So far in this series on garden soil, I have looked at soil texture, pH, and organic matter in your soil and their impact on plants. In this, the final article in the series, I will discuss soil moisture and nutrients, and why these things even matter.
Water
Some of our native plants need dry, well drained soils. At the extreme end are plants like Prickly Pear Cactus (Opuntia spp). Because these plants have evolved strategies to retain water and have lost their mechanisms for eliminating excess water, too much water will cause the plant to drown because it is not able to get enough air to the roots. In cactus, in particular, too much water will lead to rotting roots because they do not have the ability to fight of moisture loving fungi and bacteria.
We all know that plants need water to grow. How much they need (or will tolerate) depends on a variety of factors, not the least of which is the conditions under which they evolved.
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At the other end of the spectrum are aquatic and semi-aquatic plants. Plants like Soft Bulrush (schoenoplectus tabernaemontani) will quickly desiccate and die if left out of the water because they have no mechanism to retain moisture (such as the thick walled cells often found in plants like cacti). But why don’t they drown? In order to get the necessary oxygen down to the roots that are in waterlogged soil, these plants have specialized cells, called aerenchyma, which form tubes to conduct air down below the water level.
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But most of our native plants fall somewhere between the two extremes. In average garden soil, there will be periods that the soil is very wet, such as after the spring thaw and after a period of rain, and times when the soil is quite dry. The moisture retaining capacity of the soil, as we saw in the first two articles in this series, has to do with soil texture and the amount of organic matter present. And if you grow plants that are suited to the soil conditions in your garden, they will easily tolerate these variations in soil moisture.
We know plants need water, but what role does water actually play in plant growth. For starters, water is essential for photosynthesis. Photosynthesis is the plant’s superpower – being able to convert water (H2O), carbon dioxide (CO2) and sunlight into nutrients for the plant, then discarding the excess oxygen (O2) as a waste product – which is very convenient for oxygen breathers like us. During photosynthesis, water provides the hydrogen (H) atoms required to make glucose (C6H12O6) – the simple sugar that serves as the primary source of energy for the plant.
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Water also dissolves nutrients – both organic and inorganic – in the soil, making them available for uptake by the plant’s roots. It is also the medium through which the glucose and soil nutrients are distributed to various parts of the plant.
Finally, water also plays a role in the plant’s structure and shape by creating a constant pressure, called turgor, on cell walls. When a plant doesn’t have enough water, the cell walls contract, causing the wilting that we see in plants that need watering. If allowed to remain dry for too long, the cells reach a point where they can no longer reabsorb the water and the plant dies.
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Matching your plants to the appropriate soil moisture can be a tricky task. Fortunately, most of our native plants are very forgiving and will tolerate short periods outside of their moisture comfort zone. After all, we have dry years and wet years, as well as those crazy years than bring a pile of rain followed by two months of drought. If plants couldn’t cope with these vagaries, we’d have very few plants.
Plant Nutrients
There are 17 nutrients that are essential for plant growth and these can be divided into two categories: macronutrients – those nutrients that are used in large amounts – and micronutrients -those that are used in small amounts by the plant. The macronutrients hydrogen, oxygen, nitrogen and carbon contribute to over 95% of a plant’s entire biomass. Micronutrients are present in plant tissue in quantities measured in parts per million. Carbon, oxygen and nitrogen are all absorbed from the air, while the other nutrients (including H in the form of water) are primarily taken up from the soil through the plant’s roots.
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Each of these nutrients plays an important role in the plant’s growth. We’ll first look at the role of the three principle components of fertilizers – Nitrogen, Phosphorus and Potassium (N, P & K) – followed by a quick look at the other macronutrients.
NPK
Nitrogen is used by the plant to create amino acids, the building blocks of protein, and is used by chlorophyll in photosynthesis to convert sunlight into energy for plant growth, among other roles. Insufficient nitrogen can result in leaf yellowing and stunted growth.
Nitrogen is a water-soluble element that is primarily available to the plants from soil water in the form of nitrite (NO2-), nitrate (NO3-), or ammonium (NH4+) ions. Bacteria in the soil convert nitrogen gas into ammonia, which plants can use. Lightning also converts atmospheric nitrogen into ammonia and nitrate, which enter the soil with rain. When plants and animals die, or when animals excrete waste, the nitrogen in their organic matter returns to the soil. Compost, which is primarily the excrement of soil organisms, is a great source of nitrogen.
