Inorganic Soil Amendments

Inorganic soil amendments are used to supplement the organic matter that is already in the soil. It is meant to improve the texture of clay soil, and help sandy soil by increasing the sandy soil's capacity to retain water and nutrients. In a nutshell it is meant to make clay soil and sandy soil more like loam soil in texture and character. Vermiculite/ Peat, and perlite are examples of inorganic soil amendments. Both are used to improve the water and nutrient retention of the soil. Vermiculite/Peat and perlite are usually used on small scale in potting soils and small plant beds.

Other inorganic soil amendments include lime and gypsum. Both these inorganic soil amendments are used to improve the quality and texture of clay soil. They do this by causing the clay particles to group together into larger units which further results in improved aeration and drainage. These inorganic soil amendments can become an invaluable part of your drainage system in your backyard landscaping because water drainage can be a very important factor in determining the types of plants that you will be able to cultivate successfully.

The use of gypsum and lime is determined by the soil pH. In areas with high rainfall and especially in acid soil, lime is used as the inorganic soil amendment of choice. Lime alters the soil pH to make the garden soil less acidic and supplies calcium to the soil. In areas with low rainfall and usually neutral or alkaline soil, gypsum is the inorganic soil amendment of choice as it supplies both calcium and sulfur to make your garden soil less alkaline.

Checking your soil pH

As a gardener, even if you’re a novice gardener, you can never take for granted that the essential elements present in a soil will automatically be available to your plants. Availability of these elements is dependent on the soil pH level. Plants need the correct pH (acidity/alkalinity) level which controls how well plants utilize the nutrients available in the garden soil. Most garden plants thrive in soil that measures 6.0 to 7.0 on the pH scale as it is at this level that all essential elements present in the garden soil are readily available to the plants.

You can have your soil pH checked by a professional or you could invest in a good soil pH tester and test the pH level of your soil on a regular basis. Soil test kits are available for purchase from garden centers and most leading nurseries; alternatively they might even offer soil testing services by supplying their own soil testers on contract. Soil acidity and alkalinity is measured on a scale of 0.0 (very acidic) to 14.0 (very alkaline). Soil that measures 7.0 on the pH is scale is neutral soil. Soil that measures 7.5 or higher on the pH scale is alkaline and soil with a pH of 4.0 and 5.0 are regarded as acid soil.

All plants have a pH preference. Roses for instance have special preference for slightly acid soil and thus it is important to know the pH level of your soil. You can then choose plants with the same pH preferences, avoid those that will not do well in your garden soil, or know how to go about supplying their special growing needs. By testing your soil pH, you determine its condition so that you can adjust the soil pH more accurately, effectively and economically.

Adjusting the soil pH

Lowering the soil pH basically means using inorganic soil amendments to acidify alkaline soil. The best way to acidify alkaline soil is to make use of Sulfur or "Flowers of Sulfur" as it is also known. Sulfates in the form of Iron Sulfate or Aluminum Sulfate (alum) or Ammonium sulphate can be used to lower the soil pH. Apart from inorganic soil amendments to lower soil pH, you can also make use of organic soil amendments in the form of peat moss, leaf mold, and well-composted sawdust to lower the soil pH. Lime is generally used to correct soil acidity, as well as to improve the physical condition of the soil and also encourage bacterial activity. In much the same vein, gypsum is used for reclaiming 'alkali' soils or land from the sea and improving the structure of heavy black clay soils. Hence, the term soil amendments when referring to treating the garden soil with these materials to adjust the pH level.

Raising the soil pH basically means to make acid soil more alkaline. It is best to make use of agricultural lime or dolomite to lower the pH level of acid soil. The Calcium Carbonate in agricultural lime or dolomite will not react with fertilizer when applied at the same time. A word of caution though — you may make use of slaked lime, but the calcium hydroxide in slaked lime will definitely react with fertilizer and might result in bad side effects.

Now, over the years many myths relating to garden soil and pH have found its way into the gardening sphere. All gardeners are well-meaning and these myths were not really meant to harm the garden soil and the plants, but these myths have unfortunately led to some damage in many a well-meaning gardener's domain.

