Restoring Soil Nutrients
Here are twelve critical soil nutrients your plants need for healthy growth and how to supply them.
By Jim Hildreth
Your garden will thrive with the right balance of soil nutrients.
Summer puts soil fertility to the test. Intensive plantings of warm-weather crops remove nutrients at a fast clip, and when plants hunger, you should be the first to know it’s feeding time. Replacing soil nutrients keeps plants happy right up to the harvest.
I keep a watchful eye on plants when the weather becomes hot. Anything less than vigorous, lush growth raises suspicion and when leaves become discolored or misshapen, I take action. A first step might be a general feeding by scratching leaf mold into the soil. Summer is a bit early for gathering leaf mold, but there’s usually some available and it’s loaded with earthworms that further enrich the soil. Then I gather all the compost I can spare, and encircle each plant with a few inches of mulch. Finally, I spray all leaf surfaces with a solution of fish emulsion and liquid seaweed. This general feeding often helps the plants through the summer, but if problems persist, I look for specific nutritional deficiencies.
The Big Six
Abundant nitrogen immediately comes to mind when thinking of fertile soil. It’s the green giant your plants need for lush, sturdy growth. It forms plant proteins and it’s probably the nutrient most familiar to gardeners. Nitrogen deficiencies cause leaves to lose their healthy, green color, and this chlorosis (yellowing) usually begins near the base of plants. As chlorosis works its way up, the plants appear weak and spindly.
Supplying soil and plants with nitrogen supplements presents no difficulties. Give plants a foliar feeding of fish emulsion and a root feeding of dried blood, rabbit manure, cottonseed meal, or a commercially prepared organic fertilizer for an immediate pick-me-up. Compost enriched with manures, tea and coffee grounds, feathers, garden wastes, and kitchen wastes makes an excellent long-term source of nitrogen.
Too much nitrogen is as bad as too little. The most obvious signs of excessive nitrogen include plants with weak, water-filled tissues and stems that break easily during windy conditions. Aphid infestation is another symptom. Studies show that these pesky insects prefer to feed on plants that receive excessive nitrogen.
The P of the N-P-K formulation, phosphorus, proves critical to photosynthesis, plant maturity, healthy roots, and energy transfers within plants, but it’s difficult to apply effectively in a soluble form. Elements with charges opposite that of phosphorus, such as calcium and iron, capture the nutrient and render it unavailable to plants. Remedy the problem by spreading phosphate rock on the soil. It’s barely soluble, remaining stable until soil organisms and plant roots release it in a form that plants use.
Rock phosphate should also be supplied to the compost pile. Sprinkle it lightly whenever adding raw materials. Compost receives phosphorus from those items, too. Wood ashes, bone meal, citrus wastes, cottonseed meal, manures, fish wastes, and dried blood all supply phosphorus.
Watch for purple leaves, veins, and stems. They strongly indicate a phosphorus-deficiency. Leaves of corn and white pea beans turn yellow at blossoming time when they don’t receive enough phosphorus. The deficiency also causes the undersides of tomato leaves to turn purple. Radish leaves develop a reddish color on the undersides, and they yellow progressively during maturity. That condition may appear to be caused by a lack of nitrogen, but if kernels don’t fill the rows or ear tips and leaves yellow, read the problem as a lack of phosphorus.
Potassium is the familiar K of the N-P-K trio. It remains in solution after being absorbed and flows through plants completing several important functions necessary for good health: (1) It helps the manufacture of sugars and their movement within plants, which has a direct bearing on a plant’s ability to resist diseases. (2) It adjusts the openings of leaf pores (stomata) to make them open widely when moisture is available and close tightly during drought.
Potash (potassium compound) availability (3) increases photosynthesis by increasing the amount of chlorophyll in leaves. That results in plants that better utilize available light. Your plantings in partial and full shade are more apt to thrive in a potassium-rich environment.
Plant leaves provide some of the most reliable indications of potash deficiencies. Their edges will yellow, turn brown, and curl. Corn leaves show yellow streaks between veins and brown edges and tips develop. The spaces between leaf nodes will also grow unusually small, giving the leaves an appearance of being tightly packed. Curled carrot leaves indicate a potassium deficiency, especially when a below-ground inspection reveals stunted, misshapen carrots that have an “off” flavor. The carrots may be long but of a small diameter.
Excellent natural sources of potash for a summer quick fix include a foliar feeding of fish emulsion and liquid seaweed and root feeding with wood ashes, well-rotted cow manure, cottonseed meal, aged-poultry manure, and compost enriched with corn stalks. Granite dust and glauconite (greensand) provide potash over the long haul.
