by Tami Schlies
To many people, dirt is dirt, but to those of us who grow things, dirt is a dirty word. The key word for us is soil, and there are many different properties to soil, all of which make our job as growers easier or harder. Soil texture, profile, components, chemistry, and nutrients are a few of the ways we as gardeners can talk about our growing medium, but these may be terms not every gardener is aware of. To better understand how soil works, this article will cover some of the basics of these issues, trying to focus on our needs as fruit growers.
Most plants in Alaska utilize only about the top twelve inches of soil because our soils tend to be so cold. The texture of this soil is very important to promote the best physical and chemical properties for optimum plant growth. Texture determines how much water the soil holds, how easy it is to till, and how well it holds nutrients available for plant use.
There are three size categories is soil texture. Clay is the smallest, silt is medium sized, and sand is the coarsest. In the soil solution, clay is the most chemically active of the three sizes, having a negative charge that bonds with and holds many positive nutrient ions available for plant use. Conversely, sand has little charge and therefore barely aids soil fertility. All soils consist of these three grades in varying ratios, the predominant particles giving the soil its textural designation. For example, clay soils are usually more than 40% clay, loam is an equal mixture of all three size grades, sandy loam has a bit more sand, etc. An easy way to determine ratios in your own garden is to fill a quart jar halfway with garden soil, then fill to the top with water. Cover and shake the jar until it is well mixed, then allow it to settle for twenty four hours. The soil will form into three layers, with sand on the bottom, silt in the middle, and clay on the top.
While soil texture is important to gardeners it cannot be markedly changed without taking drastic measures. Instead, we focus on changing the soil structure – the way in which the particles are assembled. We can change the structure of the soil by cultivating, draining, liming, or adding organic matter. The goal is to get your soil structure to a good, loose, crumbly state that drains well, yet retains enough water for plant use, and that does not clump up or crust over. The easiest way to achieve this is by incorporating organic matter, which we shall discuss next.
Besides the sand, silt, and clay, which are all mineral components, good garden soil also contains organic matter, air, and water. Healthy garden soil is roughly 50% pore space, which can be filled with either air or water. Water is needed not only for plant uptake, but it is also the way in which nutrients are taken into the plant. Air is necessary for respiration in plant roots and for soil microbial activity. Organic matter is partially decomposed plant and animal matter. Physically, organic matter binds mineral particles into larger clusters, making soil loose and easily workable. This increases the amount of large pores in the soil, enabling it to hold more water and air.
Humus, organic material resistant to decay, has the ability to hold great amounts of water and nutrients ready for plant use, even more so than clay, but more importantly it improves the soil structure. Compost is a great source of humus, since most of the degradable organic material has already decomposed. Decomposition is required to release (mineralize) the many nutrients that are held in the organic portion of the soil, so organic fertilizers may release their nutrients more slowly in our cold soils than they do in warmer areas of the country. Soil specialist, Professor Ray Gavlak says “Typically temperate region soils (and certainly subarctic soils) have more organic matter than soils of tropical climates due to microbial activity stimulated by warmer temperatures. Soil microbes reduce organic matter levels because they are active nearly all 12 months of the year. In our soils, they have only a few months to mineralize organic matter and that typically happens in the very surface region…”
One of the best things you can do for your garden or orchard is do a soil pH test – either at the Cooperative Extension Office, with a kit from the store, or even at some of the local greenhouses. If you buy a home pH test kit, make sure you get one that tests in gradients of point five; 5.5, 6.0, 6.5, 7, and so on. The other test may be cheaper, but can easily be read inaccurately. Soil pH is the measurement of hydrogen ions in the soil, created when water mixes with the minerals in the soil. You may have heard this referred to in terms of acidity versus alkalinity or even sour versus sweet soil. A neutral soil would have a pH of 7, an acidic soil would be less than that and an alkaline soil greater than that. The pH of the soil determines the availability of minerals, affects microbial properties such as decomposition and nitrogen fixation, and affects toxicity levels of such things as aluminum. Most plants prefer a soil pH of around 6.5 because at this level nutrient uptake is at it’s maximum for most nutrients.
Preferred pH of commonly grown fruits
apples, gooseberries 5.0-6.5
apricots, grapes 6.0-7.0
cherries, plums, pears 6.0-7.5
strawberries, raspberries 5.0-7.5
blueberries 4.0 – 6.0
Alaskan soils are generally acidic, with a pH of 4 not uncommon, though there are a few areas of the interior with neutral to slightly basic tendencies. Soils can become further acidified by what is called leaching, when base forming minerals are washed away by rain or irrigation. Most commercial fertilizers are also acid forming, and even plants themselves and microbes in the soil acidify the solution by the creation of carbon dioxide during respiration, which combines with water to form carbonic acid. In order to artificially acidify soil, you can add elemental sulfur, sulfuric acid, aluminum sulfate, or ferrous sulfate.
