If you think your farm or garden soil is not ideal, you can do a lot to improve it. Some indicator signs that soil is not as good as it should be include:
- Poor crop growth, possibly stunting, small root systems, leaf abnormalities, wrong leaf color, poor flowering and seed or fruit production (assuming weather conditions are favorable), and pest or disease attack.
- If you dig into the soil with a shovel, the topsoil or especially the subsoil is hard (often with a hardpan layer several inches down) and does not crumble into small pieces; or the soil is waterlogged and poorly drained, perhaps with a foul putrid odor; or there is a dense crust on top in dry weather.
- There is little or no evidence of earthworms (however, in some regions, earthworms are not normally present, such as arid or desert regions, and sandy soils).
These are symptoms of a variety of soil problems:
- Poor soil structure [see the link on Soil Parts and Functions].
- Deficient or out-of-balance soil nutrients [see the link on Crop Growth and Nutrition].
- Extreme soil pH (acidity or alkalinity) [see the link on Soil Parts and Functions] or high salt content.
- Low levels of beneficial soil organisms [see the link on Soil Parts and Functions].
- Toxicity from pesticides, herbicides, wrong fertilizers or waterlogging.
If you do not have a good idea of which thing or things are wrong in your soil, you may want to so additional tests. Although soil tests will require some expense, sometimes they are the only good way to diagnose soil problems. Soil tests can be done by university or private soil testing labs, agricultural consultants, fertilizer dealers, and possibly garden supply companies. You can also buy do-it-yourself test kits and meters.
The typical tests that are done are soil pH and major nutrients (nitrogen, phosphorus and potassium), with secondary and minor nutrients sometimes available at extra cost (calcium, magnesium, sulfur, manganese, zinc, copper, iron, boron and molybdenum). Tests of soil texture, nutrient-holding ability (cation exchange capacity or CEC), percent organic matter and the nutrient content of plants (plant tissue test) may also be available at private and university labs.
Usually a testing lab will need a one-half to one pound soil sample. You usually will collect several ( up to 10 to 30) small samples and mix them together to get an overall average test for a field or garden, but you may also want to collect a sample from a problem area, such as a low wet spot. You generally will want to sample the soil from the upper six inches, where most roots grow. You can use a shovel or trowel to dig samples, but do not include soil that has touched the iron tool or your fingers. Put the sample in a paper or plastic bag along with a label with your name, address and sample identification, such as Field 1, Garden NE corner, etc.
Although the results of soil tests that you get back may appear very accurate, bear in mind that different soil testing procedures or different labs can give different results for the same test, and that in your soil, conditions are constantly changing, so extremely accurate numbers don’t necessarily mean much. You are mostly interested in knowing what sort of thing might be the source of your problem.
General soil improvement
For most agricultural and garden soils, the following guidelines should allow for improved soil conditions and crop growth. However, sometimes soils can be slow to change, so it can take a few years to make large changes.
You will probably be (a) improving soil structure and loosening compact soil in order to let air in, which will (b) increase beneficial soil organisms, which will (c) increase humus content and (d) improve plant health and resistance to diseases and pests, while at the same time (e) correcting soil nutrient balances and (f) detoxifying soil, which will help in all of the above, as well as (g) reducing weeds.
Soil structure that is too compact can be improved in several ways:
- Add organic matter, either as fresh, undecomposed materials such as animal manure or plant products (crop residues, straw, hay, mowed crop plants or weeds, grass clippings, tree leaves, sawdust and shredded newspapers) or as compost (decomposed organic matter)[see the link on Composting]. The fresh materials should be chopped or shredded if they are coarse and should be worked into the upper several inches of soil. There they will decompose and become humus. Do not apply too much at a time (perhaps around one-half to one ton per acre or three to five pounds per 100 square feet) or too soon before planting crops (a few weeks in the North). On the other hand, compost can be applied in small to larger amounts, and at any time.
- Cover crops (so called because they cover the soil and reduce erosion) and grass sod can loosen soil and also add organic matter. Their roots penetrate dense soil and add humus when they die. Many types of grasses and grains can be used, as well as legumes (clover, alfalfa, vetch) and crops such as buckwheat, turnip and radish. Even weeds can do the same, as long as they are mowed before going to seed. For best soil improvement, cover crops or sod should grow for at least one growing season; then they can either be tilled into the upper several inches of soil or be mowed and left on the surface to decompose.
- Soil additives can sometimes help loosen hard soil, but some of them are rather expensive for large areas (treating a smaller problem area would be practical). Some of the soil additives (or amendments) include:
- rock fertilizers, such as soft rock phosphate (colloidal phosphate), gypsum and high-calcium lime;
- humates, a mined product resembling brown coal which is high in substances found in humus;
- seaweed products, which supply enzymes and trace minerals;
- algae and bacterial inoculants, which can stimulate soil organisms; and
- wetting agents, or surfactants, which may make hard soil more porous (but not always).
- Mechanical aeration and tillage (plowing, rototilling, disking, harrowing) will do a quick job of loosening hard soil, but if nothing else is done to improve soil structure, the soil will soon settle back to its original condition. Mechanical methods can be used along with adding organic matter, for example.
Correcting soil nutrient deficiencies and/or imbalances may involve:
- Adding minerals or nutrients that are in short supply [see the link on Fertilizers].
- Improving soil structure and the number of beneficial soil organisms, which should allow plant roots to obtain more soil nutrients.
- Correcting major pH problems (too acid or too alkaline). Increasing humus levels often helps to improve pH. Very acid soil pH (below 6.0) can be improved (raised) by adding some kind of liming material, such as ground limestone [see the link on Soil parts and functions].
In some regions there are soils that present special problems:
- High organic matter, extremely acid soils are typically found in cool climates where organic matter builds up to form peat and muck soils, which are very acidic. Adding liming materials should reduce acidity, and frequent aeration by tillage (and drainage if waterlogged) will reduce excess organic matter.
- Alkaline and saline (salty) soils are common in arid regions, where salts of sodium, calcium, potassium, magnesium and others are at high levels. Crop growth is poor or impossible. These soils must be well-aerated and drained to be corrected. Adding sulfur-containing materials such as gypsum, iron sulfate and pure sulfur will reduce high alkalinity, and drainage after irrigation will leach out salts. Adding organic matter, especially compost, to the upper soil will reduce toxicity and improve soil structure.
- Prairie and dry soils, found west of the Mississippi River, are usually slightly alkaline, high in calcium and often low in humus. Adding compost and leaving fields fallow with a stubble residue should help. Adding deficient nutrients will give improved crop growth.
- High rainfall soils are found in the southeastern United States and near the Pacific coast of Washington and Oregon (tropical regions usually have similar problems). High rainfall causes leaching of certain nutrients (calcium, nitrate, sulfate), along with soil erosion. High temperatures cause depletion of organic matter, and the pH is very acidic. These soils need lime to correct acidity and organic matter to improve soil structure. Deficient nutrients will need to be added periodically.