Calcium, Magnesium, & Sulfur
Calcium, magnesium, and sulfur are often thought of as secondary elements. Plants and soil may not need large supplies of these nutrients, but the roles they play prove as essential to growth as those of nitrogen, phosphorus, and potassium.
Calcium serves several critical functions and concentrates primarily in leaves. It builds the cell walls of plant tissues and neutralizes acids produced by plants as toxic by-products of metabolism. It regulates the availability of other nutrients, builds plant proteins, and prevents magnesium toxicity.
Your tomato plant’s upper leaves yellow from a calcium deficiency. That symptom differs from a lack of nitrogen, which causes yellowing of lower leaves. Stems of tomato plants that lack calcium feel soft to the touch, and fruits exhibit blossom-end rot. The roots grow short and display a brown rather than a fleshy color.
Red patches appear on pea leaves when calcium levels are inadequate. The patches appear near the centers of the leaves and eventually spread to the outer margins. The plants lack vigor and appear dwarfed.
Limestone is the most widely used calcium-rich material. When purchasing limestone, shop for dolomitic limestone because it contains magnesium as well as calcium. Bone meal, oyster shells, wood ashes, and compost are other natural sources, the latter two materials supplying it the fastest.
The next secondary element that you should familiarize yourself with is magnesium. Photosynthesis would be impossible without magnesium because chlorophyll depends on its presence. Magnesium aids in a plant’s use of other nutrients, including nitrogen, phosphorus, and sulfur. What may appear as a phosphorus deficiency, for example, may be a magnesium deficiency that results in a plant’s inability to absorb phosphorus.
Lack of magnesium is characterized by discoloration of leaf tissue between veins, which remain green. The leaf tissues gradually turn yellow, often beginning at leaf margins, which gives them a streaked appearance. The leaves of some plants turn a reddish or purple color while the veins remain green.
Squash leaves, cucumbers, carrots, and lima bean leaves mottle and brown when the plant’s supply of magnesium proves inadequate. Corn leaves show yellow and/or white stripes; turnip leaves form brown margins and yellow mottling on inner surfaces; tomato leaves develop a yellow color that darkens away from veins, and the leaves become brittle and turn upward. Once a magnesium deficiency has been determined, it’s easily rectified by applying dolomitic limestone. The magnesium in the limestone’s dolomitic component is all that’s needed to put sufficient amounts of the element back into the soil.
Sulfur is the third secondary element needed for healthy plant growth. It helps plants produce proteins and enzymes, but there’s an inherent problem with retaining sulfur, especially in sandy soil. Unlike many other elements, sulfur isn’t readily held by soil particles. Instead it leaches past the root zone where, in time, it becomes unavailable to plants. The high porosity and low organic content of sandy soil amplify the problem.
Chlorosis is the primary result of sulfur deficiencies. The yellowing caused by a lack of sulfur differs from that of a nitrogen deficiency. The entire leaf doesn’t dry and become brittle as when lacking nitrogen. The soil’s need for sulfur strikes your legumes first because of those plants’ great demand for it. The typical yellow leaves develop, increase in numbers, and growth becomes noticeably interrupted. Yellowing cabbage leaves is another good indicator of insufficient sulfur, cabbage being another crop heavily dependent upon adequate supplies.
Don’t be quick to suspect a sulfur deficiency in the soil, although it’s never out of the question. The fact is that sulfur finds its way into the soil by other means. Acid rain sometimes contains enough sulfur to negate any deficiencies in the soil. Water supplies, too, often contain sulfur in quantities sufficient to supply the small amounts needed by plants when watering.
If a soil test indicates a sulfur deficiency, purchase sulfate of potash-magnesia. Containing approximately 27% sulfur, the product is derived from ancient marine deposits.
Nine nutritional elements exist as trace elements, and plants need them only in minute amounts. Oxygen, carbon, and hydrogen come from air. That leaves the necessary six: zinc, boron, manganese, iron, copper, and molybdenum.
Zinc is a catalyst that aids in several chemical reactions in plants, including the creation of amino acids. Zinc deficiency results in tomato leaves that appear smaller than usual. They turn yellow and mottled, with scattered dead spots, and leaf internodes grow shorter than normal. On corn plants, older leaves lose their color prematurely.
Zinc shortages can occur in most soils, acidic and alkaline. Manures and compost easily remedy zinc deficiencies, although ground, raw, phosphate rock contains traces of zinc, too, as does seaweed.
Boron is a trace element responsible for at least sixteen plant functions. It influences cell development and division, fruiting, flowering, stem growth, and a host of other plant activities.
