Soil and Nutrient Conservation for Slopeland Areas

John Jeff Palmer
Director
Asian Rural Life Development Foundation and
Mindanao Baptist Rural Life Center, 1998-09-01

Introduction

The uplands of the world are among the most fragile ecosystems known today. This pertains particularly to the uplands of the tropical, wet areas of Asia and the Pacific. However, unadulterated by high populations and predetermined exploitation, most upland ecosystems can meet an equilibrium in which man and nature live in harmony. For instance, in many cases of upland tribal minorities throughout the Philippines, sustainable and harmonic systems have been created for utilizing and maintaining the balance of nature and thus nutrients in their respective areas.

In Asia, in particular, high population densities and high demand for natural resources and production have resulted in a rapid degradation of the natural resources and reduction of the possibility for sustainable ecosystems. This paper, in particular, will try to analyze the topic of sustainable upland ecosystems from the perspective of nutrient balancing and conservation. Moreover, a special emphasis will be placed on crop-livestock integration as it relates to this nutrient balancing.

To have a common frame of reference, a few terms need to be defined.

Slopelands/uplands. Any sloping land with a slope of over 18% or 10 degrees. This is a standard Philippine Government definition.

Agroforestry. A type of farming system utilizing both agriculture and forestry components (trees) within the same growing area. In short, trees grown together with agricultural crops.

Sustainable System. Any system giving acceptable production while at the same time conserving the resources upon which that production depends.

Nutrient Balancing in a Sustainable Farming System

In any sustainable system, the laws of nature teach that the inputs of that system should be greater than, or equal to, but never less than, the outputs of the given system (Fig. 1).

There are two major types of nutrient outputs from the upland ecosystems where man is involved in agricultural production: loss and removal. Nutrient loss includes those due to soil erosion, leaching, and volatilization, among others. These are generally considered detrimental outputs and should be minimized whenever possible.

Nutrient losses due to removal include crop harvest, timber/fuel wood harvest, and animal feed harvest, among others. These forms of nutrient removal are usually considered beneficial to human survival and are generally encouraged to be maximized for greater benefit to the upland inhabitants.

As nutrient loss are non-beneficial, they pose the greatest threat to upland ecosystems. Documented cases of annual losses of 200 mt/ha per year have shown instances when tremendous amounts of N, P, and K, not to mention micronutrients and other soil properties, have been lost to upland system.

Nutrient inputs to upland (as well as other) ecosystems basically fall into two categories: imported and natural. Imported nutrients generally come from sources such as commercial fertilizers, animal manures/waste products produced off-site, and mulching/ compost materials grown or produced off-site. Natural inputs of nutrients or those readily occurring on-site via nature and natural processes include nutrient additions via air, dust, lightning (in the case of N-fixation), animal manures and nitrogen fixation. Additional nutrients sometimes occur from physical causes such as erosion.

In returning to the definition and laws of a sustainable farming system, any system which minimizes nutrient losses while maintaining acceptable removal output and providing an adequate level of nutrient inputs, can be considered a system moving towards sustainability.

Minimizing Nutrient Losses

The primary cause of nutrient losses in the uplands is soil erosion. The Universal Soil Loss Equation (USLE) was developed to help estimate the amount of soil erosion under given conditions. For our purposes here, let us utilize the equation to explore erosion and its causes, and then apply this knowledge back to the problem of stopping nutrient loss in the uplands.

A = RKLSCP, where . . .
A = Annual Soil Loss in mt/ha
R = Rainfall factor
K = Soil erodibility constant
LS = Slope percentage and length factor
C = Cropping/cover factor
P = Cultural practice

The rainfall factor increases with the amount and intensity of rain. Thus, the higher the amount or intensity of rainfall, the greater the given potential for soil erosion and thus nutrient loss. In the strictest terms, the raindrop itself is the initiator of the erosion process. Anything that can be done to stop the impact of the raindrop on the bare soil surface will greatly reduce the chances for erosion. Research in many parts of the world has shown that high levels of mulching on steep slopes with no-tillage farming methods have yielded almost zero erosion.

The soil erodibility constant is dependent on the type of soil undergoing the erosion process. The higher the erodibility of a soil, the higher the potential for soil erosion. For instance, a tightly-bound clay has a much lower erodibility factor and thus is more resistant to the external forces favoring erosion than a sandy soil.

