Why do soils need amending?

Soils take a long time to form. Some soils are thousands to millions of years old. During that formation process, a number of important processes occurred, which are called weathering. Weathering leads to the breakdown of the parent material (rocks, minerals, wind or water moved sands and sediments), and the creation of soil.

Soil, Basalt, Tropical soil, soil profile, red soils, Maracaí, Brazil.

The photo portrays the beginning of soil formation over basalt rock in Maracaí, Brazil. Credit: G Alves

And depending on where you live, your soil can be “young” or “old”!

In the Midwestern US or the Pampas of Argentina, most soils tend to be relatively young, and not highly weathered. Many of the initial soil minerals present in the parent materials haven’t been lost. Also, in many cases the soils haven’t been farmed intensively for long periods, with the resulting removal of nutrients in harvested crops. The productivity of the soil is still relatively high, compared to the much older soils found in many areas of the tropics.

Tropical soils tend to be much older geologically, were usually formed under both higher temperatures and rainfall. The weathering these soils have endured has been much more intense. As a result, the natural levels of fertility have been reduced in tropical soils, as compared to the Midwestern US or the pampas. But that is not to say that the older more highly weathered soils can’t be just as productive. Growers can achieve high levels of productivity with appropriate crops and cropping systems, and the prudent use of soil amendments.

Weathering never stops. Weather events, like precipitation or high winds, create conditions for erosion and leaching – both of which make your soil lose some of its nutrients. Weathering can also change the soil pH (more on pH here: https://soilsmatter.wordpress.com/2016/05/01/what-is-soil-chemistry-and-what-does-that-mean-to-me/) which can also change the availability of the nutrients in your soil to your plants.

Plant life, whether trees, grass, flowers or field crops depends on soil to provide its nutrients. But, along with weathering, plants use up nutrients, and don’t usually replace them. The more intensively crops are raised, and the greater portion of the plant removed, the faster soils become depleted of natural fertility.

And when soils lose their natural fertility, farmers and gardens turn to amendments to replenish the soil. What are some common soil amendments?

After you’ve tested your soil, and you know you have nutrient depletion, or you pH doesn’t fit your gardening needs, you have several options. Growers faced with acid soils (pH less than 7) use ground limestone to neutralize the acids and replace calcium and magnesium. Limestone is quarried, ground, and applied to maintain appropriate slightly acid to neutral pH (5.5 to 7) depending on the crop being grown and soil.

Today, producers commonly use chemical fertilizers rather than other materials, because they allow a more precise addition of nutrients needed. For example, if a soil test says the soil is low on nitrogen, growers and homeowners can add just the correct amount of nitrogen needed. In this case, if your soil had enough phosphorus, if you added more, that would be wasteful. In addition, you could contribute to pollution by adding phosphorus that might just run off your property. Fertilizers can supply any of the essential mineral elements that may be limited or in short supply. They can limit the additions of nutrients not needed to improve plant growth.

Animal manure or composts are also excellent sources of many nutrients. However, manures are complex materials, which contain varying amounts of many nutrients. So, if you add manure, but you only need one nutrient, say nitrogen, you’re also adding many other nutrients that aren’t needed. Growers can determine the right mix of manures, compost or fertilizers to ensure the right amounts of nutrients are applied, and over application of others is avoided.

ground phosphate rock

Ground phosphate rock. Source: orj.cc

What about other “natural” products such as ground phosphate rock as nutrient sources? There are places to use these products effectively. But, like manure or compost, they are bulky and expensive to transport. In many cases, the nutrients in them are only slowly available and require large application rates to meet the crops nutrient needs. Thus, application rates may be much higher than the concentrated commercial fertilizers made from the same material. Early in the 20th century, growers used rock phosphate as a phosphorus source extensively in the Midwestern US.  Many of the soils that received “phosphate rock” are still supplying phosphorus in significant amounts to crops today – so the application of this type of amendment has long-term consequences not originally understood. When considering the use of these products make sure you know not only the total analysis of the materials, but also the plant available analysis so that you use appropriate application rates, and to make sure to avoid unwanted byproducts or contaminants.

By Dave Mengel, agronomist, certified crop adviser and farmer

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2 responses to “Why do soils need amending?

  1. I’m a little confused. I know some plants deplete nutrients from the soil but I thought most plants add nutrients in the form of organic matter.

    • What a good question! If left to decay (and not pulled up and out), all plants eventually add carbon back into the soil. Nitrogen fixing plants, like legumes, also add nitrogen back to the soil upon decay. Depending on the crop grown, the soil will need amending – or not! It’s always best to have your garden soil tested now and then to see what amendments you might need. Thanks for being a reader. SF

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