Simpson), and turnips (Purple Top) all exhibited more than 50% reduction in germination and shoot weight with SBM sprinkled on the soil surface at 877.5 lb/acre. SBM incorporated into the media also reduced collard shoot weight by 50% at 877.5 lb/acre. Summer squash (Sunray) and cucumber (Conquest) were not affected by SBM at any rate, either sprinkled on the soil surface or mixed into the media. The shoot weight of sweet corn (Wizard) was reduced by 25% with SBM sprinkled on top at 4387 lb/acre while sweet corn shoot weight was not affected by SBM mixed into the media. From these initial tests, it appears that broadcasting and incorporating SBM at the time of planting can significantly inhibit the germination and growth of small seeds.

Treatments for Greenhouse Seed Study

• No soybean meal
• 877.5 lb/acre
• 1755 lb/acre
• 3510 lb/acre
• 4387 lb/acre

Cucumber	Lettuce
Turnip	Sweet Corn

Soybean Meal

7 % Nitrogen
2 % Phosphorus
1 % Potassium

• Commonly available animal feed
• Cost $9.50/lb or $360/ton at time of study
• Allowed by National Organic Program

Transplant Burn — Field Experiment

Three rates of soybean meal were broadcast on 5 ft. centers, bedded, then incorporated and covered with black plastic. Pepper starts were transplanted the next day. Soil tests revealed the mid and high rates of SMB had high levels of soluble salts.

When used at high rates under black plastic, soybean meal burned sweet pepper transplant roots, inhibiting growth and/or leading to death. Fertilizing at least two weeks before planting should allow the soybean meal to break down

Transplants

Mid rate
2,150 lb/acre
150 lb N/acre

No soybean meal

High rate
4,333 lb/acre
300 lb N/acre