I live in and have primarily worked in subtropical.  High rainfall areas with frequeny of storms have potential to flush or move or relocate nutrients such as in flooding.  Higher temperatures increase rates of geological and orgacic weathering, speeding nutrient cycling rates.  Where trees and vegetation are removed, what is remaining to store nutrients?  The trees, plants and organics on soils are the primary nutrient storage mechanisms.  Where evidence suggests declines due to cropping, no till or adjusted tilling may help.  Where trees are harvested, partial harvest or rapid replanting.  Compaction and other modifications may not affact fertility, but impact production in other ways, such as confining rooting depth and plant health.  Some soils have natural fertility issues, and others such as spodisols have limiting factors that even fertilization is unlikely to improve.  

I would say pH. In tropical lands, due to high preciptations levels and organic matter concetrations, there is a trend to low pH levels. Almost all native species are adapted to this condition. However, when we think in agriculture use of land, the pH has a high influence in productivity, due to the metal leves disponibility in soils.

In degraded lands the soil exchange capacity has a high importance too, due to the nutrient wash, by the preciptation. I hope it helps. Best regards

 I appreciate the good question of Dr.Ibrahim and responses of Drs.Hansen and Fengler.I agree with many good points of Hansen.In tropical soils, soil erosion is a major problem as year after year  a few inches of fertile top soil is lost.If soil erosion is controlled both soil and water are conserved.Because of high temperature the soil organic matter gets oxidized.Maintaining good amount of organic matter in soil is a challenge in tropical soils.Because of high intensity rains the basic cations may also be leached leading to decrease in soil pH.Also because of high temperatures and consequent high evapo-transpiration the salt may also builds up in some soils.Maintaining continuous crop cover or vegetation cover on land is important. But the limitations of low rainfall and high summer temperatures may pose problem.

Destructive factors for fertility in tropical soils are

i) high microbial decomposition rate of organic materials because of suitable temperature and soil moisture,

ii) high rainfall and thus erosion loss of soils

iii) higher cropping intensity

iv) imbalanced fertilizers use by the farmers

v) removal of crop residues

vi) addition of inadequate amounts of fertilizers than taken up by the crops

Causes for declining soil fertility are

1.Continuous cropping

2.Excessive free grazing

3. Little inorganic and organic fertilizer/manure addition

4. Little or no agronomic management, particularly  cropping systems

5. Soil erosion

6. Land Feature

Most destructive factor,  I feel pH,  influenced so heavily by the amount of rainfall , geomorphology and mineralogy  , thereby , soils are highly acidic , low CEC characterized by low activity clays ( Predominantly kaolinitic in nature) , sandy loam-loan texture with varying depths . But , despite all , richest centres of biodiversity as hot spots globally.

Dear all,

I thank you very much for your very useful comments/answers.

As Professionals, you are a wonderful people.

let's keep the flag flying.

El-ladan.

Tropical soils are least managed and over exploited. Principles of rotation (cereal-legume) not followed. Return of residue in soil is negligible. Every cycle a part of soil carbon is lost. As soil carbon governs the soil physical, chemical and biological properties, hence temperature is the major concern in destructing soil.

The fertility deteriorating factors differ in different soil environments varying from  arid ,semiarid ,sub-humid  to humid tropics.We have to group them according to magnitude of their effect or problem in the different environments.

Interesting . Is it not so surprising , tropical soils despite having so many  soil -related  constraints , continue to be the major biodiversity centres and  supportive to major crop production centres as well. How does it happen , where such resilience in soil comes from,  to be so crop responsive/supportive?

Very good question. The fertility deteriorating factors differ in different soil  and many environments factors change the soil health and fertility. The major factor is 1.Excessive use of inorganic fertilizer, long term same cropping system, excessive grazing and less use of organic manure.

 The basic destructive factors are:

1. Physical factors, e.g. loss of fertile top soil due to water or wind erosion.

2. Chemical factors e.g. depletion of nutrients or the toxicity due to acidity or alkalinity (salinization) or water logging.

3. Biological factors which affect the micro-flora and reduce the microbial activity of the soil. These factors reduce the yield.

Some other factors as deforestation, extensive cultivation on marginal land, improper cultivation practices like mono-cropping, poor manuring, misuse of fertilizers or excess use of fertilizers, excessive irrigation, over-grazing, fragility of soil, adverse weather and mining may accelerate the process of soil degradation.

Intensive cultivation of tropical soils with little or no return of organics, Indiscriminate and imbalanced use of chemical fertilizers has resulted huge loss of carbon from organic matter, resulting in to poor soil physical, chemical and particularly biological activities, resulting faster decrease in soil fertility of the tropical soils, particularly in last 60 years. The pressure on the tropical soils are much more due to increased population pressure. 

High rainfall, temperature and excessive use of land for agricultural production.

Dear Ibrahim,

In addition to various factors highlighted by other colleagues, I wish to emphasize on the following fertility destructive factors of tropical soils:

(1) Soil erosion resulted from growing crops on steeper slopes has lowered soil productivity - both in the hills and in the valleys - to a point where the populations could no longer survive in that area. Today only empty ruins remain. Billions of tons of soil are being physically lost each year through accelerated erosion from the action of water and wind and by undesirable changes in soil structure.

(2) Soil compaction is another destroyer of the soil fertility in tropics. Sometimes it results from repeated passes over the same field with heavy machinery, particularly when the field is wet. It can also result from the hooves of grazing animals pounding down the soil too often in the same area. Compaction is not easy to correct.

(3) Salinization, waterlogging and pollution: Many soils of the tropics are being degraded by salinization, waterlogging, and/or by pollution through the indiscriminate application of chemical and industrial wastes. Salinization or the accumulation of salts in the topsoil, can also have a deleterious effect on soil productivity and crop yields. In extreme cases, damage from salinization is so great that it is technically unfeasible or totally uneconomic to reverse the process. Today a more serious problem in several highly industrialized countries is the indiscriminate dumping of chemical wastes, some of which are extremely toxic to plants, animals, and man, and the growing use of sewage sludge, some of which contains dangerous heavy metals which can be taken up by plants.

I hope you and my other RG friends find this input interesting and useful.

Great answer by Dr. Kundu. Compaction is a huge phenomenon where puddling happens to be a continuous phenomenon during rice cultivation. In rabid/winter season mechanical manipulation further breaks the aggregates and results faster degradation of the soil organic matter, resulting faster deterioration of the productivity of the tropical soils. 

Some good responses from Dr Malhotra , Dr Kundu , worth appreciating . Endorsing the comments from both of you , loss biological properties of soil ( could be triggered by the factors what our colleagues have narrated) , the major drivers of soil fertility . The another ramification of such loss in soil fertility is very often seen for the loss of biodiversity as well, in  any given location .