Welcome to our free glossary!
Click on the tabs below to read and learn more about chemical properties and how they impact your soil and crops. This free soil testing glossary will help you better interpret your test results.
SP: Saturation Percent
This is the amount of water (by weight) that the soil can hold at saturation. This is approximately twice the field capacity and about five times as much water as in the soil at the wilting point. This figure cannot be improved, but the value must be taken into account when farming the soil. The ideal level is 35 to 40 (Loam soil). Higher than this is clay; lower than this is sandy.
Production is highest where the SP is high, but so are the problems. If the SP is above 45 then all other factors must be controlled. Production is lower where SP is low but the problems are minimized.
This is a valuable diagnostic measure. It indicates whether soil is acidic (PH1-7), neutral (PH7), or basic (PH7-l4). The PH is defined as the logarithm of the reciprocal of the hydrogen ion concentration. The equation is: PH = -log10 [h+] (mol/l)
As the concentration of hydrogen ions in a soil solution or water increases, it becomes more acid. Conversely, as the concentration of hydrogen ions decrease, it becomes more alkaline. A normal PH for
soil and water ranges from 6.5 to 8.O; a PH beyond this range can result in nutritional and growth problems for grape vines.
Three processes are largely responsible for soil acidification:
leaching of bases (NA+, CA++, MG++, K+) removal of bases by the crop and repeated application of acid-forming fertilizers, such as those containing ammonium. Fine-textured soils or those containing free lime (calcium carbonate), (as many California soils do) have considerable buffering capacity that reduces the effects of acidification. However, sandy soils low in cation exchange capacity and without free lime may acidify rapidly (especially with use of acid-forming fertilizers) and should have their PH monitored regularly.
When acidification occurs, topsoil is affected first. Soil should be sampled in 6- to 2-inch increments down to 2 to 3 feet for PH. Determination to establish the depth to which acidification has occurred helps in estimating the liming requirement. When soil PH values fall below 6.0, liming generally improves crop production.
The lime requirement (LR) to raise the PH can be estimated by laboratory analysis. The amount of lime necessary to raise the soil PH one unit varies with soil texture. The approximate amount of finely-ground limestone needed to raise the PH of a 7-inch layer of soil one PH unit from an initial PH of 4.5 or 5.5 is about one half-ton for sandy soil up to about two tons for clay loam soil. Usually, only the first 12 inches of soil will have become acidic enough to require liming.
Most of the soils in the San Joaquin valley have a high PH and must be treated with a soil amendment to decrease the PH, not raise it. Sulfur or sulfuric acid is normally the product of choice, but gypsum will lower the PH if it is above 7.7 (though it may be more economical to use the former). If the PH is between 6.8 and 7.7, sulfur or sulfuric is the only choice. Reduction of PH will increase production, improve the quality of the soil and will cut the cost of fertilization (if the PH is above 7). Watermelons and potatoes are among the few crops that benefit from PH values even lower than 6. Most crops will do best at a PH range from 6 to 7 and will have a production decrease at any PH outside this range. The amount of decrease varies, but is in the range of 20% for each PH unit outside this range. For potatoes, this could be 100 sacks for each PH unit above 6 (approximately 1,000 dollars per acre net profit).
EC: Electrical Conductivity
This is a measure of the dissolved solids in the soil solution. All salts are measured if they are in
solution. Some salts (calcium, magnesium, sulfate) are helpful salts while others (sodium and chloride) are harmful. There is no way to tell from the EC alone whether the salts are good or bad, but the level must be kept within certain limits. The acceptable level is from 1.2 to 2.8 (mmhos/cm). If the level falls below 1.2 there will probably be penetration problems, and if the level climbs above 2.8 there may be a loss of production due to excess salt. The tolerance level varies greatly between crops. The level can be reduced by leaching with good quality water and can be increased by adding gypsum (or some other soil amendment).
This is probably the most important element after N, P and K. This is usually reported in meq (mill-equivalent) or percent of soluble cations. The amount of calcium needed varies with the SP and the EC, but should always be from 50 to 65 percent of the total cations (the positive part of the soluble salts). Where sodium is a problem, the calcium must be increased to buffer it. Calcium has a controlling influence on sodium, as well as an improving influence on the soil (a flocculation agent). Gypsum is the most common source of calcium, though lime or limestone are also used.
This is an important element for a variety of reasons. If magnesium is not present in the soil solution in the amount of 1 meq (12 parts per million), it will cause a deficiency as a nutrient. If it is present in amounts up to 5 meq (60 ppm), it Behaves much like calcium and actually improves soil tilth (the
ability of the soil to flocculate). If it is present in quantities above 5 meq it will behave more like sodium and create water penetration problems.
Dolomite (or dolomite limestone) is the normal source for the addition of magnesium. Leaching with good quality water and increasing calcium is the normal way to treat an excess.
Sodium is the most harmful salt for agriculture because it is present in almost every soil and material, including irrigation water. Where the level is below 3 meq (about 60 ppm) there is little or no problem. There will be increasing problems as the level climbs to about 9 meq (about 190 ppm). Above 200 parts per million there are only a few salt-tolerant crops that will still give good production (some barley, sugar beets or cotton may be profitable at considerably higher levels).
The problems that sodium cause are two-fold: First is the toxicity. When the sodium level increases, the osmotic pressure increases preventing the roots from absorbing nutrients. When this occurs, reducing the level of sodium is the only solution. The second problem caused by sodium is the soil tilth. This
is the ability of the soil to flocculate well and allow water penetration. When the sodium level climbs, the structure of the soil is destroyed. Increasing the calcium will help to alleviate this. If The calcium percentage is more than the saturation percentage, there will probably not be enough sodium to harm the soil structure.