The length/slope factor of the USLE basically says that the steeper and the longer a slope is, the more potential there is for erosion.

The "C" in the USLE represents the cover factor. Certain plants give a better cover than others in protecting the soil against erosion. Those ecosystems with a higher C factor are the ones that are considered more erosive than others. For instance, a virgin tropical rain forest has a very low C factor (0.001) as opposed to a field cropped with conventional soybeans (0.373). A cover factor of 1.0 signifies a bare soil exposed to all the elements of nature.

The "P" factor represents the cultural practices applied to the measured ecosystem. Is the soil tilled or not? If tilled, is it done on the contour or up and down the hill?

Setting aside the USLE, in its most basic form in the humid tropics, soil erosion is a result of the combination of the raindrop splash loosening the soil and moving water with the loosened soil away from its primary site of origin. Thus the two major factors contributing to the loss of soil and thus nutrients in the uplands are: 1) the raindrop splash which loosens the soil and 2) the moving water which carries the soil and nutrients.

Therefore, to minimize and control soil erosion practice two basic steps: 1) stop the raindrop splash from contacting bare soil and 2) slow down the moving water from the sloping lands and allow infiltration. Anthony Young calls these two methods to erosion control as the cover and barrier approach ( see Fig. 2).

Most conventional soil conservation schemes heavily rely upon the barrier approach to control soil erosion. When talking about soil erosion control, most people envision terraces, vegetative barriers, contour ditches, etc. However, it should be noted that this is only one approach to soil and nutrient conservation in the uplands. In fact, the more important control factor may lie in the methods of cover approach which, while protecting the soil from erosive tendencies, enriches the soil in terms of the physical factors (e.g., soil organic matter, soil moisture and soil organism activity). The list below enumerates soil erosion control measures in the uplands:

The Mindanao Baptist Rural Life Center (MBRLC), mainly through the SALT technologies and agroforestry techniques, recommends a balanced barrier and cover approach. Along the contour, grow the N-fixing plants such as Flemingia macrophylla and Desmodium rensonii in double hedgerows. This is to act as a barrier to soil erosion and serve as a mulch source which in turn can be applied to the ground surface for enhanced soil cover.

Whatever the choice of erosion control measure to halt nutrient loss from the slopelands, it is highly recommended that both the barrier and cover needs of an effective erosion control system be addressed.

Maximizing Nutrient Inputs

In maximizing nutrient inputs, only those naturally occurring will be discussed in this portion. However, commercial fertilizers can be highly beneficial if used properly. The effect of their continued use on the sustainability factor may be questionable. Nitrogen-based fertilizers are formed through the burning of fossil fuels. Phosphate and potassium admendments are derived through mining and importing of minerals. Therefore, the main bulk of the input discussion will focus on locally available and sustainable inputs to the upland farmer.

The four main methods of natural nutrient input are: 1) air, dust, etc., 2) animal manures, 3) biomass (primarily nitrogen fixing plants), and 4) crop residues. Since the inputs coming from air, dust, etc. are fairly fixed per ecosystem and fairly non-adjustable, the major concern here will be the other three items.

Animal manures.

Animal manure can provide excellent sources of nutrient inputs into an upland ecosystem (Table 1). Many cultures have proven for years the sustainability and wisdom of utilizing animal wastes for increased production. Nutrients converted primarily from plant derivations can be readily applied to and absorbed by most systems. However, a question would be how much manure can be produced and what would be the most suitable animal husbandry systems for utilizing the manure generated as a main nutrient input source? MBRLC has focused for a number of years on the cut and carry system which holds great potential for making manure readily available to a farming system.

Biomass of nitrogen-fixing plants.

Any plant biomass can potentially add nutrients back to a system. However, deep rooted plants and nitrogen-fixers have a much higher and proven potential. Deep rooted plants have been documented to "mine" nutrients from lower levels and bring them to the surface where they can be more readily useful in the production system. Nitrogen-fixing plants have been well documented in giving additional nitrogen to production systems.

