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Everything you needed to know about soil fertility

What is Soil Fertility?

What is Soil Fertility?
by Dr.
Ieuan R. Evans

June 1, 1999

Soil fertility for plants in reality is a very simple affair but difficult to explain. There are no miracle fertilizers, only plain and simple chemical nutrients, that are absolutely essential for plant growth. In moist sand loams plant nutrients are generally more accessible than in silt or clay soils although clay soils contain higher fertility resources. Nutrients became restricted or unavailable, in very wet or very dry soils for obvious physical reasons such as a lack of root absorption by most crop species.

Plant nutrients, divided into macronutrients and micronutrients are indistinguishable, whether they are from organic or chemical sources. All must be water soluble in order to enter plant root systems.

Macronutrients are nitrogen (N), phosphate (P), potash (K), sulphur (S), calcium (Ca) and magnesium (Mg). The oxygen (O), hydrogen (H) and carbon (C) come from air and water and make up 95 to 99% of a plant’s weight. For some strange reason phosphate is expressed as P205 (43% actual P) and potash as K20. (83% actual K).

In almost all soils above pH 6 calcium and magnesium are generally present in non-limiting quantities.

In fertile soils typical of the prairies, following repeated crop removal, nitrogen, sulphur and phosphate are the first to become growth limiting. In acidic soils below pH 5, typical of higher rainfall areas potassium, calcium and magnesium may also become growth limiting. When soil pH’s are on the acid side (below pH 5) we add Ca in the form of lime, calcium hydroxide, limestone (calcium carbonate) or dolomitic limestone (a combination of calcium and magnesium carbonate), in tons per acre, to bring up the pH to 6 or higher.

Micronutrients are needed in only very small quantities but they are every bit as essential as macronutrients to normal plant growth. Plant essential micronutrients are boron (B), chloride (Cl), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo) and zinc (Zn) and if deficient they are applied to soil in pounds per acre.

Typically a 60 bushel crop of wheat, producing two tons of straw would need the following nutrients in pounds per acre (a bushel of wheat weights 60 lbs. \ 60 x 60 = 3600 lbs).

Nutrients removed by the wheat crop and the yield component of the grain and straw (lbs/acre)

Crop

Yield per acre

Yield component

Nitrogen
N

Phosphate
P2 05

Potash
K20

Sulphur
S

Wheat

60 bu

grain

75

39

24

5

Wheat

2 t

straw

30

9

65

7

 

Calcium
Ca
Magnesium
Mg
Boron
B
Copper
Cu
Iron
Fe
Manganese
Mn
Zinc
Zn
2 9 .06 .05 .45 .14 .2
9 5 .02 .03 .15 .26 .08

The 60 bushel wheat crop removes 75 lbs. of nitrogen and 39 lbs. of phosphate, almost half a pound of iron, and only a twentieth of a pound (less than an ounce) of copper. The straw, left in the field, returns 30 lbs. of nitrogen and as much as 65 lbs. of potash as well as measurable levels of micronutrients. If you take off both the wheat and the straw, then you combine both columns, i.e. 75 + 30 lbs. of nitrogen. Molybdenum levels aren’t recorded here but requirements are in the order of one hundredth of a pound or less per acre and chloride requirements are highly variable. Legumes need molybdenum for nitrogen fixation and cole crops for optimum growth, molybdenum is more likely to be deficient on acidic soils. Whip tail of cauliflower caused by molybdenum deficiency is a common problem in Ontario. The role of chloride in plant growth is unknown.

Good soils are generally understood to be sandy loam soils high in organic matter (4-10%). Poor soils are sandy soils low in organic matter (1-2%) with poor water holding capacity or heavy clay or silt clay soils which may be low in organic matter and difficult for plant root penetration. Problem soils such as flooded, saline or solozenetzic soils are not under discussion at this time.

When the unbroken prairie sod was first ploughed under, the soil organic matter was as high as 10% (in the top 6") for good soils and around 3-5% for sandy and clay soils. The key to soil fertility was this organic matter (humus) built-up in the prairie soils over thousands of years. One percent of this humus is equivalent to 11,000 lbs. of carbon, 1000 lbs. of nitrogen and 100 lbs. of sulphur (per acre stored in this organic form). Similar fertility buildups can be calculated for other soil macro and micronutrients in uncropped soils. Typically in uncropped soils everything is recycled on the soil surface - plants and animals including manures make their way back into the soil.

