How much is the organic matter (OM), the cation exchange ratio (CEC) and the nutrient content of my existing soil is crucial for my saplings to thrive. Inadequate soil nutrient and soil physical parameters will require application of fertilization and soil conditioning.
Durian trees can be grown successfully on a wide range of soils, but optimum growth and fruiting occurs on fertile, deep (without subsurface bedrock), well-drained sandy loam or clay loam which is rich in organic matter.
The most commonly used type of soil are alluvial soils derived from basaltic rocks but durian trees grown on red lateritic soils (which is the case with my land) can also be successful when adequately fertilized with NPK fertilizers and heavily amended with organic matter.
A pH range of 5.5 to 6.8 is considered ideal. At higher pH levels, iron and zinc deficiencies are commonly observed while at lower pH levels, some nutrients could not absorbed by the root system due to low CEC value which I will discuss in the below paragraphs.
I've sent an email to the Malaysian Agriculture Research and Development Institute or better know as MARDI asking them for a quote on the soil test services they provide and procedures for sampling. What you see below is the first page of the test application form.
I had chosen to test all parameters under Major Elements, Trace Elements and Soil criteria as per the below 2nd page of the form. MARDI Lab charges for each test parameters meaning the more parameters you choose to test, the more you need to pay.
The test results will normally be available after 4 weeks of submitting the soil sample. As the laboratory only conducts testing, the interpretation and conclusion of the tests shall be brought forward to the MARDI Soil Science Department for further discussion. This is however optional and is not included in the test price.
My land consist of three (3) different soil types in terms of physical appearance and location. The samples are prepared in accordance with MARDI Lab soil test submission requirements as described:
Soil sample weight shall be at least 200 grams or more
Soil sample shall be sent dry and not wet or damp
Soil sample shall be sent in closed containers and marked
Any non-compliance with the above prerequisites which causes additional task or usage of lab consumables not normally required to be use, the lab will impose additional charges.
Soil Location: On terraces
The first soil type can be observed on all the terraces. Physical appearance is orange in color, sandy with friable and crumbly texture. Smells a bit chemical compound like and does not smell anything like a forest floor.
Not high on organic content considering the color is not dark. Easy to work with but may be a tad water logged if it rains heavily. This soil is an underneath layer of the actual topsoil. The topsoil was removed during the terracing works by the excavator.
Soil Location: On terrace slopes
The second soil type is the native topsoil. This soil is observed at the slopes of the terraces which was untouched by the excavator during the earth work and terracing.
Coloration is grayish with obviously higher organic content. Sandy loam, friable and crumbly to the touch. Organic debris are mixed with the soil. Smells quite the same as the first type but less chemical like.
Soil Location: At the valley
Highest sand content of the above two types of soil with also the highest organic decomposition as seen by the naked eye. Dark in color during damp or wet condition but turns to grayish when the soil is dry.
The soil is mixed with small gravel and silt content.
A sample of the soil test result for one of the test samples can be seen below. Depending on the test parameters that was requested on the test form, this result sheet will be different.
For clarity, this result sheet is for the soil sampled from one of the terrace:
Soil Test Parameter: Cation Exchange Capacity (CEC)
It can be seen from the result page that the CEC value is 8.50 meq/100gm. The CEC unit used by soil testing laboratories to report this parameter is milliequivalents per 100 grams of oven-dry soil.
Soils with a high CEC tend to be more fertile and are capable of providing more nutrients to crops. A high CEC value (>25) is a good indicator that a soil has a high clay and/or organic matter content thus can hold a lot of cations.
A soil with a low CEC value (<5) is a good indication that a soil is sandy with little or no organic matter thus could not hold many cations. The implication of this is that nutrient exchange between the root surfaces and the soil is greatly reduced or is impaired. In other words, the roots could not feed-off the soil/fertilizer nutrients.
Based on the above argument, applying fertilizer heavily will nor proportionately increase the nutrient update of the roots. The fertilizer will just leach away and that would be a waste in terms of fertilizer costs.
As a conclusion, my soil CEC is at the below mid range value. In order to improve on this parameter, there are 2 common ways to do it based on my technical readings:
Increasing the pH value of the soil. This can be made by applying horticultural lime around the radius of the tree canopy. Application is normally at 6 months intervals. It could be applied more often on a case by case basis.
Applying finish compost or mulch heavily around the tree canopy. Soils which are high in earthworm population will decompose mulch fairly quick, mulching should then be applied regularly.
Soil Parameter: Carbon
Plants mostly acquire its carbon requirements for growth from carbon dioxide in the air. Some of the carbon is also absorbed from organic carbon in the soil.
These carbon makes up most of the building blocks for the plant to grow new leaves, stems and roots.
Practicing mulch application and applying compost will increase the organic carbon content in the soil. Naturally occurring decomposition of forest debris also does this.
Burnt soil which is a by product of organic matter open burning could also be scattered on soil surfaces to introduce carbon. It is however, not recommended to use fairly new burnt soil since the pH is quite alkaline which is good to use if you have very acidic soil.
I normally gather burnt soil either from my own house backyard where I burn plant material or soil underneath the burn pile at the orchard. A good practice is not to mix household garbage with plastics to the burn pile so that the soil harvested is not contaminated with synthetic or chemical compounds.
Biochar could be applied to the same affect as well. In Malaysia, biochar is mostly produced by burnt rice husks.
Based on my limited reading on the matter, plants rarely have insufficient carbon requirements for growth as such, the carbon content test parameter is not vital as nitrogen which is discussed below.
Soil Parameter: Nitrogen, Potassium, Phosphorus
These 3 major elements are the most consumed nutrient by plants.
The source of nitrogen is taken from the soil but for nitrogen fixing plants such as legumes, this major nutrient is sourced from the air and kept in the roots.
Lack of nitrogen will result in chlorosis of the leaves since plants use nitrogen to produce chlorophyll.
The soil test result shows the nitrogen content to be at 0.21% which is very low.
Potassium has many different roles in plants. It is use by plants to regulate the opening and closing of the leaves stomata therefore regulates the CO2 uptake.
Based on the soil test results, the potassium content of my soil is almost non-existent.
The situation is the same with the phosphorus content. Phosphorus is use by plants to grow roots and help converts other nutrients into usable building blocks for which the plant grows.
The routine method to increase nitrogen, phosphorus and potassium levels in soils is to apply either chemical N-P-K granular fertilizers or chemical foliar sprays. Organic based fertilizer will also have the same effect.
Other method that I used to increase phosphorus content is by applying rock phosphate. The type of rock phosphate that I use is Christmas Island Rock Phosphate or better know as CIRP.
Another method that I used less often to increase both the potassium and phosphorus content of the soil around the radius of the trees or saplings canopy is by applying MKP (monopotassium phosphate) fertilizer. It is however, advisable to apply MKP as a foliar spray as opposed to soil application since the risk of overdosing is higher when soil applied.
I discussed in detail the use of MKP in my blog entry here.
The effects of N-P-K fertilizers or chemical foliar sprays on the plants could be seen within a few days whereby for organic fertilizers the effect will be seen after 3 weeks in most cases.
In conclusion, as a durian grower I shall proceed to continuously amend my soil with various sources of organic matter while at the same time to periodically drop organic, chemical compound fertilizers and foliar sprays for the immediate feeding needs of the durian trees and saplings.