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  • M'Li Hoki

Soil Fertility

Soil fertility is very important in maintaining healthy plants. Utah soils generally contain all the nutrients plants need, but occasionally there can be a nutrient deficiency. Today we will cover the nutrients that are the most important to landscape plants and vegetables and how to diagnose deficiencies.

Plants need many nutrients and minerals. Some are classified as macro-nutrients and some are micro-nutrients, and some are nutrients that are supplied by our air and water. Carbon, oxygen and hydrogen are naturally available through air and water. Macro-nutrients are elements that naturally occur in nature and are very important to plant health. They are not more important to plant health than micro-nutrients, they are just needed in higher quantities. Here are the 3 main macro-nutrients to pay attention to.

Nitrogen (N)

1. The first macro-nutrient we will discuss is nitrogen. Plants use nitrogen by absorbing it through their roots in the form of nitrates or ammonium. Most of the nitrogen is used by the plant to produce proteins (in the form of enzymes) and nucleic acids. It is important for growth, dark green leaves and photosynthesis. Nitrogen is soluble in water, can leach out of the soil easily and is readily used by plants so it needs to be replaced regularly. There are both chemical and organic sources of nitrogen and knowing which to choose depends on the speed of results needed. Organic sources of nitrogen need to be broken down into the same form of nitrogen as the chemical forms by soil microbes so the plants can use it. This process takes time, so the results are seen over a longer period. Chemical sources of nitrogen are already in a form that plants can easily take up so you will see the results faster. Chemical fertilizers can have both slow release and quick release forms depending on your needs. Some good sources of nitrogen are compost, urea, ammonium sulfate, blood meal, nitrogen fixing legumes (after they die or are cut back and can release their stored nitrogen), complete fertilizer blends and manures (be careful of adding too much because of salts and extra phosphorus).

Nitrogen is readily transported through the plant from older tissue to younger tissue. A plant deficient in nitrogen will show yellowing in the older leaves first and will have smaller leaves due to poor growth.

Phosphorus (P)

1. The next macro-nutrient is phosphorous. Phosphorous promotes root, fruit and seed production and is usually abundant in Utah soils. When soils become heavy, cool and wet, phosphorous is less available for plants to use. It is helpful to add phosphorus when planting annuals in the spring and when fertilizing potted plants. Perennials, trees and shrubs can usually access the phosphorus available in our soils so there is no need to add additional phosphorus in those cases.Some good sources of phosphorus are compost, complete fertilizer blends, phosphate, rock dust and bone meal.

Phosphorus deficiency is a little more difficult to diagnose as sometimes the only symptom is a general stunting of the early growth. Some plants, like corn, will develop a purplish hue to their leaves. In more mature plants, phosphorus deficiency can affect the development of seeds.

Although rare, phosphorus can accumulate in the soil enough to reach toxic levels. That usually happens when too much manure-based compost is used on a yearly basis. Phosphorus does not readily move through soil like nitrogen does and can accumulate to toxic levels. One way to avoid this issue is to only add a couple of inches of compost per year and not use purely manure based mixes. Here are a couple of good articles discussing this issue.

Potassium (K)

1. Potassium is important to the growth of plants in many ways. One of the ways it helps plants is by regulating water and CO2 uptake and release. Potassium is widely available in Utah soils and water, but soils with high amounts of sand may need extra sources of potassium as it can easily be leached out. Clay soils hold potassium ions tightly and it does not easily move through those types of soils. Good sources of potassium are compost, complete fertilizer blends, kelp meal, and potash. Wood ash is a good source of potassium but also increases the alkalinity of soils, so it is not good for Utah soils. Adding a small amount of wood ash to compost will not hurt but be careful not to overdo it. A couple of shovelfuls in a pile of compost is plenty. Potassium can also increase to toxic levels so be careful of adding too much to your soil.

Potassium deficiency shows up as brown, curling leaf tips. It can also cause yellowing between leaf veins.