In terms of nitrogen balancing, the nitrogen-fixing plants (NFPs) may have the most overlooked potential in all of agriculture. Whether grown and applied directly or incorporated with the soil as a mulch, or whether fed to animals and returned to the system as manure, nitrogen-fixing plants have the greatest potential in the humid tropics to act as a source of renewable nutrients to the fragile upland ecosystem. They can be used as a barrier in soil erosion control. Their nitrogen-rich mulch can help alleviate the problem of reliance on costly inorganic nitrogen-based fertilizers. Moreover, being a proven source of excellent animal feed, they can be incorporated into the system as forage.

Some of the best examples of upland farming systems based on nitrogen-fixing plants are the SALT technologies generated by the MBRLC. For over twenty years, MBRLC has been utilizing plants such as Leucaena sp., Flemingia macrophylla, Gliricidia sepium, and Desmodium rensonii to promote sustainable upland systems where nutrient input and output are balanced to give long-term, acceptable production.

At the MBRLC, a typical SALT hedgerow system for one hectare can produce an average of 30 to 40 mt of fresh, nitrogen rich biomass per year. This biomass, when calculated into actual nutrients, can yield an additional 230 kg of N, 22 kg of P, and 72 kg of K per hectare. These nutrients are adequate to sustain production of 2.5 mt/ha of maize annually.

Crop residue management

Crop residues do not necessarily give positive inputs back into the production system. However, mismanagement of crop residues can have a detrimental effect on the system. Going back to the law of sustainability, any nutrient output from a system results in a corresponding need for nutrient inputs equal to or greater than the output. Therefore, when crop residues are burned, utilized for animal feeds, or in any other way removed from the system, there will have to be the equivalent replacement of nutrients. Table 2 shows the potential nutrient removal by showing the actual N, P and K in certain crop residues.

Any removal of the crop residues from a system will result in a nutrient deficit requiring a corresponding nutrient input. Therefore, even though use of crop residues for animal feed may be beneficial to animal production, manures and other sources of biomass need to be returned to the system in sufficient quantities to assure nutrient balancing and therefore an increased chance for sustainability.

Summary and Recommendations

Maintaining a balance between nutrient output and input is crucial to the idea of sustainability in the uplands. Since sustainability is dependent upon acceptable production with maintenance of the resources giving that production, all detrimental nutrient losses must be minimized while nutrient inputs of the natural type must be maximized.

The primary culprit of nutrient loss in upland systems is soil erosion. Sustainable upland systems should concentrate on minimizing the effects of soil erosion, at the same time utilizing methods of sustainable nutrient inputs. Nitrogen-fixing trees provide an excellent potential for use as erosion control barriers, as well as good sources of nutrient inputs to cropping and animal systems.

General Recommendations

1. Education of people on nutrient balancing, especially as regards upland farming systems of the humid tropics, is recommended.
2. In worldwide thinking of erosion control, there needs to be a shift from barrier control to a balanced barrier and cover control.
3. More attention should be given to nitrogen fixing plants and their potential in contributing to sustainable farming systems.

References

El-Swaify, S. A., Moldenhauer, W. C., and Lo, Andrew (eds.). 1985. Soil Erosion and Conservation. Soil Conservation Society of America. Akeny, Iowa, USA.
International Institute of Rural Reconstruction. 1989. Agroforestry Technology Information Kit. IIRR, Silang, Cavite, Philippines.
MBRLC Editorial Staff. 1991. How to Farm Your Hilly Land without Losing Your Soil: Sloping Agricultural Land Technology (SALT 1). How to Series No. 1. Davao del Sur: MBLRC. 24 pp.
MBRLC Editorial Staff. 1987. How to Make FAITH (Food Always in the Home) Garden in your Homeyard. How to Series No. 2 Davao del Sur: MBLRC. 21 pp.
MBRLC Editorial Staff. 1989. How to Farm Better. How to Series No. 3. Davao del Sur: MBLRC. 67 pp.
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MBRLC/MF-WN/IIRR. 1990. Resource Book on Sustainable Agriculture for the Uplands. Davao del Sur: MBLRC.
Palmer, J. Jeff. 1996. Sloping Agricultural Land Technology (SALT): Nitrogen Fixing Agroforestry for Sustainable Soil and Water Conservation. A publication of the Mindanao Baptist Rural Life Center (MBRLC). 59 pp.
Young, A. 1989. Agroforestry for Soil Conservation. UK: CAB International.