2011-04-08 Göte Bertilsson

A continuing project. Please comment and contribute.


Soil and crop management for today and tomorrow.


This is not a basic textbook. It is an attempt to summarize and integrate knowledge and  ideas for our present situation, where the challenge is to meet both production demands and environmental goals at a global level.






We have deep knowledge in soil and crop  science as well as in environmental sciences and ecology.  It forms a base for advanced models on the behaviour of soils and crops, the growth processes and environmental consequences of various kinds.  Continuous development is going on, The main problem is the variability of  soil and weather parameters.


We have field experiments giving a base for recommendations to farmers.


This is transformed to actions in the fields, by advisors and farm managers. Their  main task and interest is to optimize the economic result of the farming enterprise.


We also have a global and political environment listing demands or in any case environmental problems to work on:

Food production should increase

Water is scarce

Climate gases should be reduced

Nitrogen flows should be reduced.

Chemicals in general should be reduced

Agricultrual land may be needed for bioenergy

Biodiversity needs consideration.


The practical agricultrure is influenced by economic factors, various administrative measures and, not least, by the knowledge and interest of the farmer

The intention with this work is to try to summarize and discuss this complex and ways to handle it.


The base: the soil.


In fact: soil is not needed for growing crops. It can be done in water culture, in mineral wool or gravel beds. But in that case continous control of water and nutrients is needed.


With the soil as base, the plant almost always has water and nutrients available, more or less. The soil is a wonderful and complex system, almost a living organism.

A skeleton of mineral particles gives support and creates a porous system which can store water and allow passage of air and other gases.

Organic matter, humus, combines with the soil particles and builds up a  structure favouring water characteristics and giving an environment friendly for root growth.

Living organisms, from bacteria and fungi to various animals live on plant residues and keep the process going. They are breaking down soil organic matter and forming new. The soil organic matter is dynamic and changes according to environmental conditions and agricultural management.

The minerals in the soil are subject to weathering and may release soluble minerals giving a base for plant nutrition.

Further – the pH of the soil and the drainage characteristics ave very important for the function of the soil.


The soil profile.

The roots of many crops go surprisingly deep. The roots of winter wheat, for instance, normally are at more than one meter deepth already in the Autumn, if the structure of the soil profile permits. This is important. A fertile, highproducing soil has a deep active profile. That gives a more reliable supply of both water and nutrients.


Soil fertility – of crucial importance also today.

It might be believed that with our technical resources with machinery and fertilizers we should be more independent of the soil. That is only partially true. For high yields we need a well functoning soil giving reliable supply of water and nutrients. We can express it this way: a medium yield of  5-6 tons can be achieved even with some temporary shortages of water or nutrients, but for 9-10 tons the function must be on top all the time. The soil determines the possible outcome.


Threats to the soil.



Heavy machinery is a threat. Soil compaction means reduced permeability for water and air and less favourable growth conditions. Compact soil restricts root development. In the top soil compaction can be reapaired by plowing and tillage, but compaction of the subsoil persists for a long time. Trials with deep cultivation have not been very encouraging. Subsoil compaction threatens the functioning of the entire soil profile. Heavy loads on moist soils are damaging.


Avoid heavy machines. For instance – there are techniques for distributing and spreading slurry by pumping.

Avoid traffic on moist soils.

Minimize field traffic.

Use fixed tracks on the fields.

Use deep-rooted crops in the rotation.


Organic matter.

The content of organic matter is in long-term equilibrium with the cultivation system. But the processes are slow. A hundred years or so are needed to approach equilibrium, at least in north European conditions.

For Scandinavian conditions a common system with cereals and oilseeds seems to tend to an equilibrium content of abour 1,5% organic carbon, provided straw and residues are returned.

Leys in the rotation give a higher value, row crops as potatoes, beets and maize a lower.


1.5% organic carbon is too low, according to available knowledge. On soil with less than 2% organic carbon measures to improve the organic matter content will give yield increases.



Leys in the rotation

Manure and other sources of organic matter are favourable.

Cover crops are positive, especially if they are left until late autumn or preferably spring.

Delayed or reduced autumn tillage in general.

High yields are positive, especially when residues are left in the field.

Combinations may amplify the effect, for instance high yields+residues+reduced tillage.


Lime and pH.

In Norhern Europe we have a precipitation surplus and in general a natural leaching and loss of lime. Crop production and fertizers are also contributing. Mostly ”acidifying” nitrogen fertilizers are used. However, the real mechanism is complex. The acidifying effect is to a great extent caused by the increased production and resulting increased uptake and removal of minerals from the soil. Also biological nitrogen fixation is causing acidification.


What soil pH is needed? That depends on the soil and the crops. Crops have different demands and also soils react differently. For clay soils a fairly high pH (6,5-7.5) is important for the structure and general functioning. It is important to keep in mind that lime losses occur and need compensation. A high pH means higher losses and a higher maintenance demand. In natural soils the pH are often low, the lime losses low and may be in equilibrium with the weathering of soil minerals. But every use of the system, removal of material, leads to mineal losses and increased acidification.


Nutrients in soil.

This is an extensive and complicated topic. Here is only a brief overview.


Nutrients – that is nitrogen, phophorus and potassium, but also magnesium, sulphur and several micronutrients.

With fertile soils we mostly mean soils rich in lime and minerals. The parent rocks often are lime stone or shales of different kinds.

