Soil isn’t simply dirt! It is a dynamic, three-dimensional substance with various functions, including being a vital medium for plant growth. But there are tens of thousands of different types of soil, some of which are better suited for plant growth than others.
We all need healthy garden soil to grow plants, especially fruit and vegetables. More often than not, we need to amend what we’ve got, adding organic matter and fertilizers to improve soil productivity. Much depends on the primary soil type you have in your garden and what you plan to grow.
The Soil Science Society of America defines soil as an “unconsolidated mineral or organic material on the immediate surface of the Earth that serves as a natural medium for the growth of land plants.” You and I might say that this is an excellent definition of good, old dirt.
But, essentially, all soil is a mixture of components including gravel, sand, clay, silt, air, and water. These ingredients vary, which has a major effect on its cohesiveness and how well it will hold together.
The Department of Agriculture (USDA) points out that there are more than 20,000 different kinds of soil in the U.S. alone. While the system they use for the classification of soil types is known to be a common language for scientists, it’s not easy for most backyard gardeners to understand.
Soil scientists identify five soil factors to explain how soils form. These clearly define the composition of different types of soil in different areas.
The soil-forming factors are:
- The parent material of the soil is the material in which it forms.
- Climate affects temperature and amounts of moisture in the soil.
- Topography, in the form of slope and aspect, also affects moisture as well as the quality of the soil.
- Biological factors, including microorganisms, animals, plants, and humans affect the formation of the soil and ultimate soil fertility.
- Time is a critical, ever-changing element in terms of soil formation.
Whether you are farming on a large scale or growing vegetables in your backyard, it is essential to ensure soil health and soil fertility. If it isn’t healthy, productivity will be threatened and you won’t reach maximum expectations in terms of plant growth.
That said, the economics for a farmer will be very different from those that homesteaders and backyard veggie gardens will experience. But the principles are the same and equally important.
David Hillock of Oklahoma State University explains what soils need to be fertile. He explains that most soils need both commercial and organic fertilizers for efficient, high-quality plant production.
But the conundrum is that some do very well with only organic matter, and the same applies when commercial fertilizers meet all the requirements.
However, only a few soils have enough humus and the plant nutrients soil needs for good plant growth without amendments. You need to know which fertilizers will work for plant growth in your soil.
With the right fertilizers, you will be able to improve soil fertility and provide the nutrients you need for crop plants to grow. The main ones are calcium (Ca), magnesium (Mg), nitrogen (N), phosphorus (P), and potassium (K), which is why nutrient management is so important.
Soil pH is also important when it comes to soil fertility management. It doesn’t indicate soil fertility but rather measures the acidity of the soil. As Lee Stivers, a former extension educator, in horticulture, at Pennsylvania State University says, most crops do best when the pH is between 6.2 and 6.8 because their roots absorb nutrients more easily.
As Stivers explains, soil that has a good structure has a lot of empty space between its particles. This means that water and air have access to the soil and so roots can grow.
Four Main Soil Types
Trying to identify 20,000 soil types is mind-boggling. But when you are faced with the four main types it’s not that difficult.
These four types are classified in terms of texture: loams, clays, silts, and sands. And they are usually composed of a mix of at least three particle sizes.
Sand particles are quite large while, in comparison, the clay particles are tiny. Silt, on the other hand, is a medium size. Loam has nearly equal parts of clay, sand, and silt. As the Soil Science Society of America (SSSA) states, it is very rare to have soil that is made up only of sand, clay, or silt. Instead, it is usually a combination of all three.
Loam, as already mentioned, is a mixture of sand, silt, and clay. Because it has the three textures offered by these three soil types, it works very well for drainage, water retention, air circulation, and soil fertility.
Loam soils are usually very fertile and easy to work with. However, loam is sometimes labeled according to whether it is primarily silt loam, sandy loam, or clay loam, some types being more fertile than others
Clay is a fine-grained, very cohesive type of soil. The tiny particles stick together very easily and form a sticky texture when they are dry or wet.
Generally, clay soils contain a lot of water. It expands when it comes into contact with water and shrinks when it dries out.
The particles in clay soil are generally thin and flat rather unlike sand particles which are usually round. Because it is strong when dry and highly compressible, it may be used in construction as a type of mud mortar.
Silt is a fertile type of soil that forms deposits on river valley floors, especially during floods. Unlike clay, it has low plasticity and can be formed into balls when it is wet.
Silt also forms fine, runny puddles of mud when it rains.
Sand isn’t the kind of soil we use to produce crops or backyard vegetables. But it is used extensively in the construction industry.
It consists of rock and other hard-material particles, which makes it ideal for drainage when mixed with other soil types. It also increases soil aeration qualities in soil.
Types of soil fertility
There are two types of fertilizer that we use for improving soil fertility. One is natural or organic and the other is synthetic and based on chemicals. This makes it inorganic or acquired.
Both have advantages. While many people prefer organic fertilizers, inorganic types aren’t nearly as expensive.
Also, the nutrients inorganic fertilizers provide are soluble, so quickly available to plants. Organic fertilizers need to break down in the soil before the nutrients become available.
The caveat, as master gardener Mary Jo R. Gibson from PennState Extension warns, is that over-fertilization is a lot more dangerous than under-fertilization. This is because once the nutrients are in the soil, it’s impossible to remove them.
Natural fertility relies on organic fertilizers that originate from living organisms including plant and animal by-products. In its natural form, organic fertilizer, which includes compost from plants and animal manures, is moist.
Other types of compost include humus, peat moss, and mushroom compost. If you choose to use shredded tree bark or other coarse materials like sawdust, compost these for a couple of years before digging them into the soil.
Commercial organic fertilizers are manufactured and designed to provide plants with nutrients over longer periods of time. These include bone and blood meals, dried manures, and soybean and cottonseed meals.
Acquired fertility is inorganic and is manufactured from water-soluble mineral salts. Complete inorganic fertilizers contain the vital plant nutrients the soil needs together with other elements.
By law, fertilizer packaging must indicate the percentage of N, P, and K in the product – in this order. So, for instance, a bag of 5-10-5 contains 5% N and K and 10% P.
These fertilizer formulations are great for general garden and lawn use. However, people often use formulations with less P (phosphorus), like 1-2-2 or 1-3-1.
If you’re transplanting flowers or growing vegetables, high-ratio, water-soluble fertilizers, like 1-5-1 usually work well. Non-mixed fertilizers, including N and P, may be used if only one element in the soil is deficient.
How to improve soil fertility
We can’t change the structure of the soil, but we can improve soil fertility. For example, adding compost to your soil will increase soil water-holding capacity and provide pore space that improves air circulation.
Organic matter also stops tiny, sticky clay particles from cementing into a solid mass so the roots can develop. It also promotes the growth of healthy microorganisms and helps the soil release and hold more nutrients.
A good soil test will give you very important information about soil fertility and the nutrients in your soil. These include calcium, magnesium, phosphorus, and potassium, as well as the all-important pH levels. You can also test for organic matter.
One element you shouldn’t ignore is nitrogen, which is often deficient. Unfortunately, N isn’t included in most soil tests,
This, as an article from PennState Extension states, is because nitrogen changes its form and quantities in response to outside elements like temperature and soil moisture. But N is commonly added to fertilizers and you can buy it as a non-mixed fertilizer.
Soil fertility management and soil productivity are complex topics. But you don’t need to be a soil scientist to understand the basic principles needed to improve garden soil.
To make the best of your soil, test it every three to five years and fertilize it as indicated in the soil test report. Use lots of organic matter to improve the general quality of the soil for plant growth.
Then sit back and enjoy the fruits of your labor.