Feeding the Vegetable Garden: A Primer on Soil Nutrients

picture of green tomatoes growing on the vine

Vegetables, like people, require adequate food, water and shelter to thrive. Formal classification of vegetable food is couched in the language of soil nutrients. It stands to reason that growing award winning vegetables requires an adequate supply of soil nutrients. Each vegetable has its own nutrient needs and generally the types of nutrients gardeners consider breaks down into three groups:

  • Primary nutrient refers to the big three nutrients, nitrogen, phosphorus, and potassium
  • Secondary nutrient refers to calcium, magnesium, and sulfur, which are used in moderate quantities by crops.
  • Micronutrient, trace, or minor element refers to iron, boron, manganese, zinc, copper, and molybdenum, the essential plant nutrients used in relatively small quantities.

Depending on the garden theme, and the results of a soil test, some general rules of thumbs can be applied to guarantee the soil is adequately filled with nutrients prior to the planting season. According to the Knott’s Handbook for Vegetable Gardeners:

Many states issue suggested rates of application of fertilizers for specific vegetables. These recommendations are sometimes made according to the type of soil – that is, light or heavy, sands, loams, clays, peats, and mucks. Other factors often used in establishing these rates are whether manure or soil-improving crops are employed and whether an optimum moisture supply can be maintained. The nutrient requirements of each crop must be considered, as must the past fertilizer and cropping history. The season of the year affects nutrient availability. Broad recommendations are at best only a point from which to make adjustments to suit individual conditions.

So with a general recommendation in hand and the fertilizer ready to go, it’s time to work the soil and apply half of the fertilizer . Add the reset into the top three inches of soil when the soil is set for planting. Direct application of fertilizers on garden transplants is usually not recommended because of potential fertilizer burns. It’s best to apply the fertilizer approximately six inches from transplants so that as they grow, their roots will be stronger when they reach the fertilized soil.

Most commercial fertilizers contain one or more of the three major plant nutrients: nitrogen (N), phosphorus (P), and potassium (K). When a fertilizer contains all three elements, it called a complete fertilizer. Shelves of commercial fertilizers share a common label with three numbers, indication the percentages of nitrogen, phosphorous, and potassium. For example, a bag of fertilizer with the numbers 5-10-10 contains 5% nitrogen, 10% phosphorous, and 10% potassium.

Each vegetable has its own fertilizer requirements for the growing, flowering and vegetable stages. For example, cole crops require consistent doses of nitrogen, phosphorus, and potassium. Over the counter plant starter fertilizers can be applied at the planting stage, and after three weeks and five weeks. Starter fertilizers increase yields by 20%

Individual nutrients are also readily available. For example, gardeners interested in organic fertilizers might want to check out fish emulsion (3-5%), composted chicken manure (3%) and blood meal (12-15%) as means of increasing nitrogen. Bone meal (12-24%) provides a phosphorus pick up for plants. Kelp meal (2-5%) provides a potassium boost.

Knott’s Handbook for Vegetable Gardeners lists the following organic materials according to their percentage of nitrogen, phosphorous and potassium in chart form.

Bat Guano: 10.0 – 4.0 – 2.0

Bone Meal (raw): 3.0 – 22.0 – 0

Bone Meal (steamed): 1.0 -15.0 – 0

Castor Bean Meal: 5.5 – 2.0 – 1.0

Cottonseed Meal: 6.6 – 3.0 – 1.5

Fish Meal: 10.0 – 6.0 – 0

Peanut Meal: 7.0 – 1.5 – 1.2

Soybean Meal: 7.0 – 1.2 – 1.5