We can add nutrients,  use fertilizers. Most soils store phosphorus very well, to some extent also potassium and magnesium. But for nitrogen the case is different. The nitrogen in soils (most of it) is in the organic matter, and both build up and release are governed by rate limited biological processes. The rates do not suffice for neither efficiently taking care of excess available nitrogen nor delivering to a demanding crop. Nitrogen availability must be considered annually (or on shorter basis) for each crop.

Different micronutrients need consideration. Copper and to some extent boron can be stored in the soil. Manganese and iron are seldom quantitatively deficient, but may be tied up as insoluble xompounds which are unavalable for the crop. Annual consideration is needed.

The crops also need zinc and molybdenum. For Swedish conditions the soils normally give adequate supply, but not always. Humans and animals also need selenium and cobalt. Our soils (Scandinavian) are ofter poor in selenium which needs consideration. In Finland fertiizers are enriched with a low content of selenium for human health reasons.

Also calcium is a nutrient , but that is mostly overshadowed by its importance for the lime situation and pH of soils. Chloride: the plant needs small amounts which ”Nature” provides. But chloride is often important for soils and fertilization. Most potassium fertilizers contain almost equal amounts of potassium and chloride. In coastal areas high amounts of chloride are deposited by rain.

Sodium – beets give yield increases for sodium supply. The beet plant seems to remember its origin as a shore plant on soils with high amounts of sodium and chloride. There are also cases where chloride seems to give yield increases to other crops.


About systems and recycling.


If residues and refuse are recycled to the soil at least some part of the minerals taken away will be compensated.


In Nature.

The soil receives minerals and other substances from rain and air. Nitrogen is ”fixed” from the air by different organisms. There is a loss by leaching and erosion. Nitrogen and sulphur can be lost to the atmosphere. Between these proesses the soil evolves.


With our society.

We remove products from the soil. We have created a large ”unnatural” outflow of nitrogen and minerals. This had big consequences in old times, although the impoverishment of farming soils to some extent was counteracted by inclusion of meadows in the systems. Still the societies were small and local recycling was in any case to some extent possible. Today more than half of the global population lives in cities without contact with agriculture and rural society. A recycling opportunity has developed into a waste problem.

Mineral fertilizers and lime materials compensate the soi.

There are many reasons for improving recycling, but the difficulties are great, not only concerning economy and logistics. There are substances and elements in the refuse we don't want to recycle, metals as cadmium, some organic compounds as well as infectious agents.

However, it seems important that the recycling idea is kept in mind, and that innovative solutions are encouraged.


The soil – aspects on environment and sustainability.



A good function of the soil is important for both production and environent. High yields are positive  for the soil. Farming measures aiming at long-term productivity are often win-win situations, positive both for production and environment.


Shortterm – longterm.


Soil compaction situations.

A hectic day en Spring. Slurry needs to be applied before tillage and sowing.” A little too wet but it cannot be helped. We need to sow tomorrow before the rain is coming.”

Another example:  The tyres are equipped with advanced technique for adjusting the pressure between road and field. But it takes some time. ”There is no time for that today,”


Cover crops.

Cover crops building organic substans in the autumn are positive för the soil organic matter and for

soil fertility. But initally they may involve some costs and trouble. That is directly evident, the longterm advantage has less weight, unless it is specifically stressed.


There is a need for better communication of the economy of the longterm aspects. Soft arguments like ”organic matter is important” need to be translated into hard economy to be properly considered by the farm management.



Proper lime status is also a win-win situation.  In most cases maintenance is necessary to counteract the lime losses by leaching and crop mineral uptake. Liming means loss of carbon dioxide, but that is an unavoidable consequence.

We assume a maintenance requirement of 150 kg calcium oxide per hectare and year. Calcium carbonate contains 56 percent calcium oxide and 44 percent carbon dioxide. Application of 150 kg calcium oxide means that 120 kg carbon dioxide is released and contributes to global warming. Maybe other lime materials should be used? It does not help, it only means that the carbon dioxide is released at the lime factory instead.

There is no solution, unless carbon dioxide capture is installed at the lime factory. We have to maintain the soil status, but we should not keep a higher soil pH than necessary.

Maybe research should investigate possibilities to satisfy the needs of especially lime-demanding crops (sugar beets etc) without raising the pH of the entire soil volume.

More recycling of organic material or minerals would reduce the demand for maintenance liming.


Plant nutrients.


The balance principle: put ecology in the first place and the economy will follow.


The use of fertilizers mostly is governed by economic terms: what is the cost of the fertilizer, what is the value of the yield increase and what is the most economical dose?

However, the plant nutrients applied will interact with the soil-plant system and ultimately with our whole environent.


Could we start in another way: what do we expect the soil to deliver to our harvested product? Could that be a first estimate of the need for nutrient application?


An example:

Winter wheat on a good field which has in average yielded 8 tons the last two rotations. That yield contains 160 kg nitrogen and 25 kg phosphorus (let us stop there in this example). What reason is there for not applying 160 nitrogen?  Well, last year there was a clover seed crop, which we estimate gives an extra 50 kg nitrogen. Then the application should be adjusted to 110. For phosphorus we have a high soil analysis and the soil content is unnecessary high. Therefore, we apply no phosphorus to the wheat crop. The soil delivers

The same thinking should be continued for other nutrients.


Let us call it the Balance Principle. Is it too simple? Too unreliable? At least it is a subject to discuss.


