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  • Jere Folgert

Optimize Fertilizer Applications For Garlic!

Updated: Jan 31

Garlic plants have specific needs - light, air, water, space to grow, and essential nutrients - to survive and reproduce. Let's focus on the nutrients garlic plants need to thrive. There are 17 essential nutrients that all plants need, including nitrogen, phosphorus, potassium, calcium, magnesium, sulfur, boron, chlorine, iron, manganese, zinc, copper, molybdenum, and nickel. The non-mineral essential plant elements include hydrogen, oxygen, and carbon. These seventeen essential nutrients are taken up by plants from air and water. Nitrogen, phosphorus, and potassium (NPK) are needed in larger amounts than other nutrients; they are considered primary macronutrients. Secondary macronutrients include sulfur, calcium, and magnesium. Micronutrients such as iron and copper are necessary in much smaller amounts. With the right amount of nutrients, your garlic crop can thrive and provide pounds of produce for harvest.

Phosphorus and zinc fertilizers are rarely required for garlic crops. Only moderate amounts of nitrogen are needed for top yields. (Source: the University of California Agriculture.

How do we, as garlic growers, optimize fertilizer applications so our garlic will thrive and produce large garlic bulbs? Here are a few ways to ensure optimum nutrients in the field and garden.

TEST SOILS OFTEN. Our First Task in optimizing fertilizer applications is to perform a soil test. The soil test should be performed at least once each year. The Soil Test is important for several reasons: to optimize crop production, to protect the environment from contamination by runoff and leaching of excess fertilizers, to aid in the diagnosis of plant culture problems, to improve the nutritional balance of the growing media and to save money and conserve energy by applying only the amount of fertilizer needed for your garlic crop. Determining the pH and fertility level through a soil test is the first step in planning a sound nutrient management program. The fertilizer you choose should be based primarily on soil test results and garlic plant needs, both in terms of nutrients and speed of delivery.

KNOW YOUR SOIL TYPE. We need to understand how our soil type, soil texture and soil pH can determine how nutrients are taken up by the garlic plants. Some nutrients such as phosphorus (P) moves through the soil slowly in the range of a few millimeters each year.

Nutrient availability: Every part of the country has unique soil conditions. Some soils have adequate calcium, magnesium, sulfur and micronutrients to support healthy garlic plant growth. Nitrogen, phosphorus, and potassium are the nutrients most likely to be deficient in soils and should be supplemented with fertilizers for optimum plant growth. The best method for determining what nutrients are needed in your garden is to do a soil test. A basic soil test can be ordered on-line. If possible, check with a soil lab at the University in your state. At GROeat Farm, we have used Agvise Laboratoires located in North Dakota and Minnesota. Other mail-in laboratories include groSMART, and Western Materials Testing. The analysis will provide a basic interpretation of the results and provide recommendations for fertilizing.

Soil Texture: Nutrient availability in soils is a function of many factors including soil texture (clay, loam, loamy sand, silt loam), organic matter content and pH. Clay particles and organic matter in soils will hold and slowly release nutrient ions that can be used by garlic plants. For example, sandy soils with little or no clay or organic matter have less nutrient-holding ability than soils that are finer-textured (more clay) and higher in organic matter.

pH: Soil pH is the level of alkalinity or acidity of soils. When pH is too low or too high, the chemical reactions occurring in the soil can reduce the nutrient availability and biological activity in soils. Most hardneck garlic plants grow best when the soil pH is slightly acidic to neutral, or between 6.0 and 7.0.