Most people need to raise the pH of their soil, rather than lower it, and that is where the addition of lime becomes important. Liming materials are usually calcium oxides, hydroxides, or carbonates. Dolomitic lime is a mixture of calcium and magnesium, which is desirable if you also need to raise the magnesium levels. Do a soil test before adding lime to determine the amount to add, because adding too much can raise the pH too high and result in deficiencies of iron, manganese, and zinc.
We are all familiar with the three primary nutrients of nitrogen (N), phosphorus (P), and potassium (K). These are the numbers we see on the bags of fertilizer and the nutrients plant need in the greatest amounts. But there are many more nutrients necessary to plants than just these. In fact, there are sixteen known nutrient elements essential to plant growth. Three of them – carbon, oxygen, and hydrogen – are provided by air and water with no intervention on our part. In addition to these, there are the three primary nutrients mentioned earlier, three secondary nutrients, and seven micronutrients. Many nutrients available for plant use are of a positive charge in the soil solution, held for use by the negatively charged clay and organic particles. The few that are negatively charged, such as nitrate, sulfur, and molybdenum are not held by soil particles and therefore are susceptible to leaching.
Lets start with a few facts on the primary nutrients. Nitrogen is needed in larger amounts than any other element because it is what facilitates the growth of leaves. Too little results in stunted growth and yellowish leaves, too much causes excessive growth and little or no flower and fruit development in fruiting plants. Though the air we breathe is mostly nitrogen, it is not useable in gaseous form to most plants. It is only useable by plants in two forms, either as nitrate or as ammonium. Nitrate is easily leached from soil and is a major cause of ground water pollution. It is also converted to gas in waterlogged soils and lost into the atmosphere. Ammonium is held by organic matter in soil. It is used by soil microbes as a nutrient, and they in turn transform it into nitrate during a process called nitrification. In unfertilized soils, 95% of the nitrogen is held by organic matter. A few plants, such as legumes and alders, do have the capability to transform atmospheric nitrogen to a usable form with the aid of symbiotic microorganisms on their roots. Growing such things as clover around the base of your fruit trees can increase the soil nitrogen levels for your trees to use, as well.
The second primary nutrient, phosphorus, is essential to good root development, flowering, and fruit development. A deficiency may result in red, purple or very dark green leaves and stunted growth. It works in conjunction with magnesium, each increasing the other’s plant uptake. PH plays a critical role in the availability of phosphorus: above a 7.5, and calcium binds with it, making both unusable; below 5.2 and aluminum binds it. Phosphorus is of major importance to Alaskans because it has a very slow uptake by plants in cold soils. Not only that, but some soil minerals – especially those in volcanic ash – bond with phosphorus and make it unavailable to plants. Areas of Point McKenzie, the Kenai, and the Susitna Valley are high in volcanic deposits in the soil. The addition of organic matter decreases the ash’s ability to bond with the phosphorus and may help with plant uptake, decreasing the need to add large amounts of phosphorus. In the soil system phosphorus is very insoluble, meaning it does not leach, but that it is also not easily available to roots if it is not worked into the root zone. The common fertilizer recommended for Alaska, such as 8-32-16, is high in phosphorus, but over time, phosphorus levels can become excessive at this level. Doing a soil test can be very beneficial and even save you money if you discover you no longer need to use a high phosphorus fertilizer.
The final primary nutrient is potassium, which aids root and stem development, increases the overall vigor and disease resistance of plants, and increases the quality of yields by aiding the plant’s production of sugars, starches, and oils. It helps reduce water loss through the leaves, helping plants endure drought, and acts as an antidote to excess nitrogen. A lack is indicated by an increased susceptibility to disease, thin skinned, small fruit, and weak stems. Excess potassium causes course, poorly textured fruit and lowered ability to absorb calcium and magnesium. The most limiting factor for plant uptake of potassium is lack of moisture, since it is not highly effected by pH. We add potassium in our fertilizers because though it is held in soil in great amounts as part of the mineral structure, it is released too slowly to accommodate plant growth.
Like potassium, the most limiting factor in plant use of the secondary nutrient calcium is lack of soil moisture. Such things as blossom end rot in tomatoes, tip burn in strawberries, and bitter pit in apples is caused by lack of calcium due to moisture deficiency in the soil. Keep those plants evenly watered! In the soil system, calcium is the dominant ion and acts as a buffer to pH changes. It is usually present in ample amounts, except in very acid soils, and it is also present in most liming materials that we add to our soils. It is necessary for cell wall structure.
Magnesium, the next secondary nutrient, also acts as a pH buffer in the soil solution, being least available in acidic soil. It is used for plant photosynthesis. Though phosphorus works with magnesium, calcium and potassium compete with it for plant uptake, so liming materials that are all calcium and no magnesium may cause an insufficiency in the soil solution. A good way to add magnesium to the soil is with dolomitic lime, Epsom salts (magnesium sulfate,) as sulfur is also limited in cold soils, or by MagAmp, which adds lots of phosphorus along with the other primary nutrients. Magnesium is in good supply in soils in Alaska’s interior, but forage crops in Point McKenzie have benefited from its addition. Lack of magnesium causes small, poor quality fruit, premature fruit drop, and yellow leaves while the veins are still green.