A few conditions:

It is not a pure replacement. If considering a perhaps low yield last year and replacing that offtake it will not work. Consider instead the actual realistic yield goal and plan after that. Another important quesion: is the yield potential attained? If we do not look forward there will be no progress. 8 tons is high, but 9 is higher. Your own experience, neighbouring results and perhaps field experiments might give informatin.

Further: is the soil in reasonable equilibrium?  Perhaps phosphorus is low and the soil needs improvement? Then this must be considered.


Replacement fertilization has great limitations. It cannot improve the situation. But when an improvement phase has matured, the situation is different. Probably a balance principle will gain in importance. It has a solid longterm base. But it cannot be dogmatic. It has to be adjusted according to knowledge and analyses available and adapted to local knowledge.


Probably we will see that the principle of economic marginal profit as a base for fertilizer recommendations will be difficult or impossible to use in a situation with strongly fluctuating and high prices.


However, we cannot neglect the prices. Some principles are discussed below for respective nutrients.


Tools for the Balance Principle:


Fieldwise records, preferable 10 years or so. Crops, yields, nutrient inputs. Protein analyses give important information about the nitrogen situation and functioning


Soil analyses. Phosphorus, potassium, magnesium, boron, copper, pH, organic matter. These are not needed every year, but every 5-10th. Also, control analyses of plants or soils during the growing period to check the direct nutritional status for for instance manganese and other nutrients.


Zero nitrogen plots. A few square meters without any nitrogen application shows the capacity of the soil profile to deliver nitrogen, the system background. The uptake can be measured to get a quantitative value or the stand can be probed by a portable sensor. Still – only visual observations tell great deal.


Sensor guided application and other developments in the area of precision farming.


With all these modifications, what remains of the Balance Principle? Most important is that that the base for the nitrogen recommendations is an estimate of the demands of the crop, not the marginal economics. This more ecological base gives automatically a more longterm consideration.




Nitrogen in the soil.


Organic matter and mineralization.

The soil organic matter is a large nitrogen reservoir. A normal agricultural soil in Northern Europe contains 5 tons of nitrogen per hectare in the soil organic matter, give or take a few tons. The organic matter is broken down by soil organisms, the nitrogen is mineralized, which releases 50-100 kg nitrogen per year for the crops to use or as loss to the environment.

The content of soil organic matter varies, the mineralization is about 1-2% per year. Thus, the contriburion of plant available nitrogen varies. It can be estimated by measuring the nitrogen uptake in small plots sheltered from application of nitrogen fertilizera, ”Zero plots”.

Great efforts have been made to develop some laboratory method for determining the mineralization, but so far there is no practical method available.

Some soils have organic matter also in the subsoil, which somewhat contributes to the nitrogen delivery. This will also be included by a Zero plot method.


Mineral nitrogen.

Ammonium and nitrate are also present in the soil. Excess fertilizer nitrogen or mineralization in the autumn not absorbed by crops can result in considerable amounts in the soil profile, contributing to available nitrogen for the next crop, if it survives the winter. This might be important and there are methods for measuring this factor and include it in the fertilizer recommendation procedure (N-min, nitrogen profiles). It is important that the soil sampling also includes the subsoil, to 60 or maybe 90 cm. This has been used in Swedish agriculture, but it has been showed that for normal crops this component is of less importance than variations in the mineralization. A zero plot method includes also mineral nitrogen in the soil profile.


Nitrogen forms.

In the agricultural soils available nitrogen is converted to nitrate, often within a couple of weeks. It is a microbial process, nitrification. With fertilizers nitrogen is applied as ammonium, nitrate or urea. Nitrate is the final form in the soil in any case, bur for ammonium and urea the availability is somewhat delayed. Urea nitrogen might also be subject to losses by volatilization of ammonia if left on the soil surface.

Manure and other organic fertilizers also contain organic nitrogen. This must be mineralized to be effective, which takes time. Alro for organic nitrogen the final form in the soil will be nitrate. It should be mentioned, however, that the plants can readily use ammonium nitrogen when the roots reach it. The problem is the transport in the soil because ammonium is absorbed on soil particles. Also small organic molecules may be absorbed by plants, but this is of small practical importance in agriculture.

Biological fixation of atmospheric nitrogen is more diverse than often believed. Legumes are important for the agriculture, peas, beans, clover and luserne. However,  some ”freeliving” soil bacteria also fix nitrogen, especially in the rhizosphere of plants. For rice, sugar cane and grasses in the tropics this is of great importance. Symbiotic nitrogen fixation provides nitrogen to the actual plant but with roots and residues important contributions may be passed over to following crops. There are some reports that in mixed stands with for instane legumes and grass som nitrogen contribution is given to the grass.


Nitrogen to the crop.

Proteins and other substances containing nitrogen are important constituents in plants. Cereal grains contain about 15-20 kg nitrogen per ton. A crop giving 6 tons contains 120 kg nitrogen in the grain which is removed and in addition maybe 30 in the straw and 30-50 in the root system. In total 180-200 kg nitrogen is engaged.

The soil has a limited capacity to deliver nitrogen. The store may be large but the delivering capacity is limiting. If the crop cannot obtain the 200 kg needed there will be no 6 tons of grain, but less. In this way it can be said that nitrogen availability governs the yield formation. But the nitrogen availability can only constrain the yield, it cannot promote it above what is allowed by other factors as water, climate, stand development, diseases etc. The nitrogen fertilization should be adapted to the yield capacity of the stand and the soil.

