FIMBY: How to 'feed' your vegetables for healthy growth

A common question from new gardeners: What do I feed my tomato plants?

The answer to that question needs two corollaries: When and how?

Because tomato plants – like many vegetables – need different nutrients at different times in their development. How those nutrients are delivered makes a difference, too.

Bill Maynard, Sacramento’s longtime community garden guru, had this advice for all new gardeners: Think of your garden as a pet. It needs constant attention; you can’t just plant and forget it. It’s dependent on you for water and nutrients. Create a watering and fertilizing schedule, then stick to it.

Instead of kibble, plants “eat” fertilizer; they absorb nutrients through their roots, and then produce their own food through photosynthesis. For plants to take up those nutrients, water must be present in the root zone. (That’s why maintaining soil moisture is so vital.)

Plants need a wide range of nutrients for healthy growth, but three have the greatest impact: Nitrogen (N), Phosphorus (P) and Potassium (K). Also called macronutrients, the big three are essential for growth and overall vitality. They’re listed on every fertilizer package as a percentage of that product, such as 10-10-10. (In this example, that product contains 10% each of nitrogen, phosphorus and potassium.)

Low numbers on fertilizer (such as 5-1-1 on fish emulsion)  indicate an organic, slow-release fertilizer. It's not better or worse for the plant, but it is better for the soil than chemical fertilizer. Read the package to learn what you're using.

Which macro does what? Think of these nutrients as supporting different parts of the plant and different stages of growth:

Nitrogen = stems and leaves.

Phosphorus = flowers and fruit.

Potassium = roots.

That’s an oversimplification, of course, but it makes it easier to visualize how your plant is using its food.

Nitrogen supports green growth such as leaves; it’s what a plant needs most in spring during rapid development. Because turfgrasses are mostly leaves, lawn fertilizers are heavy in nitrogen.

Phosphorus is key to flower, fruit and seed production; plants need more when they’re ready to flower and set fruit. Adding more phosphorus to a plant’s diet will promote earlier flowering as well as bigger and more plentiful fruit.

Potassium relegates the flow of water and nutrients within the plant, starting at the roots. It’s necessary for every cell to do its job, so potassium always needs to be part of a plant’s balanced diet.

Those macronutrients are all water-soluble and can be leached out of soil over time. They’re also what plants demand the most. That’s why gardeners (and farmers) replenish these macros in soil before planting. These macros also can be added during growth, right up until the first red tomato.

Most micronutrients – the other things plants need for growth and development – are present in local soils and don’t need additives. That list includes copper, boron, sulfur, zinc and manganese.

The exceptions are iron and calcium; those micros may be added as needed. Plants will show signs of iron deficiency, such as yellowing of leaves between veins. (This is especially true for shrubs.) Calcium deficiency can lead to blossom-end rot on tomatoes. (A little powdered oyster shell or other calcium supplement can be added -- but the plants must be watered properly to absorb the calcium. Inconsistent soil moisture is the primary cause of blossom-end rot.)

Use fertilizers sparingly; a little goes a long ways. A double dose doesn’t result in double growth; it can hurt (or even kill) the plant. (Excess fertilizer also causes other issues such as runoff into waterways.)

Too much nitrogen can result in too-rapid growth and weak cell walls – inviting aphids and other pests to dig in. It will also produce a tomato plant that's all plant and no fruit.

Instead, apply fertilizers at half strength twice as often. That encourages slower but stronger and more resilient growth.

Now back to our original question: What to feed tomatoes?

* Start by digging in well-aged compost or manure into the soil. That adds nitrogen and other nutrients as well as organic matter (important for retaining soil moisture).

* To each planting hole, add ¼ cup bone meal or rock phosphate. (That’s a good helping of phosphorus and will help that bush bloom sooner.)

* After transplanting, spread more well-aged compost around the young plants; it acts as mulch as well as adding more nutrients to the soil.

* Every four weeks or so, up until plants are setting fruit, apply a half-dose of balanced (same amount of N-P-K such as 6-6-6) fertilizer as a side dressing.

* Make sure plants are watered before adding fertilizer, then water again after application. That assures enough moisture in the root zone for the plant to absorb those nutrients.

* Plants on drip irrigation can be “fertigated” by adding liquid fertilizer to the drip system. That delivers water and nutrients directly where plants can use them most efficiently – at the roots.

* When fruit begins to form, sprinkle around the plant a little calcium (such as the aforementioned oyster shell).