The last secondary nutrient is sulfur, which is also used as a soil acidifier. Most sulfur is tied up in the soil organic matter and its release is related to decomposition and rainfall. Like nitrogen it is very water soluble, therefore, susceptible to leaching. A lack of it shows up in plants very much like a nitrogen deficiency. Here in Alaska, plant uptake of sulfur can be affected by our cold soils, but care must be taken in already acidic conditions, as an excess of sulfur will burn plants because it lowers pH.
Micronutrients are called that because they are needed in such small amounts by plants. Not all the functions of micronutrients in plant growth are yet understood, but they are very necessary to plant growth. The most limiting factor to micronutrients is not the quantity available in the soil, but soil pH. If plants show symptoms of deficiency in these nutrients, simply adding micronutrients to the soil will not suffice, since the new nutrients will also be bound and made unusable under high pH levels. If for some reason it is impossible to lower an alkaline soil’s pH, chelated micronutrients can be used, which is a form more readily useable by plants, or a foliar spray of the nutrients can be applied. Most micronutrients are held in the organic matter of the soil.
Four of the seven micronutrients, manganese, zinc, iron, and copper, become severely unavailable to plants as the pH rises. Manganese deficiency shows up in plants as a mottled chlorosis of the leaves and stunted growth. Excess can cause small dead areas with yellow borders in the leaves. It is highly affected by the water content of the soil and can easily reach toxic levels in improperly drained, water logged soils. Zinc is not only deficient in soils with a high pH but also if phosphorus levels are too high. A need for zinc makes plants have small, thin yellow leaves with green veins and results in low crop yields. Iron is highly present in most soils, and a lack in plants shows up as a yellowing of the leaves while the veins remain green. Overliming soil to a pH above 7.5 may result in iron unavailability. Copper is so tightly bound in organic matter that it is less available to plants the higher the levels of organic matter (yes, you CAN have too much organic matter!) as well as high pH. A lack results in multiple budding, gum pockets, and pale young leaves with brown tips. Excess stunts plant roots and prevents the uptake of iron.
Molybdenum behaves differently from other micronutrients because it, like nitrate, holds a negative charge that the soil repels rather than holds. It is most likely to be deficient in acidic soils, particularly sandy ones, and it is easily leached like nitrogen. We in Alaska need be concerned especially if we are growing crucifers, as lack results in what is called whiptail. It is also essential to the nitrogen fixing abilities of legumes. In other plants it may result in yellow new leaves with green veins. According to Ray Gavlak, “Mo deficiency in trees (perennials) should occur much less frequently than in some of the cole crops which actually are the only crops on which the deficiency symptoms have been observed in the Matanuska Valley.” Do not attempt to correct a molybdenum deficiency in your soil without professional help, for it is easy to overdo the amount and ruin the soil. Ray Gavlak of the Palmer Re
Chlorine is rarely deficient in soils, especially if you water with chlorinated water. In fact, there is more of a problem with excess chlorine. Little is known about the function of chlorine in plant development, but it likely aids in a cold and drought tolerance of a plant.
The final micronutrient to discuss is boron, which is critical for the meristematic tissue in fruit trees – the flower buds, root tips and shoot tips where active cell division occurs. It can be insufficient in soils around the state, and affects the growth of vegetables such as beets, cabbage, carrots, and celery. A lack can cause small, twisted leaves, heart rot, corkiness, and multiple buds. In excess, boron may cause plant leaves to turn a yellowish red. However, the difference between deficient and toxic levels in the soil is very fine, so do not add boron without a soil test. Strawberries in particular are very sensitive to excess boron. As a molecule, boron does not hold either a positive or a negative charge, so it is only lightly held in the soil and is easily leached. It is also less available to plants as the pH goes above 6.5.
This is by no means a complete report on all the things there is to know about soil. I hope that this will help people understand soil and the terminology used to discuss it. For further reading, I suggest starting by picking up some of the soil publications at your Cooperative Extension office.
Gavlak, R.G., Fertilizer Nutrient Sources and Lime, Crop Production and Management Series, Alaska Cooperative Extension, 1998
Falen (Johnson), C.L., Organic Fertilizers, Crop Production and Management Series, Alaska Cooperative Extension, 1998
Johnson, C.L., Soil Fundamentals, Crop Production and Management Series, Alaska Cooperative Extension, 1998
Walworth, J.L., Soil Fertility Basics, Crop Production and Management Series, Alaska Cooperative Extension, 1996
Gavlak, Ray, Palmer Research Center
Hedla, Lenore, The Alaska Gardener’s Handbook (Anchorage, Alaska: High North Press, 1994), p. 91
The Big Book of Gardening Skills, (Pownal, Vermont: Garden Way Publishing, 1993)
Rodale’s Garden Answers, (Emmaus, Pennsylvania: Rodale Press, 1995)
Baldwin, Keith, “Improving Clay Soils”, Kitchen Gardener, April/May 2000, p.22-24