In the present example the fertilizer application may be 120, the soil ontributes with 80. 120 is removed with the grain harvest. Straw and roots remain on the field and pay back the soil contribution. In addition we have a contribution from the atmosphere (10-20) , nitrate leaching (20-30) and also additional soil mineralization after the ripening of the crop. There are also some nitrogen losses by denitrification, which means that nitrogen gas and a small amount of dinitrogen oxide (climate gas)  is released from the soil.

Some important conclusions:

Nitrogen availability may constrain the yield but it cannot lift it above what other factors allow.

In well functioning systems there can be almost balance between input in fertilizers and removal by the harvest.

It is necessary to keep in mind that surplus nitrogen is environmentally harmful.


A practical situation.

The following diagram describes the results from an experiment in Norway, as part of a Scandinavian project (Lindén et al, 1992).

Increasing nitrogen rates (horizontal axis) have affected the yield (vertical axis). The experiment has been repeated 3 years, 1984 – 1986, on adjacent areas of the same field . These 3 years and their average are shown in the diagram. The soil and the crop (Spring Barley) is the same. Still, the yield results are very different.









The optimum for the average (price relations 10 kg grain pay for 1 kg N) is 90 kg nitrogen and that fits well for 1985, But 1984 the optimum is 120 and 1986 only 30 kg nitrogen. The reason is not differences in the soil delivery. There are only small differences in the zero treatments, and should we consider those more nitrogen should be needed in 1986, quite the wrong direction. Other unfavourable circunstances depressed the barley in 1986 and then more nitrogen fertilizer does not help at all.


This is a very evident example of the problems encountered in practice. The weather, diseases (1986 is third year barley), weeds, problematic establishment etc affect the crop. Averages from experiments give a general guidance but we saw in the example above that it did not work well in 2 years of 3. But now better methods are available for adapting the nitrogen rate the individual year. By means of sensor technology the application rate is adjusted according to the state of the crop and as it varies in the field. This involves the following steps:

  1. Study the crop history of the individual field: yields, protein contents, observations, fertilization. Zero plots or -observations are valuable.
  2. With this background, estimate the yield goal and corresponding probable nitrogen demand.
  3. Plan a two-step strategy. Apply 50-70% of estimated demand at ”normal” time, make a complement later, preferably with sensor guidance. But even without sensor this two-step strategy is important. Stand observations (preferably plant or tiller counting) and the seasonal climate may give background for adjusting the previously planned fertilizer rate.


In this way unnecessary overdoses can be considerably reduced the problematic years and the high yield potentials fulfilled the good years. There are economic gains, although these should be weighed against the costs and trouble invoked by the two-step method.


The environmental aspect is important as will be expanded below. In the practical example described above in 1986 about 100 kg residual nitrate was found in the soil. Most of that will be lost to the environment in different ways and cause both local and global problems.


Nitrogen in the environment.


Nitrogen leaching gives eutrophication in streams, lakes and seas (together with phosphorus).

Ammonia losses distribute nitrogen in nature and increases eutrophication in general.

Nitrogen turnover in soils and waters produces some dinitrogen oxide, which is a powerful climate gas and also a threat to the ozone layer.

In general, the human society should aim at reduction of the nitrogen flows. This is a hard task as we have an expanding population in itself needing more and more nitrogen. It is really an urgent task to use nitrogen efficiently in all respects. The agriculture has an important role here.

Preliminary ecological estimates say that the global nitrogen flows are too large already today. The global ecology is negatively affected.


Nitrogen has many ways. An example. An overdose of nitrogen increases residual nitrate which is leached to rivers and the sea. Plants and algae are fertilized and increase. Eventually the nitrogen is released, and some of it is in the form of ammonia och dinitrogen oxide affecting the environment. The nitrogen surplus also gave higher nitrogen concentration in the soil giving increased formation of dinitrogen oxide there, and also higher nitrogen content in straw and roots, which also increase

 these losses. Further, the harvested grain will have unnecessarily high protein content, which might increase losses in the utilization chain. Still further, the production of the unnecessary fertilizer led to unnecessary emissions of climate gases and unnecessary energy use. This is an example of a cascade effect. Introduction at one point leads to many consequences downstream in the chain .

We need to straighten up nitrogen use where we can.


Marginal effects of nitrogen.

For especially nitrogen the marginal economy is often stressed. You shoull apply up to a level where the cost of an additional kg N is compensated by the value of the yield increase. Especially at high product prices this is almost a dangerous principle. With the balance principle discussed above we avoid this. But the marginal philosophy is one of the ground pillars in the economy of today and it is also extensively used for calculation of nitrogen optima. So – let us take it one step further and look at marginal ecology..


We use the practical example from above: the experiment at Ås, Norway. We use the results from 1984, the best year.

GHG = GreenHouse Gases, carbon dioxide equivalents. Nitrogen fertizer manufacture+turnover in soil

Prices: SEK 12 per kg N and 1.90 per kg grain. Relevant for Spring 2011. Net is grain value minus fertilizer cost.




SEK/ha net

GHG kg


Kg N

Energy GJ/ha netto


Aver. per kg grain

Stepwise per kg






























































































With the prices given 150 N is the most economical treatment.  The Balance principle would have given about 110 as recommendation.

What happens when you go from 120 to 150?