The primary role of Phosphorus in plants is storage and transfer of energy produced by photosynthesis for growth and reproductive processes. It is necessary for seed germination, photosynthesis, protein formation, flower and fruit formation, and is particularly important for good root development. Without adequate phosphorus, plants will be slow to mature, will have poor flowering, will drop flowers and fruits prematurely, and their growth will be stunted.
Phosphorus in its mineral form is not readily available to plants – it requires bacteria to convert it to a plant available form. Ideally, soil pH should be in the range of 6-7.5 – soils more acidic or alkaline than this will result in P becoming bound to other elements in the soil and not available for plant use. Chicken and horse manure, bone meal, fish emulsion and rock phosphate (used in the manufacture of organic fertilizers) are all good sources of phosphorus for plants.
Potassium plays a vital role in photosynthesis and regulates water usage by the plant. It has also been shown to improve disease resistance in plants, improve the size of seeds, and enhance fruit quality. Insufficient K can result in leaf yellowing between the veins of leaves and curled or scorched leaves. Potassium is very mobile in the plant, and plants move it to the younger, needy tissue when it becomes limiting. As a result, potassium deficiency first shows up as a mottled chlorosis of the older leaves and eventually the leaf edges become brown.
Potassium is mined as Potash (cool fact: the world’s largest potash mines are in Saskatchewan) and wood ash. Manures, compost and other organic materials are also potassium sources, even though the concentration of potassium in them is pretty low, but these materials are typically applied generously enough to contribute a sufficient amount of K.
The Other Macronutrients
Sometimes called “secondary nutrients”, the elements of carbon (C), hydrogen (H), and oxygen (O) are absorbed from air and water, while Magnesium (Mg), calcium (Ca) and sulphur (S) are taken up from the soil.
Carbon, in addition to its role in the formation of glucose for the plant, is an essential building material that gives strength to cells. Adding organic matter, such as manure or decomposing plant parts (rich in carbon– or the browns in compost), to the soil surrounding growing plants is an effective way to provide C for the plants.
Hydrogen, made available from water during photosynthesis, in addition to being used to form glucose molecules, plays a key role in plant respiration, and recent research suggests that hydrogen also plays a role in mitigating plant stress as well as promoting root growth.
Oxygen is used by plants in cellular respiration – to break down food molecules and release energy for growth. In addition, the amount of oxygen available to a plant’s root cells affects its growth rate, water and nutrient uptake, as well fruit yield and quality.
Magnesium plays a critical role in the production of chlorophyll and is a key driver in photosynthesis. It is also involved in the transportation of carbohydrates from leaves to actively growing tissues of plant roots, shoots and reproductive organs. A deficiency of Mg can reduce root or shoot growth, and potentially seed weight, and can appear as yellow bands between the leaf veins. Although Mg is normally sufficient in most garden soils, if necessary, supplemental Mg can be added through compost, Epsom salts (magnesium sulfate), or dolomitic lime.
Calcium is an important structural component of cell walls. It is necessary for cell growth and division, and influences water movement in cells. In some plants, calcium is required for the uptake of nitrogen. Calcium is found naturally in most limestone-based soils but can be supplemented with crushed eggshells, ground oyster shell or dolomite lime. Calcium can also help to neutralize acidic soils. If Mg levels are sufficient in the soil, adding extra calcium could lead to soil toxicity and cause further problems. If in doubt get a soil test done.
Sulphur also plays a key role in photosynthesis and the formation of chlorophyll, and in the production of plant proteins. It is also a great tool to help acidify soils for those acid loving plants like blueberries. Sulphur deficiency can resemble N deficiency – leaves can become light green or pale yellow due to the lack of chlorophyll production. Manure, compost, and decomposing plant matter are all sources of sulfur for the garden.
To learn more about nutrients (and nutrient deficiencies), check out the following websites, or simply google the subject.
Summary
Most soil conditions across the world can provide plants adapted to that climate and soil with sufficient nutrition for a complete life cycle, without the addition of nutrients as fertilizer. In fact, here in the southern Great Lakes region, we are blessed, for the most part, with great nutrient rich soils. When growing native plants, adding additional fertilizers or compost will often simply result in tall, lanky plants that fall over in the garden. And, speaking from experience, it can take several years for the excess nutrients to be used up. So unless you are trying to grow plants in an industrial wasteland, it is highly unlikely that you will need to add extra nutrients to a native plant garden. The deep roots of many of our native species are able to tap the depths of the soil and find all that they need.
But it is also important to do your homework on the moisture needs of the plants you are growing, and plant those with high moisture needs together and those that like it drier separately. In this way you will greatly increase your success in the garden, and provide habitat and food for many insects and for those that rely on them.
Happy Native Plant Gardening.