Properly fertilization in the vegetable garden can mean the difference between a meager or an abundant crop of vegetables. The type and amount of vegetable garden fertilizer you'll use will depend on several different factors: the type of soil, types of plants being grown, and whether or not you want to use synthetic or organic fertilizers. Since the produce from my vegetable garden will end up on the dinner table, I always go the organic route. Why not?
Fertility requirements differ between growing seasons and among soil types. For example, tomatoes, beans and other vining vegetable plants generally require less fertilizer, root vegetable plants like larger amounts. If your soil is rich with organic matter plants may require little if any fertilizer. Organic matter, such as composted manures or your own home made compost, improves soil quality and helps release nitrogen and supply essential nutrients your plants will love. And they'll pay you back with more produce!
The two primary reasons for fertilizing vegetable plants are to encourage growth, and to create healthy, vigorous, and productive plants. That being said, there is often a temptation to over-fertilize in the hopes of producing an even healthier, larger and more productive plant. But be careful! If you force a plant beyond its natural growth rate by over-fertilizing, you can cause it to grow too quickly. This can result in structural problems, predispose the plant to insect or disease infestation, reduce tolerance to drought or temperature extremes, and less produce.
So, to grow healthy vegetable plants, where to start?
Evaluate Soil Conditions
Before fertilizing your vegetable plants, the best starting point is a soil test. Soil tests are done to determine soil pH and essential nutrient levels. Your local Extension Service might provide soil testing services or you can test soil yourself with a soil testing kit or soil pH testing probe.
Keep in mind that construction and many other factors often result in soils that differ from one landscape to the next, and even from one area in a landscape to another. In other words, the soil in your front yard may be much different than the soil in your backyard.
Once you know the soil pH you can either look for shrubs and trees that will grow well in it, or you can adjust the pH to meet the needs of specific plants.
Soil pH is Important!
Most vegetable plants like a slightly acid to neutral soil pH ranging from 6.0 to 7.0 on the pH scale. Irish potatoes are a notable exception with a desired pH of 5.0 to 5.5. Soil pH is a measurement of the alkalinity or acidity of soil and is measured on a scale of 1-14, with 7 as the neutral mark. Any measurement below 7 indicates acid soil conditions, and anything above 7 indicates alkaline.
If you're unsure about the pH of your soil, and whether or not it's suitable for growing vegetable plants, it's a good idea to test the soil pH in the planting area. You can quickly test soil pH with a inexpensive soil pH tester probe.
To raise the pH (make more alkaline) you can add pelletized limestone to the soil. To lower the pH (make more acid) you can apply Soil Sulfur, Aluminum Sulfate, or Chelated Iron. Adding organic compost to the soil or using compost as mulch can also help to increase acidity and maintain acid soil conditions.
Learn More: What is Soil pH and How To Adjust It?
Soil Nutrients
When selecting a fertilizer, the first question to answer is, "What type fertilizer do I need?" Determining what fertilizer to use can be an overwhelming task.
Which formulation do you need for your particular situation? Should you get the bag of 10-10-10 or 5-10-15, go with a specially formulated vegetable food, or maybe a natural or organic plant food? If you did a soil test, the fertilizer you choose will be based on the test results and/or the needs of your plants.
What do those three numbers on a package or bag of fertilizer mean anyway? It may seem a little confusing, but you can figure it out.
The analysis of fertilizer is the three large numbers you see on every fertilizer label, such as 10-10-10. These numbers represent the percentage (by weight) of the three major nutrients required for healthy plant growth. These three major elements are always in the same order: Nitrogen-Phosphorus-Potassium (N-P-K). Each of these nutrients affects plant growth differently.
The first number represents nitrogen (N). This element provides plants with the ability to produce more chlorophyll, which in turn sponsors more rapid growth to the part of a plant that is above the ground: the foliage. With each additional nitrogen application, plants should grow taller and greener. Typically, fertilizers that are high in nitrogen content are not recommended for use on most vegetable plants. That being said, it's good to know the needs of each type of vegetable plant you're growing. For example, when corn is knee-high, many farmers side dress the rows with ammonium nitrate (34-0-0).
The second number represents phosphorus (P). Phosphorus aids in root development and therefore can help to increase bloom production and therefore more fruit. If you have vegetable plants that aren't blooming as they should, or not at all, and your soil indicates a phosphorus deficiency, you might need to add phosphorus to the soil, such as Super Phosphate.
The third number represents potassium (K). Potassium has many functions. It guards the plant against diseases and aids in drought protection and cold tolerance. It also serves a role in improving root development, and helps in the process of photosynthesis.
You might have noticed that all the numbers in the fertilizer analysis don't add up to 100 percent. That's because there are other nutrients and fillers often added to the mix. Fillers might be included to help apply the nutrients more evenly over an area.
NOTE: The only true way to determine what fertilizer or nutrients are lacking in the soil is by way of a soil test. As mentioned, most states, and many counties, offer soil tests through their Cooperative Extension Service at no charge (or for a small fee). Or you can buy your own soil test kit. The results of a soil test will indicate nutrient levels in the soil and pH. This will help you choose the right fertilizer and other nutrients or minerals needed.
Fertilizing Instructions
Synthetic or Organic?
When it comes to feeding plants I plan on eating, I always go with organic fertilizers. There are several benefits from using organic fertilizers. Most organic fertilizers are mild and won't burn your plants. Most organic fertilizer won't cause undesirable accelerated growth. Vegetable plants roots can more readily absorb organic fertilizers and nutrients better than synthetics. So there's no waste. You may not need to apply near as much of an organic fertilizer as you would a synthetic. Likewise, the nutrients in the organic vegetables you grow will be much more available to your body than those which have been grown non-organically (with synthetic fertilizers and toxic chemicals).