Lack of boron causes a variety of slow, irregular growth habits such as dwarfing and bushiness because the growing plant tips die, which encourages lower leaves to sprout. Tomato stems blacken at the tips, and the young leaves on main stems yellow, then die quickly. Top growth halts and side growth replaces it, giving the plant a dwarfed, bushy appearance. Boron deficiency leads to blackheart disease in many root crops and, although phosphorus or potassium can cause it, the disease’s presence in beets and turnips indicates a lack of boron.
Correct a boron deficiency with compost that received phosphate rock, seaweed, or small amounts of sawdust, plenty of oak leaves, peat moss, or other acidic organic material. A liquid seaweed foliar feeding provides immediate results.
A manganese shortage causes plant disorders with symptoms such as chlorosis that may appear to be caused by shortages of other nutrients. Manganese differs from most trace elements in that heavy applications of organic matter into the soil does not assure its availability. Alkaline soil can halt the nutrient’s availability, making increased soil acidity a goal when manganese deficiency is suspected. Organic matter (especially seaweed) supplies all of the manganese required by plants, but a correct soil pH ensures that plants will use it.
When the lush green color of spinach changes to gold with white, dead areas, suspect a shortage of manganese. Tomato leaves turn light green with yellow showing farthest from main veins. The yellow areas develop white dead spots similar to those on spinach leaves. The entire plant dwarfs and lacks blossoms and fruits.
The leaves of snap beans turn yellow when soil lacks manganese and brown spots often appear between leaf veins. The stems of cucumbers, cabbage, broccoli, cauliflower, peppers, and eggplant grow small and spindly from a manganese deficiency. The leaves gradually turn yellow, then white, but veins remain green (an easily recognized symptom).
Carrot tops are excellent suppliers of manganese. Add them to the compost pile after the harvest. Leaf mold is another quality supplier of manganese.
Next up is iron. The trace element plays major roles in the production of chlorophyll and in the nitrogen-fixation process. It also helps reduce nitrates to ammonia for the synthesis of plant proteins.
Chlorosis is a primary symptom of an iron shortage, but it’s not a reliable indicator without a soil test because yellowing indicates other nutrient deficiencies as well. Additional symptoms include new shoots that stop growing, and in severe cases, top growth dies. Fruit-tree leaves turn yellow and develop brown areas, and fruits generally
lack flavor. Excessive lime and/or phosphate may inhibit a plant’s intake of copper, so apply small amounts when copper or other trace-element deficiencies exist.
Chelation is a process that makes iron available to plants. In short, iron in an insoluble form is attracted to organic chelates that plants readily absorb. When plants absorb chelates, they also take in the captured iron.
The best way to put iron and chelates into the soil is by the addition of organic matter. Manures (especially chicken manure), garden and kitchen wastes, greensand, dried blood, and/or seaweed added to the compost pile provide all of the iron that plants need.
Copper, like several other nutrients, tends to act as a catalyst. The metal activates plant respiration and iron usage. Plants lacking copper develop a condition known in the Northeast as withertip. A look at the leaves on stem tips reveals wilting that watering doesn’t revive. Fruit trees suffering from withertip are especially reliable indicators of copper deficiency.
Tomato shoots exhibit abnormal growth and less-than-vigorous root systems develop when a lack of copper exists. Blossoms don’t form and leaves curl upward with bluish-green coloring. The edges of lettuce leaves whiten when copper is unavailable, and the bleached appearance eventually encompasses the entire plant.
Applications of compost or well-rotted manure correct a copper shortage in summer soil. Liquid seaweed sprayed on leaves, especially on growing shoots, helps the plants along until they can absorb it in chelated form from compost or manure.
Molybdenum is responsible for nitrogen fixation by Rhizobium bacteria on legume roots. Nonlegumes use molybdenum to reduce complex soil nitrates to ammonia, which provides plants with a simple form of nitrogen that they use readily. Molybdenum becomes scarce when it becomes chemically bound to other elements in acidic soils. Suspect your potato patch and other acidic areas as candidates for molybdenum shortages.
The early stages of a molybdenum deficiency often disguise themselves as a nitrogen deficiency because molybdenum helps plants absorb nitrogen. The yellowing and mottling of older leaves beginning near the bases of plants and plants that appear weak and spindly signal a nitrogen deficiency possibly caused by a lack of molybdenum. Particularly susceptible to molybdenum shortages are many of the brassicas (broccoli, cauliflower, brussels sprouts, kale, etc.) and lettuce. The leaf material between veins doesn’t develop properly and a leafs midrib overdevelops instead. That gives the leaf a whiplike appearance, a condition sometimes referred to as whiptail.
Remedy a molybdenum shortage by planting crops such as vetches that store the micronutrient. Turn the plants under after they mature. The stored molybdenum is released to the soil as the crops decompose. For instant relief, side dress plants with several inches of compost and spray leaves with liquid seaweed.