The economic gain is SEK 20 (less than the value of 2 kg N).

GHG increase from 1776 to 2028, 252 kg. That is more than the normal emissons from the tractor per hectare. You get 200 kg in increased production which means 1,26 GHG per kg grain. This is more than would be expected from plowing up old grassland. The actual Swedish carbon dioxide tax is SEK 3000 per cubic metet oil, which means about SEK 1 per kg GHG. If we use that figure we have a cost for climate gas emission of about 250 for this step. The leaching is estimated to inrease by 7 kg N. In other situations the society is prepared to pay SEK 50-150 per kg N leaching reduced. If we use 50 we have a leaching cost of 350.

The fundamental economic maximization led to a gain for the farmer by SEK 20 and emission costs for the society of more than 500. If we scrutinize the table we see that around 100-120 kg N gives a good combination of economy and ecology. There is a very small  theoretical loss of income which should be discussed with the farmer.

We should not over-emphasize marginal economics for fertilizer recommendations, There are traps in the system, especially when broad averages and response functions are used. We should look more to the actual demands and use either a balance principle with adjustments ot, preferably, a stepwise practice with sensor guidance. Methods and technology are available.

More details from experimental data.


Nitrogen and soil fertility.

In Swedish agriculture nitrogen fertilization is a positive factor for soil fertility. It gives high yields and also increased production of crop residues and is positive for soil organic matter. All longterm experiments give evidence in this direction.


But you can of course speculate: how would agriculture work without nitrogen fertilizers? There would have been more leys, more direct combination of animal and crop production. Lower intensity, lower yields, much lower production. There would probably be some advantage as concerns landscape and diversity. But it is not without reasons we have had the development we have seen. The reasons are at least two: rural standard of living and feeding the urban society at reasonable cost.

It is possible that we had a low mark concerning soil fertility development during the 1960s and 1970s. Straw burning was common, manure was not used optimally, the yields were mediocre.  The agriculture of today with high yields and proper residue management has changed that.

There is another step to take. With optimal use of cover crops and reduced tillage we can improve soil fertility and create resources to better take care of biodiversity and landscape issues.


There are a few reports internationally that nitrogen fertilizer reduces organic matter and soil fertility. Of course this may happen in cases where the soil microbial community is nitrogen starved, or when cropping with legume cover crops is used as comparison. In northern European conditions nitrogen is a positive factor, but this does not mean that considerations concerning rotation and soil organic matter maintenance can be forgotten.


Nitrogen as a resource.

Nitrogen as such is no resource issue. The reserve, nitrogen in the atmosphere, is inexhaustible. However, it is an issue of energy and environment.If it costs more energy and emissions to recycle nitrogen than to produce new fertilizer recycling is unfavourable. If cropping with biological nitrogen fixation in the end costs more energy and emissions than more intensive fertilized systems it is of no general advantage. Such comparisons must be made considering the whole system, and there is no general answer. When comparing nitrogen fertilizer with legume nitrogen you get one result in Sweden and probably another for smallholders in inland Africa.

Often in the general discussion a reduction of nitrogen fertilizers is seen as a goal in itself. This is logical only if there is a general over-use of fertilizers. If that is not the case there is the question about the consequences of the impact of alternative nitrogen sources or of decreased production. The crops, the food production, need nitrogen, and  the task is to manage this as well as possible considering all aspects.

If we compare organic agriculture with properly managed normal agriculture no advantages have been shown for the organic. But there are some key words: ”properly managed”. That is not always the case. Let us work on it.


One important issue on comparisons. The nitrogen industry has improved considerably the last few decades. The energy consumption and the emissions have been greatly reduced. Many ”old truths” are not true anymore.


On energy: nitrogen fertilizer is in fact an energy source, or an energy amplifier. Properly used one kg nitrogen in fertilizer (energy cost 35 MJ) gives at least 30 kg dry biomass (practical energy content about 12 MJ per kg). This means an amplification of about a factor 10, marginally. If we upgrade this bioenergy to refined biogas there is still an amplification factor of 4-5.



Phosphorus in the soil.

A normal Swedish soil contains 1 -2 tons of phosphorus in the top soil layer. In Sweden the P-AL method is used for estimation of ”available phosphorus” and P-HCl for ”reserves”. These fractions are in certain equilibrium.  In addition there is still less soluble  phosphorus in minerals and in soil organic mattter.


Long term experiments give evidence about the soil P dynamics.  Here is an example from the soil fertility experiment Orup in South Sweden.

The treatments are as follows:

A. Without P

B. Replacement of exports according to analyses.

C. R + 15 kg P/hectare and year.

D. R + 30 kg P/hectare and year


A comprehensive soil analysis was made after 27 years. We know that D has received 810 kg P more than B and C 405 more than B. The export from A is about 400 . What do we find?


The following figures are expressed as kg/hectare in the topsoil. Order: Organic P, Reserves, Available and Sum.


A: 989,820,50, sum 1850

B: 1040,860,100, sum 2000

C: 1040, 1060, 200, sum 2300

D: 1060, 1260, 320, sum 2640.


In D, 810 had been added compared to B, 220 is found in Available, 400 in reserves and 20 in Organic, sum 640. Some would have been brought to the subsoil by rroots. There are of course uncertainties in soil sampling, soil density estimation and analysis, but the main conclusions are crystal clear.

We see an interaction between Reserves and Available. The reserves are truly active, take care of surpluses and deliver when needed.