If a typical prairie soil with 5% organic matter is worked or fallowed for 1 year the resulting aeration and lack of crop growth will result in the release of release 50 lbs. of nitrogen and 5 lbs. of sulphur, i.e. 0.1% of the organic matter is broken down. In tropical soils this process may be up to 10 times faster. If the soil has been tilled for 20 years it’s a 2% breakdown, for 50 years it’s a 5%breakdown, then theoretically almost all the organic matter will be lost if crops (including grazing animals that are sold from the farm) are continuously removed resulting in a poor (rundown) impoverished soil. Typically people would say that the soil is worked out and needs a rest when in reality its just lost its plant essential nutrients. Of course with cereal crop removal (grain and straw) we take out the 50 lbs. of nitrogen 5 lbs. of sulphur that is released annually in cropped soils, along with phosphate, potash, calcium, magnesium and the 6 essential micronutrients. So after 100 years of crop removal an original 10% organic matter soil with no added fertilizer, may become almost totally depleted in nitrogen, sulphur and possibly many of the other essential nutrients.

If we expect to crop, say, depleted (poor) sand/loam soils with potatoes and expect optimum yields our first move is to take a soil test (top 6"). Typical soil test results (lbs/acre) for such sandy soils could be N(2); P (30); K 200; S 5 (Ph 6.5); B 0.1; Cu 2.5; Fe 20; Mn 10; Zn 0.2 (to convert to parts per million (ppm) divide pounds per acre by 2. (N = 1, P = 15, etc.). Remember an average loam soil is said to weigh 2 million pounds per acre for the top 6" although heavy clay soils would be more, organic soils much less because of the variable bulk density. So one pound of sulphur in an acre (top 6") would be 0.5 ppm. Deep soils 15 to 24" would contain far more nutrient (fertilizer) reserves than shallow (6" or less) soils.

If we’d like to get 20 tons of potatoes from this acre for the tubers we’d need to have at least N (228), P (66), K(298) and S (18) available in the soil (lbs/acre).

These numbers are the calculated pounds per acre of macro-nutrients removed in 20 tons of potatoes. Don’t forget the potato vines - for these we should add 1/3 more of each macronutrient. The macronutrients in the vines will normally be left behind in the soil (recycled). When fertilizers are added to soils, nitrogen fertilizers are only 70% available, phosphate 15-30%, potash 30-60% and soluble sulphur 50% available. Therefore the actual added NPKS should be at least 300 lbs. of N, 150 lbs. of P, 300/lbs. of K and 30 lbs of S since not all the fertilizer added to soil is plant available. Soil analysis may have shown adequate copper, iron and manganese but there’s a call for boron and zinc at 1 lb. actual and 5 lbs. actual respectively to ensure an optimum potato crop (assuming adequate rain or irrigation).

If you were to grow 1/10 of an acre of potatoes at the same 20 ton an acre yield simply divide the nutrient levels by 10. It seems like a lot of fertilizer but it is based on fact. Potato growers in Washington’s Columbia river basin (irrigation) regularly add 600N, 300P, 600K and 100S per acre to achieve their goal of 40 tons of potatoes to the acre along with ample supplies of micronutrients. How many bags of fertilizer? One hundred lbs. of ammonium phosphate fertilizer contains 16 lbs. of nitrogen and 20 lbs. of phosphate and 5 lbs. of sulphur. <?

An additional fact to remember is that crop varieties and species vary greatly in their ability to utilize soil micronutrients

For example:

1.) 0.5 ppm acre of zinc is considered deficient for an optimum bean crop but fine for wheat production.

2.) 0.5 ppm acre of copper can be very deficient for high yielding onion or wheat crops but adequate for a good pea crop.

3.) 0.5 ppm acre of either copper or zinc is fine for all cole crops.

(0.5 ppm is equivalent to 1 lb. actual available per acre).

Different crops have either an ability or inability to optimize micronutrient uptake from soil so if you are in a good rainfall area relatively non-mobile nutrients such as copper or zinc should be maintained at a minimal to optimal range of 2.0 and 4.0 lbs. or ideally at 4.0 lbs. for copper and 8.0 lbs. for zinc per acre.

