Fertilizer Formulations

— Written By Kate Holt
*Examples of soluble fertilizers

*Examples of soluble fertilizers. -Rayburn

In my last newsletter, I discussed our native soils and how they interact with three of the plant nutrients needed in the highest levels by plants. This month, I will cover fertilizer formulations and how they behave in the soil.

Fertilizers are available in three forms: soluble, granular and slow release. Each form has its’ own particular benefits and place in the garden.

Soluble fertilizers may come as either a liquid that can be diluted or as a crystal that is dissolved in water. Most readers will be familiar with the blue crystals of Miracle-Gro fertilizer that are mixed to make a fertilizer solution. There are organic versions of soluble fertilizers as well – fish emulsion and compost tea are two examples (although it is worth noting that these solutions have a very low concentration of fertilizer nutrients when compared to other fertilizers). Whatever the source, soluble fertilizers make nutrients quickly available to the plant. This makes them a great choice early in the growing season or to provide undernourished plants with a quick boost. However, these nutrients are also susceptible to leaching and can wash out of sandy soils or plant containers very quickly. Soluble fertilizers usually need to be applied every couple of weeks if they are used as the only source of fertility in a sandy soil or containers.

Granular fertilizers are applied before planting time and tilled in or may be applied alongside the growing plant. When irrigation or rainwater dissolves the granular fertilizer, the nutrients are made available to the plant. Granular fertilizers are a good option on lawns when growing vegetables in the ground. They are also a good option on heavier soils that contain a lot of organic matter or clay. Organic matter and clay both improve the ability of the soil to hold on to plant nutrients and prevent leaching. Granular fertilizers are usually the cheapest source of plant available nutrients based on the price per pound of the nutrients that you are applying. Depending on the crop being grown, granular fertilizers may be applied 1-4 times during the growing season, at approximately one-month intervals.

Slow release fertilizers have a special coating that is designed to release the nutrients over a given period of time. Common formulations release over 2-3 months. Slow release fertilizers are a good option for sandy soils, raised bed gardens and in containers. Slow release fertilizers are often more expensive than granular fertilizers but for some gardeners, this is offset by the ease of use and the fact that one application will often carry you through an entire growing season.

The fertilizer formulation you choose will be based on your values – cost, ease of use, and what fits best in your gardening system. However, there are some general guidelines that will help guide your decisions. Of course, I always recommend soil sampling every 2-3 years and following the specific recommendations provided in your soil sample report. This is the only way to know for sure what the current nutritional status of your site is and what fertilizer ratio will best meet the needs of the crop that you are growing.

Your soil sample report will provide a fertilizer and, if needed, lime based on pounds of product needed per 1000 square feet. You will need to measure your garden or lawn and calculate how many square feet you are treating. At that point, you will scale the amount of fertilizer up or down accordingly.

Vegetables grown in the ground

For larger vegetable gardens grown in the ground, granular fertilizers offer the most cost-effective form of nutrient application. Following your soil sample report, spread the recommended amount of granular fertilizer uniformly over your garden plot. Incorporate it 4-6 inches into the soil before seeding or transplanting. This reduces the potential for salt injury to germinating seeds or young transplants. Fertilizers are salts and applying too much can damage plants. Certain vegetable crops require additional nitrogen during the growing season.

Sidedress the extra nitrogen at the rates and times specified below:

Crop Pounds of Nitrogen/1000 sq. ft. Number of Applications Timing
Tomatoes 0.5-1 2 Monthly intervals after 1st bloom
Potatoes 1.5-2 1 1 month after emergence
Sweet corn 1.5-2 1 1 month after emergence
Cabbage 0.5-1 1 1 month after transplanting
Squash 0.5-1 1 After emergence
Okra 0.5-1 1 When plants are 2 ft high
Beans 0.5-1 1 1 month after emergence
Peppers 0.5-1 1 1 month after transplanting

These recommendations are based on pounds of actual nitrogen, not pounds of total fertilizer. The amount of fertilizer you need will depend on the fertilizer analysis. Common sources of nitrogen include calcium nitrate, ammonium sulfate, and urea.

If you are using calcium nitrate, 7 pounds of fertilizer per 1000 sq. ft. will provide 1 pound of actual nitrogen per 1000 sq. ft. If you scale it down, 2 pounds of calcium nitrate per 100 ft of row will provide you with the equivalent amount of nitrogen. If you are using ammonium sulfate or urea, these fertilizers are a more concentrated source of nitrogen so you would only need 3 pounds of fertilizer to provide 1 pound of actual nitrogen per 1000 sq. ft. or only 1 pound of fertilizer for 100 ft of row.

Fertilizer % Nitrogen Pounds of fertilizer per 1000 sq. ft. (to provide 1# N/1000 sq. ft.) Pounds of fertilizer per 100 ft of row (to provide 1# N/1000 sq. ft.)
Calcium nitrate 15.5 7 2
Ammonium sulfate 34 3 1
Urea 34 3 1
10-10-10 10 10 3
15-0-15 15 7 2
Blood meal 12 8 3

Be careful as it is easier to burn plants with more concentrated fertilizers. Make sure that plant leaves are dry, apply fertilizer to the soil surface, brush any stray fertilizer off the plant leaf and crown and irrigate following application to ensure movement of nutrients into the root zone.

Fertilizing vegetables grown in containers

Vegetables grown in containers have a smaller soil volume to root through. Combine this with the fact that potting mix may not hold on to nutrients as well as mineral soil and more frequent watering can result in nutrient deficient plants pretty quickly. Slow-release fertilizers are a good option in containers. Apply a slow-release fertilizer at the rate recommended on the label for the size container and crop that you are growing. Short season crops (harvested within 90 days or so of planting) may only need one application (depending on the formulation selected), while longer season crops (like indeterminant tomatoes) may need a second application to get them through the growing season.

Fertilizing vegetables in raised beds  

Depending on the size of the bed and the media used to fill the bed, you can use granular or slow-release fertilizers. If the bed consists mainly of actual mineral soil (topsoil from your site or topsoil that you bought in), granular fertilizers will work well, particularly if the soil has a good amount of clay or organic matter in it. Beds that are comprised mostly of manufactured topsoil blends (the sort of topsoil that you might buy in a bag or by the scoop at the garden center) or bagged garden soil will often do better if fertilized with a slow release fertilizer. These mixes do not hold on to nutrients as well and granular fertilizers may be quickly leached through the bed.

Fertilizing hanging baskets and flowering containers

During production, hanging baskets and flowering containers are usually fertilized with a soluble fertilizer injected through the irrigation system. Once these plants have been out of the greenhouse for a couple of weeks, they will be eager for some additional fertility. Use a soluble fertilizer every 2-3 weeks or apply a slow release fertilizer at the rate recommended for the size container you have the plants in.

* Use of brand names are for educational purposes and not intended for product endorsement.