Leaching and other losses? The mean phosphorus loss from Swedish agricultural soils is 0.3 kg per hectare and year. In 27 years that would mean less than 10 kg, to be compared with the figures above. These losses are unimportant for the soil and agriculture but have a large influence on water bodies.


This picture seems to be normal for most soils. We can use the reserves as a buffer, consider the phosphorus fertililization over a rotation period and apply were most efficient.


Phosphorus to the crop.

A good crop takes up about 25 kg P, and as we discussed above there is more than a ton in the topsoil only. In addition there are considerable amounts of P in most subsoils. Why is it necessary to add P in fertilizers? But experiments, also the Orup experiment we discussed above, show high yield increases for fertilizer P. It is not the total amount that matters, it is the availability which in Sweden is estimated by the AL method.  There are other methods elsewhere.


As a rule, the plant roots must seek out the phosphorus. The concentration in soil water is very low, and the phosphorus in the water the roots take up is completely insufficient. But when the root tip is boring its way in the soil it sends out root hairs which make close contact with soil minerals and absorb P.  A kind of tunnel of P depleted soil is established around the root. So, it is understandable that the root development is important. Mycorrhiza can add considerably to the contact plant – soil.


Crops differ in ability to utilize soil phosphorus. Grasses, clover, rye and winter wheat are are fairly efficient. Beets, potatoes and maize are less efficient and have greater demands and higher fertilizer requirements. Spring cereals and oilseeds are in between. New varieties and cultivation techniques influence recommendations and continuous updating is necessary.


Phosphorus fertilization should be planned with long term consideration of the change in soil P pools. For example, winter wheat takes up about 25 kg P but can use soil P if the values are not too low. The direct response to additions in fertilizers is low. But the barley or rape which may follow respond better and it is more favourable to concentrate the phosphorus fertilization to these crops. An optimizaton for the whole crop rotation is advantageous.


Phosphorus and environment.


The phosphorus outflow to the Baltic Sea is seen as one of the main environmental problems caused by agriculture. Still, the specific loss of P from agricultural soils is low, about 0.3 kg P per hectare and year in average. But P is strongly affecting water bodies. The agriculture should do what is possible to further reduce the outflow of P.


Part of the outflow is caused by erosion, soil material carried away by flowing water. Measures as keeping a vegetation cover in the autumn and winter,  leaving harvest residues on the soil surface and adapting the soil management are important.


Plant material may release P, especially after freezing. Here we see a conflict. Plant residues on the soil surface reduce erosion but may increase P losses from the residues themselves.


Some P is dissolved in water. The concentration is partly dependent of the soil P status. Above P-AL 10 (mg P/100 g soil)  there is a marked increase of the P concentration in outflowing water. For environmental reasons this level should not be exceeded. Is this a constraint for agriculture? The answer is no. For all normal crops we have full production at P levels below P-AL 10. However, farms whith animal production have manure to take care of and that complicates the issue. In Sweden there i a limit of 22 kg P per hectare in animal manure as average over the farm. That gives a certain balance but not always possibilities to adjust an unnecessary high phosphorus status.


If the phosphorus status is suitable for the cropping system it has to be maintained by replacing the export. It seems that replacement is suffient on most soils. However, certain soils may continue more than replacement to keep the status as measured by P-AL and monitoring by soil analysis is necessary. Some decades ago, during 1950-80, we had phase of soil fertility build up. Recommendations considerably higher than replacement were used at that time.


It is important that the P fertilization is done efficiently. Naturally, it is positive to get as high yield increase as possible for the maintenance fertilization given. Placemant is an efficent method especially if NPK fertilizers are used.


Phosphorus and sustainability.

Phosphorus is essential for life and nothing can replace it. The discovery of the secret of plant nutrition gave us P fertilizers and the possibility to increase the crop production and secure the food supply. It is hard to see that our society can continue development without having contros of the P supply.

Phosphorus is a fairly common element, but it is tied up in minerals mostly in low concentrations. The richer deposits we know of today do not last for ever. If the most recent estimate of reserves is divided by the present consumption we arrive at about 400 years. In addition there are known resources which are more expensive to use but give the figure 2000 years. The estimates of both reserves and resources have been considerably increased recently. Previous alarm reports saying the phosphorus sources are depleted in about 100 years are not valid any more. If the phosphorus costs rise more sources will be economical to use.

The phosphate deposits contain many elements, also some we do not want to have in fertilizers. Some purification process will be increasingly necessary. Pilot scale data indicate that this is not a critical issue for the P supply.

Nevertheless, it seems important to economize with our phosphorus reserves. To continue the single direction flow of P from agricultural soils to waste deposits and waters is not sustainable. Only a fraction is recycled today.

Another factor is the continuous enrichment of the biosphere. Phosphorus is a very active element and fetching it from locked geological reserves to the biosphere has consequences. Recycling would reduce this flow.

So – in spite of the upgraded P reserves improved recycling should be developed and encouraged. There are some alterntives and more in development.




Potassium in the soil.


Potassium is an exchangeable ion and is temporararily adsorbed on the surface of mineral particlis. It is also a constituent of soil minerals, and especially clay soils contain large amounts of potassium which can be successively released. When the plant takes up potassium, soil solution and exchangeable K are used firsthand. This causes background reserves to be mobilized. Clay soils can deliver K during long time, depending on clay minerals.