Lime: Limestone (Normally reported in tons per acre)
Limestone is generally a good source of calcium in the soil. If the PH of the soil is within tolerable limits, the limestone will generally be present in small amounts (2 to 4 tons per acre). Limestone is normally insoluble and does not contribute to the PH of the soil, or to any flocculation problems. However, if the PH is below 7, limestone or lime will increase the PH by reaction with the acids in the soil, thus using up the lime. Addition of lime to a soil with a PH above 7.5 will generally have no affect.
SAR: Sodium Absorption Ratio
This is a measure of the soil imbalance and will predict any water penetration problems. This value should be as low as possible (hopefully below 1) but any value below 2 is acceptable. As this increases, expect tighter soil and more clodding.
ESP: Estimated Sodium Percentage
This is a measure of the same elements as SAR and can be interpreted in the same manner. Lowering
The PH, increasing the calcium and/or reducing sodium will lower this value and the harm to the soil.
NO3-N: Nitrate Nitrogen
The most common fertilizer is nitrogen and the most quickly-utilized form of nitrogen is the nitrate form. It is generally reported in the soil analysis because it is the only form of nitrogen that can be depended upon to be available. It can be lost by leaching (since it is very soluble) and can be converted to ammonia and lost by volatilization. If a large amount of NO3-N is present in the soil (over 20 ppm) it is likely that production is stopped by some other problem such as alkalinity or salt buildup. CAN 17 (calcium nitrate) or UN 32 (urea nitrogen 32%) are both good sources of the nitrate form of nitrogen.
PO4-P: Phosphate Phosphorus
Phosphorus is the third most necessary fertilizer and the phosphate form is the normal method of reporting its presence since that's the way it appears in the soil. Calcium and zinc both form insoluble salts with phosphorus, thus making it unavailable. At PH below 7 this occurs hardly at all, so correct PH will stop phosphorus deficiencies from occurring that would occur at higher PH. If this condition continues for a long period of time, the insoluble salt gets coated with precipitations of limestone and will not be available even when the PH is corrected.
When phosphorus is added to soils where the PH is higher than it should be, the additions should be made as close to use as possible. That is, the additions should be made in the spring and not the fall before. An acid form of phosphorus is always preferable to another form.
Potassium is a fertilizer that has been overlooked to a large extent in the west, because most of our soils had plenty of it present when we started farming. However, after over half a century of farming, the native potash has been used up, so more must be added. The amounts necessary for production vary, but normally is approximately twice the amount of nitrogen. Potassium is quite soluble and would not pose a problem of availability, but the soil affinity is such that the soil is a prime competitor of the plant for potassium. The potassium must be in the neighborhood of 10 times the amount necessary for the crop (in the root zone) to supply the plant. This means that the level reported in exchangeable parts-per-million must be above 200 to 400 in the top 3 feet to ward off a shortage. Levels up to 1000 ppm have not shown to be harmful, but would be expensive to maintain. Good levels of potassium are sometimes offset by high levels of sodium. When sodium levels are very high, the plant sometimes uses it in places where potassium should go, so production suffers.
Additional potassium will sometimes alleviate the problem, but more often we must reduce the sodium by leaching.
Zinc is one of the micro-nutrients that plants need. The level in the soil must be from 5 to 15 ppm of DTPA extractable. The analysis of total zinc is not a useful figure, since many of the zinc salts are insoluble and therefore useless to the plant. Excess zinc can tie up phosphorus (and excess phosphorus can tie-up zinc), so proper levels should be maintained and annual additions are better than one-time large additions.
Another one of the necessary metals, and the one that should be more abundant than the others. The level of iron (DTPA extractable) should be from 50 to 200 ppm. Massive amounts added do not seem to harm, since iron does not seem to interfere with other nutrients or micro-nutrients, though almost all the other metals can and do interfere with iron.
This is not to be confused with magnesium. Manganese is a metal that is almost never found to be deficient in the valley (though there are a few spots) but quite often found to be high enough to interfere with iron. The levels in the soil should be from 30 to 80 but preferably less than the iron. The only thing that can be done when it is too high is to limit the addition (by mistake) in other fertilizers and foliars.
One of the most overlooked micro-nutrients in the valley in the past is copper. Copper is one of the most toxic of the metals, but part their natural usefulness is to protect the plant from pests and competitors, utilizing copper to form toxic chemicals. It is therefore necessary that copper be made available to the plant, although the amount necessary is very small. The amount of copper necessary is 5 to 15 ppm and should be added as the sulfate annually as copper sulfate (bluestone) about 25 lbs per acre. Testing the soil is very important so that toxic amounts are not built up.
One of the ways to insure that toxic amounts of an addition are not made is to add nutrients in the form of sulfate. The chloride or other anions are quite often soluble enough that toxic amounts can be present before precipitation occurs. Sulfur is also an important nutrient that is necessary in small amounts. In the soil the sulfate sulfur should be from 100 to 1000 ppm.
The value of organic material is vastly overrated. The organic matter is important as a means of supplying humic acid and bacteria present in the soil. If all other factors necessary to good production are corrected, earthworms and bacteria will become prolific and increase the organic matter content. Other factors become the important factors, and not the organics.
As with sodium, the chloride is harmful because of the toxic effect of the salts but will also harm the plant due to the toxic quality of chloride itself. Chloride and the other anions do not contribute to the problems of water penetration as do the cations.
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