Adjusting the Soil pH for Acid-loving Plants

Gardeners used to say that you need to fertilize acid-loving plants with peat and often one could find gardeners buying bags of peat and fertilizing their plants like Azaleas and Camellias with peat. Peat is NOT a fertilizer nor is it a fertilizer type. Peat is chemically inert with none or little nutrient value for plants. Peat does not feed plants. In fact peat is an inorganic soil conditioner that consists of compressed fibres of bog mosses. Peat can be added to soil and soil mixes for many reasons. These reasons being:

  • Peat has an acidic pH. Peat can thus be used to lower the pH of alkaline soil. Peat is thus used as a soil amendment for alkaline soil.
  • Peat has excellent moisture retention and nutrient retention properties. This means that peat can be used to keep soil moist and to hold valuable food for plants. This makes peat a valuable soil amendment in areas where water is scarce and places with hot and dry climates. Since most acidic plants have fine surface roots they need to be kept from drying our and peat will do well as an inorganic soil amendment.
  • Peat is lightweight and makes an excellent additive to potting soil.

Acid loving plants, like all other plants require feeding. Fertilizer is thus essential. Under the usual circumstances a basic fertilizer would suffice, especially a basic fertilizer such as a 3:1:5 Slow Release Fertilizer. However, in alkaline soil it would be best to make use of specifically formulated types of fertilizers for acid-loving plants. At nurseries and garden centers you will come across these specialized types of fertilizers such as: Wonder Azalea, Wonder Camellia and Wonder Hydrangea fertilizers. In some cases you might even get a type of fertilizer specially formulated for most acid-loving plants. The one aspect these types of fertilizers have in common is that they all contain sulphur and ammonium sulphate, both which serve to lower the pH of alkaline soil.

Using Pine Needles and Pine Bark

Most people and gardeners used to believe that pine needles and pine bark will result in rendering garden soil acidic. And in general most gardeners used to believe that any part of any coniferous tree will subsequently be acidic.

However, in its raw state, all coniferous trees are not acidic. This means that the pine needles that gardeners use to apply as mulch on top of soil around a plant, or even in the planting hole will not alter the pH of the garden soil. In fact it is only AFTER the pine needles have been properly composted that it will result in changes in the pH of your garden soil.

Nowadays it is not surprising that the seasoned gardeners have found that pine needles and pine bark do not break down into compost easily. It is quite a lengthy process if you want to incorporate pine and pine products into the ingredients of your compost recipe. This is thus also the reason why pine needles and pine bark makes for excellent, good, long, lasting mulch that will keep the soil moist and cool the roots of plants.

This means that pine needles and pine bark is not great additives to use as soil amendment when you want to alter the pH of your garden soil. You would serve your garden far better by making use of good compost which generally has an acid pH. Thus if you want to change the pH of your soil, you should rather plant with a good compost or acidic peat.

If you need to lower the alkaline level of your garden soil to make your soil more acidic then you should plant with plenty of good compost, or use an acidic soil/compost mixture which has the advantage of composted bark incorporated into the mixture.

Garden Soil Under Pine and Coniferous Trees

Many gardeners used to believe that the soil under pine trees or coniferous trees is too acidic to grow any plants in. They have this believe due to the poplar believe that the pine needles that get dropped by the pine tree makes the soil acidic. This is simply not true. Even acid loving plants such as Azaleas, Camellias and Fuchsias will not readily grow in acidic soil under a pine tree.

The fact of the matter is that the main reason why even acid loving plants will not grow well under a pine tree or a coniferous tree is that those treesare fast growing trees and as such are thus greedy feeders. They take up most of the water and valuable nutrients in the soil. To compound the problem for other plants even more, coniferous and pine trees have a large network of fibrous roots and the smaller plants do not stand a chance. This is also true of other fast growing trees and not only of the pine trees and coniferous trees.

However, it might be an eyesore not to be able to grow any kind of plants under pine and coniferous trees. There is a way to remedy the situation. First dig out as much soil around the base of the tree as possible. Then spread a layer of black plastic, punch a few holes in the plastic for drainage and fill in the new garden bed with good soil. All shade loving plants will then be able to grow under a pine or coniferous tree simply because you would be helping them by taking the competition it would have with the pine or coniferous tree for water and nutrients out of the equation. Alternatively you could create a raised garden bed with walls of brick or even railway sleepers in which to grow your shade-loving plants.