Available K is in Sweden estimated by means of K-AL, which roughly corresponds to exchangeable K, the reserves by K-HCl.


For potassium the reserves do not function well as a flexible buffer. Soils low in K usurally are poor in clay and potassium surplus is prone to leaching or luxury uptake by the crop.


Potassium to the crop.

The plants need substantial amounts of potassium, 100-300 kg K per hectare. Potassium is present mainly in the vegetative parts of the crop. If only the grains are harvested only a few tens of kilos are removed, with straw harvest considerably more. High yields of leys remove large amounts of K and the balance needs consideration. 10 tons of crop dry matter contains at least 200 kg potassium.

Like for phosphorus the uptake of potassium from the vicinity of the roots is important. Even if there are large reserves and the removal is small the acute demands of the crop need to be satisfied for a good yield. The fertilizer recommendations are based on field experiments and consider this aspect.

Potassium interacts with other nutrients, especially magnesium. Potassium fertilizer may increase the risk for magnesium deficiency. This interaction is considered in many fertilizer recommendations, the K/Mg ratio.

Recent experimental results in Sweden show unexpected yield increases for placement of NPK compared to placement of NP also in soils with high K status. The reason is not clear. Speculations:

It may be an advantage if all nutrients are available in the same soil volume, especially for rapidly growing crops as spring barley.

Another possibility is a direct or indirect chloride effect. We talk about potassium addition, but in fact nearly the same amount chloride is added if normal fertilizers are used. It can influence both the plant and the transport in the soil.

Our knowledge is incomplete. The work must go on.


Potassium and the environment.

No environmental problems caused by potassium in agriculture are reported.


Potassium and sustainability.

The raw material for potassium fertilizers are geological deposits of salts. The reserves are large.




Magnesium in the soil.

Magnesium is an exchangeable ion, just like potassium and calcium. It can be stored in the soil, better the higher the clay content. Soil analysis: The Swedish method is Mg- AL, which roughly corresponds to the exchangeable K.

Magnesium containing lime is a suitable magnesium source for soils needing lime. Some crops may benefit from extra magnesium in fertilizers.


Magnesium in the crop.

Magnesium is a central building block for chlorophyll.




Sulphur in the soil.

Sulphur is present in organic substances and as sulphate. Microbes transform organic sulphur to plant available sulphate. Sulphate is weakly absorbed in the soil and is lost by leaching, although more slowly than nitrate. Sulphate cannot be stored in the soil and must be considered on annual basis. Some decades ago air pollution gave so much sulphur to air and precipitation that sulphur deficiency was rare. This pollution is now so much reduced that it is important to consider the sulphur situation when planning the fertilizer use.


Sulphur in the crop.

Sulphur is a constituent in essential proteins. The plant content is in the same size of order as for phosphorus.


Sulphur and environment.

Sulphur from normal agricultural soils give no environmental problems. It is not acid-forming. However, acid-forming sulphur in air and precipitation causes acidification of soils and waters. The great threat is now very much reduced,  but some sensitive areas are still affected.



For North European conditions mainly manganese, boron and copper are of practical importance. For boron and copper soil analyses are available

The availability of manganese and boron is reduced at higher soil pH values.


General issues of plant nutrition.


Focus on the actual crop or the soil for fertilizer planning.

The actual crop: nitrogen, sulphur, manganese.

The crop in combination with soil status: phosphorus, potassium, boron.

Mainly the soil: magnesium, copper


Foliar application.

Plants can absorb small amounts of nutrients via the foliage. It works best for micronutients as manganese and boron. Smaller doses of nitrogen can also be used, in which case urea works best.


Timing and application.

Mostly it is most efficient to apply nutrients in close harmony with the crop demand, that is in Spring or during the growing season. Autumn application av phosphorus and potassium has been common, but there is a drawback of increased leaching losses of P and on light soil losses and reduced effect of potassium. NPK in Spring is more efficient, especially as placement or side-dress.


Long term consideration.


This is mentioned earlier, but needs to be stressed again. Agriculture should not be only a shorttime business. Agricultural advice and research should focus on helping agriculture with more longterm considerations. It could be about crop rotations, soil organic matter, soil management or the whole organization of the farm production. It is important, in fact necessary, to put prices on as much as possible, for instance rotation  and soil organic matter effects and include this in the economic planning.

There is at least a start in the advisory programme Focus on nutrients.



Crop rotations, soil organic matter and soil life.


Data from soil fertility experiments.

We have in Sweden 8 so called Soil Fertility Experiment, now about 50 years old. There are 2 rotations, one with ley and manure, and one with only crop production. There are also 4 levels of nitrogen and 4 of phosphorus+potassium, all in a complete factorial arrangement. Let us look at time series for crops in common for the two rotations, cereals and sugar beets. We see that the yields in the rotation without ley is lagging behind in most experiments. The difference is now between 4 and 9 percent in different experiments, which means an income difference of more than SEK 1000 per hectare, and the trend is increasing. However, this difference is not found in experiments where the soil organic carbon content is more than 2%.

How can we interprete this? It is no nitrogen effect, which we can deduce from the different nitrogen treatments as well as from protein contents in the crops. Is it pure rotation effects of the ley or a special effect of manure? If so – why not also at the sites with higher soil carbon? There is a small difference in organic carbon between the systems, a couple of tenths of percent after 50 years of different systems. This shows that there is a difference in soil carbon management and consequently in soil life.


Soil life.