Calcium (Ca)

1. Calcium is important to the development of cell walls and membranes. It also helps with the uptake of other nutrients and plays an important role in the maturation of fruit. Plants move water from the soil to the top of the plant through a process called transpiration. This means they take up water from the soil through the roots and move it to the top of the plant by pressure which is created when the moisture is released from the leaves into the atmosphere through holes in the leaves called stomata. Calcium is only upwardly mobile in plants through the transpiration process. Once the plant takes the calcium from the soil and moves it up into its leaves it will not move to other parts of the plant. For that reason, calcium needs to be continuously available. Anything that interferes with the transpiration process will cause calcium deficiencies. In Utah calcium is abundant in our soils, but can become unavailable to plants through drought, uneven watering, too much water or saline soils. Sometimes calcium is in a form that plants can’t use and that will also cause calcium deficiency.

Plants can’t move calcium from older parts of the plant to newer parts so you will first see deficiencies in the new leaves. It can also show up as stunted roots. Some signs of calcium deficiency are stunted growth, brown spots on the leave, curling tips on young leaves, blossom end rot in tomatoes and death of root tips.

Ways to avoid the signs of calcium deficiency in Utah include correct watering practices, use mulch to keep even moisture in the soil, and do not over fertilize as this can decrease the availability of calcium and other plant nutrients.

Magnesium (Mg)

1. Magnesium is important in photosynthesis, metabolizing carbohydrates and stabilizing cell membranes. It is abundant in Utah soil’s, so it is not necessary to add more. If you feel your soil is deficient in magnesium or your soil test indicate you need to add more, some good sources of magnesium are chelated micro-nutrient fertilizers, cottonseed meal, and Epson salts.

Magnesium deficiency shows up as yellowing between the veins on leaves where the veins stay green.

Sulfur (S)

1. Sulfur has several uses in the garden. First it helps in the formation of proteins. It also is converted to sulfuric acid by soil bacteria and can help lower the PH of the soil. Utah soils generally have a high PH so sulfur can be used to temporarily lower the PH in the area it is applied. Our water also has a high PH which will eventually counteract the effects of the sulfur. Sources of sulfur are complete fertilizers and elemental sulfur. Very small amounts of sulfur can be added to the soil by the addition of organic matter.

Sulfur deficiency shows up as an over all yellowing of the plant. It shows up the most in young leaves where the leaves start to die off at the tips.

Macro-nutrients are just as important to plant health, but they are not needed in high quantities. Some of the important macro-nutrients are:

Boron (B)

Chlorine (Cl)

Copper (Cu)

Iron (Fe)

Manganese (Mn)

Molybdenum (Mo)

Zinc (Zn)

Utah soils generally have sufficient amounts of most of these micro-nutrients and they will not be discussed here except for iron. Iron can pose special problems in Utah.


1. Iron is a micro-nutrient that is found in abundance in Utah soils. Despite its abundance iron chlorosis is a regular problem that Utah gardeners face. Utah soils are alkaline and that tends to keep iron from being converted into the form that plants can use. Soils that are low in oxygen, such as water logged or clay soils can also keep iron from being converted into usable forms. This creates iron deficiency in plants called iron chlorosis. Plants that suffer from iron chlorosis have yellow leaves with green veins. When there are advanced stages of chlorosis, the leaves will turn almost white.

Trees found in Utah that tend to suffer the most from iron chlorosis are Silver Maples, Amur Maples, Maples from the rubrum family such as October Glory and Autumn Blaze. There are a whole host of other plants that suffer from iron chlorosis. Most of them are plants that are adapted to the more acidic soils found in areas of the country with more acidic soils and higher amounts of rainfall.

Ways to help counteract the effects of iron chlorosis are planting iron efficient plants (plants that can tolerate alkaline soils and heavier soils), amending with compost, mulching, proper watering, fertilizer blends with iron that is in a form more available to plants and adding elemental sulfur. Adding sulfur to the soil will lower the PH for a short time and only in the area that the sulfur was added. It will need to be reapplied on a yearly basis. It will also be counteracted by Utah’s alkaline water so it will not make plants that are not appropriate for our area grow here in Utah.

If you want to learn more about your own soil, you can send a soil sample to either the USU or BYU soil labs. They will analyze your soil and let you know your soil’s PH and if you are lacking in any nutrients.

If you have any questions that you can’t find the answer for, call us and we will be happy to help.

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