MATCH FERTILIZER APPLICATION BASED ON SOIL TEST AND CROP. There are many options for fertilizers and sometimes the choices may seem overwhelming. If you paid for a soil test, many laboratories will recommend a fertilizer application based on your crop and the results of the soil test. When submitting soil to be tested, be sure to let them know you are growing garlic. The most important thing to remember is that plants take up nutrients in the form of ions, and the source of those ions is not a factor in plant nutrition. Garlic plants take up nitrogen compounds through their roots. When plants and animals die or when animals excrete wastes, the nitrogen compounds in the organic matter re-enter the soil where they are broken down by microorganisms, known as decomposers. Plants get nitrogen via NO3- (nitrate) or NH4+ (ammonium), and those ions can come from either organic or synthetic sources and in various formulations (liquid, granular, pellets or compost). All commercially available fertilizers have what is called a guaranteed analysis, meaning the percent of each major nutrient, by weight, in the fertilizer must be listed on the package as N-P-K. For example, 10 pounds of 20-18-17 garden fertilizer contains 2.0 pounds of nitrogen, 1.8 pounds of phosphorus oxide (P2O5 – phosphate), and 1.7 pounds of potassium oxide (K2O – potash). If you cannot find a fertilizer with the exact N-P-K ratio that is recommended by your soil test report, you should select a product with a ratio that most closely matches. It’s more important to match the nitrogen recommendation than the phosphorus or potassium recommendation, but try not to exceed the phosphorus recommendation because of water quality concerns.

If Our Garlic Plants Lack Certain Nutrients, What Will I See? Plants lacking nitrogen will likely show yellowing on older, lower leaves. Too much nitrogen can cause excessive leafy growth and smaller garlic bulbs. We do not want our garlic to be as tall as a corn plant! Because nitrogen promotes leafy growth, too much nitrogen late in the season can minimize bulb creation. Nitrogen is considered to be mobile in the soil, meaning that it moves with water, so it’s best to apply nitrogen when plants will be ready to use it. Plants lacking phosphorus may show stunted growth or a reddish-purple tint in leaf tissue. A potassium deficiency can cause browning of leaf tissue along the leaf edges, starting with lower, older leaves. Phosphorus and potassium are not mobile in the soil and need close contact with plant roots in order to be taken up. They should be applied and incorporated prior to planting, either in spring or fall. Among other things, phosphorus supports root growth, while potassium supports disease resistance and plant hardiness. A calcium deficiency often leads to “tip burn” on younger leaves. However, calcium deficiencies are often not a result of low calcium in the soil, but are caused by uneven watering, excessive soil moisture, or damage to roots. Lack of sulfur on sandy soils can cause stunted, spindly growth and yellowing leaves. One of the most important active compounds in garlic is alliin. Sulfur (S) fertilization was shown to significantly increase the alliin concentration in garlic cloves, while high nitrogen (N) levels had an adverse effect. The two forms of Sulfur include sulfate S (SO₄²⁻) and elemental S (S⁰). Sulfate sulfur is available immediately to the crop while S⁰ oxidizes for late-season sulfur uptake by the plants. Plants can absorb S only through their root systems in the SO₄²⁻ form. Too much sulfur can be toxic to garlic plants, so don't add too much. Sulfur Toxicity in Plants results when too much sulfur is in the soil. Sometimes, high concentrations of sulfur can cause an increase in salts. This can accumulate and cause your plants to become stunted and dark in color. Excess sulfur in the soil that creates fertilizer salts can also burn the roots of the plants.

Phosphorus and zinc fertilizers are rarely required for garlic crops. Only moderate amounts of nitrogen are needed for top yields. (Source: the University of California Agriculture.

USE THE 4 "Rs" AS A GUIDE. These include "Right Source", "Right Rate", "Right Time", and "Right Placement". The results of your soil test should provide you with simple guidance on the rate, timing, source, and placement of any required fertilizers.

ADD ORGANIC MATTER TO YOUR SOIL. Regardless of which fertilizers you choose. Adding organic materials to your soil can most certainly result in bigger garlic. Incorporating organic materials can actually reduce the need for supplemental fertilizers. Organic matter such as compost support overall soil health. Carbon-based materials such as organic fertilizers, compost, leaves, grass clippings or cover crops provide a range of benefits to both plants and soil. One of the reasons we add organic materials to soils is because they contain other essential plant nutrients in addition to N, P and K. Organic materials provide food to feed soil microorganisms, which also increases soil organic matter. Adding organic materials to soil also helps the soil retain moisture and facilitates drainage and aeration in many soil types. In simple terms, you want to feed your garlic plants, but you also want to make sure that the microbiome, those tiny little critters living in your soil, are happy and have a reason to party. Building healthy soil solves many parts of the equation.

In conclusion, as garlic growers, we should optimize fertilizer applications so our garlic will thrive. We begin by understanding that we need to use fertilizer wisely: Too much fertilizer is not only a waste of money, but it can damage plants and harm the environment.

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