A plausible interpretation of the results:

Soil life is more important than generally appreciated. Soil life gives soil structure stability and robustness. And if soil life is maintained automatically the soil carbon management is improved. Visible changes in soil carbon content takes a long time to establish, but only by working towards that goal there is a reward. The practical road-map:

recycling of harvest residues


manure etc

cover crops or autumn crops keeping up production of organic materials also in the autumn

reduced soil management.


Approximate ”critical level” for soil organic carbon: 2% C.

All Swedish soil fertility experiments which are below 2% C  give yield increases for improved soil carbon management as mentioned above.

An international scientific summary report about ”critical soil level”  arrived at the figure 2%.


The interpretation should not be that soils below 2% C are inferior to others. The fact is that in Sweden our most productive soils are low in carbon. But they perform still better with improved carbon management.


High-yielding soils are most dependent on good carbon management and soil life.


This is not what would be expected, firsthand. However, it could be seen like this: To get 810 tons of grain everything must be on top the whole vegetation period, from establishment to grain filling. Neither water nor nutrient should ever be critical. Carbon management and soil life helps.

If 5v tons is the normal yield there are more serious limitations, perhaps the soil profile, which is not easily improved.


Mechanisms för soil organic matter and soil fertility.

Textbooks often say that organic matter helps with water and nutrient management. This is true to some extent but it is not enough. At least for clay soils the effects on soil structure is most important. And that is an effect caused not only by soil organic matter but of soil life. It gives hyphae stabilizing soil aggregates, it gives polysacharides helping with the same thing. In reduces aggregate distruction, increases permeability and aeration.


Cover crops and soil organic carbon.

The Swedish catch crop programme is about catching nitrogen. It should be caught in organic material, in the catch crop growing. When the catch crop is plowed under the soil gets an addition of organic material. A secondary effect is improved soil carbon management. What does that mean? Evidently, it depends on the amount of catch crop material. But it should be noted that in addition to the vegetation we see there is an extra root system which means a lot. Further, a catch crop often means thet the soil management is delayed, which reduces the mineralization of soil organic carbon.


Internationally, there is a important work on cover crops and soil fertility: Denmark, Germany, France, USA. In Norway experiments show important soil carbon effects which gives promise also for Middle Sweden.

One could wish for more data from Swedish experiments. Unfortunately, there are no reliable soil organic carbon data from Swedish catch crop experiments, and many years are needed for such work. But there is the general base for soil organic carbon development, supported by the international work mentioned above.


Soil organic matter, soil carbon management and the environment.

Soil organic matter improvement is environmentally advantageous, as long as it is not excessive. It increases the stability of the soil, reduces soil erosion and in consequence phosphorus losses.

Excessive soil organic matter improvement is almost a theoretical issue for the crop producing agriculture. But if the soil is used for deposision of organic refuse the nitrogen losses might increase. Animal intensive agriculture may approach this situation. But improved carbon management in crop producing agriculture is as a whole environmentally positive.

There are details to discuss, as early incorporation of nitrogen rich organic material, early killing of catch crops etc. Carbon can be managed more or less environmentally friendly.


Agriculture and energy.

This issue is a pedagogical challenge. The food chain as a whole is not energy efficient. Roughly one tenth of the input energy ends up in the food we eat. One main goal of the food production is to give energy to teh population, and against this background the effeciency is regrettable low.

Let us see what happens in different steps of the food chain. We assume an input of 100 MJ as fuel and fertilizers.


The crop production gives about 6 times the input, so we get 600 MJ in plant products.

The animal production consumes energy. We get meat and milk, but only 150 MJ remains

Then we have food industry, logistics, trade and cooking. We get prepared food but only 20 MJ remains.

20 MJ can be put on the table. But there are remains and losses, and only half of this is actually consumed.

Which means 10% of the input.

But the odd thing is that ”the energy demanding fertilizers” often are blamed for this inefficiency.


Let us instead look at prospects.

With one kg of nitrogen in fertilizers, which has cost about 40 MJ to manufacture, we produce 30-50 kg extra plant matter. Let us assume 30. Total energy content about 500 MJ. We assume that it is used for producing refined biogas. With available technology we get 250 MJ of biogas. We take 40 MJ to compensate for the fertilizer and 210 remain for the society.


Nitrogen fertilizer, properly used, helps catching solar energy and is a kind of energy amplifier. By means of biogas or other bioenergy technology agricultural lands can be an important energy producer. And – that can be achieved without compromising the ordinary agricultural production. Three resources are available in addition to the system of today;


  1. Cover crops giving an extra production in the autumn. In south Sweden 3-4 tons dry matter can be produced with proper management.
  2. Cover crops are incorporated in the soil and helps up the soil carbon balance to the extent that harvest residues can be used for bioenergy.
  3. Break crops for bioenergy in the crop rotation. They cost a year of ordinary agricultural production, but give important compensation by improving yields of the other crops.


It might be added that energy production (ley harvest) can be possible on marginal lands where open landscape is a part of the production. This could be a complement to grazing animals.


1 000 000 ha * 2000 kg * 2 kwh = 4000 000 000 kwh =4 Twh



The very idea of agriculture is to replace the natural diversity of plants with i crop which can be better utilized by man. Main task: to fight biodiversity, at least on the cultivated fields.

However, biodiversity is of value for the society and is one of the Swedish environmental goals.

It has also some positive components for agriculture: pollination, more robust biological systems