If you intend planting an orchard the best kind of soil would be a well drained sand/loam or clay/loam since you must control fruit tree growth of saskatoons, cherries, apples and reduce or eliminate winter injury. Why? If you work up a loamy, high organic soil (10% organic) and plant fruit trees - then in 1 year expect a release of 100 lbs. of nitrogen per acre particularly if you work (aerate) the soil to control weeds. A newly established orchard requires less than 1/10 of that released nitrogen. The result is prolonged, vigorous lush green growth on the establishing fruit trees that make them very susceptible to fall frosts and winter kill. If there is moderate leaching or denitrification perhaps 50 lbs. of nitrogen stays available per acre in the soil overwinter. The next years weed cultivation releases another 100 lbs. of nitrogen per acre into this soil. By the fall of the third year your young growing fruit trees could have access to 200 lbs. or more of nitrogen. This is not counting the fact that you could have fallowed the land for a year prior to planting. That is the amount of nitrogen you would get if you applied 40, 50 lb. bags of ammonium sulphate per acre by the end of the third fallowed year on this 10% organic soil. Such soil is common around Edmonton, Bon Accord, Ardrossan, Stoney Plain and Ellerslie areas. This is the perfect formula for fruit tree winter-kill. Exceptions to these high nitrogen problems are strawberries, raspberries, and black currents that can produce fruit early on and use up much of the nitrogen annually because of denser plantings. If you must establish an orchard on high organic soil (5%-10% organic matter) then consider zero tillage, black cloth covers and burn-off herbicides such as glyphosate. This will greatly reduce N release.

Ideally a good soil for establishing a fruit orchard should be well drained, have a pH of 6 to 7, an organic content of 2 to 3% and the following idealistic levels of available nutrients on a per acre basis (ppm or times 2 for lbs./acre).

In lbs. acre, Nitrogen (N) 10; (P)50; (K) 200; (S) 10; (B) 1.0; Cu 1.5; (Fe) 16; (Mn) 12; (Mo) 0.2; (Zn) 4.0.

At these levels the nitrogen is crop limiting - the very element that we MUST CONTROL to enhance fruit tree hardiness and prevent or minimize winter injury. When needed we can always step up the N levels for enhanced growth. N addition though should only be done in early spring so that there is little or no nitrogen left to keep fruit trees growing past late August. There is some evidence that very high levels of applied copper (15-30 lbs. actual per acre (60-120 lbs. of bluestone) while not harming the young fruit trees will suppress nitrogen release from the organic part of the soil (humus). Such high levels of copper are routinely applied to the horticultural muck (peat or fen) soils of the Eastern United States and Canada to slow down organic decomposition of these very productive soils. Copper and zinc chelates at less than 1/4 lb. actual are often sprayed on strawberry and raspberry crops in Europe in September to speed up maturity and consequently improve winter hardiness.

I cannot say that I have made this crystal clear but maintaining good or adequate levels of plant essential macro- and micronutrients while controlling soil nitrogen levels should optimize orchard establishment. In soils with pH levels over 6 there should be more than adequate levels of calcium and magnesium available.

Foliar feeding i.e. spraying plant fertilizers onto green foliage only works for micronutrients not macronutrients, since only minute amounts actually get into the plant system enough for micro needs but far too little for the macronutrient needs of a growing crop..

Whereas plants only need six soil supplied macro nutrients (N,P,K,S,Ca,Mg) and six or seven micronutrients (B, Cl, Cu,Fe,Mo,Mn and Zn) minerals for growth, animals and man need many more micronutrients for proper growth. They are chlorine (Cl), cobalt (Co), chromium (Cr), fluorine (F), iodine (I), nickel (Ni), selenium (Se), silicon (Si), sodium (Na), tin (Sn) and vanadium (V). These eleven minerals have no defined function in plant growth or development but they must be present at trace levels in all plant parts as contaminants so that they become part of the food chain for man and animals. Sometimes one or more of these minerals may be deficient in certain areas of the prairies and we have to supplement our diet, i.e. salt for our sodium chloride needs along with traces of iodine to prevent a deficiency disease called goitre. Next time you take a multi-vitamin/mineral tablet read the label on the bottle.

In an established orchard you can assess the amount of fruit removed and if possible determine the total of the plant essential minerals removed in the fruit crop. It is always the seeds that contain the highest mineral nutrient component. If you remove 5 tons of strawberries an acre how much mineral have you removed in the form of N, P, K, S? We need facts on nutrient analysis per unit weight for each of our fruits such as blackcurrents, chokecherries, saskatoons, raspberries, and strawberries so that we can calculate the crop fertilizer needs for optimum production.

Are you any the wiser now?

 
 

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