No Soil Test?
In the absence of a soil test, I would recommend fertilizing with an organic vegetable food as directed on the product label. If you garden in a region where soils are typically acid (such as red clay) I would recommend applying a fast-acting, pelletized limestone at recommended rates on the bag. There are new, specially formulated limestone products that cover 5 times the area as standard pelletized lime.
How to Apply
Garden Beds
In the vegetable garden, I usually use a slow-release granular vegetable fertilizer and top dress with it. This is done by hand with granular fertilizers. You can either spread fertilizer down each row or around each plant, extending to the drip line (branch perimeter). Rake the fertilizer gently into the soil and water. Be careful not to add too much fertilizer as this can be more harmful than too little. Excess fertilizer accumulates as salt in the soil and causes damage to plant roots.
Container Vegetable Gardens
In container gardens, I usually go with a water-soluble plant food or a slow-release organic plant food. Follow application instructions on the product label.
When to Apply
The following guidelines apply to the use of granular fertilizers. When using an organic vegetable plant food, you can fertilize plants at planting time. When in doubt, follow application instructions on the product label.
Beans, green: Not necessary.
Beets: When tops are 4 to 5 inches high. Go light on nitrogen.
Broccoli: Three weeks after transplant. Go light on nitrogen.
Cabbage: Four to six weeks after transplant.
Carrots: Three weeks after plants are well established.
Cauliflower: Four to six weeks after transplant.
Corn: Three weeks; again at 10 inches with nitrogen; lastly when tassels appear.
Cucumbers: When they first begin to run; again when blossoms set.
Eggplant: Three weeks after planting.
Lettuce: Three weeks after transplant; again when head forms
Muskmelons: When they begin to run; a week after blossom; 3 weeks after that.
Onions: Three weeks after planting; again when tops are 6 to 8" inches.
Peas: Not necessary.
Peppers: Three weeks after transplant; again after first fruit set.
Potatoes: When plants blossom.
Pumpkins: When plants start to run; again at blossom set.
Radishes: Not necessary.
Spinach: When plants are 3 to 4" inches tall.
Squash, summer: When plants are 6" tall; Again when they bloom.
Squash, winter: When plants start to run; again at blossom set.
Tomatoes: 2 weeks after transplant; after first picking. Go light on nitrogen.
Mulching The Vegetable Garden
Mulching serves several purposes in vegetable gardening including reducing weed growth, conserving soil moisture and nutrients, regulating soil temperature, helping prevent soil erosion, and reducing water splash on plants, which keeps them cleaner and reduces the spread of disease.
Too, as it decomposes, mulch increases the amount of organic matter in the soil that your vegetable plants will love. Almost any organic matter can be used successfully as mulch. This can include things such as old hay, straw (I use wheat straw), leaves, sawdust, paper, or bark. Just make sure that if you use a wood mulch that it's been aged in a pile for at least 1 year. Never use fresh saw dust or wood mulch.
Some precautions...
- Mulches should not be applied too early in the spring because this can delay soil warming. Wait until the soil has warmed up to above 60 degrees F.
- Avoid materials that may have a lot of seed such as fresh-cut hay or overgrown grass clippings.
- Fresh material, particularly sawdust, can rob your soil and thus your plants of nitrogen, so make sure it's aged.
- Avoid material that may be contaminated with toxic chemicals or herbicides because these may damage your plants and end up in the food you put on the table.
- If you use solid materials such as newspapers these should be weighted with soil to prevent them from blowing away.
- Avoid putting the mulch too thick. Apply mulch at a maximum of 2 inches thick. Any thicker could prohibit water or applied nutrients from reaching roots.
- Keep all mulches 2 to 3 inches away from the stems of plants.
- If at all possible it's best to do a soil test to determine nutrient deficiencies and soil pH.
How To Water Vegetable Plants
There are many, many types of vegetables, so, without going into details about each type, here's some general guidelines and tips for watering.
Most common vegetable plants like a consistently moist or damp soil, but not one that is constantly soggy or wet, which can lead to harmful plant diseases and often death of the plant.
I often use observation to determine whether my vegetable plants need water. If the leaves of a plant start to wilt, I check the soil moisture. If during the heat of summer wilting occurs while soil is still moist, it is a sign that there might be root damage from too much water previously. So make sure to check soil moisture before watering. The plants I have growing in pots wilt more often than those growing in garden beds, which is common. If plants are not wilted chances are there is still sufficient moisture in the soil.
If your vegetable plants growing in pots are wilting too much and require more frequent watering than you can handle, consider moving them to a location where they'll get some filtered sun during the hottest part of the day. Or arrange some taller plants around them to provide some shade. Keep in mind though that most vegetables like a lot of sun. 6 to 7 hours of direct sunlight per day is recommended.
Too much direct sun on some types of vegetable plants growing in pots can cause root temperatures to rise to a level where metabolism slows dramatically and water movement to the rest of the plant is seriously impeded. Growing plants in light colored containers can help keep soil temperatures down. Plants growing in cooler ground temperatures usually don't have this problem.
In general, whether you're watering vegetable plants in garden beds or containers, if you keep an eye on your plants, and use the finger-test to check the soil to a depth of at least a couple inches each time before you water, you'll soon develop a feel for when water is needed. If the soil is dry, provide water. If moist, leave the plant alone...even if the leaves are wilting.
Err on the side of watering less, not too much, as it's much easier to kill plants by overwatering than by underwatering.